Medial Temporal Region
Last Updated: August 20, 2020
OBJECTIVE: To describe the surgical anatomy of the anterior, middle, and posterior portions of the medial temporal region and to present an anatomic-based classification of the approaches to this area.
METHODS: Twenty formalin-fixed, adult cadaveric specimens were studied. Ten brains provided measurements to compare different surgical strategies. Approaches were demonstrated using 10 silicon-injected cadaveric heads. Surgical cases were used to illustrate the results by the different approaches. Transverse lines at the level of the inferior choroidal point and quadrigeminal plate were used to divide the medial temporal region into anterior, middle, and posterior portions. Surgical approaches to the medial temporal region were classified into four groups: superior, lateral, basal, and medial, based on the surface of the lobe through which the approach was directed. The approaches through the medial group were subdivided further into an anterior approach, the transsylvian transcisternal approach, and two posterior approaches, the occipital interhemispheric and supracerebellar transtentorial approaches.
RESULTS: The anterior portion of the medial temporal region can be reached through the superior, lateral, and basal surfaces of the lobe and the anterior variant of the approach through the medial surface. The posterior group of approaches directed through the medial surface are useful for lesions located in the posterior portion. The middle part of the medial temporal region is the most challenging area to expose, where the approach must be tailored according to the nature of the lesion and its extension to other medial temporal areas.
CONCLUSION: Each approach to medial temporal lesions has technical or functional drawbacks that should be considered when selecting a surgical treatment for a given patient. Dividing the medial temporal region into smaller areas allows for a more precise analysis, not only of the expected anatomic relationships, but also of the possible choices for the safe resection of the lesion. The systematization used here also provides the basis for selection of a combination of approaches.
The medial temporal region is the site of the most complex cortical anatomy. It is hidden deep within the remainder of the temporal lobe and ventricular system in the margin of the basal cisterns and is surrounded by vascular and neural elements that, unless required for treatment, must be preserved during surgery. Selecting and completing a surgical approach to the medial temporal region remains a challenge because of this anatomic complexity and deep location. There are many articles focusing on the anatomy, physiology, and surgical approaches to the medial temporal region (1, 2, 4, 7–11, 13–28, 30, 33, 34, 36–38, 40–42, 45–48, 52, 53). This study had three goals: to examine the surgical approaches to the area, to divide the approaches into groups based on their anatomic characteristics, and to review the advantages and disadvantages of the approaches in relation to the part of the medial temporal region to be accessed.
MATERIALS AND METHODS
Ten adult, formalin-fixed, cadaveric heads and 10 brains were studied. Coronal cuts, at the level of the apex of uncus and inferior choroidal point, were performed in the 10 brains and distances between the temporal lobe surface and temporal horn in each approach were measured. Ten silicon-injected heads, dissected using the magnification of the surgical microscope (×3–40; Carl Zeiss, Inc., Thornwood, NY), were used to demonstrate the surgical approaches. Clinical examples of selected approaches are presented.
The surgical approaches to the medial temporal region, based on the temporal surface, through which the approach is directed, are divided into four groups: superior, lateral, basal, and medial. The superior group includes only the transsylvian-transinsular approach. The lateral group includes the approaches through the sulci and gyri on the lateral surface of the temporal lobe and anterior temporal lobectomy with amygdalohippocampectomy. The basal group is comprised of the approach through the occipitotemporal, collateral, or rhinal sulci or through the fusiform and parahippocampal gyri. The medial group is subdivided in an anterior variant, the transsylvian transcisternal approach, and posterior variants, the occipito interhemispheric and supracerebellar transtentorial approaches.
The temporal lobe is located on the lower part of the hemisphere and rests on the floor of the middle fossa and the tentorium (Fig. 1). The lateral surface of the temporal lobe forms the lower portion of the lateral convexity. It sits medial to the temporal fossa, the site of the temporalis muscle, and below the superior temporal line along which the muscle attaches. All of the middle and anterior parts of the lateral surface sit within the area outlined by the squamosal suture, deep to the squamosal part of the temporal bone, but its posterior part extends back beyond the squamosal part of the temporal bone into the area deep to the parietal bone (Fig. 1A). The temporal horn is positioned approximately 2 cm above the zygomatic arch, deep to the squamosal part of the temporal bone and the middle temporal gyrus. The inferolateral border of the temporal lobe, which marks the transition between the lateral and basal surfaces, is positioned at the level of the superior border of the zygomatic arch and supramastoid crest (Fig. 1, A and B). The shape of the floor of the middle fossa changes from being a spoon-shaped concavity anteriorly to flat with a gradual superior inclination from lateral to medial in its posterior portion. The position of the anterior tip of the temporal horn, as viewed through the basal surface, approximates a coronal line that passes at the level of the foramen ovale laterally and the entrance of the oculomotor nerve into the roof of the cavernous sinus medially. The concavity of the middle fossa adds to the difficulty in exposing the basal surface of the temporal lobe and, if not managed appropriately, may result in retraction injuries to the lobe.
Temporal Lobe Anatomy
The temporal lobe is separated from the frontal lobe by the stem and posterior ramus of the sylvian fissure; from the parietal lobe by the posterior ramus of the sylvian fissure and the extended sylvian line, which extends backwards along the long axis of the sylvian fissure; from the occipital lobe laterally by the inferior part of the lateral parietotemporal line, which runs from the impression of the parieto-occipital sulcus on the lateral surface to the preoccipital notch, and on the lower surface by the basal parietotemporal line, which extends from the junction of the calcarine and parieto-occipital sulcus to the preoccipital notch; and from insula by the inferior portion of the limiting sulcus of the insula (Fig. 2). The temporal lobe has four surfaces: medial, superior, lateral, and basal, each of which is the site of one of one or more approaches to the medial temporal region. The four surfaces meet anteriorly at the rounded temporal pole.
The medial surface of the temporal lobe is the most complex cortical area (Figs. 2–4) (12). It is formed predominantly by the rounded medial surfaces of the parahippocampal gyrus and uncus and is limited laterally by the collateral and rhinal sulci. The medial surface is composed of three longitudinal strips of neural tissue, one located above the other, which are interlocked with the hippocampal formation and amygdala. The most inferior strip is formed by the rounded medial edge of the parahippocampal gyrus, the site of the subicular zones; the middle strip is formed by the dentate gyrus, a narrow serrated strip of gray matter located on the medial surface of the hippocampal formation; and the superior strip is formed by the fimbria of the fornix, a white band formed by the fibers emanating from the hippocampal formation and directed posteriorly into the crus of the fornix. The parahippocampal and dentate gyri are separated by the hippocampal sulcus, and the dentate gyrus and the fimbria are separated by the fimbriodentate sulcus. The amygdala and hippocampal formation lie just beneath and are so intimately related to the mesial temporal cortex that they are considered in this section.
The parahippocampal gyrus deviates medially at the site of the uncus, which commonly projects above the tentorial edge and is separated from the remainder of the basal surface by the rhinal sulcus. The uncus, the medially projecting anterior part of the parahippocampal gyrus, when viewed from above or below has an angular shape with anterior and posterior segments that meet at a medially directed apex. The anterior segment of the uncus faces anteromedially and the posterior segment faces posteromedially. The anterior segment has an undivided medial surface, but the posterior segment is divided into upper and lower parts by the uncal notch, a short sulcus that extends from posteriorly into the medial aspect of the posterior segment (Figs. 2E, 3B, and 4B). The medial face of the anterior segment faces the proximal part of the sylvian and carotid cisterns and the internal carotid and proximal middle cerebral arteries. The posterior segment faces the cerebral peduncle and, with the peduncle, forms the lateral and medial walls of the crural cistern, through which the posterior cerebral, anterior choroidal, and medial posterior choroidal arteries pass. The optic tract passes above the medial edge of the posterior segment in the roof of the crural cistern. The amygdaloid nucleus, commonly referred to as the amygdala, forms almost all of the interior and comes to the medial surface of the upper part of the anterior segment (Figs. 1, F–H and 4, D–F). The upper part of the posterior segment is formed largely by the medial aspect of the head of the hippocampus. The apex, where the anterior and posterior segments meet, points medially toward the oculomotor nerve and posterior communicating artery. The head of the hippocampus reaches the medial surface in the upper part of the posterior segment at the anterior end of the dentate gyrus. Within the ventricle, a small medially projecting space, the uncal recess, situated between the ventricular surface of the amygdala and hippocampal head, is located lateral to the uncal apex (Fig. 1, F–H).
The lower surface of the superior lip of the uncal notch is visible from below only after removing the lower lip formed by the parahippocampal gyrus (Fig. 4C). The posterior segment is occupied by several small gyri that are continuations of the dentate gyri. The inferior choroidal point, the lower end of the choroidal fissure along which the choroid plexus is attached, is located just behind the upper edge of the posterior uncal segment, immediately behind the head of the hippocampus, at the site where the anterior choroidal artery passes through the choroidal fissure to enter the temporal horn (Figs. 1–5). The anterior choroidal artery arises near the midlevel of the anterior segment and hugs its surface, sloping gently upward, unless extremely tortuous (Figs. 3 and 5). It continues to ascend as it courses posteriorly around the uncal apex and reaches the upper part of the posterior segment, where it passes through the choroidal fissure at the inferior choroidal point. The dentate gyrus, named for its characteristic toothlike elevations, extends posteriorly from the upper part of the posterior segment and has the most prominent denticulations anteriorly.
The amygdala is located largely within the boundaries of the uncus (Figs. 1 and 4). It forms the anterior wall of the temporal horn. Superiorly, the amygdala blends into the claustrum and globus pallidus without any clear demarcation (Fig. 3D). The upper posterior portion of the amygdala tilts back above the hippocampal head and the uncal recess at the anterior edge of the roof of the temporal horn. In coronal cross-sections, the optic tract sits medial to the junction of the amygdala and globus pallidus (Fig. 3D).
The hippocampus, which blends into and forms the upper part of the posterior uncal segment, is a curved elevation, approximately 5 cm long, in the medial part of the entire length of the floor of the temporal horn (Fig. 1). It has the dentate gyrus along its medial edge and a curved collection of gray matter in its interior that is referred to as Ammon’s horn. It sits above and is continuous below with the rounded medial surface of the parahippocampal gyrus referred to as the subicular surface. Ammon’s horn is characterized in transverse sections of the hippocampus by its reversed c, or comma shape, associated with its tightly packed pyramidal cell layer.
The hippocampus is divided into three parts: head, body, and tail (Fig. 1). The head of the hippocampus, the anterior and the largest part, is directed anteriorly and medially and forms the upper part of the posterior uncal segment. Its upper surface is the site of three or four shallow hippocampal digitations, making it resemble a feline paw and giving it the name pes hippocampus. The initial segment of the fimbria and the choroidal fissure are located at the posterior edge of the hippocampal head. Superiorly, the anterior part of the head of the hippocampus faces the posterior portion of the amygdala that is tilted backward above the hippocampal head at the anterior edge of the roof of the temporal horn. Anterior to the hippocampal head is the uncal recess, a cleft located between the head of the hippocampus and the amygdala. The body of the hippocampus extends backward along the medial part of the floor of the temporal horn, narrowing into the tail that disappears as a ventricular structure at the anterior edge of the medial wall of the atrium. The fimbria of the fornix arises on the ventricular surface of the hippocampus behind the head and the lower end of the choroidal fissure.
The temporal horn extends forward from the atrium below the pulvinar into the medial part of the temporal lobe to the anterior edge of the hippocampal head and just behind the amygdala (Figs. 1, 4, and 5). The temporal horn ends approximately 2.5 cm from the temporal pole. The floor of the temporal horn is formed medially by the hippocampus and laterally by the collateral eminence, the prominence overlying the collateral sulcus. The medial part of the roof is formed by the inferior surface of the thalamus and the tail of the caudate nucleus, which are separated by the striothalamic sulcus. The lateral part of the roof is formed by the tapetum of the corpus callosum, which also sweeps inferiorly to form the lateral wall of the temporal horn. The tapetum separates the temporal horn from the optic radiations. The only structure in the medial wall is the narrow cleft, the choroidal fissure, situated between the inferolateral part of the thalamus and the fimbria of the fornix. The inferior choroidal point, at the lower end of the choroidal fissure, is located just behind the head of the hippocampus and immediately lateral to the lateral geniculate body.
The superior surface of the temporal lobe forms the floor of the deep sylvian compartment and faces the sylvian surface of the frontal and parietal lobes and the insula. The superior surface posteriorly is formed by the planum temporale and anteriorly by the planum polare (Figs. 2F and 5B). The planum temporale is composed of the transverse temporal gyri, the most anterior and longest of which is Heschl’s gyrus. Heschl’s gyrus and the adjoining part of the superior temporal gyrus serve as the primary auditory receiving area. The transverse temporal gyri seem to radiate anterolaterally from the posterior insular margin, widening as they progress toward the cortical surface.
The superior surface, anterior to the planum temporale, is formed by the planum polare. The planum temporale has a more horizontal orientation than the planum polare, which, from lateral to medial, slopes downward and conforms more to the convexly rounded anterior part of the insular surface. In front of Heschl’s gyrus, the medial edge of the planum polare is separated from insula by the inferior portion of the limiting sulcus of the insula, often referred to as the inferior limiting sulcus, an important site in positioning the incision in the transsylvian-transinsular approach. The mean distance from the anterior part of the limiting sulcus to the roof of temporal horn is 9.3 mm at the level of the apex of the uncus and 8.2 mm at the inferior choroidal point. Table 1 shows the range and standard deviation for these measurements. The temporal horn is located 45 degrees medial to the sagittal plane through the anterior part of the limiting sulcus; thus, to reach the temporal horn, the incision must be directed approximately 45 degrees medially from the limiting sulcus toward the midpoint of the lateral wall of the cavernous sinus. Failure to direct the incision medially will result in the incision crossing Meyer’s loop of the optic radiations. We have reviewed the relationship between the inferior insular sulcus and the optic radiations in another article (3, 31).
The lateral temporal surface is divided into three parallel gyri—superior, middle, and inferior temporal—by two sulci, the superior and inferior temporal sulci, all of which parallel the sylvian fissure (Figs. 1 and 2). The anterior portion of all of the sulci and gyri can be used for ventricular entry, although the superior temporal gyrus is infrequently used for this purpose. The superior temporal gyrus lies between the sylvian fissure and the superior temporal sulcus and is continuous around the lip of the fissure with the transverse temporal gyri. The posterior end of the superior temporal gyrus blends upward into the posterior part of the supramarginal gyrus. The angular gyrus, a parietal lobe structure, caps the upturned posterior end of the superior temporal sulcus. The middle temporal gyrus lies between the superior and inferior temporal sulci. The temporal horn and the ambient and the crural cisterns are located deep to the middle temporal gyrus. The inferior temporal gyrus lies below the inferior temporal sulcus and continues around the inferior border of the hemisphere to form the lateral part of the basal surface. One or more of the temporal gyri are frequently separated into two or three sections by sulcal bridges, giving the related gyri an irregular discontinuous appearance. The variation is greater with the middle and inferior temporal gyri than with the superior temporal gyrus. The inferior temporal gyrus is often composed of multiple fragmented gyri and may blend into the middle temporal gyrus without a clear sulcal demarcation.
Because the temporal horn is located at the level of the middle temporal gyrus, an inferior trajectory is needed to access the horn from the superior temporal gyrus or sulcus, a superior trajectory is followed from the inferior temporal gyrus or sulcus, and a straight trajectory, parallel to the middle fossa floor, is used when the middle temporal gyrus is chosen as the starting point (Fig. 1D). The entry point for temporal lobectomy for epilepsy is usually located below the superior temporal gyrus. The distance from the superior temporal gyrus to the temporal horn averaged 31.4 mm at the level of the apex of uncus and 34.2 mm at the inferior choroidal point (Table 1). The distance from the depth of the superior temporal sulcus to the lateral wall of the temporal horn averaged 12.4 mm and averaged 11.2 mm at the level of the inferior choroidal point. The distance from the middle temporal gyrus to the temporal horn averaged 26.3 mm at the level of the apex of uncus and averaged 23.8 mm at the inferior choroidal point. The average distance from the inferior temporal sulcus to the temporal horn was 13.1 mm at the level of the apex of uncus and was 14.3 mm at the inferior choroidal point. The average distance from the inferior temporal gyrus to the temporal horn was 20.2 mm at the level of the apex of uncus and was 21.9 mm at the inferior choroidal point.
The basal surface of the temporal lobe is traversed longitudinally by the collateral and rhinal sulci medially and the occipitotemporal sulci laterally, which divide the region from medial to lateral into the parahippocampal and occipitotemporal (fusiform) gyri and the lower surface of the inferior temporal gyrus (Figs. 2C, 4, and 5, C–F). Any of the gyri or sulci can be selected as the site of entry into the temporal horn. The anterior end of the basal surface projects medially to form the uncus without a limiting border between the uncus and parahippocampal gyrus. The basal surface of the parahippocampal gyrus forms the medial part of the inferior surface. It extends backward from the temporal pole to the posterior margin of the corpus callosum. Posteriorly, the part of the parahippocampal gyrus below the splenium of the corpus callosum is intersected by the anterior end of the calcarine sulcus, which divides the posterior portion of the parahippocampal gyrus into an upper part that is continuous above, posteriorly with the isthmus of the cingulate gyrus, and below and posteriorly with the lingual gyrus.
The collateral sulcus, one of the most constant cerebral sulci, courses between the parahippocampal and the occipitotemporal gyri. The collateral sulcus may or may not be continuous anteriorly with the rhinal sulcus, the short sulcus, extending along and marking the lateral edge of the uncus. The collateral sulcus is located below and indents deeply into the basal surface of the temporal horn producing a prominence, the collateral eminence, in the floor of the temporal horn on the lateral side of the hippocampus. The temporal horn can be exposed from below by opening through the deep end of the collateral sulcus.
The temporal horn is located superior to parahippocampal gyrus; thus, the more medial the basal temporal corticectomy, the more vertical the surgical trajectory needs to be to reach the temporal horn (Figs. 4 and 5). The average distance from the deep end of the occipitotemporal sulcus to the temporal horn is 7.9 mm at the level of the apex of uncus and 6.0 mm at the level of the inferior choroidal point (Table 1). The average distance from the fusiform gyrus to the temporal horn is 11.9 mm at the level of the apex of uncus and is 13.6 mm at the inferior choroidal point. The average distance from the depth of the collateral/rhinal sulcus to the temporal horn is 3.9 mm at the level of the apex of uncus and is 6.4 mm at the inferior choroidal point. The average distance from the parahippocampal gyrus to the temporal horn is 8.5 mm at the level of the apex of uncus and is 14.6 mm at the level of the inferior choroidal point.
Subdivision of the Medial Temporal Lobe
De Oliveira et al. (8, 9) and Tedeschi et al. (43) divided the medial temporal region into three parts: anterior, middle, and posterior (Figs. 2, E and F and 5A). The anterior part extends posteriorly from the anterior end of the rhinal sulcus to a transverse line at the level of the inferior choroidal point. The middle part extends posteriorly from the inferior choroidal point to a transverse line passing at the level of the quadrigeminal plate. The posterior part extends from the quadrigeminal plate to the level of the basal parietotemporal line, which connects the preoccipital notch to the lower end of the parieto-occipital fissure.
Each part of the medial temporal region has cisternal and ventricular components (Table 2). The anterior part includes the cisternal surfaces of uncus and the adjacent portion of the parahippocampal gyrus. The anterior portion of the uncus faces the carotid and sylvian cisterns, and the posterior segment faces the cerebral peduncle across the crural cistern. The anterior portion is the site of the amygdala and head of the hippocampus and the uncal recess, located between the amygdala and the hippocampal head. This anterior portion of the temporal horn is located in front of the inferior choroidal point and the attachment of the choroid plexus.
The middle portion of medial temporal region is formed medially by the cisternal surface of the parahippocampal and dentate gyri and fimbria. The ventricular surface of the middle part is formed by the body of hippocampus, the collateral eminence, the choroid plexus, and choroidal fissure. This part is located posterior to the uncus and crural cistern at the level of the ambient cistern. The choroidal fissure in the temporal horn is located behind to the uncus and the inferior choroidal point. The anterior choroidal artery enters the temporal horn and the inferior ventricular vein exits the roof of the ventricle to enter the basal vein at the level of the inferior choroidal point, which is the limit between the anterior and middle portions of the medial temporal region. Entrance into the temporal horn directed through the rhinal sulcus on the lateral side of the uncus will expose only part of the temporal horn anterior to the choroidal fissure.
The posterior portion of the medial temporal region extends from the level of the quadrigeminal plate to the basal parietotemporal line. The quadrigeminal plate sits at the junction of the ambient and quadrigeminal cisterns. The lateral wall of the quadrigeminal cistern is formed by the posterior part of the parahippocampal gyrus, isthmus of cingulate gyrus, and the anterior-most portion of lingula. The ventricular surface of the posterior portion is composed of the tail of the hippocampus, the collateral trigone overlying the collateral sulcus, the choroid plexus, and the choroidal fissure.
The operative approaches to the medial temporal region may be classified into four groups: superior, lateral, basal, and medial, based on the surface through which the exposure is directed. The superior approach, the transsylvian-transinsular approach, is directed through the floor of the sylvian fissure near the anterior inferior part of the limiting sulcus of the insula. The lateral approach may be directed through any sulcus or gyrus of the lateral face of the temporal lobe, although the superior temporal gyrus is used rarely unless it is involved in the lesion. The approach is frequently directed through a resection of the anterolateral temporal cortex, referred to as an anterior temporal lobectomy. The basal approach may be directed through any sulcus or gyrus on the basal face of the temporal lobe. The medial approach is divided into an anterior variant, the transsylvian transcisternal approach is directed through the sylvian and crural cisterns, and a posterior group is directed through the occipital bone either above or below the tentorium in the occipital interhemispheric or supracerebellar transtentorial approaches. The advantages and disadvantages of each surgical approach to the medial temporal region are summarized in Table 3.
An understanding of the anatomic characteristics of the temporalis muscle, which occupies the temporal fossa, and the techniques available to deal with it during temporal lobe exposure is essential to achieving optimal exposure with minimal retraction (Fig. 6). The temporalis muscle resembles an open fan. It extends from the superior temporal line to the coronoid process of mandible. Its fibers converge and descend between the zygomatic process and the lateral side of the cranium to a tendon that attaches to the coronoid process. The muscle’s outer surface is covered by the temporal fascia, which is comprised by a single layer over the posterior three-quarters of the muscle and divides into superficial and deep layers on the outer surface of the anterior quarter of the muscle. The superficial layer attaches to the lateral border and the deep layer attaches to the medial border of the orbital rim and zygomatic arch. A fat pad and a temporal vein fill the space between the two fascial layers.
The frontal branches of the facial nerve course in a fat pad in the anterior temple area between the superficial layer of temporalis fascia and the galea, where they could be damaged during in the subgaleal elevation of the scalp flap. To protect the nerve, the superficial layer and sometimes both the superficial and deep layers are opened at the upper edge of the fat pad and the nerve with the fat pad and superficial layer of temporalis fascia are folded downward with the galea to prevent the damage to the nerves that would occur if the dissection was directed into the fat pad on the outer surface of the fascia. The approach is referred to as interfascial if only the superficial and not the deep fascia is elevated and is referred to as subfascial if both layers of the temporal fascia are elevated and reflected downward.
Once exposed, the temporal muscle may be mobilized purposefully in several different ways to reach specific areas of the temporal fossa. In all of these techniques, the temporal muscle must be detached from the calvarium. Careful subperiosteal elevation of the muscle using a sharp periosteal elevator offers the best chance of preserving the neural innervation, arterial supply, and venous drainage of the muscle, which courses directly on the periosteal surface (Fig. 6D). Use of a hot cutting current to elevate the delicate neural and vascular structures on the deep surface of the muscle may result in atrophy of the muscle and a poor cosmetic result. Preserving the vascular and nerve supply will reduce muscle atrophy, cosmetic deformity, and masticatory symptoms.
To expose the anterior temporal fossa area, the muscle must be detached from its anterior and superior insertion and folded downward and backward, as in the classic pterional approach. If a more posterior exposure is needed, it may be necessary to section a small anterior portion of the muscle parallel to the zygomatic arch, but this may result in masticatory symptoms and cosmetic deformity. Freeing the muscle from its posterior and superior insertions and mobilizing it anteriorly allows exposure of medium and posterior part of the middle fossa only, whereas detaching the muscle from its superior, anterior, and posterior attachments, plus sectioning the zygomatic arch and displacing it downward with the masseter, allows the temporal muscle to be reclined downward through the breach in the arch, exposing all the temporal fossa (Fig. 6H).
A common step in most temporal lobectomies is entry into the temporal horn, whether the approach is through the lateral, superior, basal, or medial surfaces (Figs. 1–5). The temporal horn is encountered approximately 2.5 cm from the temporal pole. There are several steps in completing the medial temporal resection after exposing the temporal horn. The steps, which can vary in order depending on the approach, include medial, anterior, lateral, and posterior disconnection of the hippocampus and removal of the portion of the amygdala located below the optic tract. The medial disconnection of the hippocampus can be achieved by opening the choroidal fissure, the narrow cleft between the fimbria of the fornix and the thalamus, along with the attached choroid plexus. The choroid plexus is attached to the tenia thalami on the thalamic side of the choroidal fissure and to the tenia fimbria on the forniceal side of the fissure. The tenia are thin, fragile ependymal membranes. The fissure is opened by dividing the tenia fimbria rather than the tenia thalami because opening the tenia thalami risks damaging the veins passing through it that drain the optic radiations and sublenticular part of the internal capsule. The veins passing through the tenia fimbria are very small compared with those draining through the tenia thalami.
The anterior disconnection includes separating the head of the hippocampus from the amygdala by using the uncal recess as a landmark for carrying the exposure through the medial aspect of the uncus. The posterior disconnection involves sectioning the hippocampus and parahippocampal gyrus as far posteriorly as indicated by electrophysiologic and neuroradiologic studies. The partial amygdalar resection usually is completed using subpial dissection in front of the uncal recess. The superior part of the amygdala, the part in close apposition to the optic tract, branches of the anterior choroidal and posterior cerebral arteries, and the lower surface of the lentiform nucleus, is preserved (Figs. 3 and 4).
The transsylvian-transinsular approach is directed through the superior surface of the lobe (Fig. 7). The head is turned 30 degrees toward the contralateral side with the posterior ramus of the sylvian fissure parallel to the surgeon’s line of sight to provide a good viewing angle from anterior toward the posterior elements of the medial temporal region (Figs. 6G and 7). The approach is directed through the anterior part of the fissure. Excessive rotation of the head to the side opposite the approach is avoided because that shifts the line of view over the highest upward projecting part of the temporal lip of the sylvian fissure. A frontotemporosphenoidal (pterional) bone flap is elevated and the roof and lateral wall of the orbit are thinned with a drill up to the lateral part of the superior orbital fissure. After opening the dura, the sylvian fissure is opened widely from lateral to medial, beginning 2 cm behind the pars triangularis and exposing forward to the chiasmatic, carotid, and crural cisterns (Fig. 7, B and C). The limen, the anterior third of the lower part of the insula, and the proximal part of the M2 is exposed. Usually, the inferior trunk of the middle cerebral artery must be elevated away from the inferior segment of the limiting sulcus of the insula. Small branches that originate from the inferior trunk and pass through the limiting sulcus may need to be sacrificed. A 1.5-cm incision directed downward and medially approximately 45 degrees toward the midpoint of the cavernous sinus lateral wall is completed in the anterior sector of the inferior portion of the limiting sulcus of the insula (Fig. 7, D and E). An alternative that carries less risk to the optic radiations in the roof of the temporal horn is incising the planum polare on the medial side of the limen and entering the amygdala from that trajectory. The incision in the anteroinferior part of the circular sulcus, limen, and the planum polare medial to the limen exposes the amygdala in the anterior uncal segment. Removal of the lower and lateral parts of the amygdala provides entry into the temporal horn. The anterior uncal area is removed using subpial suction, taking care to preserve the anterior choroidal and posterior communicating arteries, the oculomotor nerve, basal vein, and optic tract, which are visible through the pia arachnoid. After entering the temporal horn through the amygdala, the choroid plexus is displaced toward the roof of the temporal horn, and the choroidal fissure is opened through the tenia fimbriae in the area lateral to the cerebral peduncle while preserving the anterior choroidal artery, optic tract, and basal vein. Opening the choroidal fissure through the tenia fimbriae avoids damaging the veins that cross the thalamic side of the fissure and drain the optic radiations and sublenticular part of the internal capsule (Fig. 7E). Opening the choroidal fissure exposes the structures in the ambient cistern. The laterally directed branches of the anterior choroidal artery and the P2 segment of the posterior cerebral artery that pass to the hippocampal sulcus are obliterated and cut, whereas all those that pass laterally beyond the collateral sulcus must be preserved. The next step is hippocampal disconnection. An incision around the hippocampus and the uncal recess is extended downward into the collateral and rhinal sulci. The posterior edge of the resection of the hippocampus and parahippocampal gyrus is located approximately 1 cm behind the inferior choroidal point, which corresponds to the approximate level of the lateral geniculate body, posterior border of the cerebral peduncle, lateral mesencephalic sulcus, and the ascension of the fimbria to form the crus of the fornix (Fig. 1G). With disconnection, the head and body of the hippocampus and part of the parahippocampal gyrus are removed en bloc (Fig. 7F). The last steps are removal of the remaining amygdala at the anterior edge of the exposure and the subpial removal of the remainder of the uncus, using the optic tract as the superior limit of the resection. The upper medial part of the amygdala adjacent the claustrum, optic tract, and lentiform nucleus is not removed.
The advantage of the superior approach is that it preserves the lateral and basal temporal cortex involved in higher cortical functions and language and can be combined with the transsylvian-transtentorial approach. Disadvantages are the small working area, greater technical skill required to complete en bloc resections, and the risks to optic radiations in the roof of the temporal horn (3).
Most approaches directed through the lateral surface are performed for epilepsy. The classic standard lobectomy that involved a large cortical incision extending 6 cm behind the temporal tip on the nondominant side and 4.5 cm on the dominant side have given way to more limited lateral resections because most ictal events are triggered in mesial structures (5, 39). The anterolateral approach of Spencer, in which the lateral cortical resection includes 3 to 3.5 cm of the lateral cortex below the superior temporal gyrus, was the model used in this study (Fig. 8). The temporal horn is entered after the superficial block of tissue is removed. The head is rotated approximately 30 degrees to the contralateral side of the approach and the vertex is tilted downward to facilitate the view into the temporal horn and along the choroid fissure. A pretemporal craniotomy is completed to expose the anterior two thirds of the temporal lobe, the sylvian fissure, and the floor of the middle fossa (Figs. 6E and 8B). The lobe can be entered through any gyrus or sulcus on the lateral temporal surface; however, the superior temporal gyrus usually is avoided because Heschl’s gyrus, the site of the primary auditory projection area, is on the upper surface and the insula is on the medial surface of the gyrus (Fig. 2, B and F). Magnetic resonance imaging scans of coronal sections will aid in selecting a route to the temporal horn, which is opened through its lateral wall in a lateral to medial trajectory (Fig. 8C). The medial temporal resection is completed using the same steps used during the transsylvian-transinsular approach. The subpial resection of the superior portion of uncus and amygdala is the last step in the procedure (Fig. 8, D–F). To visualize the most posterior structures of the temporal horn, it is necessary to direct the view of the microscope from anterior to posterior along the anteroposterior axis of the head and temporal horn. A useful landmark for the upper boarder of this resection is a line traced from the bifurcation of the internal carotid artery to the inferior choroidal point (carotid-choroidal line or Wen’s line) (48) because there is no clear demarcation between the gray matter of the amygdala below and the basal ganglia above. Coronal cross-sections through the amygdala reveal that it blends directly into the globus pallidus above (Fig. 3D).
Advantages of the lateral approaches that have made it the favored approach for many epilepsy surgeons are that the lobectomy provides a good window for en bloc resection of the hippocampus and adjacent structures, the easy access to the hippocampus, and the fact that it is less technically demanding than the other routes to the area. The disadvantages are that it produces lesions of the lateral temporal cortex, damages the optic radiations if extended posteriorly, and requires greater depth to the temporal horn than through the basal or superior approaches.
The head is rotated approximately 75 degrees to the contralateral side of the approach (Fig. 9). The craniotomy is similar to that described for the lateral approaches, with generous exposure of the middle fossa floor to reduce brain retraction and to improve the angle of view through the basal surface of the lobe (Fig. 9, B and C). A low exposure is facilitated by section of the zygomatic arch and downward retraction of the temporal muscle, which facilitates the view from lateral to medial and from inferior to superior. This approach is referred to as the pretemporal-zygomatic approach. After opening the dura, the temporal lobe is elevated to expose the basal surface. The vein of Labbé should be preserved. The basal cisterns may be opened and cerebrospinal fluid aspirated to facilitate the exposure. The temporal horn can be entered through the collateral and rhinal or occipitotemporal sulci or the fusiform or parahippocampal gyri. The more medial the site of the corticotomy, the more vertical will be the trajectory to the ventricle. Even if the collateral and rhinal sulci are chosen, the direction of view through the temporal horn will be from medial to lateral. Once inside of the temporal horn, the medial disconnection directed between the fimbria and the choroid plexus is completed to expose the ambient cistern (Fig. 9, D and E). In the basal approaches, the lateral disconnection at the level of the collateral eminence and lateral edge of the hippocampus is completed at the time the temporal horn is being entered through the lower temporal surface. The anterior and posterior disconnection of the hippocampus and resection of the amygdala and remaining uncus are similar to the lateral approach (Fig. 9F).
Advantages of the basal approach are the entry into the temporal horn through the floor with reduced risk to the optic radiations, the shorter route to the temporal horn than with the lateral approaches, and the possibility of accessing the posterior part of the medial temporal lobe depending on the position of the vein of Labbé. The disadvantages are the retraction required to reach the basal surface, especially medially, the language areas in the region of the fusiform gyrus of the dominant hemisphere, and the risk to the vein of Labbé.
The approaches directed through the medial surface are divided into an anterior route, the transsylvian-transcisternal approach, and two posterior routes, the occipital interhemispheric and the supracerebellar transtentorial approaches. The posterior approaches are used commonly for arteriovenous malformations, tumors, or cavernomas of the middle and posterior portion of the mesial temporal region and less frequently for resections for epilepsy.
The craniotomy is the same as that used for the transsylvian-transinsular approach (Figs. 6G and 10). After extensive opening of the sylvian fissure and basal cisterns, the bridging veins from the temporal pole that enter the sphenoparietal or cavernous sinuses are obliterated and divided to allow mobilization of the temporal pole. The arachnoid fibers between the uncus and the oculomotor nerve, vascular elements of the carotid and crural cisterns, and tentorium are cut to grant access to the anterior part of the medial temporal region (Fig. 10, B and C). The temporal pole is retracted to expose the anterior sector of the medial temporal region. The anterior choroidal artery is followed distally from its origin from the internal carotid artery and as far as possible around the uncus (Fig. 10D). It is difficult to visualize the inferior choroidal point where the anterior choroidal artery enters the temporal horn before resection of the medial part of the uncus because it is hidden behind the apex of the uncus. An imaginary line is traced from the anterior end of rhinal sulcus, which corresponds to the anterior limit of the uncus and separates it from the planum polare, to the most distal visible point of the anterior choroidal artery. The subpial resection of part of the uncus medial to this line provides access to the most medial and anterior part of the temporal horn (Fig. 10E). The steps necessary to complete the medial temporal resection resemble those accomplished in other approaches (Fig. 10F). Opening the floor of the stem of the sylvian fissure by dividing the limen insulae will enlarge the basal and posterior view a few millimeters.
Advantages of the medial approach are avoidance of the lateral and basal temporal cortex and optic radiations, access for proximal control of the internal carotid, anterior choroidal, posterior communicating, and posterior cerebral arteries, and the view of the medial temporal surface before entering the temporal horn. The disadvantages are lack of access to the posterior part of the medial temporal region, the required retraction of the temporal pole and apex of the uncus, the risk to the oculomotor nerve, and the fact that the approach through the cisterns is more technically demanding.
Occipital Interhemispheric Approach
The patient can be positioned in the sitting, prone, or park bench position. An occipital craniotomy is performed, exposing the superior sagittal and transverse sinuses. The occipital pole is retracted laterally and superiorly to expose the falx, tentorium, straight sinus, and arachnoid of the quadrigeminal cistern (Fig. 11, A–C). Usually, there are no bridging veins entering the posterior portion of the superior sagittal sinus below the lambdoid suture. The tentorial incision is made parallel to and 1 cm from the straight sinus, and the quadrigeminal and ambient cisterns are exposed (Fig. 11B). Opening the tentorium allows the medial edge of the tentorium to fall away from the lower surface of the parahippocampal gyrus and aids in exposing the posterior and middle parts of the medial temporal region and the adjacent quadrigeminal and ambient cisterns. The retraction of the occipital pole can be extended forward to expose the posterior portion of the medial temporal lobe. Changing the angulation of the microscope and retracting the tentorial surface of the cerebellum caudally exposes the middle and posterior parts of the medial temporal region, including the posterior portion of the parahippocampal gyrus, the isthmus of the cingulate gyrus, and the anterior portion of the lingula (Fig. 11, B and C). The posterior approaches infrequently involve en bloc cortical resection and most commonly involved direct cortical incision over lesions such as cavernoma and tumors or resections around arteriovenous malformations after exposure and management of feeding arteries and draining veins.
Supracerebellar Transtentorial Approach
The patient is placed in the sitting, prone, or park bench position. The craniotomy extends from the transverse and sigmoid sinuses to just above or into the foramen magnum to provide multiple working corridors and access to the cisterna magna with cerebrospinal fluid drainage to relax the cerebellum (Fig. 11, D–H). Initially, the approach can be directed along the midline. Bridging veins between the tentorium and cerebellum should be obliterated and cut if it seems that they may be stretched and torn. Opening the arachnoid membrane of the quadrigeminal cistern exposes the Galenic venous complex. The angle of vision should be redirected laterally toward the junction of the quadrigeminal and ambient cisterns. The arachnoid covering the cisterns is opened, exposing the trochlear nerve and the superior cerebellar and posterior cerebral arteries (Fig. 11F). Dividing the tentorium from below starts at the free edge and extends posteriorly. The free edge is retracted laterally to expose the middle and posterior portions of the medial temporal region, the ambient cistern, and the posterior part of the crural cistern (Fig. 11, G and H). The approach from here is based on the lesion type.
The advantages of the posterior approaches are that they leave the lateral and most of the basal cortex and the optic radiations undisturbed, avoid temporal lobe retraction, and provide access to the posterior and middle parts of the medial temporal region. The disadvantages are the occipital lobe or cerebellar retraction, risk of hemianopsia with occipital lobe retraction, difficulty in accessing the anterior parts of the mesial temporal area, the lack of proximal arterial control, early exposure of the venous drainage, and greater working distance.
Important anatomic and functional structures confronted in the surgical approach to the medial temporal region include the lateral and basal temporal cortices; Wernicke’s area; the optic radiations, including Meyer’s loop; the vein of Labbé ; the structures in the sylvian fissure; and the carotid, interpeduncular, crural, and ambient cisterns, including the middle cerebral, internal carotid, posterior communicating, anterior choroidal, and posterior cerebral arteries, the superficial sylvian, deep middle cerebral, and basal veins, and the oculomotor and trochlear nerves. Lesions in this area include a wide range of pathological features, including epileptogenic areas, arteriovenous and cavernous malformations, tumors, and traumatic contusions that are the subject of a broad range of neurosurgical specialties.
Most of the approaches to the medial temporal region were devised for the treatment of epilepsy (4, 13–15, 17, 21, 23, 24, 27, 28, 34, 40, 45, 52) or arteriovenous malformations (2, 7–10, 16, 18, 22, 23, 26, 38, 41–43, 47). The complex anatomy of the medial temporal region and position of the various pathological features have led to approaches that reach it through the lateral, basal, medial, and superior surfaces of the lobe (8, 9, 43). The approaches through the lateral and superior surfaces have been predominately for resections for epilepsy and tumors of the anterior and middle portions of the medial temporal lobe. The approaches directed through the medial surface from posteriorly and through the basal surface have been carried out predominately for arteriovenous malformations and tumors in the middle and posterior portions of the medial temporal lobe, in which cortical incisions directly over mass lesions or early vascular control is more important than cortical and en bloc resections as required in epilepsy.
Partial temporal lobectomy was first described for the surgical treatment of epilepsy in 1925 (12). In 1958, Niemeyer (24) first described selective amygdalohippocampectomy through the middle temporal gyrus. In 1985, Yaşargil et al. (52) introduced selective resection of amygdala and hippocampus through a transsylvian-transinsular approach. These approaches either dealt with the lateral temporal cortex or produced occasional lesions of the optic radiations. The approaches through the basal surface were proposed as a way to avoid the dominant lateral temporal cortex involved in higher cortical functions and the optic radiations in the roof and/or lateral wall of the temporal horn (11, 17, 23, 25, 27, 36). Experience with these approaches, however, revealed them to require significant cerebral retraction, risking injury to the vein of Labbé and the temporal cortex (10, 45). In addition, cortical mapping has revealed areas involved in language function along the dominant basal sector of the temporal lobe, especially along fusiform gyrus (20, 33).
In 1998, Vajkoczy et al. (45) described the transsylvian-transcisternal approach, which entered directly into the medial temporal surface. This route through the sylvian fissure and basal cisterns leaves the lateral and basal temporal cortexes untouched, and access through the roof and lateral wall of the temporal horn was avoided, thus, preserving the optic radiations. The approach was used in 32 patients with medial temporal lobe epilepsy, half of whom had lesions such as cavernomas or tumors. Disadvantages of this approach are that it accesses predominately the medial aspect of the medial surface, the difficulty in visualizing the medial temporal area behind the apex of the uncus, and the risks to the structures in the basal cisterns, especially the oculomotor nerve. The recent work of Sincoff et al. (36) and Rubino et al. (31), emphasizing the relationship between the optic radiations and the superior and lateral walls of the temporal horn, support the role of basal and medial approaches in preserving the optic radiations (31).
The posterior approaches that are directed through the medial temporal surface are the occipital interhemispheric and the supracerebellar transtentorial approaches (8, 9, 19, 29, 32, 35, 37, 46, 49, 53). Both provide satisfactory exposure of the posterior portion of the medial temporal region and, with further retraction, the middle sector also can be reached. The supracerebellar transtentorial approach was reported by Voigt and Yaşargil (46), in 1976, for removal of a cavernoma into the parahippocampal gyrus. Yonekawa et al. (53) used the same approach to treat 16 patients with lesions in or around the posterior portion of the medial temporal region. The occipital interhemispheric approach was proposed by de Oliveira et al. (8, 9) to treat arteriovenous malformations of the posterior portion of the medial temporal region, although they noted the disadvantages in vascular surgery of difficulty in gaining proximal control of arterial afferents and premature access to the venous drainage of the lesion. Smith and Spetzler (37) also used the occipital interhemispheric approach to treat seven patients with lesions in the posterior part of the medial temporal region and noted that the uncus could be reached (44).
Controversy remains as to the best approach for the treatment of temporal lobe epilepsy. Currently, the two most common approaches are the anterior temporal lobectomy with amygdalohippocampectomy and the transsylvian-transinsular selective amygdalohippocampectomy (40, 48, 52). The controversy between tailored resection versus anatomic resection, anterior temporal lobectomy versus selective amygdalohippocampectomy, and selective transsylvian versus selective transcortical resection, continues (4, 14, 15, 21).
In dealing with vascular lesions, an additional consideration is gaining proximal arterial control, and in the case of an arteriovenous malformation, locating the position of the venous drainage. In 1982, Heros (16) used a transcortical approach through the inferior temporal and fusiform gyri to treat three patients with arteriovenous malformations of the medial temporal region. In 1986, Solomon and Stein (38) described a similar approach through the inferior temporal, fusiform, or parahippocampal gyri. In 1994, de Oliveira et al. (9) not only divided the medial temporal region into an anterior, middle, and posterior portions, but also proposed the pretemporal approach to treat arteriovenous malformations of the anterior portion, the subtemporal approach through the occipitotemporal sulcus to reach lesions in the middle portion, and the occipital interhemispheric approach for arteriovenous malformations of the posterior portion of the medial temporal region. In 2002, Ikeda et al. (18) described a transchoroidal approach through the inferior temporal gyrus to treat a patient with a medial temporal arteriovenous malformation. In 2004, Du et al. (10) proposed a tangential resection of medial temporal arteriovenous malformations through the orbitozygomatic approach, presenting 10 patients, nine of whom obtained a complete resection of arteriovenous malformations of medial temporal region. These authors affirm that the success of the technique depended on the orbitozygomatic approach; however, we agree with other authors that good results after medial temporal arteriovenous malformation resection depend more on the circumferential microsurgical dissection and wide opening of the cisterns than on the orbitozygomatic trajectory (6, 51).
Combination of the approaches mentioned above have been reported. Yaşargil and Abdulrauf (50), de Oliveira et al. (9), and Ulm et al. (44) proposed the combination of a transsylvian-transcisternal approach with a transsylvian-transinsular approach. In 2004, Miyamoto et al. (23) reported the use of a combined subtemporal-transtemporal transchoroidal approach to treat 21 patients with medial temporal lesions. Combination of approaches can be a useful surgical solution and should be considered when the lesion involves two portions of the medial temporal region.
Each of the several techniques developed to treat medial temporal lesions have technical or functional drawbacks that should be considered in treating a given patient. Dividing the medial temporal region into subsections allows for a more precise analysis, not only of the anatomic relationships, but also of the possible choices of approaches capable of safely delivering the lesion. The systematization used here provides a rational selection of one or a combination of approaches.
Contributors: Alvaro Campero, MD, Gustavo Tróccoli, MD, Carolina Martins, MD, Juan C. Fernandez-Miranda, MD, Alexandre Yasuda, MD, PhD, and Albert L. Rhoton, Jr, MD
Content from: Campero A, Tróccoli G, Martins C, Fernandez-Miranda JC, Yasuda A, Rhoton AL, Jr. Microsurgical approaches to the medial temporal region: an anatomical study. Op Neuro 2006;59:ONS-279–ONS-308, 10.1227/01.NEU.0000223509.21474.2E. With permission of Oxford University Press on behalf of the Congress of Neurological Surgeons.
The Neurosurgical Atlas is honored to maintain the legacy of Albert L. Rhoton, Jr., MD.
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