Temporal/Subtemporal Craniotomy Free
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Temporal craniotomy is a simple approach that has vast applicability to intra-axial and extra-axial pathologies. The subtemporal approach provides a wide operative corridor to the floor of the middle fossa and upper petroclival territories and their associated cisterns. More specifically, this corridor reaches the anterior upper brainstem through the anterior petrosectomy.
The lateral neocortical temporal lobe and more specifically the dominant superior and posterior parts of the middle temporal gyri affect important functions such as language. The exact function of the anterior middle temporal gyrus is unknown, but it may be involved in processes such as contemplating distance, recognition of known faces, and accessing word meaning while reading (see Wikipedia).
The inferior temporal gyrus is involved with visual processing, associated with the representation of complex object features, such as global shape. It may also process face perception and the recognition of numbers (see Wikipedia).
Indications for the Procedure
Temporal craniotomy is beneficial for resection of mid to posterior intraparenchymal and convexity temporal lobe tumors. This route also affords access to mid hippocampal lesions through the transsulcal approach and reaches lateral thalamic tumors and basal cisterns through the transcortical transventricular transchoroidal pathway. The exposure of the Sylvian fissure is limited.
A subtemporal craniotomy is more versatile and exposes various neoplastic and tumorous intradural and extradural pathologies of the middle fossa floor, anterior basal cisterns, and upper clivus. Middle fossa meningiomas, small acoustic tumors and trigeminal schwannomas, low-lying basilar caput/upper basilar artery aneurysms, and upper petroclival meningiomas (through the anterior petrosal approach) are some examples of lesions readily reached via this route.
In addition, this approach allows repair of the superior semicircular canal dehiscence. The subtemporal approach can be used in combination with the extended pterional route for access to complex vascular abnormalities of the interpeduncular cisterns and fibrous multicompartment menigniomas filling the medial incisural and parachiasmatic space.
The location of the vein of Labbe and its drainage site into the transverse sinus should be estimated preoperatively based on magnetic resonance (MR) venogram for cases that require posterior subtemporal exposure. This drainage site is about 1cm superior to a line parallel to the superior border of the zygomatic arch and 2 to 5 cm (mean 2.9 cm) posterior to the opening of the external auditory meatus.
The displacement of other arterial and venous structures along the medial tentorium should be defined. The potential need for a combined approach to tumors with both supratentorial and infratentorial extensions should be planned preoperatively.
Mannitol (1g/Kg) should be administered during skin incision if a “tight” brain is expected. If cortical stimulation for functional mapping is contemplated, cold lactated Ringer’s solution should be available. In expectation of a subtemporal approach, I generally place a lumbar drain for most patients, regardless of their tumor size, in order to minimize the risk of temporal lobe retraction injury during intradural or extradural elevation of the lobe. Since the basal cisterns are not reached until after elevation of the lobe, alternative routes for early cerebrospinal fluid (CSF) drainage is beneficial.
The surgeon is positioned at the head of the operating room table with the surgical technician at either the right or left, depending on the handedness of the surgeon. The anesthesiologist may be situated at the foot of the table, allowing more working room for the assistant and microscope if necessary.
Depending on the skin incision, the superficial temporal artery may or may not be involved in execution of the approach. The artery typically runs one finger-breadth anterior to the tragus and bifurcates into its frontal and parietal branches approximately 5 cm superior to the zygoma. The temporal branch of the facial nerve is located more anteriorly over the zygoma and superficial to the two layers of the anterior superficial temporal fascia.
The root of the zygoma is an essential landmark to define the level of the middle fossa floor. This landmark should be used for planning the initial burr hole and subsequent craniotomy. Preservation of the vein of Labbe is essential. Its anatomic relationships are illustrated in the following images.
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Appropriate head position is paramount for surgery on deep skull base lesions. The patient’s head position should direct the surgeon to the region of interest through a path that allows adequate exposure of the lesion, minimizes brain retraction, and affords flexible working angles. Furthermore, the patient’s head position should enable a comfortable ergonomic body posture for the surgeon during the operation.
The exact location and size of the lesion will determine the corresponding skin incision. In general, smaller lesions that are within the superior or middle temporal gyri are amenable to linear incisions, whereas large subtemporal lesions benefit from a horseshoe-shaped incision. For lesions that require access to the anterior temporal pole, a small reverse question mark incision would be appropriate.
After placing a generous single burr hole just above he root of the zygoma, I use a #3 Penfield dissector to mobilize the dura away from the inner table of the calvarium in preparation for the craniotomy. If an extradural approach to the middle fossa is planned, it is essential to avoid early injury to the dura in order to protect the lobe during extradural subtemporal dissection and petrosectomy.
If the dura is adherent to the inner skull bone, I place numerous burr holes. The lumbar drain is used to remove ~30 to 40cc of CSF gradually (in 10–20cc aliquots) to relax the brain. This drainage facilitates dissection of the dura from the calvarium and reduces the risk of a dural tear. A craniotome is then used to complete the craniotomy.
Alternatively, an extradural dissection along the middle fossa may be resumed for skull base lesions. Importantly, the location of the vein of Labbe should be estimated preoperatively. The dural opening and extradural temporal lobe elevation should be adjusted for protection of this vital venous structure. Additional CSF may be released through the lumbar drain to further relax the lobe.
Please refer to the Anterior Pterosectomy chapter for further steps regarding extradural middle fossa dissection and petrosal osteotomy.
Extended Subtemporal Transtentorial Approach to the Ventrolateral Upper Brainstem
This expansion of the subtemporal approach towards the ventrolateral brainstem is especially worthwhile and should be employed when neccessay to replace more radical skull base approaches such as an anterior petrosectomy. The following two key steps allow anterolateral reflection of the medial tentorial flap:
- dissection of the trochlear nerve from its dural canal up to its entrance in the cavernous sinus and
- extension of the tentorial incision up to the Meckel's cave.
Two retraction sutures are placed along the tentorium to mobilize it effectively. With fine microscissors, the medial border of the dural canal of the trochlear nerve is opened. The nerve is not adherent within the canal and a microdissector is used to gently mobilize it from the rest of the canal and towards the brainstem. The tentorial incision is continued and oriented toward the superior petrosal sinus at the area just over the entrance of the trigeminal nerve into the Meckel's cave. The mean distance from the trochlear nerve’s entrance in its dural tunnel to its entrance into the cavernous sinus is about 8 mm (range, 3.4-14.9 mm; standard deviation, 3.9 mm).
With this technique, the trochlear nerve is free of the tentorial flap, creating a supra- and infratrochlear working window. If the tentorial incision is extended to the most lateral aspect of the Meckel's cave, the root of the trigeminal nerve and the anterior inferior cerebellar artery (AICA) origin can be seen.
In the presence of air cells, I prefer a watertight dural closure primarily or secondarily using a piece of dural allograft. Adipose tissue with its globular texture is one of the best barriers against CSF leakage. In the case of subtemporal skull base exposures that require removal of the tumor-infiltrated dura and bone, strips of adipose tissue are placed across the dural opening to seal the dural defect. Before placement of the adipose grafts, all air cells must be meticulously waxed.
Alternatively, a vascularized muscle flap prepared from the posterior aspect of the temporalis muscle may be rotated to fill the defect within the bone or dura. This latter method is used during repeat operations for patients who have previously undergone radiation treatment.
Any additional mastoid and temporal air cell are rewaxed. Finally, the bone flap is replaced and the scalp is closed in anatomic layers.
Postoperatively, the patient is admitted to the ICU for neurologic and blood pressure monitoring and pain control. Frequent and careful neurologic exams are paramount because temporal lobe hematomas can occur due to lobar retraction injury or venous drainage compromise, leading to rapid brainstem compression.
The patient is usually transferred to the regular ward on the first or second postoperative day. Lumbar drainage may be continued if there is a high suspicion of CSF leakage. Due to manipulation of the temporal lobe, the use of prophylactic antiepileptic medications for at least one week after surgery is highly recommended.
Pearls and Pitfalls
- During positioning, tilting the patient’s head toward the floor is a key maneuver to maximize the use of gravity retraction and obtain appropriate subtemporal exposure.
- The upper edge of the zygoma is a good landmark for locating the level of the middle fossa floor.
- Removal of the overhanging inferior edge of the craniotomy is important for preparing an obstructed operative trajectory toward the middle fossa floor.
- The location of the vein of Labbe should be estimated preoperatively. Dural opening and extradural temporal lobe elevation should be adjusted for protection of this vital venous structure.
Apuzzo. M, Brain Surgery: Complication Avoidance and Management, Volume 1. Churchill Livingstone, 1983.
Campero A, Tróccoli G, Martins C, Fernandez-Miranda JC, Yasuda A, Rhoton AL Jr. Microsurgical approaches to the medial temporal region: An anatomical study. Neurosurgery. 2006;59(Suppl 2)S279-308.
Rhoton AL Jr. The cerebral veins. Neurosurgery. 2002;51(Suppl 4)S159-205.
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