Posterior Interhemispheric Transcallosal Intervenous/Paravenous Variant
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In this chapter, our specific region of interest is the posterior segment of the third ventricle. The region around the aqueduct of Sylvius and the suprapineal recess demarcate this segment. The structures composing this region include the posterior commissure, pineal body, and habenular commissure.
Diagnosis and Evaluation
For a general discussion of diagnosis and evaluation for ventricular tumors, see the Principles of Intraventricular Surgery chapter.
Patients who harbor a tumor in this region often present with obstructive hydrocephalus because of the location of the tumor near the third ventricle and aqueduct. If the tumor is large or intratumoral hemorrhage or pineal apoplexy occurs, the patient may develop Parinaud’s syndrome.
Indications for Surgery
The goals for surgery in this region may be gross total resection, biopsy alone, or reestablishment of cerebrospinal fluid flow. During preoperative planning, I consider the goals of the surgery based on the size and location of the tumor and its invasion of the neurovascular structures.
Of course, the surgeon’s experience and comfort level with various approaches are imperative in operative planning. In addition, the texture of the tumor (soft and suckable versus firm and vascular) must be considered in the decision to proceed with a particular approach. Firm and vascular tumors require wider operative corridors.
Two methods that are most commonly chosen to access the posterior third ventricle/pineal area are the supracerebellar-infratentorial route and the occipital interhemispheric-transtentorial route. I have predominantly used the infratentorial supracerebellar pathway because of its minimal disruption of the normal anatomy. I do not prefer the occipital interhemispheric transtentorial route because it places a number of supratentorial structures at risk.
The disadvantages of the occipital interhemispheric-transtentorial route include the vulnerability of the occipital lobe, the straight sinus and its anastomosing veins, and the corpus callosum (splenium).
Parinaud’s syndrome can result from injury to the quadrigeminal plate from the use of the supracerebellar-infratentorial route. The paramedian supracerebellar transventricular approach for small posterior third ventricular lesions is addressed in its own dedicated chapter.
Overall, based on the exact location of the lesion, the following routes have been described:
- Posterior interhemispheric transcallosal intervenous/paravenous route (described in this chapter,)
- Paramedian supracerebellar transventricular approach
- Occipital interhemispheric transtentorial route
- Anterior interhemispheric transcallosal interforniceal approach.
For a general discussion of the indications for surgery in patients with ventricular tumors, see the Principles of Intraventricular Surgery chapter.
A careful study of the preoperative magnetic resonance images (MRI) determines the location of the internal cerebral veins, veins of Rosenthal and Galen in relation to the tumor. This important information will determine the pathway of the dissection between or around the internal cerebral veins (intervenous versus paravenous). Superior and anterior dislocation of the veins indicates a need for a posterior approach, preferably via the supracerebellar route.
Parasagittal veins are more indispensable in the posterior frontal and parietal regions, and their sacrifice and manipulation may predispose the patient to venous infarction. The preoperative MRI or a computed tomography (CT) venogram may help determine the side of approach that avoids dominant parasagittal veins along the desired interhemispheric operative corridor.
The use of an external ventricular drain in the presence of non-communicating hydrocephalus is advised so that interhemispheric dissection is pursued safely. In the absence of hydrocephalus, I use a lumbar drain to facilitate smooth dissection before entering the ventricle.
POSTERIOR INTERHEMISPHERIC TRANSCALLOSAL INTERVENOUS/PARAVENOUS VARIANT
The posterior interhemispheric transcallosal intervenous/paravenous variant is appropriate for lesions that lie primarily within the posterior third ventricle, quadrigeminal plate, and pineal region. For this approach, the center and bulk of the tumor should be within the ventricle, rather than in the pineal region. This approach provides a unique and suitable surgical field for the area anterior to the internal cerebral veins, straight sinus, vein of Galen, and pineal region.
The more posterior location of the craniotomy places the sensorimotor cortices at risk. Therefore, the surgeon must be especially cautious during exposure and interhemispheric dissection to prevent undo manipulation and retraction of these functional cortices. All parasagittal veins in the posterior extent of the craniotomy must be preserved.
Decompression and diversion of CSF greatly facilitates the work within the interhemispheric fissure.
Navigation should be used to guide the extent of the callosotomy in relation to the tumor’s anteroposterior margins. Most of the splenium must be spared to prevent the risk of disconnection syndrome. The surgeon should continue the callosotomy and dissection following the midline plane; this maneuver will lead the operator to the avascular membrane that separates the internal cerebral veins. If the septum has not been invaded by tumor, I continue dissection between the leaflets of the septum pellucidum, avoiding the lateral ventricles, if possible.
The septum pellucidum has been usually invaded by the tumor, and its anatomy is distorted. In these cases, I enter the right ventricle (for a midline pathology) or the ipsilateral ventricle (for a paramedian pathology). For the technical tenets of this stage of the operation, please refer to the Transcallosal Interforniceal Approach chapter.
After the tumor has been removed, I use angled mirrors or endoscopes to inspect the operative blind spots for residual tumor that may lie just below the splenium, under the edges of the corpus callosum, or under the folds of the surrounding tissues and veins.
Paravenous-Interforniceal Variant for a Paramedian Pathology
Using navigation, I incise the corpus callosum in the paramedian plane, directly over the pole of the tumor near the lateral ventricle. Preoperative MRI allows me to reliably identify the course of the fornix, internal cerebral veins, and the available operative trajectory.
Normal ependyma may or may not cover the capsule of the tumor. Intratumoral debulking mobilizes the tumor into the resection cavity while dissection progresses from a lateral to a medial direction. The internal cerebral veins may have been displaced by the tumor to the contralateral side over the capsule, or they may course through the tumor.
If these veins are over the capsule, I can debulk the tumor more rapidly. However, I regularly reconfirm the exact location of the veins with extracapsular dissection and direct visualization.
When the veins are engulfed by the tumor, only a debulking procedure is completed, based on the route of the veins within the mass. In these cases, aggressive tumor removal is not advised because its risks outweigh its benefits.
Closure and Postoperative Considerations
For a detailed discussion of recommendations for closure and the postoperative care of patients with ventricular tumors, see the Principles of Intraventricular Surgery chapter.
The following case describes the operative events for removal of a recurrent large posterior third ventricular epidermoid via the posterior interhemispheric transcallosal intervenous route.
The Posterior Interhemispheric Subsplenial/Transsplenial Route
This approach is rarely employed but is worth mentioning so that it can be considered when the venous anatomy is favorable. This approach is reasonable for more posteriorly and superiorly located targets that separate the internal cerebral veins and provide a wide a intervenous corridor. After the interhemispheric approach is complete, the edge of the splenium is moblized and the tumor is brought into view. If the splenium is very attenuated by the tumor, it may be partially transected to expand the operative working space.
Pearls and Pitfalls
- The posterior interhemispheric transcallosal intervenous/paravenous approach is reasonable for exposure of large posterior third ventricular lesions that displace the deep veins posteriorly. The applications of this approach are limited.
Contributors: Frederick A. Boop, MD and Paul Klimo Jr., MD, MPH
*Redrawn with permission from Tew JM, van Loveren HR, Keller JT. Atlas of Operative Microneurosurgery, WB Saunders, 2001. © Mayfield Clinic
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