Preoperative Considerations
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The transcallosal approach allows midline exposure of the anterior two-thirds of the third ventricle.
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Preoperatively, in addition to standard MR imaging, venography is often helpful to assess the pattern of cortical venous drainage in order to help determine the side of the craniotomy.
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When venous drainage permits and when the pathology is appropriate, an approach on the non-dominant side is preferred.
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Preoperative neuropsychological evaluation is recommended in those patients with preoperative symptoms of cognitive impairment (e.g. memory deficits), owing to potential risk of injury to the fornices during the approach.
Contraindications
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Contraindicated in patients with “crossed dominance” where the hemisphere controlling the dominant hand is contralateral to the hemisphere responsible for speech — such patients are at risk for a “disconnection syndrome” and patients may develop writing and speech deficits postoperatively (particularly with larger or posteriorly positioned callosal incisions).
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This approach provides limited access to the anterior part of the frontal horn, posterior trigone or temporal horn. The transcortical approach is a safe alternative for those lesions ( Figure 14.1 ).
Figure 14.1
Management of tumors of the anterior third and lateral ventricles. Transcortical and transcallosal routes. The transcortical route allows access to more anterior or posterior areas compared to the transcallosal approach, which is more limited.
Reproduced with permission from Recinos, P.F., Lim, M., 2011. Parasagittal approach. In Jandial, R., McCormick, P., Black, P. (Eds.), Core Techniques in Operative Neurosurgery. Saunders, Elsevier Inc., Philadelphia.
Surgical Procedure
Patient Positioning
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Patient is positioned in either the supine or lateral decubitus position. In supine, the head can be turned approximately 45° contralateral ( Figure 14.2 ). This allows for the ipsilateral hemisphere to fall away from the falx with gravity in addition to allowing the surgeon to work with their hands side by side versus on top of each other. With good positioning, the falx acts as a natural retractor.
Figure 14.2
The patient can be placed in the supine position (A) with the head rotated towards the contralateral side (B). An alternative is the lateral decubitus position.
Reproduced with permission from Lee, M., Steinberg, G.K., 2011. Subcortical arteriovenous malformations: corpus callosum, lateral ventricle, thalamus and basal ganglia. In Jandial, R., McCormick, P., Black, P. (Eds.), Core Techniques in Operative Neurosurgery. Saunders, Elsevier Inc., Philadelphia.
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The disadvantage of lateral positioning compared with supine positioning is an increased distortion of the midline anatomy caused by gravity. This can jeopardize correct midline orientation, which is essential in this surgery.
Skin Incision
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Bicoronal, lazy-S, straight or U-shaped incisions can be tailored to the planned craniotomy site.
Craniotomy
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Usually the craniotomy is made paramedian to the superior sagittal sinus along the non-dominant (right) hemisphere and when necessary going across the superior sagittal sinus ( Figure 14.3 ).
Figure 14.3
A paramedian craniotomy is made crossing the superior sagittal sinus. The craniotomy is typically centered such that two-thirds of the flap are in front of the coronal suture. However, the craniotomy can be tailored to the position of the lesion.
Reproduced with permission from Recinos, P.F., Lim, M., 2011. Parasagittal approach. In Jandial, R., McCormick, P., Black, P. (Eds.), Core Techniques in Operative Neurosurgery. Saunders, Elsevier Inc., Philadelphia.
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To minimize the chances of superior sagittal sinus/bridging veins injury, the craniotomy is typically centered such that two-thirds of the flap are in front of the coronal suture. Placement of the bone flap primarily anterior to the coronal suture minimizes risk to the venous tributaries providing drainage for the motor cortex/supplementary cortex, which often enter the sinus 2–3 cm behind the coronal suture.
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Also, at least the ipsilateral half of the superior sagittal sinus is exposed. Partial exposure of the sinus is necessary to allow for complete dural reflection such that the microscope can be aligned parallel to the falx to achieve better lighting.
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The craniotomy can be tailored to the location of the tumor:
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Lateral ventricle, posterolateral lesion: Place the craniotomy more anteriorly to angle the trajectory.
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Although the non-dominant hemisphere is preferred (right), a left hemisphere approach may be considered if better preservation of the draining veins can be accomplished.
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Dural Opening
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A U-shaped dural flap based on the sinus is created.
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During dural incision, be cognizant of the venous tributaries; more often than not, they can be dissected off the dura to permit elevation of the dural flap. Occasionally, these tributaries may drain into a dural venous lake or an extreme lateral extension of the sinus; in these situations, incisions can be made leaving a small amount of dura attached to the vein.
Interhemispheric Dissection/Callosotomy ( Figures 14.4 and 14.5 )
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A combination of hyperventilation, diuresis and gravity retraction is critically important to aid in the interhemispheric dissection. Avoiding placing a brain retractor is recommended.
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Arachnoidal dissection is first performed medially over the cerebral hemisphere to separate it from any arachnoidal granulations. Once this is done, the axis of visualization is downwards, parallel to the falx.
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A combination of blunt and sharp dissection is used to develop the plane along the falx and the hemisphere. When dissecting down towards the corpus callosum, several key structures are identified sequentially: the callosalmarginal arteries, the cingulate gyrus and then the pericallosal arteries.
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A common mistake is to consider the cingulate gyrus the corpus callosum and to unintentionally make an incision in this structure. The corpus callosum is identified by its bright white color and relative hypovascularity in relation to surrounding gyri. The surgical navigation can also aid in guiding the dissection towards the corpus callosum.
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The pericallosal arteries over the corpus callosum (and their laterality) are confirmed.
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When performing the callosotomy, the surgeon must be cognizant of the midline orientation. For a transcallosal–transforminal approach (as opposed to a trans-septal–interforniceal trajectory), the goal is to enter the ipsilateral ventricle. Hence, the callosal incision is made eccentric to the ispilateral pericallosal artery with bipolar coagulation at a low setting with a 6- or 7-French suction.
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The callosotomy should be restricted to approximately 2–3 cm in length. Under the microscope, an adjacently placed retractor or cottonoid can serve as a reference to determine the appropriate length.
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If any additional anterior exposure is needed, a trans-septal–interforniceal approach can be performed. This approach is barely used due to the high risk of bilateral damage to the fornices and consequent devastating memory impairment.
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Note that the thickness of the corpus callosum can be variable depending on the size of the tumor and degree of hydrocephalus — where in more severe cases, the corpus callosum can be thinned.
