Transcallosal Approach

Indications

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Tumors of the lateral and third ventricles

Contraindications

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This approach is contraindicated if the patient is medically unstable and would not tolerate surgery.
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The transcallosal approach, although it provides exposure to tumors in the lateral and third ventricle, is limited in providing satisfactory access to tumors in the posterior trigone, temporal horn, or superior frontal horn. Patients with these tumors are best approached by the transcortical route, with its own set of indications and complications.
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Although a partial callosotomy (usually anteriorly located) generally does not lead to significant neurologic deficit, serious impairment may arise because of poor patient selection, inattentive consideration of the vascular anatomy, or inadequate techniques.
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Crossed dominance, wherein the hemisphere controlling the dominant hand is contralateral to the hemisphere controlling speech and language, is a contraindication. Crossed dominance can arise after cerebral injury during childhood that resulted in cortical functional reorganization. These patients may develop writing and speech deficits postoperatively. Special consideration should be given to cases in which a more posterior callosotomy (splenium) is required, increasing the risks of cognitive dysfunction (e.g., alexia), particularly in patients with established preoperative visual field cuts (e.g., homonymous hemianopsia).

Planning and positioning

Patient Selection

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Patients who present with symptoms of cognitive impairment, such as memory deficits, should have preoperative neuropsychologic evaluation owing to potential risk of injury to the fornices.
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A preoperative vascular anatomy study is often helpful to assess the cortical and deep venous drainage and the relative risk of venous engorgement associated with a protracted hemispheric retraction and a meticulous surgical manipulation.

Patient Positioning

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For a parasagittal approach, the patient can be positioned in a neutral supine position or alternatively in a lateral decubitus position.

Figure 8-1:

For supine positioning, the vertex is elevated 45 degrees from the horizontal.
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The lateral decubitus position allows for gravity to help pull down the hemisphere away from the falx, allowing for greater midline exposure with less retraction on the hemisphere. Some surgeons prefer lateral positioning because the greater exposure allows access to a greater portion of the corpus callosum. The disadvantage of lateral positioning compared with supine positioning is the greater amount of midline distortion caused by gravity. Maintenance of a midline reference plane helps with operative orientation.

Procedure

Craniotomy

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The location of a given lesion is an important factor in planning the positioning of craniotomy. For better visualization of a lesion in the posterior lateral ventricle, a more anterior craniotomy is used. Most often, a craniotomy is made paramedian to the sagittal sinus along the nondominant (right) hemisphere. Preservation of draining veins takes priority, and consideration should be given to a left hemisphere approach if preservation of veins can be accomplished. A modified bicoronal incision (usually shorter and centered on the midline) is used to create a skin flap that can be distracted in the anteroposterior dimension.
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Exposure of the interhemispheric region requires the use of an anterior parasagittal craniotomy extending to or encompassing the midline. The midline craniotomy is over the superior sagittal sinus. The bone flap is positioned in relation to the coronal suture. To minimize the chances of injuring the sinus or parasagittal veins feeding the sinus, care should be taken to position the posterior margin of the bone flap no more than 2 to 3 cm posterior to the coronal suture. This is done to avoid venous tributaries, which often enter the sinus approximately 2 to 3 cm behind the coronal suture. The anteriormost edge of the bone flap can be made up to 4 to 5 cm in front of the coronal suture depending on the extent of exposure needed. For definitive understanding of individual variations in venous tributary anatomy, obtaining preoperative computed tomography (CT) venography or magnetic resonance venography is recommended.
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On exposure of the parasagittal region, we make a rectangular craniotomy using a variable number of burr holes according to the condition of the underlying dura. It is of paramount importance to dissect free the dura of the superior sagittal sinus, and this can be done by placing two burr holes on the paramedian ipsilateral edge of the sinus or three burr holes (two ipsilateral and one contralateral) on each side of the sinus. In each case, the interhemispheric region is generously exposed with a covered sinus in the first case and a visualized sinus in the second. Time should be taken to dissect the dura carefully from the inner table working away from the sagittal sinus. The burr holes are then connected with either a craniotome or a Gigli saw.

Interhemispheric Dissection

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A semicircular or trapezoidal dural dissection is made based on the lateral edge of the sagittal sinus. As the flap is retracted, an effort should be made to preserve bridging veins. The objectives of the interhemispheric dissection are to prevent venous infarction and to ensure minimal retraction on the brain. To prevent venous infarction secondary to overretraction, one must be cognizant to limit retraction to no more than 2 cm along any part of the corridor. Pauses of 2 to 3 minutes should be observed after every advancement of the retractor blade down the interhemispheric fissure. This pause allows for the ventricular pressures to equilibrate in the face of forces exerted by the retractor itself.
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Initially, arachnoid granulations along the medial hemisphere are opened with sharp dissection. A combination of blunt dissection with the blunt end of a No. 1 Penfield and advancement of the retractor blade should enable for adequate dissection down the midline. Before arriving at the corpus callosum, the inferior falx, inferior sagittal sinus, cingulated gyri, callosomarginal arteries, and pericallosal branches of the anterior cerebral arteries should be identified. The corpus callosum can be identified easily because of its glistening and relatively hypovascular aspect.

Collosotomy

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The length and site of the callosotomy incision depend on the location of the lesion one is trying to approach. The corpus callosum should be split down the midline with the use of microinstruments and microirrigators. With ventricular masses, there may be midline distortion of the corpus callosum. It is important to anticipate asymmetry by thoroughly reviewing preoperative imaging. After the trunk of the corpus callosum is split, the callosotomy is widened with bipolar coagulation and a microsuction (5F) tip. Care must be taken at this stage to ensure proper hemostasis to prevent intraventricular hemorrhage.
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After the callosotomy is made, the retractor can be advanced to expose the lateral ventricular anatomy. If the foramen of Monro is open, a physical barrier should immediately be placed at its entry to prevent blood from pooling into the third ventricle. If the contralateral ventricle is entered, fenestration or excision of the septum pellucidum can open access into the ipsilateral lateral ventricle. Fenestration of the septum also allows for the alternative pathway of cerebrospinal fluid flow. The fornices travel across the base of the septum and must be preserved. Identification of normal ventricular anatomy should reveal the septal vein, septum pellucidum, fornices, thalamostriate vein, internal cerebral veins, choroid plexus, and head of the caudate. Following the thalamostriate vein, septal vein, fornices, or choroid plexus reliably guides the surgeon to the foramen of Monro.