These approaches enter the third ventricle through the lamina terminalis (20, 21). They are suitable for small tumors but do not allow sufficient vision of the upper and posterior compartments of the third ventricle. When this is needed, a combination of anterior transcallosal route with subfrontal or pterional approaches may be used,(24, 44, 45).
- Supine: The patient is positioned supine with the head up about 30 degrees, slightly turned to the opposite side, and the neck extended in such a way that the frontal lobe tends to fall back as a result of gravity (28).
- Scalp incision: A bicoronal incision is made starting 1 cm anterior to the tragus at the zigomatic arch, extending contralaterally until it is possible to turn a scalp flap that exposes the entire right orbital ridge and the insertion of the zygoma into the frontal bone. Usually the temporal muscle is included in the scalp flap and not separated from it so that the two are retracted as a single unit.
- Craniotomy: A right-sided craniotomy is typically used (unless the surgeon is left-handed). A rectangular bone flap whose medial edge is the cranium’s midline is cut. The inferior edge of the craniotomy should be cut as flush as possible with the orbital roof. A bifrontal craniotomy may be useful in some cases. In these cases the superior sagittal sinus and falx should be divided as far anteriorly as possible (28). If the frontal sinus is entered, its mucosa is pushed downward before the internal wall of the sinus is removed, and the sinus should be obliterated during closure. When needed, the orbital roof can be removed by incorporating it into the frontal flap as a single piece (17).
- Easy: Easier exposure of the lamina terminalis is obtained.
- Low risk: No major neural or vascular structures are on the way.
- Anosmia: There is a high incidence of anosmia.
- Entry into frontal sinus: The frontal sinus frequently blocks this approach, thus necessitating its entry.
Frontolateral (Pterional) Approach
- Supine: The patient is positioned supine with the head elevated 20 degrees and rotated 30 degrees toward the opposite shoulder. The neck is extended so that the head’s vertex is tilted down, bringing the upper part of the zygomatic process to the highest point.
- Scalp incision: An incision starting 1 cm anterior to the tragus and extending superiorly just behind the hair line to the midline is made. The temporal muscle may be reflected with the scalp in a single flap, or isolated, after an interfacial dissection (44).
- Craniotomy: First a bur hole is drilled at the keyhole and another at the temporal squama. Additional bur holes, if wanted, can be made along the superior temporal line coronal suture. The sphenoid ridge is then drilled off until the orbitomeningeal artery is found. The craniotomy can be enlarged by removing the orbital roof.
- Dural opening: The dura is opened in a curvilinear fashion and reflected over the sphenoid ridge.
- Retract frontal lobe: The frontal lobe is then retracted and elevated adjacent to the sylvan fissure, following the sphenoid ridge, in a gentle and progressive fashion. The ipsilateral olfactory nerve and optic nerve are identified, and the arachnoid over the optic chiasm and carotid arteries is opened to allow CSF to drain from the cisterns.
- Retract temporal lobe: Next, the temporal lobe’s tip is retracted posteriorly to expose the tentorial edge and third nerve.
- Open the sylvian fissure: The arachnoid of the sylvian fissure is then dissected medial to the sylvian vein, and the fissure is opened to reach the carotid bifurcation by following M1. At this point the arachnoid is usually tough and has to be incised with sharp instruments.
- Follow A1 to lamina terminalis: The A1 segment of the anterior cerebral artery is followed to the anterior communicating artery. The lamina terminalis is found below the A1-anterior communicating artery complex and is often bulging. It is opened with a blunt instrument immediately anterior to the anterior communicating artery and posterior to the optic chiasm, from one optic tract to the other (27, 34).
- Short distance: This approach represents the shortest distance from the scalp to the midline.
- Avoids frontal sinus: This approach is well lateral to the frontal sinus.
- Poor visualization: Opposite optic nerve and carotid artery are poorly visualized. Ipsilateral carotid arteries and optic nerves are in the way.
Combined Subfrontal or Pterional/Anterior Transcallosal Approach
The two approaches described above can be combined with an anterior interhemispheric transcallosal approach when a larger exposure is required, such as when large tumors extend up to the foramen of Monro causing either unilateral or bilateral ventricular enlargement (24, 44, 45). The combined pterional/anterior transcallosal approach developed by Yasargil (44, 45) will be briefly described below.
- Supine: The patient is positioned supine with the head elevated 20 degrees, flexed slightly forward, and turned 30 degrees to the contralateral side.
- Scalp incision: A bicoronal or fronto-temporal incision is made.
- Craniotomy: Two separate right-sided craniotomies are performed in order to approach both the frontolateral region and the anterior interhemispheric fissure, as previously described. The first step in the pterional approach is an extradural removal of the sphenoid wing.
- Dural opening: The dura is reflected over the sphenoid ridge.
- Follow A1 to lamina terminalis: Retract frontal lobe and open the sylvian fissure as described above. Follow A1 to the lamina terminalis as described above.
- Entry into third ventricle: The second step is the anterior intehemispherical transcallosal approach and entry to the third ventricle through the foramen of Monro.
Advantages of the Subfrontal Approach
- Easier exposure of the lamina terminalis
- No major neural or vascular structures on the way
Disadvantages of the Subfrontal Approach
- High incidence of anosmia
- Frontal sinus on the way
Advantages of the Pterional Approach
- Shortest distance from the scalp to the midline
- Avoids opening the frontal sinus
Disadvantages of the Pterional Approach
- Opposite optic nerve and carotid artery poorly visualized
- Ipsilateral carotid arteries and optic nerves on the way