Posterior Approaches to the Third Ventricle in Children

Posterior Interhemispherical Transcallosal Approach

• Indications: Recommended for lesions that extend superiorly from the trigone or involve the splenium of the corpus callosum (19, 33, 45).

Positioning

• Three-quarter prone or sitting:  In the prone position the side to be operated on is  downward. 

Surgical Approach

• Posterior interhemispherical transcallosal approach: The initial approach is that described for entry into the posterior lateral ventricle via an interhemispheric, transcallosal approach.
• Expose floor of lateral ventricle: The opening through the splenium in the midline exposes the roof of the third ventricle. The internal cerebral veins and their junction with the great vein of Galen are found below the splenium. Lesions in this region tend to displace them laterally, but sometimes they must be separated from the lesion.
• Entry into third ventricle: Opening the layers of tela choroidea will expose the third ventricle.

Approach Advantages

• No bridging veins blocking entry interhemispheric fissure: There are no bridging veins connecting the posterior parietal lobe to the superior sagittal sinus
• Good view of tumors around splenium: The approach is useful for tumors with upward extension through the posterior part of the splenium or tumors that lie above the deep venous system.

Approach Disadvantages

• Retraction injury: Parietal lobe retraction may cause neurological deficits.
• Risk bridging veins anteriorly: Anteriorly placed bridging veins can be damaged by excessive retraction.
• Hemialexia and disconnection syndrome: Sectioning of the splenium may cause hemialexia and disconnection syndromes.
• Potential blockage by Galenic veins: Deep cerebral veins (Galenic system) may be on the way.

Occipital Transtentorial Approach

Positioning

• Three-quarter prone or sitting:  In the prone position the side to be operated on is  downward. 

Surgical Approach

• Scalp incision: A parieto-occipital scalp incision is made extending across the midline. It should be large enough to identify the sagittal and lambdoid sutures.
• Craniotomy: A bone flap is turned with its two medial bur holes placed at the midline 6 – 8 cm apart, with the lower one just at the margin of the torcular herophili,. The upper margin of the transverse sinus should be exposed.
• Dural opening: The dura mater is opened with two pedicles, one toward the superior sagittal sinus and the other toward the transverse sinus.
• Retraction of occipital lobes: The medial occipital lobe is then retracted away from the falx. Retraction is usually minimal due to the gravitational effect from the head positioning. As the retractors are advanced, the free edge of the tentorium’s incisura will come into view.
• Division of tentorium: The tentorium is divided from the incisura posteriorly in a lateral direction, parallel to the straight sinus towards the transverse sinus. Aberrant venous sinus are sometimes seen and should be clipped or coagulated. The medial flap is retracted by sutures, and the top of the cerebellum is exposed.

 

 

• Entry into cisterns: The arachnoid over the ambient and quadrigeminal cisterns is usually thick and should be incised sharply. The dissection should start near the cerebellum. The precentral veins are identified in the midline, coagulated, and divided. This allows the cerebellum to be retracted, making the final approach to the tumor easier.

 

 

Approach Advantages

• Pineal region tumors: The approach is suitable for tumors of the posterior third ventricle, extending to quadrigeminal cistern especially at the pineal region.
• No bridging veins: There are no bridging veins on the way.
• Lessens risk retraction injury: Minimal or no retraction is required.
• Good exposure: Wide exposure of the tumor and access to the superior cerebellar vermis and splenium are possible, if required.

Approach Disadvantages

• Blocking veins: The overlying deep venous system may make tumor removal difficult.
• Injury to occipital lobe: Hemianopsia due to inappropriate retraction of the occipital lobe may occur.