Anterior Approaches to the Third Ventricle in Children

Transforaminal Approach

Positioning

  • Approach dependent: The initial anterior approach to the third ventricle is via the lateral ventricle, so positioning is similar to that used to approach the anterior lateral ventricle.

Surgical Approach

  • Approach to lateral ventricle: This portion of the approach is covered in the section above that describes approaches to the lateral ventricle.
  • Identify landmarks: First, the foramen of Monro is identified using landmarks such as the choroidal plexus, thalamostriate, and septal veins.
  • Fenestration of septum pellucidum: Both foramina of Monro can be exposed by fenestrating the septum pellucidum in its midsection. This will allow bilateral exploration of the third ventricle (42).
  • Transforaminal entry into third ventricle: The foramen of Monro can be used to access the third ventricle. It may be enlarged when needed by incising the ipsilateral column of the fornix at the anterosuperior margin of the foramen of Monro, but this may produce sequelae (4). Additionally, when the tumor protrudes through the foramen of Monro, it will usually enlarge the foramen and help the surgeon gain access to the anterior third ventricle. The foramen can also be enlarged by opening the choroidal fissure as far as the junction of the anterior septal vein and internal cerebral vein (44, 45).

Approach Advantages

  • Good anatomical landmarks: Anatomical landmarks lead to the foramen of Monro and third ventricle.
  • Tumors within foramen of Monro easily exposed:TIdeal for tumors emerging through the foramen of Monro. The foramen of Monro can be enlarged.

Approach Disadvantages

  • Poor access to posterior third ventricle: There is limited vision and access to the posterior part of the third ventricle.
  • Forniceal columns vulnerable: Forniceal injury may cause memory deficit.
  • Venous injury: Venous bleeding from inside the third ventricle may be troublesome.

Transchoroidal Approach

Positioning

  • Approach dependent: The initial anterior approach to the third ventricle is via the lateral ventricle, so positioning is similar to that used to approach the anterior lateral ventricle.

Surgical Approach

  • Approach to lateral ventricle: This portion of the approach is covered in the section above that describes approaches to the lateral ventricle.
  • Identify landmarks: The choroidal fissure is located underlying the choroidal plexus of the lateral ventricle. The plexus is attached along the choroidal fissure, between the fornix and thalamus.
  • Open the choroidal fissure: The fissure is opened along the attachment of the choroidal plexus to the fornix (tenia fornices), beginning at the posterior edge of the foramen of Monro (41, 33). Enter the roof of the third ventricle by opening the layers of the tela choroidea. The fornix is displaced to the opposite side.

Approach Advantages

  • Accesses entire third ventricle: The approach allows exposure of the middle and posterior third ventricle.

Approach Disadvantages

  • Difficult dissection: The anatomical definition of the velum interpositum may be difficult, and there is a close relationship of the velum with the internal cerebral veins.

Interforniceal Approach

Positioning

  • Approach dependent: The initial anterior approach to the third ventricle is via the lateral ventricle, so positioning is similar to that used to approach the anterior lateral ventricle.

Surgical Approach

  • Approach to lateral ventricle: This portion of the approach is covered in the section above that describes approaches to the lateral ventricle.
  • Identify landmarks: The columns of the fornix are identified, as they will form part of the roof of the third ventricle. The septum pellucidum facilitates the identification of the midline between the two fornices.
  • Divide forniceal columns: An incision between the bodies of the fornices at the level of the foramen of Monro carried posteriorly for 1-2 cm opens the third ventricle from above (3, 4).

 

Sagittal view of velum interpositum or roof to third ventricle: Below the fornix, the internal cerebral veins, the posterior choroidal artery and its branches course between the two layers of the tela choroidea within the velum interpositum. (Courtesy of Dr. Cassius V. C. Reis, FUMG, Belo Horizonte, Brazil)

 

  • Penetrate the velum sagittal: Roof of the third ventricle. Below the fornix, the internal cerebral veins, the posterior choroidal artery and its branches course between the two layers of the tela choroidea, the velum interpositum. These structures are often displaced from the midline by the tumor.

Approach Advantages

  • Exposes entire third ventricle: This approach allows exposure of the middle and posterior third ventricle.
  • Tumor or cavum spetum pellucidum can do dissection: This approach is useful when there is either a cavum septum pellucidum or when the tumor separates the fornices.

Approach Disadvantages

  • Difficult dissection: The anatomical definition of the space may be difficult. This can result in a forniceal injury that can cause severe memory deficit.