3.**Cisternal space in this space, we must recognize:**


**Figure 7.** *Cisternal space (prepontine).*

**Figure 8.** *Prepontine cisternal space.*

#### **Figure 9.**

*Prepontine space. Lilliequist's membrane (mesencephalic portion) open spontaneously, but present forming a tent in the cisternal space.*

#### **Figure 10.**

*Floor of the 3rd ventricle. Variant in a patient with hydrocephalus and myelomeningocele. Indistinguishable mammillary bodies, transparent premammillary membrane revealing the vascular anatomy: Basilar artery and its terminal branch.*

Liliequist's membrane (mesencephalic portion) opens spontaneously but presents forming a tent in the cisternal space.

#### **Liliequist's membrane.**

Originally described by Key and Retzius in 1875, and after described by Liliequist in 1956 [13, 14] is an arachnoid membrane separating the chiasmatic cistern, interpeduncular cistern and prepontine cistern. It arises anteriorly from the diaphragma sellae and extends posteriorly separating into two sheets, The membrane of Liliequist is a partially trabecular, partially dense folded inner arachnoid membrane, and a very important anatomic landmark in the anatomy of the interpeduncular fossa and sellar and parasellar regions [15]. The membrane of Liliequist consists of two leaves: a superior diencephalic and an inferior mesencephalic one [16], these leaves are highly variable in their shape, distribution, and density, most commonly trabeculate (**Table 3**).

## **3. Anatomical injuries**

With endoscopic access, there is a risk of injuring anatomical structures that are in the surgical corridor or adjacent structures that can be directly injured or by vascular lesions.

*Endoscopic Third Ventriculostomy in the Pediatric Patient DOI: http://dx.doi.org/10.5772/intechopen.111534*


#### **Table 3.** *ETVSS.*
