**2. Endoscopic anatomy**

A preoperative MRI is almost always needed prior to surgery. From the frontal coronal burr hole, one reaches first the central part of the lateral ventricle near the frontal horn. The frontal horn is demarcated by the absence of choroid plexus. The lateral wall is formed by the nucleus with subependymal veins; medially is the septum pellucidum with septal veins. The choroid plexus and the foramen of Monro are very important landmarks for the central part of the lateral ventricle. The plexus is situated in the floor of the lateral ventricle, the thalamostriate vein lies laterally, and the septal vein's meeting point is on the medial wall; these three structures form the Y-shaped configuration necessary for orientation. The foramen of Monro is formed anterolaterally by the fornix, posteromedially by the anterior thalamic tubercle (**Figure 1**). On looking backwards with the endoscope, the body of the lateral ventricle back to the region of the trigone, with the body of the caudate laterally underlying the thalamostriate vein and the stria terminalis thalami. Adequate orientation of the morbid anatomy that can occur due to chronic hydrocephalus is needed [12–14].

**3. Indications**

ETV is considered the first management option in adult patients with obstructive hydrocephalus by many neurosurgeons. It is a reliable management option in adults with aqueductal stenosis with a success rate that reach 88% [15]. Stenosis at the aqueduct of Sylvius can be congenital or acquired. In three quarters of cases, the root cause might be unknown [16]. It is not considered as a stable condition as it can be tolerated for years, where stenosis is aggravated by trauma, subarachnoid hemorrhage, viral infection, or gradual accumulation of the CSF proximal to the aqueduct in partial obstruction [16–18]. In a previous study, clinical improvement with identifiable success of the procedure was detected in 86.4% of cases [19], where the success rate was lower in secondary ETV after VP shunt (**Figure 2**). This would be better identified in patients with previous history of multiple VP shunt revisions where the ETV failure is relatively more encountered [19, 20]. ETV is also preferred as the first-line management of hydrocephalus due

**Figure 1.** (A) Endoscopic view of the foramen of Monro (f) and its boundaries. (B) Endoscopic view of the floor of the third ventricle before perforation for ETV showing the planned site of perforation (asterisk) in between the tuber cinereum (t) and the mammillary bodies (m). (C) Endoscopic visualization of the membrane of Liliequist after perforation (arrow).

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(D) Endoscopic visualization of the basilar artery after opening the membrane of Liliequist (arrowhead).

to obstruction of the aqueduct of Sylvius with pineal tumors or tectal gliomas [21–23].

than 6 months of age, even in aqueductal stenosis [24, 25].

ETV is less successful in pediatric age groups, with the lowest success rate in children younger

The application of the ETV has been expanded to patients with hydrocephalus associated with fourth ventricular outlet obstruction, Dandy-Walker malformation [26], Chiari malformation [27–29], communicating hydrocephalus [30], and normal pressure hydrocephalus [31, 32].

By entering the foramen of Monro, the floor of the anterior part of the third ventricle is clearly identifiable, with the mammillary bodies and tuber cinereum as the two main structures needed for the anatomical orientation (**Figure 1**) [14].

Liliequist's membrane is an arachnoid leaflet situated in the basal cisterns and is a very important anatomical structure in the postsellar region.

**Figure 1.** (A) Endoscopic view of the foramen of Monro (f) and its boundaries. (B) Endoscopic view of the floor of the third ventricle before perforation for ETV showing the planned site of perforation (asterisk) in between the tuber cinereum (t) and the mammillary bodies (m). (C) Endoscopic visualization of the membrane of Liliequist after perforation (arrow). (D) Endoscopic visualization of the basilar artery after opening the membrane of Liliequist (arrowhead).
