**4. Surgical technique**

Under general anesthesia, the patient is restrained, disinfected, and draped as for a frontal burr hole; after opening the dura, a small corticectomy is performed, and a blunt obturator cannula is inserted, with free hands, directed medially toward the ipsilateral medial epicanthus and posteriorly toward the tragus of the ear. After insuring being in the ventricles by the outflow of CSF, the optical visualization system is inserted; we use the Lotta endoscope (Karl Storz, Tuttlingen, Germany) [7].

The first structure identified is the foramen of Monro, with the choroid plexus attached to the posterior margin (**Figure 1A**). The endoscope is introduced through the foramen to the third ventricle, and the floor is identified (**Figure 1B**). A small puncture is done using the decq forceps as posterior as possible to the infundibular recess and avoiding the mammillary bodies and the small arterioles running in this area. The endoscope is then advanced near to the puncture to visualize Liliequist's membrane which must be opened, after which, the CSF flow should be clearly visualized through the opening (**Figure 1C,D**).

It is important to clearly visualize the fornix before introducing the endoscope to the third ventricle. Opening the floor of the third ventricle with ballooning the fenestra aiming for its widening, with subsequent opening of the liliquist membrane, is important for a direct visualization of a naked basilar artery (BA) [25]. A delicate surgical technique is required with experienced hands during the opening of the floor of the third ventricle till the BA is clearly visualized to avoid major vascular injury [33].
