Neurodevelopmental arachnoid cysts in sup-rasellar, quadrigeminal, middle cranial fossa, interhemispheric septum pellucidum, and par-enchymal locations have all been approached neuroendoscopically. The guiding principle is to marsupialize the cyst into an adjacent normal cerebrospinal fluid chamber or pathway. Pre-operative MR with CISS is again enormously helpful in planning an approach by displaying the fluid/cyst wall/fluid interfaces in three orthogonal planes. The next important principle is, wherever possible, to approach the lesion via a normal cerebrospinal fluid space or chamber. Even if this space is smaller than the cyst, the advantage of having some normal, and hopefully recognizable, anatomy greatly exceeds the perceived difficulty in entering a space that may not be particularly dilated. Generous fenestrations of 1-2 cm are required, and are best made using cutting/coagulating diathermy. Suprasellar cysts are approached by the right frontal route to the lateral ventricle, and then via the interventricular foramen. The dome of the cyst is widely opened into the ventricular system (cysto-ventriculostomy) and the cyst then usually collapses, exposing the hitherto obstructed posterior third ventricle and aqueduct. There is debate as to whether the base of the cyst should then be opened into the interpeduncular cistern (cysto-cisternostomy) .
The author's approach is to perform both cysto-ventriculostomy and cysto-cisternostomy if the latter seems safe. However, if the area is very vascular, such that there is no very apparent safe route, then a generous cysto-ventriculostomy usually suffices. Quadrigeminal plate cysts can usually be opened into the third ventricle by an approach via the lateral ventricle and interventricular foramen, although on occasion there is an interface presenting into a lateral ventricle that can be accessed. Intraparenchymal cysts can often be marsupial-ized into a lateral ventricle. Other midline cysts may be made to communicate with the ventricular system or the subarachnoid space. Symptomatic cysts of the septum pellucidum may be approached via a lateral ventricle. If the cyst is punctured directly, the very different anatomy will warn the surgeon of the position and, with care regarding the midline vascular structures and the fornices, the cyst can be marsupialized into the third ventricle. Many of these cysts will require unique approaches and directions of attack that are not along straight lines; it is in this type of case that the flexible neuroendoscope really comes into its own and has considerable advantages over a rigid instrument. On occasion, small intraventricular third ventricle cysts of presumed ependymal origin, unsuspected from pre-operative imaging, have come to light in the course of performing a NTV; these can be readily opened up to relieve the hydrocephalus.
The place of neuroendoscopy in the management of colloid cysts of the third ventricle remains uncertain. Early reports concerned diagnostic rather than therapeutic interventions . Subsequently it became clear that some
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