Neurosurgery

infancy, and can restrict the range of direction of approach when using a rigid neuroendoscope. There may be thick, calcific subdural membranes from past subdural hematomas. The wall of the lateral ventricle may be tough and thick, especially in those patients shunted for perinatal post-hemorrhagic hydrocephalus, those with slit-ventricle syndrome, and those who have suffered ventriculitis. The internal anatomy of the lateral ventricle can be bizarre. There may be synechiae related and unrelated to the presence of a ventricular shunt catheter. The usual landmarks leading to the interven-tricular foramen may be absent, especially in patients who have suffered intraventricular hemorrhage, or ventriculitis associated with meningitis or serious ventricular shunt infections. On occasion, the lateral ventricles can be subdivided by complete or incomplete septae. The septum pellucidum may be spontaneously perforated or absent. The interventricular foramen may be completely obliterated by gliosis or may have assumed an abnormal configuration. Alternatively, patients with very large lateral ventricles due to chronic shunt malfunction can have very large interventricu-lar foramina that are so huge that the third ventricle is almost assimilated into the lateral ventricle. The third ventricle can also be very abnormal, with gliotic septae obscuring or frankly obstructing the pathways. The cavity of the third ventricle may be narrow.

The usual landmarks on the third ventricle floor may be quite unclear. The anterior part of the floor may be thick and opaque; fortunately, the vascular area that marks the recess of the pituitary infundibulum is usually preserved. The interpeduncular cistern may be densely obliterated by subarachnoid adhesions that may in themselves conceal the basilar artery and its branches and cranial nerves III and VI. Liliequist's membrane may be abnormally thick. The circle of Willis may be in an unusual position. The two most frequent variants are an unusual application of the basilar artery to the dorsum sellae and upper clivus, usually due to subarachnoid scarring, and an abnormal tortu-ousness of the anterior communicating artery, which may bulge into the anterior part of the third ventricle. Patients with intracranial tumors may have anatomical distortions due to the presence of tumor tissue or the effects of previous surgery and radiation therapy.

Regression of clinical symptoms and avoidance of an implanted diversionary CSF shunt indicate a successful outcome. Routine postoperative imaging is not mandatory. In 60% of cases ventricular size is unchanged despite relief of symptoms; ventricular volume may drop despite ventricular size remaining constant. A flow void through the ventriculostomy on appropriate MR sequences is confirmation of functional patency of the ventriculostomy and correlates with radionuclide studies; flow voids in the interventricular foramina and interpe-duncular cistern indicate active CSF flow, but signal in the prepontine cistern alone reflects basilar artery pulsation.

Results of NTV

One of the longest running and largest series is that accumulated in Sydney, Australia, which extends back to 1978. In a mixed series of 103 children and adults, there was an overall success rate of 61%, with no difference between those undergoing primary, as opposed to secondary, NTV [10]. In a purely adult series from Nottingham, UK, followed for a mean of 3 years, 80% of 63 patients were successfully treated by NTV [11]. In both of these series there was no difference between those having primary NTV and those previously shunted patients undergoing secondary NTV. In an earlier, predominantly pediatric, series from Nottingham, with a median age of 16 months, there was a success rate of 62% [1]. Smaller series have reported much higher success rates, but in highly selected cases. For example, a French center reported 33 successes in 35 previously untreated cases [12]; around 60% would seem to be the overall success rate in unselected cases across all ages. Although there is a wider range of experience for secondary NTV, most reported series from experienced operators report success in 60-80% of cases (Table 6.4). This probably reflects a

Table 6.4. Success rate of secondary NTV

Series

[ref]

n

Success (%)

Jones, 1992

[6]

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