Human vagus nerve electrophysiology: a guide to vagus nerve stimulation parameters.

The authors studied human vagus nerve electrophysiology intraoperatively on 21 patients (age range: 4 to 31 years) during implantation of a vagus nerve stimulator for seizure control. The study was performed with direct electrical stimulation of the vagus nerve with various stimulation parameters resembling those employed by the Cyberonics NeuroCybernetic Prosthesis System (Houston, TX), which is used clinically for vagus nerve stimulation for treatment of seizures. Recordings were made directly from the rostral end of the vagus nerve. The response of the vagus nerve to various stimulus parameters in patients of different ages was studied. Based on the vagus nerve characteristics, age-related adjustments for stimulus parameters were recommended.

Postural changes in intracranial pressure in chronically shunted patients.

A subset of hydrocephalic patients with indwelling shunts become symptomatic when they are upright and active. Intracranial pressure (ICP) measurements in these patients have shown a significant drop in pressure when the patient is upright with return to normal levels when the patient is supine. In 20 chronically shunted hydrocephalic patients who previously had no siphon protection devices, ICP changes in supine and upright position were studied at the time when the patient had external ventriculostomy for treatment of shunt infection. Our hypothesis was that these patients might display rapid changes in ICP from fluid shifts occurring in non-CSF compartments. To minimize the effects of hysteresis, drift and zero-point error, measurements were made using a fluid manometer rather than a strain gauge pressure transducer. The pressure-volume index was estimated using the standard technique of bolus injection. Intracranial CSF volume was estimated on CT scans. The fluid shift was calculated using a mathematical model of the CSF compartment that incorporates negative pressure and volume components that permits simulation of siphoning. Sixteen patients had small, slit ventricles; 3 patients had moderate-sized ventricles and in 1 patient the ventricular size was normal. The average intracranial CSF volume estimated on CT scan was 12 cm(3). There was a mean drop in ICP in the upright position of 159 mm H(2)O. The mean PVI of 42 ml suggested a volume displacement out of proportion to the available intracranial CSF volume. Based on these findings, we conclude that even in the absence of drainage through the shunt, chronically shunted patients still display a fall in ICP when assuming the upright position. This raises the possibility of fluid shifts other than of CSF through nonshunt pathways. Possible mechanisms involving altered CSF-venous system interaction are discussed.

Evaluation of shunt malfunction using shunt site reservoir.

OBJECTIVE: To determine the usefulness of a separate reservoir placed at the site of the shunt in evaluation of shunt malfunction. METHODS AND MATERIALS: A ventricular catheter was placed alongside the proximal catheter of the shunt and connected to a subgaleal reservoir in 17 patients, in 9 a double-lumen catheter with integrated reservoir and in 13 patients a dual catheter with a double-port reservoir was used. At presentation of suspected shunt malfunction, a standard shunt function evaluation using shunt tap, CT scan or shunt injection was performed, and subsequently, the pressure from the tap of the reservoir was obtained. RESULTS: Thirty-three patients presented with symptoms of malfunction at an interval of 2.3 +/- 3 months (range 2-429 days). The pretest probability of shunt malfunction in this population was 73%. Posttest probability of shunt malfunction was 82.5% with standard evaluation and improved to 100% by the separate reservoir tap pressure measurement. In 4 patients in whom the shunt tap was dry, shunt infection was diagnosed prior to revision using CSF obtained at the reservoir tap. In 5 patients with proximal malfunction and bradycardia, the reservoir tap allowed early ventricular decompression. CONCLUSION: This study shows that a reservoir placed at the site of the shunt remains patient even when the shunt malfunctions, suggesting that flow rather than catheter position is important in proximal malfunction. It is superior to shunt tap for detection of shunt malfunction and infection, and it allows early ventricular decompression in a sick patient awaiting surgery for shunt revision.

Adjustable antisiphon shunt.

Overdrainage from siphoning, a result of negative distal pressure, has been a major problem with the existing differential pressure shunt devices. The antisiphon/siphon control devices used to reduce siphoning often malfunction owing to fibrous scar formation around the valve. Similarly, the functioning of gravity-actuated systems is adversely affected by body movements. Constant flow shunt systems may not be superior to existing differential pressure valves, as has been shown in a recent multicenter study. In view of these difficulties, we investigated the possibility of developing a shunt valve based on the Starling resistor concept of flow regulation that allows proximal pressure-dependent and distal pressure-independent flow. The valve designed is capable of proximal pressure-dependent and distal pressure-independent flow. The valve allows for adjustable negative pressure in the vertical position.

Useful components of the shunt tap test for evaluation of shunt malfunction.

Results of shunt tap were studied in 224 clinically or radiologically suspected instances of shunt malfunction. In 130 retrospectively studied patients the results of the tap had reported the opening pressure and ease of aspiration. In 94 prospectively studied instances the shunt tap parameters described were (i) the opening pressure, (ii) the drip interval, i.e., the interval between the drops of cerebrospinal fluid when the open end of the butterfly used for tapping was placed 5 cm below the level of the valve, and (iii) the closing pressure. The efficacy of the aspiration procedure for proximal malfunction was 40.3%, compared with the efficacy of drip interval which was 95.1%. For distal malfunction, the efficacy of measurement of opening pressure was 54.3% whereas that of closing pressure was 60.6%. An in vitro model of a functioning shunt showed that the opening and the closing pressures were related to the flow rate and the level of the distal catheter tip with respect to the valve, whereas the drip interval was linearly related to the flow through the proximal catheter and was independent of the distal catheter position. The opening pressure, closing pressure, and the drip interval recorded at surgery were not significantly different from the values obtained by shunt tap. The results suggested that shunt tap accurately provides information otherwise obtained at surgery and the drip interval is most useful for evaluating a proximal malfunction. The subjective impression of the distal flow may be more helpful in diagnosing distal malfunction rather than the absolute level of opening or closing pressures.