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.