A simple and harmless method is described for detecting the patency of ventriculo-peritoneal shunts with a pair of small disk thermistors. This method is based on the principle that when a moving column of fluid is locally cooled, a drop in temperature can be recorded at some distance from the place of application in the direction of the movement.
A disk thermistor, measuring 6 mm in diameter covered with Epon 812 is taped to the chest skin just above the shunt tube 4 cm distal from the cooling point, and the other thermistor, the reference one, is also taped to the skin beside the shunt tube 4 cm distal from the cooling point. They are balanced so that, when the measuring thermistor detects a drop in temperature, a pen-deflection occurs. As a cold source we use an ordinary cube of ice in a vinyl sack for one minute.
In the experiment a shunt tube was embedded in clay, the results of which showed a significant pen-deflection between different flow velocities, ranging from 5 m
l/h to 60 m
l/h (120 m
l/24 hrs and 1, 440 m
l/24 hrs).
In clinical data there were 23 hydrocephalic children and 9 adult patients with normal pressure hydrocephalus, and they were examined 26 and 10 times respectively. Three out of 23 hydrocephalic children could not be examined because of their violent movements, but all of the adult patients could be examined.
In all 25 patients whose clinical pictures suggested functioning shunts, the proper diagnosis of shunt patency was obtained by this method. In 6 patients whose clinical pictures were dubious as to whether the shunt was patent or not, this method revealed that all shunts were patent and their CT scans and follow-up data supported shunt patency. In 2 patients whose clinical pictures suggested shunt malfunction, this method revealed no pen-deflection, and their shunt obstructions were proved by operations.
This method is a simple and safe procedure which may be repeated at frequent intervals.
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