The reliability of the "absent cistern sign" in assessing LP shunt function

Objective: One of the difficulties with lumboperitoneal (LP) shunts has bee n non-invasively ascertaining shunt function. It has been previously report ed that in the presence of a functioning LP shunt the perimesencephalic cis terns become obliterated - the "absent cistern sign". In order to more rigo rously test this association we performed a retrospective analysis of LP sh unt patients at the Hospital for Sick Children, Toronto. Methods: The CT sc ans of all patients undergoing LP shunting over a 17 year period were revie wed. The "absent cistern sign" and ventricular size were compared against t he results of either an isotope shunt study or surgical findings performed within 2 days of the CT, Results: There were 38 CT scans (27 patients) perf ormed within 2 days of an isotope shunt study and 15 CT scans (14 patients) performed within 2 days of a surgical intervention, These results give the absent cistern sign a sensitivity of 75% and a specificity of 57% when com pared to the shunt isotope findings and a sensitivity of 100% and a specifi city of 50% when compared to the surgical findings. Over 30% of the CT scan s showed ventriculomegaly in the presence of a functioning shunt and, conve rsely, nearly 45% of the CT scans had normal or small lateral ventricles in the presence of a malfunctioning shunt. Conclusions: The "absent cistern s ign" appears to reliably rule out a completely blocked shunt, but is less r eliable in detecting a normal or partially obstructed shunt. Ventricular si ze correlates poorly with LP shunt function.