Disease duration: the key to accurate CSF tap test in iNPH.

Objectives: The cerebrospinal fluid tap test for idiopathic normal pressure hydrocephalus (iNPH) is one of the good predictors of the shunt treatment, although this test has a low sensitivity. We aimed to identify key parameters that could be used to improve this sensitivity.

Materials & Methods: During 2010-2011, we recruited and then followed 93 patients with possible iNPH for 12 months after shunt. Among them, 82 patients were finally enrolled in this study. The modified Rankin Scale, iNPH grading scale, and several quantitative measurements were evaluated at entry, after the tap test, before and after shunt. Area under the receiver-operating characteristic curves (AUCs), sensitivities, and specificities of the tap test for predicting shunt effectiveness were calculated for each measurement. They were additionally assessed after stratification by disease duration since the initial presentation of iNPH symptoms.

Results: The gait disturbance on the iNPH grading scale had the highest accurate scale at the tap test for predicting effectiveness 12 months after shunt: AUC 0.74, sensitivity 56.5%, specificity 91.7%. This AUC increased to 0.76, 0.91 and 0.94 in the subgroup of disease duration <24, <12, and <6 months, respectively. The sensitivity and specificity of the gait disturbance on the iNPH grading scale in the subgroup of <12 months' duration were 92.3% and 90.0%.

Conclusions: The shorter period of clinical symptoms, for example, <12 months, made the tap test sufficiently accurate examination for predicting improvement 12 months after shunt surgery. The findings imply that the tap test should be applied to patients being considered for shunt surgery as soon as possible.