Instrumented gait analysis for an objective pre/post assessment of tap test in normal pressure hydrocephalus.

OBJECTIVE: To present an objective method to evaluate gait improvements after tap test in idiopathic normal pressure hydrocephalus (iNPH). Tap test is often used to prognosticate shunt responsiveness, although test accuracy is limited by the lack of quantitative outcome measures. DESIGN: Retrospective analysis of gait data. SETTING: Public tertiary care center, day hospital. Gait analysis was performed before and 2-4 hours after tap test. PARTICIPANTS: 60 iNPH patients and 50 age and sex matched controls (used to obtain reference intervals). From an initial referred sample of 79 patients, we excluded those unable to walk without walking aids (n = 9) or with incomplete (pre/post tap test) gait data (n=10). Thirteen out of 60 patients were shunted and then reappraised after 6 months. INTERVENTION: Not applicable. MAIN OUTCOME MEASURES: Mahalanobis distance from controls, before and after tap test. Eleven gait parameters were combined in a single quantitative score. Walking velocity was also evaluated, since it is frequently used in tap test assessment. RESULTS: Patients were classified as: (A) tap test responders (n=22 patients, 9 of them were shunted), or (B) not suitable for shunt (n=38 patients, 4 of them were shunted). In group A, 9 out of 9 patients improved after shunt. In group B, 3 out of 4 patients did not improve. Gait velocity increased after tap test in 53% of responders and in 37% of patients not suitable for shunt. CONCLUSIONS: The new method is applicable to the clinical practice and allows for selecting tap test responders in an objective way, quantifying the improvements. Our results suggest that gait velocity alone is not sufficient to reliably assess tap test effects.