The Kuopio Idiopathic Normal Pressure Hydrocephalus Protocol Revisited: The Predictive Value of Cerebrospinal Fluid Tap Test

BACKGROUND AND OBJECTIVES: Idiopathic normal pressure hydrocephalus is a condition with gait impairment, incontinence, and cognitive decline, treated with shunt surgery. The Gothenburg scale (GBS) provides comprehensive assessment of symptoms and outcomes. The cerebrospinal fluid tap test (CSF-TT) is used as a prognostic tool, but its predictive value remains inconsistent. This study aimed to evaluate the clinical utility of CSF-TT and its association with postoperative outcomes measured using GBS, a rarely applied multidomain outcome measure in CSF-TT evaluation. METHODS: A prospective, population-based cohort of consecutive patients with suspected idiopathic normal pressure hydrocephalus was examined. Shunt surgery was recommended based on clinical and radiological criteria, with CSF-TT serving as prognostic support. Outcomes were assessed using GBS preoperatively, at 3 and 12 months. Linear mixed-effects models were applied to evaluate shunt outcomes and receiver operating characteristic analysis assessed classification accuracy. RESULTS: A total of 218 patients underwent shunt surgery, 181 completed follow-up (median age 74.6, 56% female). Mean GBS scores increased by 13.6 (95% CI: 11.5-15.6) at 3 months and 13.7 (95% CI: 11.4-16.1) at 12 months. Patients without CSF-TT improvement showed no response (mean change 5.3 [95% CI: -0.6 to 11.2] at 3 months; 2.9 [95% CI: -3.9 to 9.6] at 12 months). Classification accuracy of CSF-TT was low (3 months: area under the curve 0.56; 12 months: area under the curve 0.59). At 12 months, positive predictive value was 76% and negative predictive value 45%. CONCLUSION: CSF-TT is associated with shunt outcomes when assessed with GBS. Strict reliance on CSF-TT thresholds may inappropriately exclude patients who could benefit from shunt surgery.