Clinical characteristics and outcomes after shunt surgery in idiopathic normal pressure hydrocephalus with or without prior cerebrospinal fluid tap testing: a single-center follow-up study of 481 patients

Abstract Background The cerebrospinal fluid tap test (CSF TT) is a commonly used predictive test for selecting patients with idiopathic normal pressure hydrocephalus (iNPH) for shunt surgery, but low sensitivity rate carries the risk of excluding individuals from effective treatment. We explored clinical characteristics and postoperative outcomes of iNPH patients shunted based solely on clinical and MRI findings compared with those shunted following a positive CSF TT. Methods A total of 481 consecutive shunt operated iNPH patients were assessed in a team-based setting. Patients were categorized into two groups: those shunted based on typical clinical symptoms and MRI features without a supplementary test (NoTT, n = 390) and those shunted based on a positive CSF TT (TT, n = 91). Baseline clinical data, including comorbidities, and 5-month postoperative outcomes were assessed using the Gothenburg iNPH Scale and the modified Rankin scale (mRS). Results Baseline characteristics and clinical measures were similar across groups, except that TT patients had more prevalent other concurrent neurological conditions (37% vs. 11%) and a longer delay between diagnosis and surgery (median 198 vs. 126 days) (both p < 0.001). Overall improvement (≥ 5-point postoperative increase in the total iNPH Scale) was observed in 70.9% of NoTT patients compared with 58.6% of TT patients (percentage point difference = 12.3; 95% CI = 0.3, 24.3; p = 0.044) while postoperative deterioration (defined as ≥ 5-point decrease on the iNPH Scale) was significantly more common in TT group (22.9%) than in the NoTT (10.7%) (percentage point difference = 12.1; 95% CI = 3.5, 20.7; p = 0.006). After adjusting for concurrent neurological disorder and surgical delay, improvement in continence (percentage point difference = 13.5, 95% CI = 0.7, 26.3) was better in NoTT patients ( p = 0.039). Conclusions This real-world study shows that a majority of iNPH patients can be routinely assessed by a clinical team and shunted with favorable outcomes without a supplementary predictive tap test. The worse outcomes seen in TT patients are likely to be due to higher prevalence of neurological comorbidity and delayed time to surgery. We suggest that the tap test may be reserved for difficult patients with suspected iNPH including those who present with concurrent other neurological disorders.