Optimal Application Indices for Shunt Surgery for Idiopathic Normal Pressure Hydrocephalus with an Evans Index Below 0.3.

Background: Normal pressure hydrocephalus (NPH) is a neurological disorder characterized by three primary symptoms (gait disturbance, cognitive impairment, and urinary incontinence), accompanied by specific morphological imaging features. Idiopathic normal-pressure hydrocephalus (iNPH) is characterized by symptoms without identifiable secondary causes. The Evans index (EI) is traditionally used to assess ventricular enlargement, with values >0.3 indicating iNPH. Herein, we investigated the efficacy of shunt surgery in patients with iNPH and EI ≤0.3, who may still benefit from the procedure despite not meeting conventional diagnostic criteria.

Methods: We retrospectively analyzed consecutive iNPH patients categorized into two groups based on EI: ≤0.3 (atypical iNPH group = discovery group) and >0.3 (probable iNPH group = control group). The shunt response rate was subsequently assessed. Morphological magnetic resonance (MR) markers, including EI and the z-Evans index, were analyzed to identify predictors of shunt response.

Results: After applying the inclusion and exclusion criteria, 231 patients were eligible for analysis, including an atypical iNPH group of 48 patients and a probable iNPH group of 183 patients. The mean EI value was 0.33 ± 0.04, and VA shunt surgery significantly improved symptoms (p <0.001), with responder rates of 83.3% and 90.2% in atypical iNPH and probable iNPH group, respectively. In the atypical iNPH group, multivariate analysis identified the z-Evans index as a morphological MR marker of shunt response (p = 0.043).

Conclusion: The response rates to shunt surgery were comparable in patients with EI ≤0.3 and EI >0.3 without statistical significance. The z-Evans index is a useful tool for identifying shunt responders among those with a lower EI, potentially expanding the therapeutic options for patients previously considered ineligible.