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

BACKGROUND

Normal pressure hydrocephalus 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 markers, including EI and the z-EI, 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-EI as a morphological magnetic resonance marker of shunt response (P = 0.043).

CONCLUSIONS

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