BACKGROUND: An Evans index greater than 0.3 is a key diagnostic criterion for idiopathic normal pressure hydrocephalus (iNPH). However, some patients with iNPH symptoms and an Evans index below 0.3 achieve favorable outcomes after cerebrospinal fluid (CSF) shunt surgery. This study aimed to evaluate the clinical and radiographic characteristics and CSF shunt outcomes of these patients. METHODS: This prospective study enrolled patients presenting with iNPH symptoms and an Evans index below 0.3. Patients who showed clinical improvement after a spinal tap test underwent CSF shunt placement via ventriculoperitoneal or lumboperitoneal shunting. Outcomes were assessed using subjective tools and symptom-specific scores at baseline and at 3 days, 1 month, 3 months, and 6 months post-surgery. RESULTS: Thirty-five patients with a mean (SD) Evans index of 0.26 (0.02) underwent CSF shunt placement. All clinical domains showed statistically significant improvement at 6 months compared with baseline, except cognition. Subgroup analysis revealed no statistically significant difference in preoperative and final postoperative iNPH grading scale scores between patients undergoing ventriculoperitoneal and lumboperitoneal shunting. One patient experienced shunt overdrainage causing bilateral chronic subdural hematoma that required burr hole craniostomy. CONCLUSIONS: These findings suggest that patients with iNPH-like symptoms and an Evans index below 0.3 can benefit from CSF shunt surgery. Multiple assessment tools demonstrated substantial postoperative improvement in this group. In patients with typical iNPH symptoms, a positive spinal tap response, and supportive imaging findings, an iNPH diagnosis is justified regardless of ventricular size. Such patients may be considered surgical candidates for CSF shunt placement.
Akaraboonlertyot et al. (Thu,) studied this question.