Diagnostic Accuracy of the Relative Subcortical Atrophy Index in Idiopathic Normal Pressure Hydrocephalus

BACKGROUND AND OBJECTIVES: In neurosurgical practice, idiopathic normal pressure hydrocephalus (iNPH) presents notable diagnostic and therapeutic challenges. This study aimed to evaluate the diagnostic accuracy of the relative subcortical atrophy (RSCA) index and to investigate its potential role in assessing surgical response. METHODS: A retrospective analysis was conducted on 78 patients who underwent ventriculoperitoneal shunt for iNPH and compared with healthy controls and patients with possible iNPH. Diagnosis was established in accordance with the Japanese Guidelines for Management of iNPH, including serial assessments of cognition, balance, and gait, as well as cerebrospinal fluid diversion during a predetermined hospitalization. Neurocognitive, balance, and gait performance were assessed before and after the cerebrospinal fluid tap test. Clinical outcomes were evaluated at 1, 6, and 18 months postoperatively. The RSCA index and the maximum linear extension of transependymal edema were independently measured by 2 neurosurgeons using fluid-attenuated inversion-recovery MRI preoperatively and at different follow-up times. An independent validation team was recruited to perform the data validation process. RESULTS: A significant difference was observed in the RSCA index between patients with iNPH and healthy controls (P < .001) as well as between possible iNPH patients (P < .001). The area under the receiver operating characteristic curve was 0.959 (95% CI, 0.91-1.00). A cutoff value of 0.62 (P < .001) yielded high sensitivity (92.3%) and specificity (94.6%) in discriminating patients from controls. Postoperative values demonstrated a significant reduction compared with preoperative measurements (P < .001). Inter-rater reliability was excellent (intraclass correlation coefficient = 0.997; 95% CI: 0.996-0.998; P < .001). CONCLUSION: The RSCA index demonstrated high diagnostic accuracy for iNPH and showed significant postoperative variation, supporting its potential utility as a reliable radiological biomarker in both diagnosis and surgical follow-up.