Analyzing the relationship between specific brain structural changes and the diffusion tensor image analysis along the perivascular space index in idiopathic normal pressure hydrocephalus

Background Diffusion Tensor Image Analysis Along the Perivascular Space (DTI-ALPS) evaluates the glymphatic system in patients with idiopathic normal-pressure hydrocephalus (iNPH). However, white matter compression due to ventricular enlargement may affect the ALPS index. This study aimed to investigate the relationship among the ALPS index, white matter changes, and clinical symptoms in patients with iNPH. Methods We calculated the ALPS index in 30 patients with iNPH, aged 70 and above, using DTI data and correlated it with various clinical and imaging indices, including the Evans index, callosal angle, cognitive tests, gait assessment (timed up-and-go [TUG] test), cerebrospinal fluid (CSF) medullary pressure, and various DTI indices (axial diffusivity [AD], radial diffusivity [RD], mean diffusivity [MD], fractional anisotropy [FA]). Results Significant negative correlations were observed between the ALPS index and the rate of change in step count in the TUG test after the tap test ( r = −0.5014, p = 0.0048), as well as CSF medullary pressure ( r = −0.4651, p = 0.0096). Positive correlations were identified between the ALPS index and both AD ( r = 0.4984, p = 0.0051) and MD ( r = 0.3631, p = 0.0486). Conclusion A lower ALPS index was associated with gait improvement following the tap test as well as higher CSF medullary pressure. The ALPS index may detect subtle periventricular compression-induced changes in iNPH. Consequently, it could potentially serve as a predictor for tap test effectiveness in patients with iNPH, offering a new perspective on its application in iNPH diagnosis and treatment.