Abstract
PurposePrevious studies suggest that compression and stretching of the corticospinal tract (CST) potentially cause treatable gait disturbance in patients with idiopathic normal pressure hydrocephalus (iNPH). Measurement of axon diameter with diffusion MRI has recently been used to investigate microstructural alterations in neurological diseases. In this study, we investigated alterations in the axon diameter and intra-axonal fraction of the CST in iNPH by q-space imaging (QSI) analysis.MethodsNineteen patients with iNPH and 10 age-matched controls were recruited. QSI data were obtained with a 3-T system by using a single-shot echo planar imaging sequence with the diffusion gradient applied parallel to the antero-posterior axis. By using a two-component low-q fit model, the root mean square displacements of intra-axonal space ( = axon diameter) and intra-axonal volume fraction of the CST were calculated at the levels of the internal capsule and body of the lateral ventricle, respectively.ResultsWilcoxon's rank-sum test revealed a significant increase in CST intra-axonal volume fraction at the paraventricular level in patients (p<0.001), whereas no significant difference was observed in the axon diameter. At the level of the internal capsule, neither axon diameter nor intra-axonal volume fraction differed significantly between the two groups.ConclusionOur results suggest that in patients with iNPH, the CST does not undergo irreversible axonal damage but is rather compressed and/or stretched owing to pressure from the enlarged ventricle. These analyses of axon diameter and intra-axonal fraction yield insights into microstructural alterations of the CST in iNPH.
Highlights
Idiopathic normal pressure hydrocephalus is a clinical entity of unknown cause and is characterized by the triad of gait disturbance, cognitive deterioration, and urinary incontinence [1]
The corticospinal tract (CST) intra-axonal volume fraction was significantly higher in patients with Idiopathic normal pressure hydrocephalus (iNPH) than in the controls, whereas no significant difference was observed in the CST axon diameter
A two-component low-q fit analysis of q-space imaging (QSI) revealed that the CST intra-axonal volume fraction in areas near the ventricles was increased in iNPH patients compared with controls, whereas the CST axon diameter was unaltered
Summary
Idiopathic normal pressure hydrocephalus (iNPH) is a clinical entity of unknown cause and is characterized by the triad of gait disturbance, cognitive deterioration, and urinary incontinence [1]. It is associated with ventricular enlargement, flattening of high-convexity sulci, and periventricular T2-weighted image hyperintensity in the absence of elevated cerebrospinal fluid (CSF) pressure [2]. Previous studies conducted with DTI revealed increases of fractional anisotropy (FA) and axial diffusivity values in the CST in patients with iNPH [9,10,11,12,13,14,15,16,17], which tended to return to normal after placement of a ventriculoperitoneal (VP) shunt [9,10,11,12]. Diffusion MRI is expected to become a non-invasive method for diagnosing iNPH and predicting the response to surgery [12,17]
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