Diffusion tensor imaging detects areas of abnormal white matter microstructure in patients with partial ornithine transcarbamylase deficiency

Abstract

Background and purpose: OTCD, an X-linked disorder, is the most common of the UCDs. Neonatal onset is associated with uniformly poor outcome. Males with late-onset OTCD show deficits in executive function, motor planning, and working memory. A broad phenotype is observed in heterozygous females. A specific neurobehavioral phenotype with white matter dysfunction and impaired attention and working memory has been described. The extent to which the deficits involve specific pathways in the brain is unknown. We hypothesized that DTI would disclose white matter microstructure in OTCD correlating with cognitive deficits.

Materials and methods: Nineteen adults with partial OTCD and 18 adult control subjects ages 19-59 years participated. MR imaging was performed by using a 3T whole-body scanner. Anisotropy was calculated from the eigenvalues of the diffusion tensor by using the FA metric and was compared between the study and control groups.

Results: FA of the frontal white matter was significantly decreased in subjects, indicating changes in white matter microstructure. There was an inverse relationship between FA and disease severity, but not with age.

Conclusions: Findings of MR imaging in OTCD are often normal in patients with late-onset disease, heterozygotes, or in those not in hyperammonemic crisis. DTI was more sensitive than FSE T2-weighted imaging for detecting abnormalities in normal-appearing white matter. The extent of abnormality correlated with cognitive deficits. The location of the deficits in the frontal white matter is important because this area connects fibers that are vital to executive function, attention, and working memory.

Fig 1.

Group means for Stroop tasks. Asterisk indicates P < .05; double asterisks, P < .01.

Fig 2.

Group means for sections of the CTMT. Asterisk indicates P < .01; double asterisks, P < .001.

Fig 3.

Region of Interest. Voxels with significantly lower FA in subjects compared with controls (P < .001) overlaid on a standardized brain structural MR image.

Fig 4.

Scatterplot with linear regression showing average FA in the region of interest versus the CTMT composite percentile in subjects (R = 0.6) and controls (R = 0.2).

Fig 5.

Scatterplot with linear regression showing average FA in the region of interest versus Stroop performance for subjects (R = 0.4) and controls (R = 0.2).