Objective To determine whether quantitative measure of magnetic resonance imaging data from patients with the inherited leukodystrophy, Pelizaeus–Merzbacher disease (PMD) correlates with clinical severity or progression. Methods In our current work we have analyzed the clinical phenotypes and MRI scans of 51 male patients with PMD and 10 female carriers for whom the PLP1 genotype had been determined. In addition, we developed a 32-point functional disability scoring (FDS) system for PMD, and validated it for inter-rater reliability. Using conventional T1- and T2-weighted MRI images of the whole brain, we measured white matter and total brain volume (WMV and TBV), inter-caudate ratio (ICR), and corpus callosum area. Results There was a significant positive correlation of FDS with white matter fraction (WMV/TBV) and corpus callosum area. Also, when applying a median split based on FDS, patients with lower FDS showed reduced white matter fraction and corpus callosum area, and increased ICR compared to patients with relatively higher FDS, regardless of age. Conclusion Although this patient population is heterogeneous, with multiple genetic and molecular mechanisms causing PMD, these data imply that white matter atrophy is a major pathological determinant of the clinical disability in most patients. Development of reliable non-invasive quantitative biomarkers of disease activity would be useful not only for following the natural history of the disease, but also raising the potential for evaluating future therapies.

Pelizaeus-Merzbacher disease (PMD) is an X-linked disorder of the central nervous system (CNS) caused by a wide variety of mutations affecting proteolipid protein 1 (PLP1). We assessed the effects of PLP1 mutations on water diffusion in CNS white matter by using diffusion tensor imaging. Twelve patients with different PLP1 point mutations encompassing a range of clinical phenotypes were analyzed, and the results were compared with a group of 12 age-matched controls. The parallel (λ// ), perpendicular (λ⊥ ), and apparent diffusion coefficients (ADC) and fractional anisotropy were measured in both limbs of the internal capsule, the genu and splenium of corpus callosum, the base of the pons, and the cerebral peduncles. The mean ADC and λ⊥ in the PMD patient group were both significantly increased in all selected structures, except for the base of the pons, compared with controls. PMD patients with the most severe disease, however, had a significant increase of both λ// and λ⊥ . In contrast, more mildly affected patients had much smaller changes in λ// and λ⊥ . These data suggest that myelin, the structure responsible in part for the λ⊥ barrier, is the major site of disease pathogenesis in this heterogeneous group of patients. Axons, in contrast, the structures mainly responsible for λ// , are much less affected, except within the subgroup of patients with the most severe disease. Clinical disability in patients with PLP1 point mutation is thus likely determined by the extent of pathological involvement of both myelin and axons, with alterations of both structures causing the most severe disease. © 2014 Wiley Periodicals, Inc.