Thalamic involvement in neurofibromatosis type 1: evaluation with proton magnetic resonance spectroscopic imaging

Neurofibromatosis type 1 is a common autosomal dominant disorder associated with learning disabilities. In addition to gliomas and other tumors, T2 hyperintense lesions (unidentified bright objects or UBOs) are frequently found in the globus pallidus, cerebellum, and white matter regions. To better characterize supratentorial UBO functional significance, we studied by quantitative magnetic resonance spectroscopic imaging (MRSI) 9 male subjects with neurofibromatosis type 1 (age, 6-19 years) and 9 age-matched and sex-matched controls. Maps of the anatomical distribution of the metabolites choline (Cho), N-acetylaspartate (NAA), and creatine were calculated in four axial 15-mm slices. Absolute metabolite concentrations within UBOs, unaffected globus pallidus, and thalami demonstrated an age-related pattern, characterized by elevated Cho and relatively preserved NAA in younger subjects (<10 years) and reduced NAA and normal Cho in older subjects. These changes were found in both UBOs and thalami but were only significant for NAA, NAA/creatine, and NAA/Cho in the latter region. Decreases in NAA ratios were most severe in the thalami of subjects with UBOs in the globus pallidus, whereas UBOs showed similar but milder abnormalities than those in the thalamus. We speculate that the MRSI metabolic abnormality may represent a more generalized phenomenon, without a T2 signal counterpart in the affected brain regions. Based on the neuropathological study by DiPaolo and colleagues (1995), we postulate that Cho elevations reflect increased myelin turnover in areas of intramyelinic edema, which is followed by neuropil injury (reduced NAA). Temporal progression and behavioral correlates of these MRSI changes deserve further exploration.