[Myotonic dystrophy: magnetic resonance tomography and clinico-genetic correlations]

Myotonic dystrophy (DM) is an autosomal dominant multisystem disorder involving muscle, brain, heart, eyes and endocrine organs, among others. The molecular basis is an unstable trinucleotide repeat at the 3'-untranslated end of the myotonin protein kinase gene on chromosome 19 q 13.3, and the number of repeats correlates with the severity of muscle weakness. We performed a clinical, psychometric and MRI study on 43 patients with DM and correlated findings with the molecular analysis. Nineteen patients had mild distal muscle weakness, 17 moderate und 7 severe weakness. Thirteen had marked cognitive deficits with reduced speed of cognition, low IQ, and apathy. MRI showed pathological muscle signal in 35 cases with a characteristic mosaic involving distal muscle groups, often sparing the posterior tibial muscle. Cerebral MRI showed significant subcortical white matter lesions in 20 cases and brain atrophy in 15 cases. Clinical and MRI findings of CNS and muscle both correlated with CTG repeat length, but did not parallel each other. DM is a significant disease of the brain as well as muscle, and several aspects of the disease correlate with molecular findings, with a threshold effect for repeats exceeding 1000 trinucleotides. The individual predominance of specific organ involvement probably depends on variable somatic mosaicism of the molecular defect.