[A case of dystrophia myotonica with homozygous DM kinase abnormalities].

We report a case of dystrophia myotonica (DM) in a 38-year-old man with homozygous DM kinase abnormality. His distal muscle strength was reduced moderately; muscle biopsy showed type 1 predominance and type 1 fiber atrophy. The patient's WAIS total IQ score was under 60. Since his childhood, his cognitive deficit has been more severe than his muscle weakness. MRI demonstrated many abnormal changes in the brain of this patient, but these changes were mild in comparison to the severity of his reduced cognition and low IQ. The relationship between mental dysfunction and DM kinase abnormalities is quite different from that seen in cases of heterozygous DM kinase abnormalities. This case demonstrated severe mental changes in spite of mild DM kinase abnormalities. We suspect the homozygous DM kinase abnormality to be a cause of the different clinical presentations of this patient.