Abnormalities of nerve conduction studies in myotonic dystrophy type 1: Primary involvement of nerves or incidental coexistence?

The involvement of peripheral nerves in myotonic dystrophy type 1 (DM1) is controversial and the features of peripheral neuropathy (PN) are not well known. The aim of this study was to assess the frequency of abnormal nerve conduction findings and the electrophysiological characteristics of PN in DM1. We analyzed medical records, data from nerve conduction studies (NCS), and the results of genetic analysis of 18 patients with DM1 and 30 healthy individuals. The early changes identified in NCS were determined using the sural/ulnar sensory nerve action potential amplitude ratio (SUAR). To correlate the neuropathic changes with cardiac abnormality, we compared the corrected Q-wave/T-wave interval (QTc) with the NCS parameters. Eight of 18 patients had abnormal NCS findings. Of these, abnormal peroneal motor nerve conduction and H-reflex abnormalities were most common. Only one patient complained of sensory symptoms and had abnormal sensory and motor nerve conduction compatible with sensorimotor axonal polyneuropathy. There were no significant correlations between SUAR and disease duration, age, gene CTG repeats, or the QTc. The presence of diabetes was not related to abnormal nerve conduction or SUAR. The frequency of PN or abnormal NCS results was lower in our patients with DM1 than in previous studies. Our findings suggest that most abnormal NCS results in DM1 patients are more likely to result from myopathic changes, coincidental neuropathies, or radiculopathies than from primary involvement of the nerve.