CENTRAL AND PERIPHERAL RESPIRATORY ELECTROPHYSIOLOGICAL STUDIES IN MYOTONIC-DYSTROPHY

Acute and chronic respiratory failure is a common and potentially life -threatening feature in patients with myotonic dystrophy (MD). The cau ses may be varied and can involve both the central and peripheral nerv ous system. To evaluate the incidence of respiratory muscle involvemen t and the function of the central motor inspiratory pathway to phrenic motor neurons we performed magnetic stimulation of the cortex and cer vical spinal cord, phrenic nerve conduction studies and needle EMG of diaphragm and intercostal muscles in 25 patients with MD. The results were compared with those front 35 healthy subjects. In addition, pulmo nary function tests, blood gas analyses and static mouth pressures wer e evaluated. Abnormalities in response to magnetic stimulation, includ ing a reduced compound muscle action potential (CMAP) from the diaphra m and increased excitability threshold, indicated impaired central ins piratory drive in 20% of cases. Phrenic nerve conduction showed a redu ced diaphragmatic CMAP amplitude in 20%, and a delayed negative peak o nset latency in 4% of cases. Abnormalities in diaphragm and intercosta l muscle needle EMG were found in 76% of cases, these were mainly myot onic discharges (68%) and a decrease in the number of active motor uni ts (36%). Patients with abnormal respiratory electrophysiological para meters had a significantly lower functional vital capacity (FVC; P = 0 .005). The duration of the disease correlated negatively with diaphrag matic CMAP amplitude to phrenic nerve, but not magnetic, stimulation. Our results demonstrate that the involvement of the central inspirator y pathway is common in MD patients. Central and peripheral electrophys iological studies of the diaphragm should be considered in the diagnos is and management of patients with MD and dyspnoea.