The lack of deep reflexes in myotonic dystrophy

Clinical and electrophysiological observations have been carried out on 12 patients with myotonic dystrophy. Neurological examination showed that the tendon reflexes were absent or weak in almost all cases, whereas the cutaneous reflexes were normal. Examination of both deep and superficial sensibility gave normal results. Electromyography confirmed widespread "myopathic" activity and myotonic discharges were recorded on insertion of the needle electrode and at rest. Motor and sensory conduction velocity in the ulnar nerve and motor conduction in the peroneal nerve proved to be normal. Repetitive supramaximal nerve stimulation showed in 10 cases a decrease in potential amplitude, more evident at higher frequencies of stimulation. In the 2 other cases, by contrast, an increase in amplitude was observed, and this was suggestive of a partial presynaptic block. The jaw reflex was absent in 5 cases and reduced in amplitude in the 7 other cases. The results of blink reflex investigations were normal, with the exception of 2 cases where no early response was elicited. Spinal monosynaptic reflexes were absent in 7 cases after both electrical (H reflex) and mechanical stimulation (T reflex), whereas the response to direct stimulation of nerve motor fibres (the M response) was always present, even though reduced in amplitude. Such data lead one to reject the hypothesis that the absence of deep reflexes is due to pathological change in the muscle spindles. It seems more likely that the selective atrophy of Type 1 muscle fibres, known to be involved in deep reflex responses, is responsible for the early disappearance of the tendon reflexes.