Heat-sensitive myotonia in proximal myotonic myopathy

Two siblings with proximal myotonic myopathy (PROMM), a hereditary disorder, had predominantly proximal weakness, pain, and gait impairment aggravated by warm temperatures. EMG of the deltoid showed sparse abnormal spontaneous activity at room temperature and with cooling. Warming induced frequent myotonic discharges and fibrillations. Profuse myotonia recorded at room temperature in the first dorsal interosseous abated following cooling. Repetitive stimulation did not reveal a decrement recording from distal muscles, but recording from the deltoid, in the one patient tested, revealed a significant decrement that did not improve with edrophonium. Myopathic motor units were recorded only in distal musculature. The myotonia of PROMM is provoked by heat and diminished by cold and may have a different physiologic basis than traditional myotonic syndromes.