DECAY OF POSTEXERCISE AUGMENTATION IN THE LAMBERT-EATON-MYASTHENIC-SYNDROME - EFFECT OF COOLING

The effect of local cooling on surface recorded compound muscle action potential (CMAP) amplitude was studied in five patients with the Lamb ert-Eaton myasthenic syndrome (LEMS). The time course of decay of post exercise augmentation of CMAP amplitude characteristically seen in pat ients with LEMS was determined. We recorded the CMAP from abductor dig iti minimi (ADM) in response to supramaximal stimulation of the ulnar nerve. Thirty consecutive stimuli were delivered at 1 Hz immediately a fter a 10-second period of maximal voluntary contraction. Skin surface temperature was recorded throughout. Initial testing at approximately 30 degrees C was repeated after cooling the hand and forearm by 6 to 12 degrees C. The effects of blood flow on temperature were counteract ed by the application of a sphygmomanometer cuff, inflated above systo lic blood pressure. The CMAP amplitude following contraction decayed i n an exponential manner both during warm and cold conditions. The mean time constant for decay (1/b) in all patients was increased by approx imately 25% after cooling. This prolongation of the period of postexer cise augmentation of CMAP amplitude in LEMS after cooling concurs with patient reports of symptomatic improvement in cold weather. The mecha nism for this benefit is thought to be due to reduction in the rate of removal of calcium ions from the nerve terminal following stimulation , similar to that seen in animal models of short-term synaptic enhance ment.