Myasthenic syndrome of snake envenomation: a clinical and neurophysiological study.

In this prospective study, 65 consecutive patients with neurological manifestations after snake envenomation, were examined in order to describe the natural history of the reversible nature of muscle weakness. Snake envenoming led to a completely reversible muscle paralysis involving the external ocular muscles with sparing of the pupils, muscles of mastication, facial muscles, palatal muscles, neck and proximal limb muscles. The deep tendon reflexes were preserved with no sensory abnormalities. The muscular weakness usually set in within an hour of envenomation and lasted up to 10 days, with fatigability lasting for 12 days. Respiratory muscle paralysis led to ventilatory failure needing ventilation in severely envenomed patients. Motor and sensory nerve conduction were normal with normal resting compound motor action potentials on electromyography. Repetitive nerve stimulation gave rise to a decremental response during high frequency stimulation. The edrophonium test gave negative results. These manifestations are due to abnormalities of neuromuscular transmission and are not typical of myasthenia gravis. As the exact pathophysiology of venom-related neurotoxicity is not known, it is suggested that the neurological manifestations of snake envenoming be designated a myasthenic syndrome. Further studies to isolate the neurotoxin and its mechanism and exact site of blocking at the neuromuscular junction would pave the way for the development of a novel long-acting neuromuscular blocking agent.