[Binocular diplopia and ptosis due to snakebite (Agkistrodon blomhoffi "mamushi")--a case report]

We treated a patient who developed binocular diplopia and ptosis after being bitten by an Agkistrodon blomhoffi (mamushi). The patient was a 49-year-old man who presented with binocular diplopia after the snakebite on the second finger of his right hand. He experienced local pain and swelling and a few hours later, he developed diplopia. In the primary position he had no tropia. On the basis the ocular angle of deviation measured by a Hess chart test, he was diagnosed with paresis of the medial rectus muscle paresis. Binocular diplopia persisted for 2 weeks. The venom of A. blomhoffi venom mainly consists of hemolytic toxins, but it also contains 2 types of neurotoxins--an alpha-toxin and a beta-toxin. Neurotoxins affects the neuromuscular junction (NMJ). The alpha-toxin acts postsynaptic inhibition as a competitive inhibitor of acetylcholine and causes postsynaptic inhibition; these effects are similar to those of the anti-acetylcholine receptor antibody identified in patients with myasthenia gravis. The beta-toxin inhibits acetylcholine release by disrupting the presynaptic membrane, and thus, its effects cannot be blocked by the anticholinesterase edrophonium chloride. Although both antiserum and cepharanthine are widely used for the treatment of snakebites, there is no evidence of a specific effective therapy for the eye manifestation after snakebite. However, it these manifestation improves in about 2 weeks without any specific treatment. Our case suggested that the occurrence of subjective binocular diplopia without objective tropia could be caused by snakebite.