Envenomation from most species of spiders in the United States causes only local pain, redness, and swelling. The more venomous black widow spiders (Latrodectus mactans) cause generalized muscular pains, muscle spasms, and rigidity. The brown recluse spider (Loxosceles reclusa) causes progressive local necrosis as well as hemolytic reactions (rare).
Stings by most scorpions in the United States cause only local pain. Stings by the more toxic Centruroides species (found in the southwestern United States) may cause muscle cramps, twitching and jerking, and occasionally hypertension, convulsions, and pulmonary edema. Stings by scorpions from other parts of the world are not discussed here.
A. Black Widow Spider Bites
Pain may be relieved with parenteral opioids or muscle relaxants (eg, methocarbamol, 15 mg/kg). Calcium gluconate 10%, 0.1–0.2 mL/kg intravenously, may transiently relieve muscle rigidity, though its effectiveness is unproven. Latrodectus antivenom is possibly more effective, but because of concerns about acute hypersensitivity reactions (horse serum–derived), it is often reserved for very young or elderly patients or those who do not respond promptly to the above measures. Horse serum sensitivity testing is required. (Instruction and testing materials are included in the antivenin kit.)
B. Brown Recluse Spider Bites
Because bites occasionally progress to extensive local necrosis, some authorities recommend early excision of the bite site, whereas others use oral corticosteroids. Anecdotal reports have claimed success with dapsone and colchicine. All of these treatments remain unproven.
No specific treatment other than analgesics is required for envenomations by most scorpions found in the United States. An FDA-approved specific antivenom is available for Centruroides stings.
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