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  1. Pharmacology. To produce the antivenom, horses are hyperimmunized with Latrodectus mactans (black widow spider) venom. The lyophilized protein product from pooled equine sera contains antibodies specific to certain venom fractions as well as residual serum proteins such as albumin and globulins. After intravenous administration, the antivenom distributes widely throughout the body, where it binds to venom.

  2. Indications

    1. Black widow envenomation–induced severe hypertension or muscle pain or cramping that is not alleviated by muscle relaxants, analgesics, or sedation; consider particularly in patients at the extremes of age (ie, younger than 1 year or older than 65 years).

    2. Black widow envenomation in pregnancy may cause abdominal muscle spasms severe enough to threaten spontaneous abortion or early onset of labor.

  3. Contraindications. Known hypersensitivity to horse serum.

  4. Adverse effects

    1. Immediate hypersensitivity may rarely occur, including life-threatening anaphylaxis.

    2. Delayed-onset serum sickness may occur after 7–14 days but is rare owing to the small volume of antivenom used in most cases.

    3. Use in pregnancy. FDA Category C (indeterminate). There are no data on teratogenicity. An anaphylactic reaction resulting in shock or hypoxemia in the mother could conceivably affect the fetus adversely (see Table III–1).

  5. Drug or laboratory interactions. No known interactions.

  6. Dosage and method of administration. Generally, one vial of antivenom is sufficient to treat black widow envenomation in adults or children. The antivenom is dosed on the basis of symptoms, not on patient weight.

    1. Treat all patients in an emergency department or intensive care setting.

    2. Before a skin test or antivenom administration, insert at least one and preferably two secure intravenous lines.

    3. Perform a skin test for horse serum sensitivity by using a 1:10 dilution of antivenom (some experts prefer this method) or the sample of horse serum provided in the antivenom kit (according to package instructions). Do not perform the skin test unless signs of envenomation are present and imminent antivenom therapy is anticipated. If the skin test is positive, reconsider the need for antivenom as opposed to supportive care, but do not abandon antivenom therapy if it is needed. Even if the skin test is negative, anaphylaxis may still occur unpredictably in rare cases.

    4. If antivenom is used in a patient with horse serum sensitivity, pretreat with intravenous diphenhydramine (See Diphenhydramine) and ranitidine or another H2 blocker (See Cimetidine and Other H2 Blockers), and have ready at the bedside a preloaded syringe containing epinephrine (1:10,000 for intravenous use) in case of anaphylaxis. Dilute the antivenom 1:10 to 1:1000, and administer it very slowly in these cases.

    5. Reconstitute the lyophilized product to 2.5 mL with the supplied diluent, using gentle swirling for 15–30 minutes to avoid shaking and destroying the immunoglobulins (as indicated by the formation of foam).

    6. Dilute this solution to a total volume of 10–50 mL with normal saline.

    7. Administer the diluted antivenom slowly over 15–30 minutes. One or two vials are sufficient in most cases.

  7. Formulations

    1. Lyophilized antivenom (L mactans), 6000 units, contains 1:10,000 thimerosal as a preservative. Note: Product is ...

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