Nearly all venomous snakebites in North America are caused by snakes from the family Crotalidae (pit vipers).

DESCRIPTION

Crotalidae includes the following genera:

• Agkistrodon: Copperhead, cottonmouth, and water moccasin snakes
• Crotalus: Rattlesnakes
• Sistrurus: Massasauga and pigmy rattlesnakes

PATHOPHYSIOLOGY

• Crotalid snake venom contains dozens of components ranging from individual ions (e.g., zinc) to peptides and large-molecular-weight molecules.
• Crotalid snake bite produces three types of effects: local wound injury (pain, swelling, ecchymosis), coagulopathy (low platelet count, low fibrinogen, increased prothrombin time/international normalized ratio), and systemic effects (hypotension).
• The venom is usually injected subcutaneously; however, intramuscular (uncommon) and intravenous (rare) injection may occur.

EPIDEMIOLOGY

• The approximate 8,000 snake bites each year in North America cause about five deaths.
• The elderly and young children are at the highest risk of death.

CAUSES

• Bite on the hand is usually due to handling of the snake.
• Bite on the foot is usually accidental.

PREGNANCY AND LACTATION

Moderate to severe envenomation may cause vaginal bleeding and miscarriage.

Section Outline:

• DESCRIPTION
• PATHOPHYSIOLOGY
• EPIDEMIOLOGY
• CAUSES
• PREGNANCY AND LACTATION

DIFFERENTIAL DIAGNOSIS

• Bite by a venomous snake in which venom is not injected (dry bite)
• Bite by a nonvenomous snake
• Puncture wounds from inanimate objects (e.g., thorns)
• Swelling or ecchymosis due to unrecognized trauma or coagulopathy

SIGNS AND SYMPTOMS

Venom effects may appear immediately or develop over several hours. If no effects occur within 8 to 12 hours, a dry bite is likely. If life-threatening effects develop, they typically occur within a few hours.

Vital Signs

Heart rate and blood pressure are often increased soon after the bite occurs and may be followed by hypotension with continued tachycardia.

HEENT

Cranial nerve paralysis manifested as ptosis has been reported occasionally, usually after the bite of the Mojave rattlesnake.

Dermatologic

• The bite will show one, two, or more puncture wounds. Scratches or small lacerations also may produce envenomation.
• Faint ecchymosis may develop around the site of the bite.
• Localized pain and edema usually start early and progress proximally; however, they may be delayed for hours. Rapid progression indicates more serious envenomation.

Cardiovascular

Hypotension may occur early, although rarely. Generally, it develops after several hours due to volume depletion caused by loss of volume into the edematous limb.

Pulmonary

Noncardiogenic and cardiogenic pulmonary edema rarely develop.

Gastrointestinal

Nausea and vomiting are common. Gastrointestinal hemorrhage rarely occurs.

Renal

• Volume depletion is common and may become severe.
• Acute renal failure may occur if hypotension is prolonged or rhabdomyolysis develops.

Hematologic

Anemia, thrombocytopenia, and hypofibrinogenemia are common.

Musculoskeletal

• Fasciculation has been reported after bites by some species.
• Compartment syndrome is rare. Severe pain, distal paresthesia, pallor, and pulselessness are indicators.

Neurologic

Altered mental status may occur in cases of severe envenomation.

PROCEDURES AND LABORATORY TESTS

Essential Tests

• Complete blood count. Blood counts will decrease over the first few days; hemoconcentration may occur to third-spacing of fluid into edematous area; and the platelet count, used to monitor the effectiveness of antivenom therapy, is often decreased and may fall below 20,000/mm³.
• Serum electrolytes, BUN, and creatinine measurements are used to assess volume status and renal injury.
• Prothrombin time (PT) and international normalized ratio (INR) are often increased, sometimes dramatically. Measurement is used to monitor the effectiveness of antivenom therapy.
• Urinalysis
  • Red cells indicate hemorrhage.
  • Positive hemoglobin without cells may indicate myoglobinuria.

Recommended Tests

• Fibrinogen level is often decreased and may reach zero. Measurement is used to monitor the effectiveness of antivenom therapy.
• Fibrin split product concentration is often increased.
• The leading edge of swelling should be marked with an indelible marker every 15 to 30 minutes because progression of swelling is one indicator for use of antivenom.
• Creatine kinase concentration is measured to monitor venom-related muscle injury.
• Imaging. Chest x-ray is needed only if pulmonary symptoms develop.
• Intracompartmental pressure monitoring is necessary for all patients with suspected compartment syndrome.

Section Outline:

• DIFFERENTIAL DIAGNOSIS
• SIGNS AND SYMPTOMS
  • Vital Signs
  • HEENT
  • Dermatologic
  • Cardiovascular
  • Pulmonary
  • Gastrointestinal
  • Renal
  • Hematologic
  • Musculoskeletal
  • Neurologic
• PROCEDURES AND LABORATORY TESTS
  • Essential Tests
  • Recommended Tests

• The principle of therapy is to neutralize the venom using antivenom.
• Supportive care of local injury, coagulopathy, and systemic toxicity is performed until antivenom can be administered.
• Time of bite and any first-aid measures already performed must be determined.

DIRECTING PATIENT COURSE

The health-care provider should call the poison control center when:

• Shock or other life-threatening effects are present.
• Signs and symptoms are not consistent with pit viper envenomation.
• Underlying disease presents an unusual diagnostic or therapeutic challenge.

The patient should be referred to a health-care facility any time that a history of a venomous snake bite is obtained.

DECONTAMINATION

Out of Hospital

• Supportive care of local injury, coagulopathy, and systemic effects, with appropriate airway management, is vital.
  • The affected limb should be immobilized in a functional position at heart level or below until antivenom can be administered.
  • Aspiration of the bite site using an extractor may remove some venom. This should not involve the incision of the bite marks.
• An intravenous line should be inserted, if possible, and the patient transported to an emergency department.

In Hospital

• First aid is probably not useful after the patient has reached the emergency department. The patient should be placed in a monitored bed, and venom effects should be assessed; if progressing, an antivenom agent should be administered.
• After an intravenous line has been started, the affected limb should be elevated above heart level.

ANTIDOTES

• The principle of management is to neutralize the injected venom using an antivenom agent. Antivenom should be administered if the effects of the venom are worsening.
• Crotalidae Polyvalent immune Fab (ovine) (see SECTION III, Snake Antivenom chapter)
  • Indications include evidence of local wound worsening (e.g., progression of swelling), coagulopathy, or systemic signs or symptoms.
  • Contraindications include a history of allergy to the product (relative contraindication).
  • Dose. Initially 4 to 6 vials are administered, repeated up to two times to achieve control, and then two vials are administered every 6 hours for three additional doses. Further doses may be needed at physician discretion.
  • Administration. The dose is diluted in 250 ml of crystalloid and infused intravenously over 1 hour
  • Adverse effects. Acute adverse effects occur during infusion in 20% of patients. Anaphylaxis has not been reported to occur.
• Antivenin (Crotalidae) Polyvalent (Wyeth-Ayerst) (see SECTION III, Snake Antivenom—Crotalid and Elapid Snakes chapter, for details).
  • Indications include evidence of local wound worsening (e.g., progression of swelling), coagulopathy, or systemic signs and symptoms.
  • Contraindications include a history of allergy to the product (a relative contraindication).
  • Dose. Smaller doses than those listed may be appropriate if the bite is made by a copperhead snake.
    • No antivenom is administered if the envenomation is minimal.
    • Ten vials should be administered if the envenomation is moderate or if the envenomation syndrome is worsening.
    • Ten to thirty, or more, vials should be administered in the presence of life-threatening envenomation or if the syndrome is worsening rapidly. At least 20 vials should be given initially if the patient demonstrates cardiovascular instability.
  • Administration
    • Epinephrine and equipment for endotracheal intubation should be immediately available. A skin test should be conducted by injecting 0.02 cc of the skin test material intradermally. A positive response is a wheal and flare reaction within 15 to 30 minutes.
    • Ten milliliters of normal saline should be added to each vial of antivenom. It is mixed by rolling the vial between the hands. The mixture is diluted by injecting it into a normal saline intravenous bag (5-10 vials in 250 ml normal saline). The vials should not be shaken. The volume of saline should be reduced in pediatric patients (total volume should equal 20 ml/kg).
    • Initially, the antivenom should be administered intravenously at a slow rate (10 to 25 cc/h), then titrated upward (250 cc/hr) if no reaction occurs. The initial dose should be completed during the first hour, if possible.
  • Adverse effects
    • Acute allergy or anaphylaxis occurs in 25% of patients.
    • Serum sickness occurs in 75% of patients.

ADJUNCTIVE TREATMENT

• Adequate analgesia (e.g., meperidine or morphine) should be provided as needed.
• Hypotension is treated in the usual manner (see SECTION II, Hypotension chapter).
• A tetanus immunization should be administered, if needed.

Section Outline:

• DIRECTING PATIENT COURSE
• DECONTAMINATION
  • Out of Hospital
  • In Hospital
• ANTIDOTES
• ADJUNCTIVE TREATMENT

PATIENT MONITORING

• Continuous respiratory and cardiac monitoring should be performed in symptomatic patients.
• The patient should be assessed repeatedly for development of elevated compartment pressure, evidence of bleeding, and progression of tissue injury.

EXPECTED COURSE AND PROGNOSIS

• The effects of venom usually peak during the first day and then abate over several days. Edema may recur repeatedly for weeks to months.
• The patient should be monitored for bleeding. Coagulation abnormality may return within the first week.
• Approximately 40% of patients bitten on the hand and 15% of patients bitten on the lower extremity have long-term complaints.

DISCHARGE CRITERIA/INSTRUCTIONS

• From the emergency department. The patient can be discharged if swelling, coagulation abnormality, and systemic signs fail to develop for 8 to 12 hours.
• From the hospital. The patient can be discharged when all venom effects have clearly begun to resolve and when antivenom therapy is complete (usually within 36 hours after admission).

Section Outline:

• PATIENT MONITORING
• EXPECTED COURSE AND PROGNOSIS
• DISCHARGE CRITERIA/INSTRUCTIONS

DIAGNOSIS

Venom effects may take hours to develop; therefore, patients should be observed for 8 to 12 hours.

TREATMENT

• Delay in antivenom administration may allow irreversible venom effects to develop.
• The patient should not be left unattended during antivenom infusion.
• Fasciotomy has been used to treat compartment syndrome; however, research suggests that it is ineffective. The procedure should not be considered unless true compartment syndrome exists.
• Additional antivenom and mannitol should be administered instead.
• Electric shock treatment and tourniquets should not be used.

FOLLOW-UP

• Serum sickness may develop; it usually becomes evident 1 to 2 weeks after treatment.
• Venom effects may recur 3 to 7 days after the antivenom agent has been excreted.

Section Outline:

• DIAGNOSIS
• TREATMENT
• FOLLOW-UP

Toxic effect of other substances, chiefly nonmedicinal as to source: venom.

See Also: SECTION II, Hypotension chapter; SECTION III, Snake Antivenom—Crotalid and Elapid Snakes chapter.

RECOMMENDED READING

Gomez H, Dart RC. Clinical toxicology of snake bite in North America. In: Meier J, White J, eds. Clinical toxicology of animal venoms and poisons. Boca Raton, FL: CRC Press, 1995:619-644.

Author: Richard C. Dart

Reviewer: Katherine M. Hurlbut