Post Snake Bite - Clinical Management

Epidemiology

Types of Venom

• The Essentials of Forensic Medicine and Toxicology, 36th ed., p.564

Clinical Features

Colubrine (Neurotoxic - Cobra/Krait) vs. Viperine Bite - Key Differences

• Forensic Medicine and Toxicology, 36th ed., p.565

Local Envenomation (Viperidae)

• Massive swelling progressing over days, involving the entire bitten limb and adjacent trunk
• Linear erythema/bruising along superficial lymphatics
• Blisters (serous or serosanguinous) at bite site, extending proximally with severe envenomation

Extensive local swelling, bruising, blistering and early tissue necrosis 48 hours after a Malayan pit viper (Calloselasma rhodostoma) bite on the calf - Pye's Surgical Handicraft, p.428

Neurotoxic Envenomation Sequence (Cobra)

1. Local symptoms within 6-8 minutes (small reddish-bluish wheal, tender, burning pain)
2. Systemic symptoms at ~30 minutes: sleepiness, leg weakness, nausea/vomiting
3. Ptosis - earliest neuroparalytic sign
4. Ophthalmoplegia, diplopia, dysarthria, dysphagia
5. Ascending flaccid paralysis: limbs → trunk → facial muscles, palate, tongue, neck
6. Excessive salivation, headache, perioral paraesthesias
7. Respiratory arrest (obstruction by paralyzed tongue/vomitus, or intercostal/diaphragmatic paralysis)
8. Key test: inability to raise head in supine position = imminent respiratory failure

Rhabdomyolysis (Sea snakes, some elapids)

• Generalized muscle aches, stiffness, trismus within 0.5-3.5 hours
• Urine becomes "mahogany" / "Coca-Cola" coloured (myoglobinuria) by 3-8 hours
• Can progress to acute renal failure, hyperkalemia, cardiac arrest

Early Clues to Severe Envenomation

• Snake identified as very dangerous
• Rapid early extension of local swelling from bite site
• Tender enlargement of local lymph nodes (lymphatic spread)
• Early systemic collapse (hypotension, shock, nausea, vomiting, heavy eyelids, drowsiness, ptosis/ophthalmoplegia)
• Early spontaneous systemic bleeding
• Passage of dark brown/black urine (myoglobinuria/hemoglobinuria)

Diagnosis

1. 20-Minute Whole Blood Clotting Test (20WBCT): A few mL of fresh venous blood placed in a clean dry glass tube. If blood remains liquid after 20 minutes = non-clotting = viper bite/coagulopathy. Repeat every 6 hours to guide antivenom dosing.
2. Snake venom antigen detection: From wound swabs, aspirates, serum, CSF, or urine. By radioimmunoassay (RIA, detects 0.4 μg/L) or enzyme immunoassay (EIA).
3. Urinalysis: Myoglobinuria (positive 'stix' for blood/hemoglobin), proteinuria, dark urine.
4. Monitoring: Level of consciousness, ptosis, pulse rate, BP, respiratory rate, progression of local swelling, urine output.

Management

First Aid (Pre-Hospital)

1. Reassure the patient (panic accelerates venom absorption)
2. Immobilize the bitten limb with a splint/sling - treat like a fractured limb; firm bandaging prevents muscular contraction and lymphatic spread
3. Remove constricting items (rings, bracelets, watches, footwear)
4. Transport to hospital as quickly, comfortably, and passively as possible
5. Bring the snake if safely killed/secured (for identification)

• Apply tight tourniquets (don't work; can be dangerous)
• Incise, excise, or cauterize the wound
• Apply ice packs
• Give alcohol or stimulants (vasodilators - speed venom absorption)
• Apply suction or electric shock devices
• Give aspirin or NSAIDs (bleeding risk)

At Primary Health Centre

1. Assess for local and systemic envenomation (hourly monitoring)
2. 20WBCT + urinalysis + snake identification
3. Analgesia: Paracetamol (adults 500 mg-1 g; children 10-15 mg/kg) or codeine phosphate (adults 30-60 mg) every 4-6 hours - NOT aspirin/NSAIDs
4. Antivenom if indicated and skills/equipment available
5. IV fluid challenge (250-500 mL 0.9% saline) for shock/hypotension
6. Oxygen + consider atropine and neostigmine for respiratory paralysis; transfer to hospital

Antivenom (Anti-Snake Venom - ASV / Polyvalent Antivenom - PAV)

Indications for Antivenom

• Hypotension, shock, other cardiovascular toxicity
• Neurotoxicity (ptosis, ophthalmoplegia, paralysis)
• Rhabdomyolysis
• Impaired consciousness
• Spontaneous systemic bleeding
• Non-clotting blood (positive 20WBCT)
• Leukocytosis (WBC >20,000/μL), elevated serum enzymes
• Acidosis

• Swelling involving >half of bitten limb
• Rapid progression of swelling (e.g., crossing a joint within 1-2 hours)
• Known necrotic venom species
• Bites on digits or tight fascial compartments

Dosing (PAV/ASV)

• Lyophilized powder diluted in 500 mL distilled water/normal saline, infused over 1 hour
• Children require the same dose as adults
• In neurotoxic envenomation: repeat 10 vials after 1 hour if no response
• In hemotoxic envenomation: repeat initial dose every 6 hours until 20WBCT normalizes (liver needs 6 hours to restore clotting factors); then check every 12 hours for at least 48 hours for recurrence
• Most useful within 4 hours of bite; less effective at 8 hours; doubtful value after 24 hours
• Skin/conjunctival sensitivity tests should NOT be given (poor predictors of anaphylaxis; may presensitize)

Antivenom Reactions

• Early (anaphylactic): Itching, urticaria, tachycardia, fever, cough, vomiting within 10 min-2 hours
• At first sign of reaction (urticaria, shivering, hypotension, bronchospasm): stop infusion immediately, give IM adrenaline (epinephrine)
• Late (serum sickness): Occurs 5-10 days later; treat with antihistamines and corticosteroids

Treatment of Specific Complications

Fatal Doses and Periods (Forensic Reference)

• Forensic Medicine and Toxicology, 36th ed., p.565-566

Key Points to Remember

• "Dry bite": No venom injected - occurs in 1/3 to 1/2 of bites by venomous snakes; may be due to washing of area, bite through thick clothing, superficial bite in mobile body part
• Fright reactions (tachycardia, cold clammy skin, hypotension) from a non-venomous bite can mimic envenomation - distinguish carefully
• The most common early neuroparalytic sign is ptosis
• Neostigmine test: If ptosis reverses after neostigmine, it confirms postsynaptic NMJ blockade (elapid venom) and guides ongoing treatment
• Severely envenomed patients must be observed in hospital for several days due to late recurrence from continued venom absorption