Clinical Signs of Neurotoxic Snake Bite
Neurotoxic snake envenomation characteristically presents with progressive neuromuscular paralysis that begins with ptosis and descends to involve respiratory muscles, often with minimal or absent local tissue injury at the bite site. 1, 2
Early Warning Signs
Drowsiness is the herald symptom that signals the onset of systemic neurotoxicity in most patients and should prompt immediate concern for progression to life-threatening paralysis. 3
Classic Neuromuscular Progression
The neurotoxic effects follow a predictable descending pattern:
- Ptosis (drooping eyelids) is typically the first cranial nerve manifestation and one of the most characteristic early signs 3, 4
- Ophthalmoplegia (paralysis of eye movements) develops as cranial nerve involvement progresses 5, 3
- Frothy saliva accumulation occurs due to impaired swallowing and excessive salivation 3
- Slurred speech results from bulbar muscle weakness 3
- Skeletal muscle paralysis progresses in a descending fashion 3
- Respiratory failure represents the life-threatening endpoint requiring mechanical ventilation 3, 4
Critical Timing Considerations
Onset of neurotoxicity occurs within 8 hours in 94% of cases, but may be delayed up to 13 hours despite minimal local findings. 3, 4 This delayed presentation is a critical pitfall—the absence of immediate symptoms does not exclude severe envenomation. 1, 2
Distinguishing Local vs. Systemic Features
Minimal to no local tissue injury at the bite site is characteristic of neurotoxic envenomation, in stark contrast to cytotoxic/hemotoxic bites that cause progressive local necrosis and swelling. 1, 2 When local necrosis does occur with neurotoxic bites, it typically develops at the end of the first week, not acutely. 3
Severe Presentations
In extreme cases, neuromuscular paralysis may be so profound that patients present with:
- Locked-in syndrome (complete paralysis with preserved consciousness) 6, 5
- Unconsciousness accompanying respiratory failure 3
- Patients may be mistakenly diagnosed as brain dead due to the severity of flaccid paralysis 5
Occult Bites
Patients may be completely unaware of the bite, with no visible fang marks, particularly with krait envenomation (Elapidae family). 5 This creates diagnostic challenges when patients present with unexplained progressive paralysis without a clear history of snake exposure.
Key Clinical Pitfall
Never assume no envenomation based on absence of immediate pain, swelling, or local findings—neurotoxic bites cause life-threatening systemic toxicity despite minimal local signs. 1, 2 The critical window for antivenom administration may be missed if clinicians wait for local tissue changes that will never develop with purely neurotoxic venoms.