22.1 Envenomation
Snakebite
Introduction
Snakes are ectothermic (‘coldblooded’) reptiles, comprising around 3000 species. Venomous snakes are restricted to just four families: Colubridae, Elapidae, Atractaspididae, Viperidae (Table 22.1.1 and Figs 22.1.1–22.1.4). Fang structures and venom types vary between families, but the common theme is a bite resulting in injection or inoculation of venom through a break in the victim’s skin. In most cases, venom is injected by fangs, paired teeth evolved to deliver venom, usually through a venom groove or enclosed channel, exiting near the tip. The act of biting can leave a variety of bite marks, which may be highly visible or almost invisible. Venom need not be injected (‘dry bites’).
Fig. 22.1.2 Diagramatic representation of the head of a proteroglyph (front fanged) snake of the cobra type.
Venom actions are diverse. Some major actions are listed in Table 22.1.2. From a clinical perspective, venom effects can be divided into three major groups:
The specific systemic effects of snake venoms are the most intensely studied, partly because they can usually be ascribed to particular venom components that can be isolated and studied in detail. An overview of these components is listed in Table 22.1.2. Specific clinical findings for the effects of these components will be discussed in the sections on ‘history’, ‘examination’ and ‘investigations’.
History
There may be a clear history of snakebite, or an encounter with a snake, where an actual bite is uncertain, or no history of a snake or bite. Particularly in young children, there may be no possibility of obtaining a history. Listen carefully to the story from young children, because relevant information may be disguised by rudimentary language. Some key points are listed in Table 22.1.3.
Broad category | Question | Significance |
---|---|---|
Details of bite | Was snake seen? | Increases likelihood of snakebite |
Description of snake? | May assist identifying type of snake, so possible problems can be anticipated | |
Size of snake? | May indicate potential for severe bite if large specimen, but beware, even juvenile snakes can inflict a severe bite | |
Geographical location? | May limit types of snake to be considered | |
Environment? | May indicate likelihood of snake encounter, if no clear history of snake being seen | |
Number of bites? | Multiple bites increase the likelihood of severe effects | |
Was bite through clothing? | Clothing may soak up some venom, reduce the chance of an effective bite. This may also be a source for venom detection (Australia) | |
Details of first aid | Was first aid used? | If no first aid then nothing to impair development of effects of venom |
What type of first aid? | Some types of first aid (e.g. tourniquets, cut and suck, suction devices, snake stones, electric shock/stun guns) may make matters worse or be ineffective | |
Effective first aid (e.g. immobilisation of the bitten limb, or full pressure immobilisation bandage) may delay onset of envenoming, thus the patient may present well, yet deteriorate after removal of first aid | ||
When was first aid applied? | If applied promptly, it may be effective, delaying envenoming If applied late or after physical activity (e.g. chasing snake, running for help) it may be ineffective | |
Local effects of bite | Were any bite marks, etc. noted prior to application of first aid? | If bite marks present, snakebite more likely, but absence of visible bites does not exclude snakebite (especially for some Australian elapids, notably brown snakes) |
Is there any local pain, swelling, bleeding, blistering, skin discolouration or other local effect? | May indicate likelihood of effective bite and possibly even type of bite | |
General symptoms | Headache, nausea, vomiting, abdominal pain? | Non-specific indicators of possible systemic envenoming (or anxiety) |
Collapse? | If in association with a definite bite, is suggestive of systemic envenoming | |
Convulsion? | If in association with a definite bite is strongly suggestive of major systemic envenoming | |
Blurred or double vision experienced within a few minutes of the bite? | Common effect, not likely to indicate developing paralysis | |
Specific systemic effects | ||
Paralytic effects | Presence and time of onset of paralytic symptoms? (Early ptosis may be described as heavy or sleepy eyes/eyelids) | Cranial nerves affected first, usually ptosis. Important to pick this up, before paralysis advances too far. May also help indicate the most likely type of snake |
Myolytic effects | Presence and time of onset of myolytic symptoms? (muscle pain, tenderness, weakness; urine becoming pink, red, brown or black) | Usually takes several hours to manifest. May indicate most likely type of snake |
Coagulopathic and haemorrhagic effects | Presence of coagulopathy effects, such as persistent bleeding from bite site or cuts, gums, or bruising, haemoptysis, haematemesis, haematuria? | Indicates coagulopathy likely and probably significant. May indicate most likely type of snake |
Renal effects | Presence of anuria or oliguria or polyuria | Indicates likely significant renal damage |
General history | ||
Medications | Anticoagulants or NSAIDS? | May affect coagulation test results or increase likelihood of a major bleed if coagulopathy present |
Antihypertensives? | Though not proven for antivenoms, it is suspected that β-blockers and ACE inhibitors may increase the chance of and severity of anaphylactic reactions to antivenom | |
Past history | Past bites requiring antivenom? | Past exposure to antivenom may increase the likelihood of reactions to subsequent antivenom therapy |
Past renal problems? | May increase the likelihood of envenoming causing renal damage | |
Other past medical history? | Evaluate as appropriate |
A description of the snake and geographical location may help narrow the range of possible culprit species. This can be combined with clinical features to assist in identifying the most likely culprits using diagnostic algorithms (Figs 22.1.5 and 22.1.6).
It is important to ask about any local, general or specific symptoms that might indicate developing significant envenoming (see Table 22.1.3).
Examination
It is clearly important to look at the bite site, or look for a bite, if no site is indicated from the history. Snakebites may result in single or paired fang punctures, multiple teeth punctures or even scratches, as fangs are dragged through the skin during release (Figs 22.1.7–22.1.10). If there is a bandage over the bite site, as first aid, cut a window only to inspect. Keep the removed bandage portion, if in Australia, for possible venom detection later. If venom detection is available (Australia, New Guinea), swab the bite site with the stick provided in the test kit. Do not allow anyone to clean the bite area until it has been swabbed for venom. Look for bite marks and particularly for multiple bites. Observe for local bruising, bleeding, blistering, swelling or necrosis. If there is significant local tissue injury or swelling, check pulses, etc., to exclude compartment syndrome in affected compartments. Compartment syndrome, if suspected clinically, must be confirmed by measuring intracompartmental pressure, before any consideration of surgical intervention.
Fig. 22.1.10 Extensive bruising of bitten limb. Typical of viper bites causing coagulopathy (green pit viper bite).
Examine for specific effects, notably neurotoxicity (flaccid paralysis; check for cranial nerve paralysis first, starting with ptosis; Figs 22.1.11, 22.1.12), myolysis (muscle tenderness and weakness), coagulopathy (persistent bleeding from bite site, needle punctures, etc.; Fig. 22.1.13) or deep vein thrombosis (DVT, pulmonary embolism; Martinique crotalids only), cardiotoxicity (arrhythmias), ‘allergy’ (angioneurotic oedema; particularly European vipers).
Fig. 22.1.11 Early stage flaccid neurotoxic paralysis with mild ptosis. An important early sign, easily missed (tiger snake bite).
Differential diagnosis
Differential diagnosis can also be applied within snakebite, in determining the type of snake most likely to have caused the bite. Diagnostic algorithms have been developed for Australia (see Figs 22.1.5 and 22.1.6) and South-East Asia (Figure 22.1.14). These are based on cases with significant envenoming and will not function if the patient is not envenomed, though this hardly matters, as such a patient will not require antivenom therapy. In some regions, notably Australia, it is important to know the type of snake involved, because antivenom therapy can be targeted appropriately. A similar situation applies in some other regions, where specific antivenoms are available. In regions such as North America, this is less important, because there is only one polyvalent antivenom covering all venomous species, except coral snakes.