In most cases the cause of autopharmacological reactions is the direct toxin-induced release of biogenic amines. Immediate hypersensitivity reactions (type I) appear to be rare.

Tourniquets can cause local swelling and mimic local venom effects.

For some species of snakes local signs of envenoming are a reliable parameter for possible systemic envenoming, while for other species they provide no indication at all.

Swelling that involves large areas of an extremity or the entire extremity and that extends to the trunk carries the risk of development of hypovolaemic shock due to sequestration of large volumes of fluid.

The efficacy of antivenom with regard to local effects, in particular swelling and necrosis, is also a matter of controversy in Mexico and Central America and has even been called into question entirely. The neutralising capacity of the available antivenoms with regard to those venom components that possess haemorrhagic and myonecrotic activity is most probably small, as these effects occur rapidly after the injection of venom (Gutierrez 1990, Gutierrez et al. 1985).

The local effects of Bothrops asper venoms are myonecrosis, bleeding and oedema. Damaged musculature regenerates to some extent. However, in many cases regeneration is deficient due to changes in the microcirculation (Gutierrez 1990, Gutierrez and Lomonte 1989).

Lachesis sp. and Crotalus sp. (including former Crotalus durissus durissus, now C. simus): marked local effects with extensive swelling.

Micrurus sp.: local swelling either does not develop at all or is insignificant.

Even extensive swelling of the extremities is not necessarily an indication of compartment syndrome. There are no reports of patients with objectively identified compartment syndrome (measurement of compartment pressure). The decision to perform a fasciotomy must have a rational basis (evidence of increased intra-compartmental pressure; reduced or absent arterial blood flow).

Even coagulation disorders that are severe according to laboratory tests may only be clinically apparent to a slight degree, or not at all. There is a threat of spontaneous haemorrhage with extensive loss of blood or focal bleeding (e.g. intracranial) as long as the haemostatic defect exists (untreated, i.e. without antivenom treatment, days to weeks). The risk is even greater if a patient does not receive appropriate treatment at a hospital and is then exposed to trauma, even very minor trauma, for example while working. Loss of large volumes of blood can occur due to blood oozing from the bite wound or from injuries or due to medical or paramedical intervention.
If antivenom is effective, spontaneous systemic bleeding should cease within 15–30 min, and blood coagulability should be restored within 1–6 h. The clotting time test is a simple means to regulate the antivenom dose. The initial dose should be repeated if the blood is still not coagulable 6 h after the first dose (Warrell 1990b).

If antivenom is ineffective, one of the possible reasons may be misidentification of the snake that caused the bite, in particular if a monovalent antivenom was used.

The procoagulative components of the venom of juvenile Bothrops asper cause more marked haemostatic defects than those of the adult animals, or of Crotalus sp. and Lachesis sp., which have primarily direct fibrinogen-coagulating activity. Haemorrhagic activity has been described in particular for the venom of Bothrops sp., but also for Lachesis sp. venoms (Mandelbaum 1990).

This means that clinically significant haemorrhages are theoretically more likely to occur following Bothrops asper (and Lachesis sp.) bites than with Crotalus sp. bites. Documented cases with haemorrhagic complications appear to confirm this.

There are no controlled clinical studies of the efficacy of antivenom for the treatment of neurological effects of the venom.

On the whole, the effect of the currently available and clinically proven antivenoms on neurotoxin-induced defects is not very convincing and, if present at all, is slow to occur (Watt 1992).

Already established neurological effects of envenoming cannot be reversed with antivenom and only improve very slowly (Kitchens and Van Mierop 1987, Rosenberg 1971).

Due to the highly questionable efficacy of antivenom against neurotoxic symptoms of envenoming, the other two available treatment approaches need to be used concurrently and in a timely manner:

• Endotracheal intubation and artificial respiration: endotracheal intubation is certain to prevent any form of aspiration. Artificial respiration, even though it may have to be employed over a long period of time, can ensure survival of a patient with neurotoxin-induced respiratory failure.
• The edrophonium (Tensilon®) test should be performed immediately in every patient with signs of paralysis, in order, if the result is positive, to make the most of the improvement in neuromuscular transmission that can be achieved through use of a longer-acting acetylcholinesterase inhibitor (neostigmine). In some cases this may avoid the need for artificial respiration (see the Biomedical database entry for Micrurus sp.).

The venoms of various Micrurus species appear to impair neuromuscular transmission in different ways, and thus it can be expected that cholinesterase inhibitors will have varying degrees of efficacy (Brazil 1990).

There are no systematic investigations of the efficacy of acetylcholinesterase inhibitors. Their use is recommended (Bucaretchi 1990).

To date there have been no reports of systemic rhabdomyolysis following snakebites in Mexico and Central America. Only Crotalus durissus  from South America causes such symptoms of envenoming.

In contrast, local myonecrosis in the context of extensive local soft tissue destruction is a well-known complication of Bothrops asper and also Crotalus sp. bites (Gutierrez 1990, Gutierrez and Lomonte 1989, Ownby 1990).

There are no reports on the course of envenoming that indicate a primary cardiac effect of the venoms of Mexican and Central American elapids and crotalids. Secondary cardiac involvement is to be expected in patients with renal failure (hyperkalaemia).

• Antivenom,
• symptomatic treatment:
  
  • treatment of the acute renal failure.

There is no information regarding a primary nephrotoxic effect of venoms of Mexican and Central American snakes.

Acute renal failure can occur secondary to a haemostatic defect, arterial hypotension etc. and in most cases is the consequence of acute tubular necrosis. The prognosis is good, as long as there is adequate medical care (dialysis, fluid and electrolyte balance).

Two external characteristics of the crotalids and elapids are so obvious that identification of these snakes can be made relatively reliably, and they also make it possible to distinguish between the two genera:

• the rattle on the end of the tail: Crotalus sp. (see Find terrestrial snake by morphology (regional)),
• the markings and colouring: Micrurus sp., Fig. 4.56.

In addition, differentiation according to symptom complexes can aid regional identification (see Clinical flowchart: Mexico and Central America):

• Local swelling, incoagulable blood, systemic bleeding:
  
  • Crotalids.
• Signs of paralysis of the cranial nerves, the extremities and the respiratory musculature:
  
  • Elapids: Micrurus sp.

No controlled clinical studies on the efficacy of antivenom are available.

If the selected antivenom is not effective, 3 possible causes need to be considered:

1. correct identification of the cause, but insufficient dose administered;
2. correct identification of the cause, but inadequate efficacy of the antivenom;
3. incorrect identification of the cause → revision of identification.

Even if the desired effect of antivenom administration, namely normalisation of the parameters relevant to envenoming (findings on physical examinations, physical and laboratory investigations), is achieved quickly, this does not mean that the symptoms of envenoming may not re-occur due to continued absorption of venom from a depot in the region of the bite.

Patients bitten by species of snakes whose venom causes haemostatic defects should be kept in hospital for up to several days after initial treatment, and blood coagulability should continue to be monitored twice daily.

The same considerations apply to patients suffering from elapid bites.

• Excision of necrotic material,
• surgical debridement under general or regional anaesthesia, 
• skin grafting (split-thickness).

• Physiotherapy,
• surgical correction.

See Antivenom treatment.