Neuro- and myotoxicological signs and symptoms are significant clinical features of envenoming snakebites in many parts of the world. The toxins primarily responsible for the neuro and myotoxicity fall into one of two categories—those that bind to and block the post-synaptic acetylcholine receptors (AChR) at the neuromuscular junction and neurotoxic phospholipases A2 (PLAs) that bind to and hydrolyse membrane phospholipids of the motor nerve terminal (and, in most cases, the plasma membrane of skeletal muscle) to cause degeneration of the nerve terminal and skeletal muscle. This review provides an introduction to the biochemical properties of secreted sPLA2s in the venoms of many dangerous snakes and a detailed discussion of their role in the initiation of the neurologically important consequences of snakebite. The rationale behind the experimental studies on the pharmacology and toxicology of the venoms and isolated PLAs in the venoms is discussed, with particular reference to the way these studies allow one to understand the biological basis of the clinical syndrome. The review also introduces the involvement of PLAs in inflammatory and degenerative disorders of the central nervous system (CNS) and their commercial use in the food industry. It concludes with an introduction to the problems associated with the use of antivenoms in the treatment of neuro-myotoxic snakebite and the search for alternative treatments.

β-Bungarotoxin, a neurotoxic phospholipase A₂ is a major fraction of the venom of kraits. The toxin was inoculated into one hind limb of young adult rats. The inoculated hind limb was paralysed within 3 h, and remained paralysed for 2 days. The paralysis was associated with the loss of synaptic vesicles from motor nerve terminal boutons, a decline in immunoreactivity of synaptophysin, SNAP-25 and syntaxin, a loss of muscle mass and the upregulation of NaV_1.5 mRNA and protein. Between 3 and 6 h after the inoculation of toxin, some nerve terminal boutons exhibited clear signs of degeneration. Others appeared to be in the process of withdrawing from the synaptic cleft and some boutons were fully enwrapped in terminal Schwann cell processes. By 12 h all muscle fibres were denervated. Re-innervation began at 3 days with the appearance of regenerating nerve terminals, a return of neuromuscular function in some muscles and a progressive increase in the immunoreactivity of synaptophysin, SNAP-25 and syntaxin. Full recovery occurred at 7 days. The data were compared with recently published clinical data on envenoming bites by kraits and by extrapolation we suggest that the acute, reversible denervation caused by β-bungarotoxin is a credible explanation for the clinically important, profound treatment-resistant neuromuscular paralysis seen in human subjects bitten by these animals.

We describe 70 cases of monocled cobra (Naja kaouthia) bite admitted to Chittagong Medical College Hospital, Bangladesh. The biting snakes were identified by examining the dead snake and/or detecting N. kaouthia venom antigens in patients' serum. Bites were most common in the early morning and evening during the monsoon (May–July). Ligatures were routinely applied to the bitten limb before admission. Thirty-seven patients consulted traditional healers, most of whom made incisions around the bite site. Fifty-eight patients experienced severe neurotoxicity and most suffered swelling and pain of the bitten limb. The use of an Indian polyvalent antivenom in patients exhibiting severe neurotoxicity resulted in clinical improvement but most patients experienced moderate-to-severe adverse reactions. Antivenom did not influence local blistering and necrosis appearing in 19 patients; 12 required debridement. Edrophonium significantly improved the ability of patients to open the eyes, endurance of upward gaze, and peak expiratory flow rate suggesting that a longer-acting anticholinesterase drug (neostigmine) could be recommended for first aid. The study suggested that regionally appropriate antivenom should be raised against the venoms of the major envenoming species of Bangladesh and highlighted the need to improve the training of staff of local medical centers and to invest in the basic health infrastructure in rural communities.

The demographics, epidemiology, first aid, clinical management, treatment and outcome of snake bites causing no significant signs of systemic envenoming were documented in Chittagong Medical College Hospital, Bangladesh, between May 1999 and October 2002. Among 884 patients admitted, 350 were systemically envenomed and 534 were without signs of either systemic or significant local envenoming. The average age of patients with physical evidence of snake bite but no systemic envenoming was 26.4 years. Most had been bitten on their feet or hands. Ligatures had been applied proximal to the bite site in >95% of cases and the bite site had been incised in 13%. Patients were typically discharged at 24h. Those with clinical signs of systemic envenoming resembled the non-envenomed cases demographically and epidemiologically except that they arrived at hospital significantly later than non-envenomed patients, having spent longer with traditional healers. No non-envenomed patient was treated with antivenom and none went on to develop symptoms of systemic envenoming after discharge. The potential complications and confusing signs caused by ligatures and incision demand that all patients admitted with a history of snake bite be kept under observation for 24h after admission even if they have no signs of systemic envenoming.

Envenoming bites by Vipera ammodytes ammodytes (the long-nosed viper) can cause life-threatening neurotoxicity, particularly in children. We investigated the mechanisms of the neurotoxicity of ammodytoxin A, the principal toxin in the venom of these snakes, in isolated nerve-muscle preparations from mice. The toxin was bound selectively to the neuromuscular junction, and at concentrations similar to those likely to be found in the circulation of young bite victims, it blocked the response of the muscle to indirect but not direct stimulation. Electron microscopy showed that the toxin induced a small but insignificant depletion of synaptic vesicles from motor nerve terminals; nerve terminal mitochondria were swollen and damaged, but plasma membranes of terminal boutons were undamaged. Exposure to the toxin did not affect postjunctional acetylcholine receptors or cause structural damage to preterminal motor axons or muscle fibers. Spontaneous transmitter release was similarly unaffected. Taken together, these results indicate that ammodytoxin A is the principal agent involved in the neurotoxic activity of the venom of V ammodytes ammodytes and that the underlying cause of the failure of transmission may be the deenergization of the nerve terminal resulting from mitochondrial degeneration and subsequent impairment of coupling between the action-potential-induced depolarization of the nerve terminal and the evoked transmitter release.