Envenomation

Published on 14/03/2015 by admin

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Chapter 30 Envenomation

Shane Curran, Thomas McDonagh

Australia is home to many venomous creatures. Australian animals that are important in causing envenoming in humans include snakes, spiders, octopuses, fish and other marine creatures. The distribution of venomous creatures is wide, and each region has its own pattern of envenomation. Local knowledge is very important and local expert knowledge can be invaluable.

Resources that are available to you include the local emergency physician or the on-call toxinologist, who can be contacted via Poisons Information (tel. 131126). They are available to discuss management of patients with possible or definite envenomation.

SNAKEBITE

Australia is home to a number of the most venomous snakes in the world. Snake venom is a complex mixture of substances. Australian snakes produce venoms that have a range of clinical effects including neurotoxic (presenting as progressive paralysis), myotoxic (causing rhabdomyolysis and subsequent renal failure) or severe coagulation disturbances and haemolysis (Table 30.1).

Table 30.1 Clinical effects of Australian snake venom

Snakebite is a medical emergency. Patients presenting following possible snakebite should receive urgent assessment and management. Patients who have significant envenomation may initially appear well.

The majority of snakebites will not result in significant envenoming, and so will not require antivenom. This is because many snakebites are ‘dry bites’ where no venom is injected. The amount of venom injected by a snake depends on snake maturity, fang length, venom yield, snake temperament, the number of bites and the time since the snake’s last meal. Overall, fewer than one in four patients require antivenom.

First aid: the pressure-immobilisation technnique

Snake venom spreads via the lymphatics. The pressure-immobilisation method of first aid prevents spread of the venom via the lymphatics and can prevent clinical envenomation. It involves applying a firm broad bandage, commencing at the site of the bite, and then applying the bandage over the entire limb, extending both proximally and distally. The pressure is the same as that used for a sprained ankle. A splint is then applied to the limb, to immobilise the limb and reduce muscle contraction to further reduce lymphatic spread of the venom. The patient is then kept as still as possible and transported to hospital.

The pressure-immobilisation technique can prevent clinical envenomation if applied early and correctly. Both the limb and the patient should be kept still. The bandages are kept in place until facilities are available to treat clinical envenomation. This may mean transporting a patient to a hospital with a supply of antivenom. If deterioration occurs when the bandages are removed, the bandages should be reapplied.

It is important not to wash the site of the bite, as it contains traces of venom that are important for identifying the snake type.

If a patient arrives following a possible snakebite and has had no first aid but is well with no signs of envenoming, there is no need to apply the pressure-immobilisation technique.

If a patient arrives with symptoms of envenomation and does not have a bandage applied, then one should be applied.

Table 30.2 Use of the pressure-immobilisation technique of first aid

Pressure-immobilisation is recommended for	Do not use pressure-immobilisation first aid for
All Australian venomous snake bites, including sea snake bites	Redback spider bites
Funnel web spider bites	Other spider bites, including mouse spiders, white tailed spiders
Box jellyfish stings (if possible)	Bluebottle jellyfish stings
Bee, wasp and ant stings in allergic individuals	Other jellyfish stings
Blue-ringed octopus bites	Stonefish and other fish stings
Cone snail (cone shell) stings	Bee and wasp stings in non-allergic individuals
Australian paralysis tick envenomation	Bites or stings by scorpions, centipedes, beetles

Adapted with permission of the Australian Venom Research Unit

Venom detection kit

The venom detection kit developed by the Commonwealth Serum Laboratories (CSL) is important in the management of the patient with snakebite. The venom detection kit can detect minute amounts of snake venom if present, and is used to determine what family of snake that venom came from.

The venom detection kit does not indicate that a patient has been envenomed. It also does not determine that a patient should be given antivenom.

The venom detection kit does indicate which antivenom to use, once the decision to give antivenom is made. The decision to give antivenom is a clinical decision based on symptoms, signs and pathology testing.

A swab of the bite site is best as this is where venom is present in highest quantities. A window can be cut in the bandages overlying the bite site. When the swab has been taken, the bandage is reapplied.

Urine is the next best for venom detection as Australian snake venoms are excreted in the urine. Blood or serum is not used.

The venom detection kit is used by trained laboratory staff and takes about 20 minutes to complete.

Antivenom

Antivenom is the definitive treatment for a patient with systemic envenomation. The administration of antivenom can reverse the clinical effects of envenomation. The treatment of envenomation following snakebite involves the administration of adequate quantities of the appropriate antivenom.

Antivenom is produced by CSL from antibodies that are harvested from horses that have been injected with subclinical doses of snake venom toxins. As the antibodies are obtained from horse serum there is the risk of anaphylaxis, allergic reaction or delayed serum sickness. Prior to administration, therefore, preparations should be made to treat a possible anaphylactic reaction. Currently premedication with adrenaline is not recommended.

Antivenom is given intravenously and multiple ampoules may be necessary. Monovalent antivenom is indicated if the type of snake is known, when the venom detection kit determines the type of antivenom to be given, or in areas where the occurrence of snakes is limited to specific snake types. Monovalent antivenom is preferred as it is less expensive, is a smaller volume of foreign protein and has fewer side effects.

Polyvalent antivenom contains antibodies to venom of all groups of snakes. It consists of a large volume and is expensive, but is used when treatment with antivenom is indicated and the snake type is unknown and the venom detection kit procedure is negative or will take too long to perform.

In certain areas a mixture of monovalent antivenoms, based on the local prevalence of snake types, may replace polyvalent antivenom.

The dose of antivenom is dependent on the dose of venom injected by the snake, not patient size, so children require the same amount of antivenom as adults do.

Laboratory testing

Laboratory tests are vital to assessing the patient possibly bitten by a snake. Patients with possible snakebite should have blood taken for:

• coagulation—activated partial thromboplastin time (APTT), prothrombin time (PT), fibrinogen level, d-dimer/fibrinogen degradation products (XDP), platelet count

• creatine kinase

• renal function

• electrolytes.

Urine should be tested for myoglobin and blood.

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