Covulsions are very distressing for the owner and dangerous in the case of large animals. Depending on the cause, they can also quickly cause serious damage to the animal's CNS. They should be treated as an emergency.
In general, benzodiazepines (diazepam) should be given iv immediately after a quick preliminary assessment; this will allow safer handling of the animal and some time for further assessment and organisation of the definitive treatment. Remember to secure the airway and prevent inhalation of vomit. Placing an iv catheter is essential.
Benzodiazepines bind to GABAA receptors to increase opening, which causes an inflow of chloride ions into the neurone and hyperpolarisation. Benzodiazepines will potentiate other drugs with the same mechanism of action, ie, most other anaesthetics. They have anticonvulsant, sedative and muscle relaxing effects, and only produce respiratory depression at very high doses.
Initially diazepam should be given at 0.5 - 2 (dogs & cats) - 5 (adult horses and cattle) - 9 (foals) mg/kg. Give the lowest dose by slow iv injection, followed by more slow injections to effect. The dose is not critical as diazepam is very safe.
In dogs, iv injections are best made into the lateral saphenous vein, to avoid accidentally getting bitten. In large animals. the jugular vein is probably best, approached from behind the animal to avoid getting kicked.
If it is not possible to find a vein, diazepam solutions (Pamlin, many human products) can be given im. Diazepam emulsions (Diazemuls) should not be given im. As a last resort, both can be given per rectum, but absorption is variable.
Diazepam only lasts a short time, usually about 20 minutes. A second dose can be given, or the animal can be given an iv infusion. A maximum of 80mg/L in 5% dextrose should be used, and the mixture used within 6 hours as diazepam can stick to the plastic in infusion bags and giving sets. Animals rapidly become tolerant to diazepam and other drugs usually have to be used after the intitial control of convulsions with diazepam.
Midazolam (Hypnovel) is very similar to diazepam, but is water soluble which overcomes problems with the vehicle in injections. It is short acting, and an iv infusion is likely to be needed after the initial iv injection.
Initial dose (all species) 100 - 200µg/kg iv. This is only likely to last about 10 minutes. It can be given im if necessary.
Clonazepam (Rivotril) is often used in people for treating convulsions. Experience in animals is limited.
Dose for dogs is 50 - 200µg/kg iv
In the very unlikely event of benzodiazepine overdose causing respiratory depression, the antagonist flumazenil (Anexate) can be given at 100µg/kg iv every minute until the effects are reversed. Flumazenil is very expensive.
Phenobarbitone is more anticonvulsant than other barbiturates, but iv solutions are rarely available when needed.
Dose 3 - 6 (- 20)mg/kg slow iv. Onset takes about 20 minutes, so small doses every 20 minutes until an effect is seen is the best way of giving it. Diazepam or pentobarbitone may have to be given until it takes effect.
Overdose is likely to cause anaesthesia with respiratory depression. If this occurs, the animal should be intubated and ventilated until the effects wear off. This may take some time.
If the animal starts to convulse again as the diazepam wears off, pentobarbitone is usually given.
Dose in all species is 10 - 15mg/kg slowly iv. It has a much slower onset of action than modern anaesthetics, so top up doses to effect should not be given until 2 - 3 minutes after the first dose.
Pentobarbitone is a barbiturate, which acts on GABAA receptors in a similar way to the benzodiazepines. It is potentiated by any remaining diazepam, so dosing must be to effect. It is likely to cause significant respiratory depression and the animal should be intubated.
Anaesthetic pentobarbitone should be used; euthanasia solutions are not sterile.
If pentobarbitone or phenobarbitone are not available, anaesthesia may be induced with thiopentone and maintained with isoflurane or halothane. Propofol can also be used, but remember that propofol in unpremedicated animals often causes twitching, which can be mistaken for convulsions. Anaesthesia should be kept as light as possible.
If the muscle rigidity arises from the spinal cord rather than the brain, drugs which act at this level to cause muscle relaxation are sometimes used.
Methocarbamol was useful but is no longer available in NZ. If it can be obtained, the dose is 150mg/kg slowly iv for dogs, followed by 75mg/kg iv as necessary. For cats, 55 - 220mg/kg iv to effect. For horses, 22 - 55mg/kg to effect. For cattle, 110mg/kg to effect.
Guaiphenesin (glycerol guaiacolate (ether), GG) has been used but causes haemolysis, particularly in dogs. Its main advantage is that it is relatively cheap if the animal requires several days of treatment. Dose: dogs, 44 - 110mg/kg iv to effect, horse and cattle, 66 - 132mg/kg iv to effect, pigs, 44 - 88mg/kg iv. There is no information on withholding times. Avoid if possible.
Acepromazine has been advocated for treating convulsions, but it lowers the threshold for convulsions and can cause a variety of dyskinesias which resemble convulsions. It should not be used.
Xylazine, and probably other α2 agonists, can have some anticonvulsant effects. However, xylazine can also be proconvulsive, and can antagonise the effects of benzodiazepines and barbiturates. Avoid.
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