Friday, November 15, 2013

Seizures in dogs and cats

A chihuahua is presented for investigation of seizures. (Note the painted nails are not related in anyway. This breed commonly suffers from hydrocephalus.
Veterinary neurologist Georgina Child presented a fantastic webinar on seizures this week. Seizure disorders are common in companion animals and may be acute (often secondary to a toxicity of some kind) or recurrent. Seizures are incredibly distressing to witness and if left untreated can increase in frequency.

We tend to think characterised seizures as generalised seizures (by far the most common and the classic tonic/clonic type seizure) or focal seizures (much harder to detect). Petit-mal or absent seizures aren’t properly recognised in dogs although they are a common seizure disorder in people.

Dr Child said that her job was easier thanks to smartphones as owners can now present her with footage of seizure events, making characterisation much easier. Differential diagnoses for seizures include syncope, movement disorders, narcolepsy and some compulsive behaviours.

“Anything that presents as a stereotypic set of events that comes out of a background of normality then finishes abruptly with the animal returning to normal has to be considered a possible seizure,” she said.

Causes of seizures are divided into two main categories:

Intracranial or extracranial. 
Extracranial causes include metabolic disease and toxins.
Intracranial causes include underlying structural abnormalities which may or may not be progressive, eg tumours, previous head injury, cerebrovascular accidents. There are also idiopathic seizures for which no underlying cause is found, though Dr Child said that many such dogs had inherited the seizure disorder.

Seizures always indicate a forebrain abnormality. Focal seizures occur due to a transient disturbance in a specific area of the brain rather than the whole forebrain. Contrary to popular belief, focal seizures may also be familial in origin.

The term epilepsy refers to a syndrome of recurrent seizures – it is not a disease per se. It is the most common cause of seizures in dogs under five years old.

Commonly affected breeds include border collies, Belgian shepherds, Siberian huskies, malamutes, retrievers and collies of all kinds. Dr Child’s own border collie Tilly has epilepsy, so as she said she lives the nightmare of her clients.

According to Dr Child, the chances of finding a metabolic abnormality in dogs with a seizure disorder is less than 1 in 50 for dogs without interictal signs, but they should be looked for as these can be treated.

All animals should undergo a full neurological examination once they have recovered from the postictal period – but in some cases this can last for a day or more.

In a dog under one year old with no obvious interictal neurological signs or metabolic abnormalities, Dr Child recommends liver function tests to rule out hepatic encephalopathy which may be secondary to a portosystemic shunt. MRI may be helpful in identifying intracranial causes such as hydrocephalus.

In dogs where seizures commence between 1 and 5 years of age, the most likely cause is idiopathic epilepsy.

Previously well controlled epileptics may develop an increased frequency of seizures. This can be due to emergence of a concurrent intracranial problem, e.g. brain tumour, or may be due to pharmacoresistance which develops in about 40% of dogs with time.

Hyperlipidaemia/hyperliperproteinaemia is an hereditary problem in mini schnauzers and can be associated with the development of seizures. Fasted triglyceride level should be performed.

Interestingly, Dr Child made the comment that cerebrovascular infarcts occur more commonly in animals than we have heretofore appreciated.

In dogs over 5 years old, it is vital to take a fasting blood glucose. But these animals have a 50% probability of having intracranial disease, specifically a brain tumour. Assessment for extracranial neoplasia is warranted e.g. thoracic radiographs, abdominal ultrasound.

Dr Child always recommends performing an MRI prior to CSF tap as CSF tap is more risky in animals with clinical signs suggestive of neurological disease. If imaging is suggestive of inflammation or an infectious process, a CSF tap may be indicated.

Dr Child always considers DNA-testing in breeds with known familial neurodegenerative disease, including Staffordshire terriers.

Cats are a bit different - epilepsy is less common than in dogs; infectious CNS disease and systemic hypertension are more common.

When should anticonvulsant treatment be instituted?

This is a subject that experts disagree on, but Dr Child’s recommends treatment when:
  • Progressive intracranial disease is present
  • If there is more than one seizure every 4-6 weeks
  • There are 2 or more seizures in a 24 hour period (cluster seizures)
  • Any history of status epilepticus (seizure >5 minutes – most last 60-90 seconds).


There is no guarantee that any medication will stop seizures completely.

She discussed her drug regimes and monitoring protocols at length.

Seizure control in emergencies


Animals may present as an emergency for treatment of seizures including status epilepticus or cluster seizures. 

Drug
Dose
Route
Comments
Diazepam
0.5-1mg/kg
Intravenous

Diazepam
2mg/kg
Per rectum
Can repeat at 5 and 20 mins – no more than 3 doses in 24 hours. Can be given intranasally.
Midazolam
0.06-0.3mg/kg
IV or IM
Useful if no IV access; preferred in cats
Levetiracetam
40-60mg/kg
Intravenous; can also give via any route
Then 20mg/kg every 8 hours
Phenobarbitone
2-10mg/kg (average 5mg/kg)
Slow IV, IM, PO
Continue for up to 6-12 hours. May take 30 mins to take effect.
Propofol
Bolus to effect then 0.05-0.1mg/kg/min to a max of 6mg/kg/hr
As constant rate infusion

Thiopentone/propofol + inhalation anaesthetic



CRI diazepam/midazolam added to CRI maintenance requirement



Pentobarbitone
To effect – much less than GA dose


In all continue phenobarbitone and/or levetiracetam at maintenance doses.
HALVE ALL DOSES FOR CATS AND ADMINISTER THIAMINE.

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