Myoclonus is an involuntary jerk of the legs and/or body that can be likened in appearance to a brief electric shock.
What is myoclonus?
Myoclonus is best likened to how we would react having had an electric shock. It is possible to have a single myoclonic jerk but also it is possible to have a train of shocks. Although it is likened to an electric shock it is important to say it is not associated with the same painful feeling.
What causes myoclonus?
Myoclonus can be seen as a normal finding. For example, hiccups are a type of myoclonus as are flailing movements as we fall sleep (so-called hypnic jerks). However, when they are seen irregularly and spontaneously on a frequent basis it is likely these myoclonic jerks are something to have investigated. Frequently they can be associated with epileptic seizures.
Conditions associated with epilepsy that result in myoclonus include Lafora disease, feline audiogenic reflex seizures, juvenile myoclonic epilepsy and storage diseases (specifically neuronal ceroid lipofuscinosis).
Conditions associated with non-epileptic myoclonus include distemper virus, hemifacial spasm and startle disease.
What are the signs of myoclonus?
Sudden jerks of part or all of the body are the main feature of myoclonus when a pet is awake. It is important to be aware that seizures can occur alongside these jerks. The type of seizure most commonly seen is a generalised tonic-clonic seizure. This is a seizure that begins with contraction of the muscles and loss of awareness. A pet may fall to the side with their legs stretched out and the head back. Some pets may vocalise or have facial twitching. Vocalisations are involuntary and do not indicate pain. Often the pet may drool excessively, urinate, or defecate. This can then develop into rhythmic, relatively violent, movements. Typically this consists of clamping the jaws and jerking or running movements of the legs.
How do I diagnose myoclonus?
Video footage of the patient’s problem is the best first step to take so we can see exactly what the owners are observing at home (see video). If myoclonus is confirmed then further tests are a sensible step to take.
This should start with comprehensive blood work to rule-out problems with the blood going to the brain. It is important for these tests to be done after the patient has been starved for 12 hours so performing the blood tests first thing in the morning is a good idea.
If these results are normal then an MRI scan can help make a diagnosis. Cerebrospinal fluid analysis is also an important test to perform if it can be performed safely.
Can myoclonus be treated?
It very much depends on the underlying cause as to whether myoclonus is treatable. If it is thought to be part of an epilepsy syndrome then levetiracetam seems very effective at controlling the episodes.
If generalised tonic-clonic seizures occur then further antiepileptic medication, such as phenobarbital, may be appropriate.
What is the prognosis?
Recovery is dependent on the underlying cause. Patients that have an epileptic syndrome may not be curable but have a good chance of receiving treatment that will help manage their myoclonic jerks. If the myoclonus is due to canine distemper virus then the prognosis for recovery from the myoclonus is poor but depending on the stage of infection, many dogs can have a good quality of life in spite of the muscle jerks. If an epilepsy syndrome is the cause then the myoclonic jerks are likely to slowly worsen over months to years and it is also possible that other signs may develop that appear similar to dementia in people. These signs may include incontinence, being unsettled in the house, pacing around, visual difficulties and behavioural changes.