Myoclonus is defined as a sudden onset jerking motion. How is this different from a tremor? Tremors tend to have a creshendo, decreshendo appearance where as myoclonus has been described as "square form" impulse with sudden, rapid onset, sustained contraction, and sudden stop. Okay, fine, it's not a tremor. How do I diagnose myoclonus?
There are four common types:
1) Physiologic - the classic example is hiccups.
2) Epileptic- occur with or around epileptic seizures
3) Idiopathic - unknown origin or cause (rare in veterinary medicine...or we're just missing it.)
4) Symptomatic - due to an underlying pathology. For example, distemper virus.
Neuroanatomic lesion localization:
1) Cortical - usually associated with myoclonic seizures.
2) Subcortical (brainstem) - "falling asleep" myotonia. In this case, people or animals will suddenly "jerk" just as they fall asleep. Classically, this has been thought to occur because the cortical functions "turn off" before the brainstem functions but this may not be the complete story.
3) Peripheral - an example is hemifacial spasms, or ocular spasms (have you ever had your eyelid twitch annoyingly and you cannot stop it?)
Veterinary neurologists are often consulted for symptomatic and epileptic myoclonus. Although there is a population with idiopathic myoclonus, it is rare. Symptomatic myoclonus occurs at rest, asleep, or during motion. Symptomatic myoclonus does not stop with sleep! The animal may appear aware or unaware of the twitch. If the contraction is happening in a body part used for walking or eating it may be life limiting. If not, animals may live with a myoclonus with minimal disruption. Epileptic myoclonus occurs around a time of seizure activity and is therefore often managed with anticonvulsant management.
Distemper virus is a common cause of canine myoclonus and CNS infections (various causes) have been cited as causes of feline myoclonus. There is no treatment available for distemper virus therefore, patients are monitored for progressive neurologic signs (mentation changes, gait changes, seizures, other) and euthanized if signs are progressive. If the signs are not progressive, usually myoclonus is not a reason for euthanasia.
That's all I have for you today! I hope you enjoyed this TidBit about myoclonus and feel better prepared when you see it in your exam room. :)
Thank you for reading! May you have a wonderful, twitch-free holiday week!