Myokymia refers to continuous muscle twitching resulting in rippling muscle contractions of whole or part of the body. Some people suggest it looks like worms underneath the skin.
Neuromyotonia is a more advanced form of myokymia characterised by generalised muscle stiffness making it difficult for a pet to stand and with many collapsing into a very stiff, out-stretched posture.
What are myokymia and neuromyotonia?
Myokymia can be focal or widespread causing rippling of the muscles. Neuromyotonia is a more severe form of myokymia. The main features of neuromyotonia are persistent stiffness of the muscle without any relaxation. This causes an almost ‘cramp-like’ appearance that can result in complete collapse. Myokymia and neuromyotonia can occur separately or at the same time.
What can cause myokymia and neuromyotonia?
Myokymia affecting the head or face is something that can occur in dogs. This has been linked to problems in the brain but has also been seen in dogs with no obvious underlying cause. However, if myokymia is affecting the whole body this can be linked to diseases that affect the cerebellum which is art of the brain controlling movement. This is something seen in Jack Russell and Parson Russell terriers. Border collies and Yorkshire terriers also suffer from generalised myokymia although the cause is unknown.
What are the signs of myokymia and neuromyotonia?
A patient with myokymia alone may just have very mild ripples in the muscles and it may not be causing a problem (see myokymia video). However, this can progress and result in collapsing or severe stiffness (a likely sign of neuromyotonia – see neuromyotonia video).
How do I diagnose myokymia and neuromyotonia?
Try and obtain video footage of any abnormal episodes of collapse. You will need to take blood samples and if this is all normal then you may choose to refer to a specialist centre where the electrical activity of the patient’s muscles can be tested using electromyography (EMG). Myokymia produces a characteristic electrical pattern seen on EMG that can help with diagnosis. Neuromyotonia also produces a characteristic trace but episodes of neuromyotonia only occur very occasionally so it is unusual to be able to capture this using an EMG recording. If myokymia and neuromyotonia are confirmed then further imaging of the nervous system may be required using magnetic resonance imaging.
Can myokymia and neuromyotonia be treated?
If an underlying cause is found then it is important this is treated where possible. In some circumstances this may not be a realistic thing to do. When myokymia or neuromyotonia are severe then drugs can be given that calm the muscle rippling (e.g. procainamide and mexilitine). Pets that have episodes of severe neuromyotonia can get hot very quickly due to the severe muscle work-out their body is having. Therefore cold water baths are recommended to manage the hyperthermia that is often seen with severe collapsing episodes of neuromyotonia.
Will a dog recover from myokymia and neuromyotonia?
Sadly this disorder is often progressive and will worsen with time. Therefore medication can help initially but it is likely the problem will worsen and the episodes of collapse caused by neuromyotonia will become more frequent. Euthanasia is often recommended if these episodes occur too often as they can be life threatening due to the severe hyperthermia they can cause.