Not-So-Tiny Review: JVIM July/August 2017 – Myoclonus and Myotonia

Neurologic article reviews on this issue of JVIM will be divided into two posts. This first post is solely on the Lowrie and Garosi article on myoclonus and myotonia.

Lowrie and Garosi discussed a classification system for myoclonus and myotonia, two involuntary movement (IM) disorders, in order to aid diagnosis and treatment. Myoclonic movements are sudden, brief, shock-like IMs. They are typically positive (caused by muscle contraction) and are usually are associated with movement of the affected body part, unlike twitches in which the twitch remain within the affected body segment and movement is not observed.  Myoclonus can be subdivided into physiological myoclonus (e.g., hypnic jerks), essential myoclonus (idiopathic or hereditary), epileptic myoclonus, or symptomatic myoclonus. Most people are inherently familiar with myoclonus. Most of us twitch when we fall asleep and sometimes experience this “twitch” as part of a dream. These episodes are entirely normal and are called hypnic jerks, but they give a good indication of what a sudden, brief, “shock-like,” involuntary movement caused by muscular contraction would feel like. Hiccups are another example of physiological myoclonus. Epileptic myoclonus occurs in patients whose main complaint is one of epilepsy but who also exhibit myoclonus and also is seen with degenerative diseases such as Lafora’s disease or lysosomal storage diseases. Non-epileptic myoclonus is seen in dogs secondary to distemper virus encephalomyelitis. Levetiracetam and clonazepam appear to have some benefit in many types of myoclonus; levetiracetam works well with epileptic myoclonus. However, subcortical (e.g., familial reflex myoclonus in Labradors) or spinal myoclonus (e.g., CDV) are fairly non-responsive to either medication.

Myotonia always occurs after a voluntary movement or after a physiological or external stimulus (e.g., percussion). After a period of rest, dogs develop marked stiffness in all 4 legs, exhibiting a “bunny-hopping” gait in the pelvic limbs for the first few steps that subsides with exercise but the gait remains abnormal. In severe cases, dogs may experience episodes of stiffening and falling sideways with all 4 limbs rigidly extended. These episodes resolve rapidly with dogs returning to ambulation but with stiffness remaining.  Myotonia congenita, non-dystrophic myotonia, has been described in the Miniature Schnauzer and is inherited as an autosomal recessive trait.  The disease results from a mutation in the skeletal muscle voltage-dependent chloride channel.  Unfortunately, there are no safe and effective drugs available that act directly on the chloride channel. Therefore, treatment is aimed at sodium channel blockade to prevent the receptive activation of these channels and hence decrease the repetitive electrical activity of the myotonic muscle. Class 1 antiarrhythmic drugs such as procainamide (class 1A) and mexiletine (class 1B) affect sodium channel activation and have both been used successfully in the management of myotonia in people. In dogs, only higher doses of these medications resulted in clinical improvement, but the adverse events at these higher doses make them unsuitable as long-term treatment.

Myotonic dystrophy has only been reported in 4 adult dogs with clinical signs similar to those of congenital myotonia.  Muscle was grossly and histologically grossly abnormal, with variability in fiber size, fibrosis, rows of internal nuclei, and type I fiber atrophy. No mutation has been identified for veterinary muscular dystrophies to date.

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