Myotonic Myopathies

Myotonia is defined as a state in which active muscle contraction persists after voluntary effort or stimulation has stopped. Congenital myotonia with onset of clinical signs at about 2 months of age have been described in the chow chow,46 Staffordshire terrier,46 and great Dane,47 and recently at 5 months of age in two domestic shorthair cats.48 Stiffness is usually present after rest and characteristically disappears after exercise ("warming-out" phenomena). Hypertrophy of the proximal limb muscles, tongue, and neck may be present. Dimples result following percussion of muscles. The diagnosis is usually confirmed by the presence of characteristic trains of repetitive discharges that wax and wane in frequency to produce a "dive-bomber" sound. Muscle biopsies are usually normal or show only mild myopathic changes.

Recently, a myotonic disorder with dimpling and characteristic electrophysiological abnormalities (myotonic discharges) has been observed in miniature Schnauzers at approximately 6 weeks of age.50 Marked hypertrophy of proximal limb muscles is present. The tongue stiffens and protrude from the mouth. Dysphagia and excessive salivation are common. Facial dysmorphism, described as a beak shape to the jaw, has been present in some affected dogs. In contrast to the myotonia that occurs in the chow-chow, stiffness appears to worsen following exercise instead of improve. Muscle morphology has been unremarkable (Shelton, unpublished). No treatment is currently available.

Other Hereditary Myopathies.


Serum CK concentration may be within normal limits or mildly elevated. Electromyographic evaluation may show spontaneous activity including fibrillation potentials, positive sharp waves, and bizarre high frequency discharges. The nerve conduction velocity is normal and there is no decremental response to repetitive nerve stimulation. The diagnosis of this myopathy is confirmed by evaluation of fresh frozen muscle biopsy sections including fiber typing. While the original report of this myopathy described a type II muscle fiber deficiency,51 a wide range of morphological features may be observed in muscle biopsies from affected dogs suggesting there may be more than one disorder affecting young dogs of this breed. Neuropathic features have been present in some dogs, while in others myopathic and dystrophic features predominate. The underlying etiology of this disorder, whether myopathic or neuropathic, has not yet been clarified. No treatment is currently available. Although affected dogs are not suitable for work, they may be acceptable house pets as clinical signs stabilize between 6 months and 1 year of age. Since the molecular defect of this disorder(s) has not yet been identified and a diagnostic test is not yet available for identification of clinically normal heterozygous carriers, breeders should eliminate parents or siblings of affected pups from their breeding program.

Nemaline rod myopathy - Nemaline rods have been described in association with a congenital myopathy in a family of cats 53 and in a young blue merle border collie.54 The onset of clinical signs in the reported cats began at about 6 months of age and consisted of mild weakness and reluctance to be handled, progressing to tremor and muscle atrophy. Clinical signs in the border collie, evident at 14 weeks of age, consisted of tremors, exercise intolerance, stiff and stilted gait, and muscle atrophy. Histochemical evaluation of fresh frozen muscle biopsy sections using the modified Gomori trichrome stain demonstrated the presence of numerous rods. Weakness was progressive, ultimately resulting in quadriplegia. No treatment is currently available. Nemaline rods have also been described in association with a severe progressive myopathy in a young silky terrier with clinical signs evident at 12 weeks of age including dysphagia, choking, stiff hindlimb gait and hunched stance.55

Central core myopathy - An inherited myopathy, beginning at about 6 months of age, has been described in great danes in the United Kingdom. Clinical signs begin at about 6 months of age with progressive muscle wasting, exercise intolerance, generalized body tremors and collapse that is exacerbated by excitement.56 The diagnosis of this myopathy is dependent on evaluation of muscle biopsy sections which show well-defined dark-staining central areas within many myofibers, consistent with central cores. No specific treatment is available at this time.

Hypertonic myopathies - An electrically silent hypertonic myopathy beginning at about 3 months of age has been described in Cavalier King Charles spaniels in the UK,57,58 and muscle biopsies were evaluated from 2 affected littermates from Australia (Shelton, unpublished). All described dogs had a history of exercise and excitement-induced "collapse" that was preceded by a "deer-stalking" action. An increase in extensor tone of muscles of all 4 limbs was evident during the time of collapse with recovery occurring in about 10 minutes. Treatment with diazepam did not result in improvement. Progression of the disorder has not been reported and stabilization or improvement may occur. While mitochondrial and membranous abnormalities have been found morphologically within muscle biopsies, the pathogenesis of this condition remains obscure.

A hypertonic disorder characterized by myoclonus and extensor rigidity has been described in Labrador retrievers.59 Clinical signs, occurring at 6 weeks of age, included intermittent stimulus-sensitive contractions of the appendicular and axial muscles, and generalized contractions initiated by voluntary movements. Therapeutic trials with diazepam and clonazepam were not effective.

Muscular hypertonicity (Scottie Cramp) associated with postural and locomotive difficulties and characterized by paroxysms of muscular hypertonicity have been described in Scottish terrier dogs.60,61 Clinical signs may be observed in puppies at 6-8 weeks of age. Marked pelvic limb extensor rigidity may cause the dog to fall when running and severity may be such that ambulation is impossible. An autosomal recessive mode of inheritance is suggested. Administration of methylsergide (0.1-0.6 mg/kg orally) is effective in identifying mildly affected dogs with cramping evident within 2 hours and the effects lasting for 8 hours. Diazepam (0.5 to 1.5 mg/kg orally three times daily) may be used in the treatment of affected dogs.

Miscellaneous inherited myopathies - Scattered case reports of other inherited myopathies are in the literature. An autosomal recessive myopathy has been described in Devon rex cats.62,63 Clinical signs are observed between 1 and 6 months of age and include generalized appendicular weakness, ventroflexion of the neck, megaesophagus, and dorsal protrusion of the scapulae with normal reflexes and normal CK. Dystrophic changes have been described in muscle biopsy sections. To date, the pathogenetic mechanisms responsible for Devon rex myopathy remain unknown. A familial polysystemic disorder involving dyserythropoiesis, polymyopathy and cardiac abnormalities has also been described in 3 related springer spaniel dogs.64 The list will continue to grow as newly recognized bred-specific myopathies are identified.

Inflammatory myopathies

Masticatory muscle myositis (MMM) - This immune-mediated, focal inflammatory myopathy selectively affects the muscles of mastication and can occur in dogs less than 6 months of age. Clinical signs include some combination of masticatory muscle atrophy or swelling and abnormal jaw function, manifested generally by restricted jaw mobility. The serum CK concentration may be normal or mildly elevated. Laboratory diagnosis is made by detection of circulating antibodies against type 2M fibers, a unique fiber type present only in the muscles of mastication.65,66 Evaluation of a muscle biopsy is necessary for confirmation of the diagnosis and in prognosis for return of jaw function and muscle mass as determined by the amount of myofiber destruction and fibrosis. Immunosuppressive dosages of corticosteroids should be used until jaw function returns to normal and serum CK is within the reference range. The dosage should then be decreased until the lowest alternate day dosage is reached that keeps the dog free of clinical signs. This dosage should be continued for an additional 4-6 months since clinical signs will reoccur if treatment is stopped too soon or an inadequate dosage is used initially.
Infectious polymyositis - Inflammatory myopathy (myositis), polyneuropathy, and multifocal neurologic disease may be found in young puppies infected with Toxoplasma gondii and Neospora caninum. 69,70 Clinical signs may be present as early as 4 weeks of age and include progressive paraparesis and "bunny hopping" gait with progression to pelvic limb hyperextension and muscle atrophy. Progression to pelvic limb hyperextension is more likely when infection develops prior to 4 months of age.71 Serum CK concentration is usually elevated. Elevated concentrations of serum and CSF antibodies against N. caninum and T. gondii support the presence of infection. Muscle and peripheral nerve biopsies should confirm a diagnosis of polymyositis and peripheral neuropathy. Occasionally organisms are found within muscle biopsy sections. Treatments have included clindamycin 72 and sulfadiazine and trimethoprim.73 While some improvement may be noted in neurological function, complete resolution of pelvic limb hyperextension has not been reported to occur.

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