A myopathy associated with myotonia in the dog

Electrophysiological and histopathological findings of muscular disease suspected as myotonic dystrophy in a Shiba dog

An 8-year-old male Japanese Shiba exhibited muscle wasting and a stiff gait. A low-amplitude myotonic discharge was recorded by needle electromyography (EMG). A histopathological examination on a tru-cut biopsy sample from the muscle revealed myofiber size variations. Internal nuclei and cytoplasmic vacuoles were observed in many fibers. A type 1 fiber predominance and many hybrid type fibers were observed immunohistochemically. On the basis of these EMG and histopathological findings, myotonic dystrophy (DM) was suspected as tentative diagnosis. The cytoplasm around the vacuoles was immunopositive for cytochrome c, tom 20, and SOD-1, suggesting that these vacuoles might occur within mitochondria. Collectively, these results indicate that a mitochondrial abnormality partly play the role on the pathogenesis of present case.

An unusual myopathy in a dog

A nine-month-old Cavalier King Charles Spaniel was examined because it could not withdraw its tongue into its mouth and had difficulty eating and drinking. These clinical signs were first observed when the dog was two months of age. On percussion of the tongue a dimple could be produced and there was electromyographic evidence of myotonia. Histological examination showed replacement of muscle fibres by adipose tissue, focal areas of myonecrosis, neutrophil infiltration and proliferation of sub-sarcolemmal nuclei. These changes were considered to be consistent with a primary myopathy similar to paramyotonia in man. No related dogs were found to be affected.

Equine muscular dystrophy with myotonia

OBJECTIVES

To describe a case of equine muscular dystrophy with myotonia.

METHODS

A 5-year-old horse presented with hypertrophy and delayed relaxation of the muscles of the hindlimbs from age 2 months. Testicular atrophy developed from 2 years of age. Action and percussion myotonia was associated with weakness in these muscles, and EMG showed diffuse myotonic discharges and myopathic features. Biopsy of the gluteal muscle showed adipose and connective tissue infiltration, marked variation in muscle fibre size, and moth-eaten, ring and whorled fibres.

RESULTS

Injection of apamin, a peptide blocker of calcium-activated potassium channels, which inhibits myotonia in human myotonic dystrophy, was ineffective in blocking myotonic discharges. Discharges promptly abated with 2% lidocaine injection.

CONCLUSIONS

Myotonia in this horse is associated with dystrophic changes similar to human myotonic dystrophy, though there are some pharmacological differences.

A new inherited muscular disorder in Japanese quails (Coturnix coturnix japonica)

Thirteen adult mutant (LWC strain) Japanese quails (Coturnix coturnix japonica), between the ages of 8 and 60 weeks were examined for a progressive muscular disorder. The disorder, inherited as an autosomal dominant trait, was clinically apparent as early as 28 days of age; it was characterized by generalized myotonia, muscle stiffness, and muscle weakness. Affected birds were identified by their inability to lift their wings vertically upward and by their inability to right themselves when placed on their dorsum. Electromyographic studies in two mutant quails showed high-frequency repetitive discharges comparable to those of myotonic runs. These discharges persisted after nerve resection. The distinctive histopathologic changes in the various muscles examined were ring fibers, sarcoplasmic masses, and internal migration of sarcolemmal nuclei. A slight decrease in the size of type IIB muscle fibers and a slight increase in the size of type IIA fibers were observed in the M. pectoralis thoracicus of affected quails. In older affected birds, inter- and intrafascicular fatty infiltration with replacement of type IIB fibers by fat cells was seen in the pectoral muscles. Single fiber necrosis, nonspecific lymphorrages, and variations in the muscle fiber size and shape were also noted. The typical muscle lesions and multisystem involvement, which was manifested by testicular degeneration and atrophy in the male LWC specimens and bilateral lenticular cataracts in 6 of 13 affected mutant quails, suggest resemblance of this new inherited muscular disorder to myotonic dystrophy in man.

Skeletal muscle changes associated with equine myotonic dystrophy

A progressive neuromuscular disorder in young horses, clinically apparent as early as 1 month of age, is characterized by generalized myotonia, muscle stiffness, muscle weakness and atrophy. Myotonia is identified by percussion dimpling and myotonic EMG discharges. Changes in one case included testicular hypoplasia, cataract formation, and glucose intolerance, indicating a systemic involvement. Pathologic changes in skeletal muscles from three affected foals were examined. Sarcoplasmic masses, ringed fibers, internal positioning of sarcolemmal nuclei, and nuclear rowing were among the primary histologic changes noted. Variation in fiber diameter size, especially atrophy, and type I predominance were also prominent changes. A neurogenic involvement was indicated by type grouping changes in several muscles.

Progressive myotonia in foals resembling human dystrophia myotonica

A severe and progressive neuromuscular disorder accompanied by clinical, electrophysiological, and pathological features resembling human dystrophia myotonica was observed in three foals. This disorder was apparent as early as 1 month of age and involved progressive skeletal muscle dysfunction, initially characterized by proximal muscle hypertrophy and hypertonicity with subsequent muscle stiffness, weakness, and atrophy. Multisystem involvement was manifested in one case by testicular hypoplasia, early cataract formation, and borderline glucose intolerance. Prolonged dimpling of these large rear-limb muscles was elicited by percussion. Myotonic discharges were identified by electromyography. Percussion dimpling and the typical myotonic discharges persisted after neuromuscular blockade. Distinctive histologic muscle changes included ring fibers, sarcoplasmic mass formation, variation in fiber diameter size, and internally positioned nuclei.

A myopathy associated with muscle hypertonicity in the Cavalier King Charles Spaniel

Clinical signs of electrically silent muscle hypertonicity are described in five Cavalier King Charles dogs. Biopsies of the biceps femoris and triceps muscles, when examined with the electron microscope, revealed evidence of sarcotubular and mitochondrial abnormalities. These included enlargement of the sarcoplasmic reticulum, hydropic degeneration of mitochondria, tubular proliferations in the vicinity of the triads and vacuolar invagination of mitochondria. The exact nature of these findings is not clear and it is suggested that utilization of tracer techniques would help to explain them.

Spontaneous primary myopathies in domestic mammals: a review

By reviewing the literature concerning primary myopathies in man and animals, a classification of spontaneous primary myopathies in domestic mammals is made. This classification is based on aetiological factors. Primary myopathies are divided into metabolic, immunologic, toxic, nutritional, congenital, inherited, and inflammatory myopathies. Muscular dystrophies, in accordance with the definition in human cases, are considered as a separate entity. In the description of the different muscle disorders the emphasis is on the structural changes and the pathogenesis. Clinical signs and diagnostic criteria are considered only briefly.

Subclinical myopathy associated with hyperadrenocorticism in the dog

One of seven dogs with clinical signs of hyperadrenocorticism had clinical evidence of neuromuscular disorder. Clinical diagnosis of hyperadrenocorticism was confirmed by plasma cortisol concentrations before and after adrenocorticotrophin hormone and dexamethasone administration. Electromyographic studies showed bizarre, high frequency discharges in all dogs. Examination of muscle biopsies taken from the biceps femoris muscle from each dog revealed mild degenerative changes of fiber size variation, focal necrosis and fiber splitting. Ragged red fibers were prominent and appeared to be related to mitochondrial aggregates. Significant (P less than 0.05) type 1 and 2 fiber atrophy was found in all dogs. Fiber grouping was present in four dogs. Studies of fascicular biopsies of the peroneal nerve showed thinly myelinated fibers, onion-bulb formation, segmental demyelination and short intercalated internodes. These findings suggested a glucocorticoid-related myopathy in hyperadrenocorticism that also may have a neurogenic component.