Non-inflammatory myopathies

Non-inflammatory myopathies (NIM) are a diverse group of genetic disorders, characterized by neuromuscular weakness, fatigue, muscle wasting, and pain, due to inherited defects in proteins critical in the structural integrity and function of muscle fibre or in enzymes involved in energy production in this most metabolic tissue. The shared clinical and laboratory features (including elevated muscle enzyme levels, myopathic change on electromyography, and even inflammatory changes on muscle biopsy) of myositis and NIM frequently cause diagnostic confusion. Failure to distinguish these disorders will result in unnecessary immunosuppression, lack of screening for cardiorespiratory and other associations of NIM, and a missed opportunity for genetic counselling and potential future treatments. A strong index of suspicion is required in all patients presenting with neuromuscular syndromes if a long diagnostic odyssey is to be avoided. A clinically focused multidisciplinary approach, with a working knowledge of subtypes of NIM, is outlined in this chapter.

Incidence of Polysaccharide Storage Myopathy: Necropsy Study of 225 Horses

Muscle samples were obtained at necropsy from 225 horses and ponies 1 year of age or older. Samples were processed in routine manner and were stained with hematoxylin and eosin and with periodic acid-Schiff for glycogen. Sections were examined for abnormal glycogen content and amylase- resistant complex polysaccharide and for chronic myopathic change (excessive fiber size variation, increase in number of internal nuclei). A total of 101 horses and ponies with lesions of polysaccharide storage myopathy were identified. Age of affected horses ranged from one to 30 years, with a mean of 14.7 years. Mean age of nonaffected horses was 12 years. Incidence of polysaccharide storage myopathy varied depending on breed; Thoroughbreds had the lowest (27%) and draft-related horses had the highest (86%) incidence. Chronic myopathic changes were more severe in polysaccharide storage myopathy-affected horses than in nonaffected horses. Results of this study indicate that polysaccharide storage myopathy is a common disorder of many breeds of horses and ponies.