Sarcoplasmic masses in equine skeletal muscle

Comparison of Histopathologic Criteria and Skeletal Muscle Fixation Techniques for the Diagnosis of Polysaccharide Storage Myopathy in Horses

The purpose of the study reported here was to determine the effect of three methods of fixation of skeletal muscle biopsy specimens on the histopathologic appearance of muscle sections and to determine criteria that were most consistently associated with a diagnosis of polysaccharide storage myopathy (PSSM) in horses. Surgically excised semimembranosus muscle biopsy specimens were obtained from nine horses previously diagnosed with PSSM and from 15 control horses. Portions of each specimen were fixed in formalin, frozen immediately, and chilled for 24 hours prior to freezing. Sections stained with hematoxylin and eosin (HE), periodic acid-Schiff (PAS), and amylase-PAS were scored for histopathologic criteria by three investigators blinded to the sample origin. The presence of amylase-resistant, abnormal polysaccharide was found to be the most sensitive and specific diagnostic indicator for PSSM, and was readily detected regardless of the fixation technique or investigator. Other less-specific features associated with PSSM included atrophy and cytoplasmic and subsarcolemmal vacuoles; however, their histologic scores varied among fixation technique and investigators. Scores for subsarcolemmal and cytoplasmic amylase-sensitive glycogen in horses with PSSM were similar to those for control horses and varied among fixation techniques. In conclusion, PSSM is most accurately diagnosed in muscle biopsy specimens on the basis of appearance of amylase-resistant, abnormal polysaccharide, not amylase-sensitive glycogen, regardless of fixation technique. In general, frozen sections appeared to be better suited for studying myopathies because many histopathologic features of skeletal muscle were obscured by formalin fixation.

Comparative Skeletal Muscle Histopathologic and Ultrastructural Features in Two Forms of Polysaccharide Storage Myopathy in Horses

Polysaccharide storage myopathy (PSSM) has been found in more than 35 different horse breeds through identification of abnormal storage of polysaccharide in muscle biopsies. A dominant mutation in the glycogen synthase 1 gene ( GYS1) accounts for a substantial proportion of PSSM cases in at least 17 breeds, including Quarter Horses, but some horses diagnosed with PSSM by muscle histopathologic analysis are negative for the mutation. We hypothesized that a second distinct form of glycogen storage disease exists in GYS1 -negative horses with PSSM. The objectives of this study were to compare the histopathologic features, ultrastructure of polysaccharide, signalment, history, and presenting complaints of GYS1 -negative Quarter Horses and related breeds with PSSM to those of GYS1 -positive horses with PSSM. The total histopathologic score in frozen sections of skeletal muscle stained with hematoxylin and eosin, periodic acid Schiff (PAS) and amylase-PAS stains from 53 GYS1-negative horses did not differ from that of 52 GYS1 -positive horses. Abnormal polysaccharide was fine granular or homogenous in appearance (49/53; 92%), often amylase-sensitive (28/53; 53%), more commonly located under the sarcolemma, and consisting of β glycogen particles in GYS1 -negative horses. However, in GYS1 -positive horses, abnormal polysaccharide was usually coarse granular (50/52; 96%), amylase-resistant (51/52; 98%), more commonly cytoplasmic, and consisting of β glycogen particles or, in some myofibers, filamentous material surrounded by β glycogen particles. Retrospective analysis found that GYS1 -negative horses ( n = 43) were younger at presentation (4.9 ± 0.6 years vs. 6.7 ± 0.3 years for GYS1 -positive horses) and were more likely to be intact males than GYS1 -positive horses ( n = 160). We concluded that 2 forms of PSSM exist and often have distinctive abnormal polysaccharide. However, because evaluation of the histologic appearance of polysaccharide can be subjective and affected by age, the gold standard for diagnosis of PSSM at present would appear to be testing for the GYS1 mutation followed by evaluating muscle biopsy for characteristic abnormal polysaccharide in those horses that are negative for the mutation.

Development of polyglucosan inclusions in skeletal muscle

Muscle samples from 24 horses with polysaccharide storage myopathy were stained with periodic acid-Schiff (PAS) stain and were immunostained for ubiquitin. Abnormalities detected with PAS stain were coarse granular cytoplasmic aggregates of amylase sensitive glycogen, subsarcolemmal aggregates of glycogen, central amylase sensitive bodies, and a variety of subsarcolemmal to intracytoplasmic amylase resistant polyglucosan inclusions. All amylase resistant inclusions were positive for ubiquitin. Ubiquitin was also detected in many amylase sensitive inclusions. Based on morphologic findings and pattern of ubiquitin staining, a sequence of events, beginning with abnormal glycogen storage followed by ubiquitination and eventual development of amylase resistance, is proposed.

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.

Inherited myopathy of great Danes

A hereditary, non-inflammatory myopathy occurring in young great Danes with distinctive histological features in muscle biopsy specimens is reviewed. Onset of clinical signs is usually before one year of age and both sexes are affected. Clinical signs are characterised by exercise intolerance, muscle wasting, and an exercise-induced tremor. Although most affected dogs have a severe form of the disease, occasional dogs may have a less pronounced form and survive into adulthood with an acceptable quality of life. Litters containing affected puppies are born to clinically unaffected parents, and an autosomal recessive pattern of inheritance is likely. All recorded cases have had fawn or brindle coat coloration. Elevated serum creatinine kinase concentrations and spontaneous electrical activity in skeletal muscles are frequently found. While originally reported (Targett and others 1994) as a central core myopathy in this breed, the histochemical characteristics of the distinct cytoarchitectural structures differ from those of the well-characterised central core myopathy in human beings. In fact, these structures differ from any known myopathy in human beings and likely represents a unique non-inflammatory myopathy affecting dogs. Until this myopathy is characterised further, the name inherited myopathy in great Danes is suggested.