A possible new inherited myopathy in a young Labrador retriever

Abstract

A 5-month-old, male, Labrador retriever was evaluated for progressive weakness and muscle atrophy. Histologic evaluation of fresh frozen muscle revealed distinct cytoarchitectural changes and central mitochondrial accumulations indistinguishable from those found in the inherited myopathy described in Great Danes. Multiple male littermates and half-siblings were similarly affected.

Figure 1

Marked temporal and masseter muscle atrophy in a 5-month-old Labrador retriever with an inherited myopathy.

Figure 2

A 5-month-old Labrador retriever with an inherited myopathy stands with a ventroflexed neck and an arched spine, keeping both hind limbs tucked under the abdomen.

Figure 3

Serial fresh frozen biopsy sections from the triceps muscle were evaluated using several histochemical stains and enzyme reactions. Variability in myofiber size and multifocal areas of myonecrosis and phagocytosis were identified histologically (A, hematoxylin and eosin stain). In addition to the areas of myonecrosis, several myofibers showed lightly stained blue to purple subsarcolemmal ringed and central dense areas (B, modified Gomori trichrome stain). The subsarcolemmal ringed and central dense areas were darkly positive with the mitochondrial specific reactions succinic dehydrogenase (SDH, C) and cytochrome C oxidase (COX, D). Atrophic fibers were of both fiber types (E, ATPase reaction at pH 9.8 with type 1 fibers lightly stained and type 2 fibers darkly stained). Lysosomal activity within macrophages clearing necrotic debris was highlighted by the esterase reaction (F). Bar = 150 μm for all images.