Case 4 - 180525-4

Three-year-old, male, castrated, mixed-breed dog (presumed hound cross))

This patient developed a cough one year before presentation. Two months prior, he developed vomiting and progressive weight loss. He had exploratory surgery for a suspected foreign body, which was negative.

Upon presentation to the referral hospital, the patient was very thin and a body condition score of one, based on a score of one to nine. He underwent a swallow study where he was diagnosed with pharyngeal dysphagia, but aspirated the food bolus and developed aspiration pneumonia. He was subsequently euthanized due to poor prognosis.

A 9.5-kg, 3-year-old, male, castrated, hound-mix was examined by necropsy. The ribs and spinous processes were prominent, and there was minimal fat.

The entire length of the esophagus was dilated. The esophageal wall in the proximal one-third of the esophagus was 0.5 cm thick and the luminal diameter was approximately 1.0 cm. The luminal diameter in the distal two-thirds was approximately 2.0 cm. The esophageal mucosa was pale, tan and smooth The cranial and ventral lung lobes (approximately 50% of the lung parenchyma) were mottled pale-tan to red. The remaining portions of the lung were soft and were mottled pink to red. All sections floated in formalin. Approximately 75% of the diaphragm, particularly in the region of the esophageal hiatus, was\ nodular and up to 0.5 cm thick.

Tall three sections of diaphragm on this slide have evidence of marked polyphasic myocyte degeneration, atrophy, necrosis, and regeneration. Degenerative changes include floccular sarcoplasm and fragmentation of the myofibers. The number of satellite cells is moderately increased around many muscle fibers, and there is often internalization of nuclei and rowing of nuclei (regeneration). Often, there is contraction band necrosis, hypereosinophilic sarcoplasm and loss of striations (necrosis). The interstitium is expanded by thick bands of fibrous connective tissue, and there are thin, wispy, atrophic myofibers in the fibrotic regions. Large numbers of histiocytes are present within the interstitium, and within degenerate and necrotic myofibers. Small numbers of lymphocytes and plasma cells are also scattered within the interstitium.

N/A

Skeletal muscle (esophagus, diaphragm, temporalis, laryngeal muscle, tongue): Severe, diffuse, polyphasic myodegeneration, atrophy, and necrosis, with multifocal fibrosis and regeneration

All sections of striated muscle examined (increased severity of laryngeal muscles, esophagus and diaphragm) had similar lesions of non-inflammatory polyphasic myocyte degeneration, necrosis, and regeneration. Although the skeletal muscle of the esophagus was severely affected, there was no histologic evidence of inflammation affecting the mucosa. These skeletal muscle changes can explain the clinical signs, but the underlying cause is still unknown. It can be difficult to differentiate a primary disease of muscle versus a secondary degenerative process such as atrophy due to long-term nutritional deficiency; however in this case, having ruled out other causes such as hypothyroidism or myositis, and given the severity and diffuse distribution of the lesions, the findings are are most consistent with a primary, hereditary degenerative muscle disease. Further characterization of this pathogenesis is warranted.

We would like to thank Dr. Katie Olstad for contributing this case.

Valentine BA, Cooper BJ, Cummings JF, Lahunta A. Canine x-linked muscular dystrophy: Morphologic lesions. 1990; 97(1):1-23.