JPC SYSTEMIC PATHOLOGY
Signalment (JPC 95/96 16-1): Dog, age, sex and breed not specified
HISTORY: This dog presented for congested respiration and trouble barking. There was transient intermittent response to steroids, bronchodilators, and antibiotics. Several months later, signs had progressed to dyspnea, inspiratory stridor, vomiting, and activity-induced cyanosis. Thoracic radiographs revealed normal appearing lungs and a soft tissue density in the laryngeal area. Laryngeal examination showed a 2-3 cm mass displacing the left side of the larynx. The animal was euthanized and the laryngeal block submitted for histopathological examination.
HISTOPATHOLOGIC DESCRIPTION: Submucosa, perilaryngeal area (per contributor): Markedly expanding the submucosa and elevating the mucosa is a 1.8 x 1.9 cm, unencapsulated, densely cellular, lobular neoplasm composed of large, brightly eosinophilic, polygonal cells, arranged in solidly cellular areas and vague streams, separated by thin bands of fibrous stroma. Neoplastic cells have variably distinct cell borders, moderate to abundant amounts of granular and occasionally vacuolated cytoplasm, and round to oval nuclei with finely stippled chromatin and 1-2 variably distinct nucleoli. Multifocally, there are elongate binucleate strap-like cells and multinucleate polygonal cells. There is moderate anisocytosis and anisokaryosis. Mitoses average 1 per 10 high power fields. There is multifocal hemorrhage, hemosiderin-laden macrophages, and single cell necrosis.
MORPHOLOGIC DIAGNOSES: Submucosa, perilaryngeal area (per contributor): Rhabdomyoma, breed not specified, canine
- Rhabdomyomas are rare, benign, laryngeal neoplasms of dogs
- Suggested sex predilection for female dogs, no breed predisposition
- Laryngeal rhabdomyomas are slow-growing tumors arising from striated muscle; they rarely metastasize and rarely recur with complete surgical resection, although recurrence may develop if it is incomplete
- Can arise in any part of the body, including sites that normally lack striated muscle, but occur more often in the myocardium, skeletal muscles of the larynx, and in the head region in both humans and animals
TYPICAL CLINICAL FINDINGS:
- Large tumors may be partially obstructive, which can result in dysphonia, cough, dyspnea, stridor, exercise intolerance, syncope, cyanosis, and collapse
TYPICAL GROSS FINDINGS:
- Solitary, circumscribed, round or lobulated, red-brown, smooth nodule protruding into the larynx
- Secondary hemorrhage, necrosis, and hemosiderosis may be present
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Pleomorphic, polygonal to round cells forming a lobulated mass, separated by thin bands of fibrous stroma
- Neoplastic cells have abundant, eosinophilic, granular or foamy cytoplasm
- Nuclei are vesicular, with one or more prominent nucleoli
- Cells with cross striations, multinucleate cells, and elongated strap-like cells may be difficult to find
- Numerous mitochondria, rough endoplasmic reticulum, and glycogen granules
- Primitive myofilaments and Z band type material
ADDITIONAL DIAGNOSTIC TESTS:
- PTAH to better visualize cross striations
- Positive for myosin and muscle specific actin, myoglobin, desmin
- PAS positive
- Vimentin and cytokeratin negative
- Squamous cell carcinoma of the larynx is the most common laryngeal neoplasm in dogs
- Laryngeal neoplasms with cells that have abundant granular cytoplasm in addition to rhabdomyoma:
- Granular cell tumor - uncommon neoplasm that is recognized in a variety of organs, typically the tongue of dogs, lung of horses, meninges/brain of laboratory rodents; likely of Schwann cell origin; cytoplasm with fine PAS positive, diastase resistant granules; positive for S100 and vimentin
- Oncocytoma – Chords and nests of large epithelioid cytokerain positive cells; eosinophilic cytoplasm with numerous mitochondria that appear as PTAH positive granules
- Rhabdomyosarcoma of the larynx
- Laryngeal rhabdomyomas are very rare in other species
- Overall, primary tumors of striated muscle are rare in animals
- Malignant striated muscle tumors are more common than benign neoplasms Approximately one half of striated muscle tumors arise from sites other than skeletal muscle; almost all benign striated muscle tumors in animals are congenital cardiac rhabdomyomas; these tumors are responsible for about one third of reported striated muscle tumors
- Benign tumors of striated muscle origin are most common in heart of pigs (one case of cardiac rhabdomyoma described in canines)
- In humans, cardiac rhabdomyomas are one of the most frequently occurring primary tumors of the heart, and the most common in infants and children
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