Comparative Neuromuscular Laboratory

NEUROMUSCULAR CASE OF THE MONTH - AUGUST 2002

Myositis and dermatitis in a 10 year old female spayed Miniature Schnauzer
Contributed by Dr. Karen L. Kline
Iowa State University
Ames, IA

Clinical History
Three weeks prior to presentation, the referring veterinarian described “noisy breathing” and significant phlegm in the back of the throat that minimally improved 6 days after the first examination. Clavamox was prescribed. Ten days later, the dog developed scattered, crusty skin lesions that had a diffuse pattern. At that time the dog had a dramatically elevated creatine kinase (CK) and the owner complained of progressive weakness. Dexamethsone (unknown dosage) and concurrent use of Clavamox did not yield any improvement. Vaccinations were current and the dog had no travel history.

Physical and Neurological Examination
Upon presentation at the Veterinary Teaching Hospital, the physical examination was within normal limits with the exception of diffuse, multifocal crusty skin lesions that were non-ulcerative and non-painful. The neurological examination revealed normal mentation and a bilateral CN 7 paresis (Fig. 1) with normal CN 5 sensory function. There were no signs of spinal cord dysfunction and myotatic reflexes were normal. The dog had marked diffuse temporal and masseter muscle atrophy and moderately diffuse appendicular muscle atrophy (Fig. 2). Difficulty prehending and swallowing food was observed and there was increased salivation. Neurological localization supported a generalized lower motor neuron dysfunction. A myopathy was suspected due to the diffuse muscle atrophy and history of CK elevation.

Figure 1.

Figure 2.

Diagnostic Testing
CBC – No significant abnormalties
Serum Chemistry including bile acids – Elevated CK (11,847; reference 60-270)
Premium Thyroid Panel (MSU) – Within normal limits
Thoracic Radiographs – Mild right-sided cardiac enlargement
Abdominal Radiographs – Multiple uroliths
Ophthalmic examination – Healing corneal ulcer OD
Fungal culture of skin lesions and skin scrapings – Negative
Electromyography – Under general anesthesia with isofluorane, fibrillation potentials and positive sharp waves were found in the triceps, extensor carpi radialis, cranial tibial, laryngeal, and tongue muscles.

Muscle and Nerve Biopsies
Similar pathological changes were present within the vastus lateralis, cranial tibial and triceps muscle biopsies. There was a moderate variation in myofiber size with atrophic fibers having a round shape and surrounded by endomysial fibrosis. Most atrophic fibers contained internal nuclei. Multifocal areas of mononuclear cell infiltration were present having an endomysial and perimysial distribution with invasion of non-necrotic fibers (Fig. 3, H&E). The cell population was composed predominantly of macrophages (Fig. 4, esterase) with scattered clusters of lymphocytes. No organisms were observed. No abnormalities were identified within a biopsy from the peroneal nerve.

Figure 3. (H&E)

Figure 4. (Esterase)

Skin Biopsy
Biopsies from the haired skin showed ulcerative and suppurative dermatitis, and cell-poor interface lymphohistiocytic dermatitis. These changes were suggestive of erythema multiforme (EM), toxic epidermal necrolysis (TEN), drug reaction, or generic dog food dermatosis. The dog had received Clavamox and dexamethasone prior to admission.

Diagnosis and Conclusion
Inflammatory myopathy/polymyositis and cell-poor interface lymphohistiocytic dermatitis most likely associated with an idiosyncratic drug hypersensitivity or toxicity. TEN and EM are commonly caused by drugs. Levamisole, penicillins, cephalosporins, and sulfonamides are commonly implicated. Several drugs have also been reported to produce an inflammatory myopathy including penicillamine, cimetidine and trimethoprim-sulfa. Due to financial constraints and a very low suspicion of disease, antibody titers against infectious agents were not performed. Prednisone at immunosuppressive dosages and supportive care including syringe feeding of a liquid diet were initiated. Unfortunately, the dog was euthanitized due to poor quality of life 3 weeks after initiation of therapy according to the owner. A necropsy was not performed.