Clinical History
The dog presented with a four month history of kyphosis, stiff gait and ataxia in all four limbs as well as exercise intolerance unresponsive to phenylbutazone. Clinical signs progressed to neck stiffness and reluctance to stand. Prednisolone was prescribed at l mg/kg BID PO. Some improvement was noticed but after two months the clinical signs flared up again. A course of clindamycin was prescribed, then oxytetracycline with no improvement. At that time ulcers developed on the left side of the face (over the left eyelid and left upper lip) as well as on the tongue. The dog was intensely pruritic over the left side of the face and an Elizabethan collar was worn to prevent self-mutilation. There was excessive drooling and difficulty eating although she was interested in food. The dog was by then unable to take more than three crouched steps. The owner consulted a homeopathic veterinarian who recommended referral to a neurologist.

Physical Examination
Vital signs were within reference ranges. Ulcerations were present on the left upper eyelid and upper lip as well as on the chin and entire left side and anterior half of the right side of the tongue. Bilateral corneal lipid deposits were present but the fundic examination was unremarkable. The remainder of the physical examination was normal. Neurologic examination revealed the dog was bright and alert with normal head position and cranial nerve evaluation. The dog could not stand for more than a few seconds and was reluctant to walk although able to move all four limbs. Muscle mass was generally good except for noticeable atrophy of the anterior shoulder muscles bilaterally. Due to the abnormal stance and extreme weakness, conscious proprioception was difficult to assess. However, if fully supported, the dog could hop on all four limbs. The patellar reflex was absent in both pelvic limbs but all other spinal reflexes were normal. Withdrawal was present and strong in all limbs. There was no pain on manipulation of the spine or joints. Since there was extreme chronic muscle weakness with generally normal muscle mass, the neurolocalization was likely the motor unit involving either muscle or neuromuscular junction.

Muscle biopsy - Fresh frozen muscle biopsies were evaluated from the deltoid, triceps, supraspinatus, and omotransversarius muscles. Similar changes were present within all four muscles but varied in severity with the supraspinatus and omotransversarius most markedly affected. As shown in Figure 1 in the biopsy from the supraspinatus muscle, there were multifocal areas of mononuclear cell infiltration with myofiber atrophy, endomysial and perimysial fibrosis, and lipid accumulation. The cellular infiltrates were found by additional histochemical reactions (not shown) to be composed of lymphocytes and macrophages. The histologic diagnosis was a chronic inflammatory myopathy/polymyositis. Differential diagnosis for polymyositis includes infectious (Toxoplasma, Neospora, or tick-related disorders), immune-mediated, drug induced, or paraneoplastic disorders