Immune Mediated Polymyositis

INTERNAL MEDICINE CASE STUDIES Immune mediated polymyositis in a male neutered 4 year old Boxer A four year old male neutered Boxer was referred to the R(D)SVS Medicine Service with a 2 week history of stiffness and dyspnoea. On clinical examination, the dog was dull, weak and had a very stiff, shortened gait and was walking on his toes. The dog had reduced muscle mass but no evidence of muscle pain on palpation. Proprioception was normal but withdrawal reflexes were very weak. Patellar and triceps reflexes were absent bilaterally and the gag reflex was markedly reduced. Haematology revealed a mature neutrophilia (23.7x109/l, ref. range 3.6-12). Serum biochemistry revealed a moderate increase in creatine kinase (1242 iu/l, ref. range 50-200). Thoracic radiographs were generally unremarkable and serology for Toxoplasma and Neospora was negative. In light of the reduced reflexes and increase in creatine kinase concentration, a myopathy was suspected to be the cause of the dog’s clinical signs and so biopsies of the cranial tibial, biceps femoris, vastus lateralis and triceps muscles were taken. Histopathology of the muscle biopsies revealed a mixed inflammatory cell population of granulocytes and macrophages with mild to severe myofibre atrophy and loss. No infectious agents were seen. Consequently, a diagnosis of immune mediated polymyositis was made. The dog was treated with immunosuppressive doses of prednisolone and azathioprine, with sucralfate for gastro-protection. At a recheck 2 weeks after discharge, the owner reported that the dog was doing well, the dyspnoea had resolved and the dog was walking better but was still stiff. Primary immune mediated polymyositis is an inflammatory myopathy not associated with any other systemic or infectious causes1. There appears to be a predilection towards larger, mature dogs and commonly observed clinical signs include weakness, exercise intolerance and a stiff gait. The criteria for a definitive diagnosis of immune mediated polymyositis is poorly defined in veterinary medicine although most authors agree that the disease can be diagnosed if the following features are present 1) clinical signs of muscle weakness, 2) elevated serum creatinine kinase concentrations, 3) abnormal electromyography examination with normal motor nerve conduction velocities, 4) absence of an underlying cause based on clinical examination, diagnostic imaging and infectious agents screens and 5) a muscle biopsy which shows an inflammatory infiltrate. Canine polymyositis has been associated with systemic lupus erythematous and recent drug administration although the link between the associated condition and polymyositis is often poorly defined. Treatment of immune mediated polymyositis is based on immunosuppression, initial pain relief and supportive care. Prednisolone is the most commonly used immunosuppressive drug although other drugs such as aziathioprine may also be helpful, particularly if long term therapy is required to control the clinical signs. The prognosis for dogs with idiopathic page 1 of 2 polymyositis is poorly characterised although is generally considered to be reasonably good. Muscle fibrosis and secondary contracture is a possible consequence of polymyositis which may lead to permanent gait abnormalities. Reference 1 Evans et al (2004) Canine inflammatory myopathies: a clinicopathologic review of 200 cases. JVIM. 18, 679-91 The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336. www.ed.ac.uk/vet/hfsa-int-med page 2 of 2