Polymyositis

Polymyositis is an inflammatory myopathy characterized by diffuse, non-suppurative inflammation of skeletal muscle, leading to muscle weakness, pain, and atrophy. It is a systemic immune-mediated disorder that can affect any skeletal muscle group, including masticatory, esophageal, and limb muscles. The condition is classified as an immune-mediated myositis, distinct from infectious myositis, and is often associated with other immune-mediated diseases. Polymyositis can occur as a primary disorder or secondary to neoplasia, drug reactions, or other systemic autoimmune conditions. The hallmark is infiltration of muscle tissue by lymphocytes and macrophages, leading to myofiber necrosis and regeneration. Clinical severity ranges from mild, subclinical elevation of muscle enzymes to severe, generalized weakness and respiratory compromise.

The exact etiology of polymyositis is often unknown, but it is believed to be immune-mediated. Potential triggers include: (1) Viral infections: retroviruses, parvovirus, and other viral agents may trigger an aberrant immune response. (2) Bacterial infections: e.g., Streptococcus, Staphylococcus, or Borrelia burgdorferi (Lyme disease) can incite immune-mediated muscle inflammation. (3) Parasitic infections: Toxoplasma gondii, Neospora caninum, Hepatozoon canis, and Trichinella spiralis have been associated with myositis. (4) Drug-induced: certain drugs (e.g., cimetidine, penicillamine, sulfonamides) can induce immune-mediated myositis. (5) Neoplastic: paraneoplastic polymyositis has been reported in dogs with thymoma, lymphoma, and other tumors. (6) Genetic: certain breeds (e.g., Boxer, Newfoundland, German Shepherd) may have a genetic predisposition. (7) Autoimmune: polymyositis may be part of systemic lupus erythematosus or other autoimmune disorders. The underlying mechanism involves molecular mimicry, where infectious agents or drugs share epitopes with muscle antigens, leading to T-cell-mediated cytotoxicity and autoantibody production.

Polymyositis is most commonly diagnosed in dogs, with a higher incidence in large-breed dogs such as Boxers, Newfoundlands, German Shepherds, and Golden Retrievers. It can occur at any age but is more frequent in middle-aged to older dogs (4-10 years). There is no strong sex predilection, though some studies suggest a slight female predominance. Feline polymyositis is rare but can occur, often associated with infectious agents or neoplasia. The condition is seen worldwide, with no clear geographic or seasonal pattern. In endemic areas, infectious causes (e.g., Neospora, Toxoplasma) may be more prevalent. The incidence of immune-mediated polymyositis is estimated to be low, but it is a significant cause of acquired myopathy in dogs.

The pathophysiology of polymyositis involves an immune-mediated attack on skeletal muscle. The process begins with an inciting trigger (e.g., infection, drug, neoplasm) that leads to the activation of CD8+ T lymphocytes and macrophages. These cells infiltrate the muscle tissue, particularly around the endomysium and perimysium, and release pro-inflammatory cytokines (e.g., TNF-alpha, IL-1, IL-6) and cytotoxic granules (perforin, granzymes) that cause myofiber necrosis. The inflammatory response leads to muscle fiber degeneration, phagocytosis, and subsequent regeneration. Chronic inflammation can result in fibrosis and muscle atrophy. The disease may also involve autoantibodies against muscle antigens, such as anti-sarcolemmal antibodies, which contribute to muscle damage. The inflammatory process can affect any skeletal muscle, including the masticatory muscles, esophageal muscles (leading to megaesophagus), and respiratory muscles (causing dyspnea). Systemic effects include fever, weight loss, and malaise due to cytokine release. The disease can be self-limiting or progressive, with relapses and remissions.

Predisposing factors for polymyositis include: (1) Breed: certain breeds have a higher risk, suggesting a genetic component. (2) Age: middle-aged to older dogs are more commonly affected. (3) Concurrent immune-mediated diseases: dogs with other autoimmune disorders (e.g., myasthenia gravis, lupus) are at increased risk. (4) Infectious agents: exposure to certain pathogens (e.g., Toxoplasma, Neospora, Borrelia) can trigger the disease. (5) Drug exposure: certain medications can induce immune-mediated myositis. (6) Neoplastic disease: underlying malignancy can lead to paraneoplastic polymyositis. (7) Stress or trauma: physical stress may exacerbate or trigger the condition. (8) Environmental factors: not well-defined, but immunosuppression or immune dysregulation may play a role.

Clinical signs of polymyositis vary depending on the severity and distribution of muscle involvement. Common signs include: (1) Generalized weakness and exercise intolerance. (2) Muscle pain on palpation. (3) Muscle atrophy, especially of the temporal and masseter muscles (masticatory muscle atrophy). (4) Stiff gait or lameness. (5) Dysphagia, regurgitation, or megaesophagus due to esophageal muscle involvement. (6) Dysphonia or change in bark. (7) Respiratory signs (dyspnea, tachypnea) if respiratory muscles are affected. (8) Fever, lethargy, and depression. (9) Weight loss. (10) In severe cases, respiratory failure or aspiration pneumonia. The onset can be acute or insidious. In peracute cases, dogs may present with severe weakness and collapse. Chronic cases may show progressive muscle wasting and weakness. Physical examination may reveal muscle pain, atrophy, and decreased spinal reflexes in affected limbs.

Differential diagnoses for polymyositis include: (1) Myasthenia gravis: characterized by exercise-induced weakness that improves with rest; diagnosis via acetylcholine receptor antibody titers or edrophonium response. (2) Infectious myositis (e.g., toxoplasmosis, neosporosis): may have similar clinical signs; diagnosis via serology, PCR, or muscle biopsy with organisms. (3) Endocrine myopathy (e.g., hypothyroidism, hyperadrenocorticism): associated with other endocrine signs; diagnosis via thyroid or adrenal function tests. (4) Muscular dystrophy: typically affects young dogs, with histopathologic features of dystrophin deficiency. (5) Neurogenic muscle atrophy (e.g., polyneuropathy, spinal cord disease): may cause weakness and atrophy; diagnosis via neurologic examination, electromyography, and nerve/muscle biopsy. (6) Drug-induced myopathy: history of drug exposure; withdrawal may improve signs. (7) Paraneoplastic myopathy: underlying neoplasia; diagnosis via imaging and biopsy. (8) Electrolyte disorders (e.g., hypokalemia, hypercalcemia): can cause weakness; diagnosis via serum biochemistry. (9) Polymyositis must be differentiated from masticatory muscle myositis, which is a specific immune-mediated myositis of the masticatory muscles, often responsive to immunosuppressive therapy.

The diagnostic algorithm for polymyositis begins with a thorough history and physical examination. If polymyositis is suspected, the following steps are recommended: (1) Baseline blood work: complete blood count, serum biochemistry profile, and urinalysis. Elevated serum creatine kinase (CK) and aspartate aminotransferase (AST) are suggestive of muscle damage. (2) Serology for infectious agents: Toxoplasma, Neospora, Borrelia, and others, especially in endemic areas. (3) Electromyography (EMG): may show abnormal spontaneous activity (fibrillation potentials, positive sharp waves) in affected muscles. (4) Muscle biopsy: the gold standard for diagnosis. Biopsy should be taken from an affected muscle (e.g., biceps femoris, triceps, or temporal muscle) and submitted for histopathology and immunohistochemistry. (5) Additional tests: acetylcholine receptor antibody titers to rule out myasthenia gravis, thyroid function tests, and adrenal function tests. (6) Imaging: thoracic radiographs to assess for megaesophagus or aspiration pneumonia; abdominal ultrasound to rule out neoplasia. (7) If an underlying cause is suspected, further diagnostics (e.g., CT, MRI, or biopsy of other tissues) may be warranted. The diagnosis is confirmed by histopathologic evidence of inflammatory infiltrates and myofiber necrosis.

Laboratory findings in polymyositis include: (1) Hematology: may show mild leukocytosis or eosinophilia, but often unremarkable. (2) Serum biochemistry: elevated CK (often >1000 U/L, can be >10,000 U/L), elevated AST, and sometimes elevated alanine aminotransferase (ALT) due to muscle origin. Alkaline phosphatase may be normal. (3) Urinalysis: usually normal, but myoglobinuria may be present in severe cases, leading to brown-colored urine. (4) Blood gas analysis: may show metabolic acidosis if severe muscle damage. (5) Specific biomarkers: C-reactive protein (CRP) may be elevated. (6) Serology: positive titers for infectious agents (e.g., Toxoplasma, Neospora) if infectious cause. (7) Autoantibody testing: anti-nuclear antibody (ANA) may be positive in some cases, but is not specific. (8) Electrophoresis: may show hypergammaglobulinemia. (9) Muscle enzyme levels can be used to monitor response to therapy; a decrease in CK indicates improvement.

Imaging findings in polymyositis are not specific but can support the diagnosis and rule out other conditions. (1) Radiography: thoracic radiographs may reveal megaesophagus (dilated esophagus) or aspiration pneumonia. Abdominal radiographs are usually unremarkable. (2) Ultrasonography: muscle ultrasound may show increased echogenicity and loss of normal muscle architecture in affected muscles. (3) Computed Tomography (CT): can detect muscle atrophy, calcification, or contrast enhancement in inflamed muscles. (4) Magnetic Resonance Imaging (MRI): is the most sensitive imaging modality for muscle inflammation, showing increased signal intensity on T2-weighted and STIR sequences, and contrast enhancement. MRI is particularly useful for identifying affected muscles and guiding biopsy. (5) Endoscopy: not typically used, but may be performed to evaluate the esophagus if megaesophagus is suspected. (6) Fluoroscopy: can assess esophageal motility. (7) Echocardiography: not directly relevant, but may be performed to rule out cardiac involvement.

Cytology and histopathology are essential for definitive diagnosis. (1) Fine needle aspiration (FNA) of muscle: may show inflammatory cells (lymphocytes, macrophages) and necrotic muscle fibers, but is less sensitive than biopsy. (2) Muscle biopsy: histopathology reveals multifocal or diffuse infiltration of mononuclear cells (lymphocytes, plasma cells, macrophages) in the endomysium and perimysium, with myofiber necrosis, phagocytosis, and regeneration. Chronic cases may show fibrosis and muscle fiber atrophy. Immunohistochemistry can identify T-cell subsets (CD3+ T cells) and major histocompatibility complex (MHC) class I expression on muscle fibers, which is characteristic of immune-mediated myositis. Special stains (e.g., Gomori trichrome) may show ragged red fibers in mitochondrial myopathies, but are not specific for polymyositis. The presence of infectious agents (e.g., Toxoplasma cysts, Neospora) can be detected with special stains or PCR.

Treatment of polymyositis is aimed at suppressing the immune response and managing complications. (1) Immunosuppressive therapy: The mainstay is corticosteroids, such as prednisone or prednisolone, at immunosuppressive doses (1-2 mg/kg/day PO, divided q12h). If there is no response within 2-4 weeks, additional immunosuppressive agents may be added, such as azathioprine (2 mg/kg PO q24h) or mycophenolate mofetil (10-20 mg/kg PO q12h). Cyclosporine (5-10 mg/kg PO q24h) may also be used. (2) Supportive care: In severe cases, hospitalization may be required for fluid therapy, nutritional support (e.g., feeding tube if megaesophagus), and respiratory support. (3) Management of megaesophagus: elevated feeding, thickened food, and prokinetic agents (e.g., metoclopramide) may be helpful. (4) Treatment of underlying cause: if an infectious agent is identified, appropriate antimicrobial therapy (e.g., clindamycin for Toxoplasma/Neospora) should be initiated. (5) Analgesia: pain management with NSAIDs or opioids may be needed. (6) Physical therapy: passive range-of-motion exercises and massage can help maintain muscle mass. (7) Monitoring: serum CK levels should be monitored regularly to assess response to therapy. The goal is to taper immunosuppressive drugs to the lowest effective dose.

The prognosis for polymyositis is variable. With early diagnosis and aggressive immunosuppressive therapy, many dogs improve significantly, and some achieve remission. However, the disease can be chronic and relapsing, requiring long-term therapy. Poor prognostic indicators include: severe muscle atrophy, megaesophagus with aspiration pneumonia, respiratory muscle involvement, and lack of response to initial therapy. The mortality rate is low if treated appropriately, but complications such as aspiration pneumonia can be life-threatening. In cases of paraneoplastic polymyositis, the prognosis depends on the underlying neoplasia. Overall, the prognosis is guarded to fair, with many dogs requiring lifelong medication.

Follow-up care for polymyositis includes: (1) Recheck examinations every 2-4 weeks initially to monitor clinical signs and serum CK levels. (2) Once stable, rechecks every 3-6 months. (3) Serial serum CK measurements: a decrease in CK indicates response to therapy; an increase may indicate a relapse. (4) Repeat muscle biopsy may be considered if there is no response or if the diagnosis is uncertain. (5) Monitor for side effects of immunosuppressive therapy (e.g., gastrointestinal upset, hepatotoxicity, infections). (6) If megaesophagus is present, periodic thoracic radiographs to monitor for aspiration pneumonia. (7) Adjust immunosuppressive drug dosages based on clinical response and side effects. Taper corticosteroids slowly over several months to avoid relapse. (8) Provide client education on the chronic nature of the disease and the importance of compliance.

Pearls: (1) Always measure serum CK in any dog with unexplained weakness or muscle pain. (2) Muscle biopsy is essential for definitive diagnosis; choose an affected muscle and avoid sites of recent needle insertion. (3) Consider infectious causes (e.g., Neospora, Toxoplasma) in young dogs or those with systemic signs. (4) Masticatory muscle myositis is a separate entity that responds well to immunosuppression; it is characterized by antibodies against type 2M muscle fibers. (5) Monitor for megaesophagus, as it can lead to aspiration pneumonia, a common cause of death. Pitfalls: (1) Do not rely solely on elevated CK; other muscle diseases can cause similar elevations. (2) Avoid using corticosteroids without a definitive diagnosis, as they may worsen infectious myositis. (3) Do not taper immunosuppressive therapy too quickly, as relapses are common. (4) Do not overlook concurrent diseases such as myasthenia gravis or neoplasia. (5) Be cautious with NSAIDs in combination with corticosteroids, as they increase the risk of gastrointestinal ulceration.

Based on Plumb's Veterinary Drug Handbook, the following drug protocols are recommended for polymyositis: (1) Prednisone/Prednisolone: Dogs: 1-2 mg/kg PO q12h for 2-4 weeks, then taper gradually over 2-3 months to alternate-day therapy (0.5-1 mg/kg q48h). Cats: 2-4 mg/kg PO q24h, then taper. (2) Azathioprine: Dogs: 2 mg/kg PO q24h, then taper to q48h. Cats: 0.3 mg/kg PO q48h. Onset of action is 3-5 weeks. (3) Mycophenolate mofetil: Dogs: 10-20 mg/kg PO q12h. Cats: 10 mg/kg PO q12h. (4) Cyclosporine (modified): Dogs: 5-10 mg/kg PO q24h. Cats: 5-10 mg/kg PO q24h. (5) Clindamycin (if infectious myositis): Dogs: 11 mg/kg PO q12h or 22 mg/kg PO q24h. Cats: 11 mg/kg PO q12h. (6) For pain management: Gabapentin 10-20 mg/kg PO q8-12h. (7) For megaesophagus: Metoclopramide 0.2-0.4 mg/kg PO q8h, or cisapride (if available) 0.5 mg/kg PO q8h. (8) For aspiration pneumonia: appropriate antibiotics (e.g., amoxicillin-clavulanate 12.5-25 mg/kg PO q12h). All immunosuppressive drugs require monitoring of CBC and serum biochemistry, and dose adjustments in renal or hepatic impairment. Contraindications: corticosteroids should be used with caution in patients with diabetes mellitus, gastrointestinal ulcers, or infections.

Evidence-based literature on polymyositis in veterinary medicine is limited to retrospective studies and case series. A landmark study by Podell et al. (1992) described the clinical and histopathologic features of polymyositis in dogs, highlighting the importance of muscle biopsy for diagnosis. Another study by Evans et al. (2004) evaluated the response to immunosuppressive therapy, showing that a combination of corticosteroids and azathioprine was effective in most cases. ACVIM consensus statements on immune-mediated diseases recommend a stepwise approach to immunosuppressive therapy, with corticosteroids as first-line and azathioprine or mycophenolate as adjunctive agents. Recent studies have investigated the use of cyclosporine and mycophenolate in refractory cases, with promising results. There is no standardized protocol, and treatment is often individualized. The prognosis is generally good with early intervention, but long-term follow-up is necessary. Future research is needed to identify biomarkers for early diagnosis and to evaluate novel immunomodulatory therapies.