Polymyositis

Definition/diagnostic criteria Polymyositis is an idiopathic inflammatory myopathy characterised by symmetric proximal muscle weakness, elevated muscle enzymes, electromyographic abnormalities and muscle biopsy evidencing inflammation. The diagnostic criteria, as per the European Neuromuscular Centre (ENMC), include clinical signs of myopathy, elevated serum creatine kinase (CK), evidence of myopathic changes on electromyography (EMG), and muscle biopsy showing mononuclear cell infiltrates with muscle fibre necrosis.

Epidemiology The condition is rare, with an estimated incidence of 1 in 100,000 people per year in the UK. Polymyositis affects adults and is more common in females, with a peak incidence between 40-60 years of age. The exact aetiology remains unknown, though it is believed to be autoimmune in nature.

Diagnosis
Clinical features: The hallmark of polymyositis is progressive, symmetric muscle weakness, predominantly affecting the proximal muscles. Patients may present with difficulty climbing stairs, rising from a seated position, or lifting objects. Dysphagia and muscle tenderness may also be present.

Investigations: Investigations for suspected polymyositis include blood tests, EMG and muscle biopsy. Typical abnormalities in blood tests include elevated levels of muscle enzymes such as CK, aldolase, aspartate transaminase (AST) and alanine transaminase (ALT). Autoantibodies, including antinuclear antibodies (ANAs) and myositis-specific antibodies (MSAs), can be supportive of the diagnosis.

An EMG may reveal myopathic changes, and muscle biopsy is the gold standard for diagnosis, typically demonstrating endomysial infiltration by mononuclear cells and macrophages, along with muscle fibre necrosis.

Treatment The mainstay of treatment for polymyositis is immunosuppression. Corticosteroids, such as prednisolone, are the first-line therapy. Initial doses are typically high, with a gradual tapering as symptoms improve. For those intolerant to steroids or with refractory disease, BNF and NICE guidelines say steroid-sparing agents like azathioprine, methotrexate, or mycophenolate mofetil may be used.

The use of intravenous immunoglobulins (IVIG) can be considered in cases resistant to standard therapy. Biologic agents, such as rituximab, have been used in refractory cases although their use is not yet standard practice.

Physiotherapy plays a crucial role in managing disability and maintaining muscle strength and function.

Prognosis The prognosis for polymyositis varies. Early diagnosis and treatment initiation can lead to improvement in muscle strength and function. However, some patients may experience a chronic course with relapses. Factors associated with poorer prognosis include older age at onset, associated malignancy and delayed treatment. Complications such as interstitial lung disease, cardiac involvement and increased risk of malignancy may also affect the prognosis.

Overlap with dermatomyositis Polymyositis and dermatomyositis share many similarities, as both are idiopathic inflammatory myopathies, but they are distinguishable by certain clinical features and their associated pathologies. Dermatomyositis presents with characteristic skin manifestations, including a heliotrope rash on the eyelids and Gottron’s papules over the knuckles, which are absent in polymyositis. The pathogenesis of dermatomyositis involves a complement-mediated microangiopathy leading to ischaemic damage to muscle fibres and skin, which is not typically seen in polymyositis. On a microscopic level, dermatomyositis shows perifascicular atrophy, while polymyositis demonstrates more diffuse inflammatory changes.

Patients with overlap symptoms may present diagnostic challenges and often require a more nuanced approach to management. These patients may respond differently to treatment and have a varied prognosis compared with those with polymyositis alone. It is crucial for clinicians to recognise the overlap, as the presence of dermatologic symptoms can signal a higher risk of associated malignancies and may necessitate more aggressive monitoring and management strategies.

The treatment for dermatomyositis generally follows similar principles to those for polymyositis with corticosteroids and immunosuppressants forming the cornerstone of therapy. However, the presence of skin involvement may also require adjunctive treatments.