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Polymyositis

Polymyositis (PM) is a type of chronic inflammatory myopathy that primarily affects skeletal muscles. It is related to dermatomyositis and inclusion body myositis, and its name derives from the Greek words "poly-" (many), "myos-" (muscle), and "-itis" (inflammation). This condition is characterised mainly by inflammation within the endomysial layer of the muscles.

Micrograph of polymyositis. Muscle biopsy. H&E stain.
Micrograph of polymyositis. Muscle biopsy. H&E stain.

Introduction

Polymyositis (PM) is a type of chronic inflammatory myopathy that primarily affects skeletal muscles. It is related to dermatomyositis and inclusion body myositis, and its name derives from the Greek words "poly-" (many), "myos-" (muscle), and "-itis" (inflammation). This condition is characterised mainly by inflammation within the endomysial layer of the muscles.

Signs and Symptoms

The hallmark of polymyositis is proximal muscle weakness and/or muscle mass loss, particularly affecting the hip extensors, neck, and torso. This often leads to difficulties in climbing stairs, standing up from a seated position, and raising the arms above the head. These symptoms may be accompanied by pain and lymphocytic inflammation. Unlike dermatomyositis, polymyositis does not involve the skin.

Patients may also experience dysphagia (difficulty swallowing) and other oesophageal motility issues, low-grade fever, and enlarged lymph nodes. In advanced cases, foot drop can occur in one or both feet. Systemic involvement is common, with interstitial lung disease (ILD) and heart disease, such as heart failure and conduction abnormalities, being notable complications.

Polymyositis often presents in adulthood with bilateral proximal muscle weakness, particularly in the upper legs, which manifests as early fatigue during walking. The disease progression is generally gradual, and the weakness is progressive.

Associated Illnesses

Polymyositis is associated with an increased risk of cancer. Risk factors include older age (over 45), male sex, difficulty swallowing, cutaneous vasculitis, and a rapid onset of myositis (less than four weeks). Certain malignancies, such as nasopharyngeal cancer, lung cancer, non-Hodgkin's lymphoma, and bladder cancer, are more frequently associated with polymyositis.

Cardiac involvement, primarily heart failure, is present in up to 77% of patients. Interstitial lung disease is found in up to 65% of patients, as identified by high-resolution CT (HRCT) or restrictive ventilatory defects.

Causes

The exact cause of polymyositis remains unknown, but it is believed to involve autoimmune factors and possibly viruses. Cancer may also trigger polymyositis and dermatomyositis through immune reactions that affect muscles. Some evidence suggests an association with celiac disease.

Diagnosis

Diagnosis involves a combination of patient history, physical examination, elevated creatine kinase levels, electromyograph (EMG) alterations, and muscle biopsy findings. The primary clinical feature is proximal muscle weakness, sometimes accompanied by muscle pain and dysphagia. Cardiac and pulmonary symptoms are present in approximately 25% of cases.

Differentiating polymyositis from sporadic inclusion body myositis (sIBM) is very important, as sIBM does not respond to treatment and has a slower onset. Polymyositis generally responds well to treatment initially.

Treatment

The first-line treatment for polymyositis is corticosteroids. Specialised exercise therapy may also be recommended to enhance the patient's quality of life.

Epidemiology

Polymyositis more frequently affects females than males.

Polymyositis as a Distinct Diagnosis

The identification of myositis-specific autoantibodies has refined the diagnosis of polymyositis, particularly through the discovery of Antisynthetase syndrome, reducing the number of polymyositis cases.


Self-assessment MCQs (single best answer)

What is the primary tissue affected by polymyositis?



Which muscle areas are primarily weakened in polymyositis?



What is a common symptom accompanying muscle weakness in polymyositis?



Which of the following is NOT a common complication of polymyositis?



What is the first-line treatment for polymyositis?



Which diagnostic tool is NOT commonly used to diagnose polymyositis?



What is the gender prevalence for polymyositis?



What type of lung disease is associated with polymyositis?



Which of the following myopathies is characterised by the absence of skin involvement?



Which notable person was the first to die by euthanasia in Peru due to polymyositis?



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Excellent content clearly explained.
SJ

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