Polymyositis (PM) is a rare, chronic inflammatory muscle disease causing progressive weakness, primarily in the muscles closest to the center of the body. Diagnosing PM is a challenging, multi-step process because its symptoms mimic many other muscle or neurological disorders. To accurately identify polymyositis, clinicians gather information from a patient’s medical history, perform a physical examination, and utilize specialized laboratory, electrical, and imaging tests. The goal is to confirm the presence of muscle inflammation and the specific cellular changes characteristic of this autoimmune condition.
Clinical Presentation and Physical Exam
The diagnosis begins with a detailed patient history and physical examination, focusing on muscle weakness. Patients typically report a gradual onset of symmetric muscle weakness developing over weeks to months, affecting the proximal muscles of the limbs. This weakness often manifests as difficulty performing everyday activities, such as rising from a chair, climbing stairs, or lifting the arms above the head.
Muscle pain is not a defining feature, but a minority of patients may report muscle aches or tenderness upon examination. The physical exam also assesses for signs of associated involvement in other body systems, such as difficulty swallowing (dysphagia) or a change in voice (dysphonia) due to neck and throat muscle weakness. The doctor will also check the deep tendon reflexes, which are usually preserved.
Laboratory Markers in Diagnosis
Blood tests provide objective evidence of muscle damage and autoimmune activity. A key finding is the elevation of serum muscle enzymes, most notably Creatine Kinase (CK). CK is released into the bloodstream when muscle fibers are damaged, and in active polymyositis, levels are often significantly elevated, sometimes ranging from 5 to 50 times the upper limit of the reference range.
Other muscle-related enzymes like aldolase, lactate dehydrogenase (LD), and transaminases (AST/ALT) may also be elevated, reflecting widespread muscle breakdown. Inflammatory markers, such as the Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP), are measured to assess general inflammation. These markers are often only elevated in about half of polymyositis patients.
The presence of specific autoantibodies is particularly important as it can strongly suggest an inflammatory myopathy. These include myositis-specific antibodies (MSAs) like anti-Jo-1, which is associated with a specific syndrome that can involve the lungs and joints. Finding these antibodies helps categorize the disease and differentiates it from other conditions.
Electrophysiology and Muscle Imaging
Specialized functional and structural tests pinpoint the location and nature of muscle abnormalities. Electromyography (EMG) involves inserting a needle electrode into the muscle to record its electrical activity, which is abnormal in nearly all polymyositis patients. The test typically reveals signs of muscle fiber irritability, such as abnormal spontaneous activity, fibrillation potentials, and positive sharp waves when the muscle is at rest.
When the patient contracts the muscle, the Motor Unit Action Potentials (MUAPs) often appear short, small, and polyphasic, indicating a myopathic process. The EMG findings help differentiate a muscle disorder from a nerve-related problem, which shows different electrical patterns. However, these EMG changes are not unique to polymyositis.
Magnetic Resonance Imaging (MRI) creates detailed images of the muscles, revealing structural changes. The MRI technique, particularly using sequences like Short Tau Inversion Recovery (STIR), highlights areas of muscle inflammation and edema, which appear as high signal intensity. This imaging is valuable for identifying which muscles are actively inflamed, as inflammation can be patchy and unevenly distributed. MRI also helps guide the location for a subsequent muscle biopsy, ensuring the tissue sample is taken from an affected area.
The Role of Muscle Biopsy
The muscle biopsy is generally considered the most definitive step in the diagnostic process, providing final confirmation and allowing for the exclusion of other diseases. This procedure involves surgically removing a small piece of muscle tissue, usually guided by prior MRI or EMG findings to target a site of active inflammation. A pathologist then examines the tissue under a microscope to identify the characteristic cellular changes of polymyositis.
The defining microscopic feature is the presence of an endomysial inflammatory infiltrate. This means immune cells are found within the muscle fascicles, surrounding individual muscle fibers. These inflammatory cells consist mainly of cytotoxic CD8+ T-lymphocytes and macrophages that surround and invade non-necrotic muscle fibers. This pattern of invasion and destruction by immune cells is the pathological hallmark distinguishing polymyositis from other muscle disorders.
The biopsy is also crucial for differentiating PM from other inflammatory myopathies that present similarly. For example, dermatomyositis shows inflammation primarily around blood vessels, while inclusion body myositis features characteristic protein deposits. Because inflammation can be patchy, a sample may not always capture the full extent of the disease, requiring the overall clinical picture to be considered.