Polymyositis is a rare connective tissue disorder that causes inflammation and weakness of muscles. It is a progressive disorder that affects the skeletal muscles and commonly starts during mid-childhood and into the 20s. The exact cause of polymyositis is not known, but viral infections, genetics and cancer may play a part in its development. Symptoms may appear gradually or rapidly. Muscular weakness most commonly affects the muscles closest to the trunk – muscles of the neck, shoulders, upper arms, hips and thighs, but can reach the distal muscles as the disease progresses. Your child may find it difficult to get up from a chair, climb stairs, walk, lift heavy objects, swallow and speak, and may experience fatigue, weight loss, muscle tenderness, shortness of breath and low-grade fever.

Polymyositis is diagnosed by a thorough physical examination, blood tests, electromyography (tests the electrical activity of muscles), muscle biopsy (sample of muscle tissue is removed and tested in the lab) or MRI. There is no treatment for polymyositis, but the symptoms can be relieved with corticosteroid medication, physical therapy, speech therapy, orthotics and assistive devices, dietetic assessment and rest.