Polymyositis and Dermatomyositis

Polymyositis is the chronic inflammation of muscles. Dermatomyositis is the chronic inflammation of skin and muscles.

Presentation

The main presenting feature is bilateral, proximal (shoulders and pelvic girdle) muscle weakness, developing over weeks-months. As the disease progresses, there may be pharyngeal, oesophageal and respiratory muscle involvement, leading to dysphonia, dysphagia, and T2 resp failure, respectively.

Features more specific of Polymyositis - Muscle pain, fatigue, and weakness

Features more specific of Dermatomyositis - That of Polymyositis + Skin features:

• Heliotrope rash (lilac discolouration of eyelid with periorbital oedema)

• Gottron lesions (scaly erythematous patches) on joints

• Macular erythematous rash

Investigations

Key investigation finding is Raised CK, which can also be raised by Rhabdomyolysis, AKI, MI, Statins, Strenuous exercise.

Other investigations include:

• EMG – Confirms myopathic cause, and excludes a neuropathic cause

• Anti-Jo-1, Anti-Mi-2

For a definitive diagnosis, a Muscle biopsy, which will show inflammatory infiltrates.

Management

Corticosteroids. Immunosuppressants/DMARDs e.g. methotrexate may be added alongside this as it makes it easier for the steroids to work, therefore not needing as high a dose.