Write a short note on idiopathic inflammatory myopathies (polymyositis and dermatomyositis).

Definition

The idiopathic inflammatory myopathies are autoimmune disorders characterised by symmetrical proximal muscle weakness and inflammation of skeletal muscle.

• Polymyositis (PM) — muscle involvement without skin change; mediated by CD8+ T-cell attack on muscle fibres (endomysial infiltrate).
• Dermatomyositis (DM) — myositis with characteristic skin changes; a humoral, complement-mediated microangiopathy with perimysial and perivascular infiltrate and perifascicular atrophy.
• Related entities: inclusion body myositis, immune-mediated necrotising myopathy, and juvenile dermatomyositis.

Clinical Features

Muscle:

• Symmetrical, proximal muscle weakness developing over weeks to months — difficulty rising from a chair or squatting, climbing stairs, combing hair, and lifting objects overhead.
• Muscles may be tender; wasting occurs late.
• Pharyngeal and oesophageal involvement causes dysphagia, nasal regurgitation and aspiration risk.
• Neck flexor weakness is characteristic; respiratory muscle involvement in severe disease.
• Ocular and facial muscles are spared — a key point distinguishing it from myasthenia gravis.

Skin (dermatomyositis only) — often precedes muscle disease:

• Heliotrope rash — violaceous discoloration of the upper eyelids with periorbital oedema; pathognomonic.
• Gottron's papules — violaceous, scaly papules over the knuckles (MCP and IP joints), elbows and knees; pathognomonic. (Note the contrast with SLE, where the rash spares the knuckles and involves the phalanges between joints.)
• Shawl sign and V sign — photodistributed erythema over the upper back, shoulders and anterior chest.
• Mechanic's hands — hyperkeratotic, cracked, fissured skin on the radial fingers.
• Nailfold changes — dilated capillary loops, ragged cuticles, periungual erythema.
• Calcinosis cutis — especially in juvenile dermatomyositis.

Systemic:

• Interstitial lung disease — in up to 30%, and a major determinant of prognosis, especially with anti-Jo-1 and other antisynthetase antibodies.
• Antisynthetase syndrome: myositis, ILD, mechanic's hands, Raynaud's phenomenon, arthritis and fever, with anti-Jo-1 antibody.
• Cardiac: myocarditis, conduction defects, arrhythmia.
• Arthralgia, Raynaud's, dysphagia, constitutional symptoms.

Investigations

• Muscle enzymes: creatine kinase markedly raised (often 10–50 times normal, correlating broadly with activity); also raised aldolase, AST, ALT and LDH — note that a "deranged LFT" in these patients may be of muscle origin.
• Autoantibodies: ANA positive in most; myositis-specific antibodies — anti-Jo-1 (antisynthetase syndrome), anti-Mi-2 (classic DM with good prognosis), anti-MDA5 (amyopathic DM with rapidly progressive ILD), anti-TIF1-gamma and anti-NXP2 (strongly associated with malignancy), anti-SRP (necrotising myopathy).
• Electromyography — short-duration, low-amplitude polyphasic motor units, fibrillations and positive sharp waves; distinguishes myopathic from neurogenic weakness.
• MRI of thigh muscles — shows oedema and inflammation, and guides the biopsy site.
• Muscle biopsy — the gold standard: endomysial CD8+ infiltrate with invasion of non-necrotic fibres in PM; perifascicular atrophy with perivascular CD4+ and B-cell infiltrate in DM.
• HRCT chest and pulmonary function tests with DLCO for ILD; ECG and echocardiography; swallowing assessment.
• Malignancy screening as above.

Management

• Corticosteroids — high-dose prednisolone is first-line, continued until clinical and enzyme improvement, then slowly tapered over months.
• Steroid-sparing immunosuppressants, started early in most patients: methotrexate or azathioprine; mycophenolate particularly where there is ILD; ciclosporin or tacrolimus.
• Intravenous immunoglobulin — effective in refractory disease, in severe dysphagia, and in dermatomyositis.
• Rituximab for refractory disease, including antisynthetase syndrome.
• Physiotherapy — early passive movement to prevent contractures, progressing to graded strengthening as inflammation settles; rest during the acute inflammatory phase.
• Supportive: swallowing assessment with aspiration precautions, sun protection and topical treatment for skin disease, calcium and vitamin D with bone protection, and *Pneumocystis* prophylaxis with intensive immunosuppression.
• Treat any identified underlying malignancy — the myositis often improves with successful cancer treatment.

Inclusion body myositis should be suspected when weakness is asymmetrical, involves distal muscles (finger flexors) and quadriceps, occurs in men over 50, and fails to respond to steroids; biopsy shows rimmed vacuoles.