Polymyositis and dermatomyositis are a mixture of various inflammatory diseases of the muscles with or without inflammation of skin. Both polymyositis and dermatomyositis have an autoimmune basis and respond to immunosuppression, but the basic mechanisms for the conditions are different and they have specific clinical and histological features. Viral infection has been implicated in the form of the human retroviruses HIV and human T-cell lymphotrophic virus type I (HTLV-I), the simian retroviruses, and Coxsackie virus B. Damage to capillaries would seem to be an underlying pathology in dermatomyositis.¹ It is possible to divide the conditions into 5 basic subgroups according to aetiology:²

• Primary idiopathic polymyositis in adults.
• Idiopathic dermatomyositis in adults.
• Childhood dermatomyositis or myositis with necrotising vasculitis.
• Polymyositis associated with connective tissue diseases.
• Polymyositis or dermatomyositis associated with malignancy.

Epidemiology²

The incidence of polymyositis and dermatomyositis is 5-10 cases per 100,000 individuals.
• Juvenile dermatomyositis has an incidence of 2 or 3 per million per year.³
• There is no racial difference but there is a female preponderance of about 2:1 at all ages.
• Polymyositis tends to present between 30 and 60 years old with a smaller peak at about 15 years old.
• Dermatomyositis affects adults between 45 and 65 and there is a smaller peak incidence in children between 5 and 15 years old.

Polymyositis

History

• There is an inflammatory myopathy with onset over weeks or months and steady progression.
• Diffuse weakness in the proximal muscles develops.
• Proximal myopathy causes difficulty rising from a low chair, climbing steps, lifting objects and combing hair. Fatigue, myalgia, and muscle cramps may also be present.
• Distal muscles are spared, and so fine motor movements of the hand, such as buttoning a shirt, writing, operating a keyboard or playing the piano are affected only late in the disease.
• Pharyngeal weakness causes dysphagia.
• Weakness may vary from week to week or month to month.
• Only a third have pain. There is no rash.
• There is no family history of neuromuscular disease, evidence of endocrine disorder or history of exposure to possible toxins.

Examination

Polymyositis produces muscle weakness. It is not painful, although a few patients complain of aches or cramps.

• Proximal muscle weakness occurs with comparative sparing of distal muscles until the disease is well advanced.
• External ocular muscles are unaffected. Facial muscles are affected only in severe disease.
• Forced flexion of the neck is weak and there may be difficulty just holding the head up.
• Muscular atrophy occurs with preservation of tendon reflexes, flexor plantar response and normal sensation.
• Muscles may be tender on palpation and may have a nodular grainy feel.

Investigations

• Creatine kinase can be up to 50 times normal. It is rarely normal in active disease and the level is usually a good indicator of disease activity.
• About 20% have anti-Jo-1 antibodies. They indicate a poor prognosis with interstitial lung disease.⁴ This lung disease occurs in about a third.⁵
• Other enzymes to be elevated include aldolase, serum glutamic-oxaloacetic transaminase (SGOT), serum glutamic-pyruvic transaminase (SGPT), and lactate dehydrogenase (LDH). If SGOT is higher than SGPT, a myogenic cause should be suspected.
• The diagnosis is established by electromyography (EMG) and is confirmed by muscle biopsy. In polymyositis it is the definitive test.
• Cancer antigen 125 (CA-125) and cancer antigen 19-9 (CA19-9) may be useful markers of the risk of malignancy.²
• Autoantibody testing for myositis-specific antibody (MSA) and myositis associated autoantibodies (MAA) may be useful to differentiate the patients with underlying malignancy.⁶

Differential diagnosis

Polymyositis is a diagnosis of exclusion and other considerations include:

• Hereditary neuromuscular diseases
• Endocrine disease including thyrotoxicosis and Cushing's disease
• Malabsorption syndromes, alcoholism, cancer, vasculitis, granulomatous disease, sarcoidosis or exposure to drugs or toxins that affect the muscles

Dermatomyositis

History

Dermatomyositis affects children as well as adults. The muscle weakness has the same pattern as in polymyositis but there are other features too:

• Rash: see under Examination below.
• There is systemic upset with fever, arthralgia, malaise and weight loss. It can resemble scleroderma with Raynaud's phenomenon and dysphagia.
• Possible cardiac disease including atrioventricular conduction defects, tachyarrhythmias and dilated cardiomyopathy.
• Gastrointestinal ulcers and infections.
• Thoracic muscles may be weak but there may also be interstitial lung disease in 30 to 50%.⁷
• Children tend to have more non-muscular features, especially gastrointestinal ulcers and infections.

Examination

• The rash includes blue-purple discoloration on the upper eyelids with periorbital oedema, a flat red rash involving the face and upper trunk and raised purple-red scaly patches over the extensor surfaces of joints and fingers. Ulcerative vasculitis and calcinosis of subcutaneous tissue may occur.
• The rash may affect knees, shoulders, back and upper chest and may be exacerbated by sunlight.
• Skin lesions may produce scaling, pigmentation or depigmentation of the skin and a shiny appearance.
• Dilated capillary loops at the fingernail base are typical of dermatomyositis. The cuticles may be irregular and thickened, and the palmar and lateral surfaces of the fingers may become rough and cracked.
• Muscle weakness is proximal and can vary from mild to extreme. Sensation is preserved and tendon reflexes are normal unless atrophy is severe.
• There is muscle pain and tenderness early in the disease.

Investigations

• Elevation of creatine kinase is not so reliable although it can be extremely high.
• SGOT, SGPT, LDH, and aldolase levels may also be raised.
• Cancer antigen 125 (CA125) and cancer antigen 19-9 (CA19-9) may be useful markers of the risk of malignancy.²
• MRI is not very helpful for making the diagnosis but it can help monitor activity and guide the best place for muscle biopsy.
• EMG may be helpful but can be normal in 15%. It can also guide a suitable place to biopsy.
• Muscle biopsy can be diagnostic.

Differential diagnosis

Dermatomyositis is much easier to diagnose as the rash and subcutaneous calcification is typical.

• In systemic lupus erythematosus (SLE), the skin over the phalanges is involved and the skin over the metacarpophalangeal joints is spared. In dermatomyositis, the reverse is true.
• There can be some overlap with systemic sclerosis, rheumatoid arthritis, and Sjögren's disease but the weakness is more than can be accounted for by arthritis alone.

Associated diseases

• Polymyositis and dermatomyositis may be associated with other autoimmune diseases such as myasthenia gravis, Hashimoto's thyroiditis, systemic sclerosis and Waldenström's macroglobulinaemia.
• In both polymyositis and dermatomyositis it may be necessary to hunt for an underlying malignancy.
  • Polymyositis and especially dermatomyositis may be part of a paraneoplastic syndrome.
  • Between 10 and 20% of patients with dermatomyositis have neoplasms.
  • In elderly patients with dermatomyositis there is often malignancy.
  • Breast cancer, lung cancer, carcinoma of ovary and stomach cancer are usually implicated.

Treatment

Non-drug

• Sun blocking agents should be used.⁸
• Encourage physical activity within reason to maintain muscular strength. This may involve consultation with a physiotherapist and occupational therapist.
• Evaluation of swallowing may be required and a speech and language therapist may help with difficulties of swallowing.
• Monitor creatine kinase and clinical response but treatment can improve the former without benefiting the latter.

Drugs

• Early initiation of therapy is essential.⁹
• Steroids are the most important drugs. In mild disease, topical steroids may suffice. In more severe disease, high doses of systemic steroids are used and tapered off. Improvement is usually apparent by the 2nd or 3rd month. The usual precautions must be exercised when giving high doses of steroids for long periods.
• If steroids fail then immunosuppressive drugs such as azathioprine can be used. As an alternative cyclophosphamide is usually better than methotrexate. High-dose intravenous immunoglobulin has been shown to be beneficial.^10,11,12
• Other treatments include antimalarial agents and immunomodulatory therapies.⁸
• Patients with anti-Jo-1 antibodies need long-term immunosuppression.¹³
• For lung disease, an aggressive combination regimen including ciclosporin A or tacrolimus with cyclophosphamide is recommended to be added to corticosteroids.⁷
• There have been reviews of treatment options but with few good controlled trials.^14,15

Complications²

These tend to be unrelated to muscles.

• Gastrointestinal ulceration can cause melaena or haematemesis. Infarction of bowel may occur, especially in dermatomyositis.
• Dermatomyositis can cause subcutaneous calcification that punctures the skin with ulcerations, infection and ugly scars.
• Polymyositis: cardiac abnormalities and interstitial lung disease are common complications, especially in patients with anti-Jo-1 antibodies. Rates as high as 45% have been reported.

Prognosis

• The active period of the disease is approximately 2 to 3 years in both children and adults.²
• The duration is greater for patients with cardiac or pulmonary complications than for others.
• About 20% of patients recover completely.
• The mortality rate after several years of the disease is approximately 15%. This is higher with connective tissue diseases and malignancy.
• Response to steroids is usually good but with interstitial lung disease there is often a high mortality and extra therapy such as cyclophosphamide is required.⁵ Some patients respond poorly. Early treatment tends to produce the best outcome. Acute, fulminant disease does less well.
• If response is very disappointing the muscle biopsy should be repeated to confirm the diagnosis.
• In juvenile dermatomyositis, outcomes have become good with modern treatments, but the disease remains chronic in a large number of children and sequelae are often seen.³
• Pulmonary manifestations contribute significantly to the morbidity and mortality of the idiopathic inflammatory myopathies, ranging from intrinsic lung disease to secondary complications that include aspiration pneumonia, opportunistic infection, congestive heart failure, and hypoventilation.¹⁶
• Polymyositis and dermatomyositis can occur in pregnancy or during the puerperium. The role of pregnancy in the disease is unknown. The outcome of the pregnancy seems to reflect the severity of the disease. The more active the myositis during the pregnancy, the greater the chance of fetal loss.¹⁷