Dermatomyositis is an inflammatory muscle disease, characterized by weakness of the skeletal muscle, in particular, the proximal muscles of the arms and thighs.

The cause is unknown, and immune mediated, possibly initiated by post viral infection, and several viruses have been implicated, including coxsackie and picornaviruses. Mumps, adeno and influenza virus have also been associated. There are in addition, several drugs, including AZT, colchicine and penicillamine, that have been associated with a myositis. 

Clinical

The disease is associated with a progressive deterioration, which may fluctuate in severity from mild to severe, including paralysis. The muscles of respiration and cranial nerves may be involved, and lead to swallowing and respiratory difficulty. The patient may experience constitutional symptoms, including fatigue, tiredness and malaise, as well as a fever and myalgia, with occasional aching in the musculoskeletal system. Onset is slow over months in the majority, but some people have rapid onset and progression of the disease. 

The condition may be accompanied by a rash. This may be located in a fairly characteristic distribution, including around the eyes and face - where it is edematous and red / lilac - called a heliotrope, and in the hands, in particular with a nodular appearance around the knuckles, known as Gottrons papules. Presence of the rash constitutes dermatomyositis. Scaling of the skin may be seen. Of note is that the skin can occur well before the muscle manifestations occur.

Investigations

Biochemically, there is an increase in the muscle enzymes, with measurement of these constituting a marker for diagnosis and prognosis. The enzymes measured are the creatine phosphate kinase (CPK), and the aldolase enzymes, whilst an elevation of the lactate dehydrogenase may also be found (LDH). Occasionally, the muscle enzymes may not be elevated, especially in a situation of muscle wasting, and the urinary creatine to creatinine ratio may be of use. In addition, electrical tests on the muscles, electromyography (EMG), reveals a characteristic pattern, of irritability on needle insertion, with muscle fibrillation and short duration multiphasic action potentials. The clinical examination, the EMG and the elevated enzymes, assist in diagnosis, and classification and diagnostic criteria are defined. Suspicion of disease can thereafter be confirmed by biopsy, although, patchy involvement of muscle can occur, resulting in normal or only slight abnormality on some biopsies. Biopsy findings usually show lymphocyte and macrophage infiltration of the muscle.
Autoantibodies may also be detected on testing. These are usually against intracellular components. They include myositis specific antibodies, including an antibody called anti Jo 1, which may be associated with severe disease and lung involvement.

Classification

Polymyositis
Juvenile dermatomyositis
Dermatomyositis
Malignancy association
Inclusion body myositis

Systemic complications of the disease are frequently seen.

Raynauds phenomenon - changes to the small circulation of the hands and feet, identified as color changes in response to cold, is frequent.

Esophageal involvement with dysphagia - difficulty swallowing occurs especially in those patients with Raynauds. In addition involvement of small or large bowel can present with bloating, or even perforation of the bowel.

Joint symptoms of arthralgia, pain in the joints without swelling occurs in 25%, but more severe arthritis, including with erosions, may occur in up to 4 percent of cases, including hand involvement. Most often the muscles are painless, but are tender in 1/3 of cases.

Pulmonary involvement with inflammation of the lung tissue - the alveoli, results in a progressive breathlessness in some cases. This process is called interstitial lung disease, with lymphocyte cells infiltrating the lung tissue. Fibrosis of the lung tissue can follow.
Elevated pulmonary pressures may occur. Another common lung problem may occur with reflux of stomach acid into the lung as a consequence of esophageal involvement, causing aspiration pneumonia.

Cardiac involvement with a muscle weakness rarely may occur. This can result in heart failure, or cardiac arrhythmia.

Renal involvement is particularly rare, with protenuria and mesangial proliferative glomerulonephritis reported. Breakdown of muscle can result in myoglobin production and secondary kidney damage.

In the case of dermatomyositis, there are reports of increased incidence of visceral cancers (approximately 5% of cases), including, ovary, lung, colon and breast. Incidence in myositis without the rash is more controversial, but evidence for coexisting cancer is not proven.

Overlap syndromes with other connective tissue are also well described - especially scleroderma, or even lupus

Diagnosis

Diagnosis is made on combination of clinical, laboratory and electrical tests. Thereafter a biopsy is done preferentially of the most effected muscle - usually the thigh. The biopsy is done under local anesthetic and a small piece of muscle excised and examined microscopically including with light and electron microscopy. The problem with biopsy is that even this may be falsely negative, due to patchy muscle involvement and false negative results may be seen after institution of therapy where there is a reduction of inflammation. Some reports of using needle biopsy instead of open biopsy show good positive results and are simple to do.

The inflammed muscle shows inflammatory cells interspersed in the muscle fibres with variable evidence of scarring or fibrosis of damaged tissue.

In the case of dermatomyositis, additional screening tests to exclude coexisting cancer, are potentially indicated, including, a chest radiograph and an ultrasound of abdomen and pelvis.

Therapy

Therapy requires induction of remission and thereafter, maintenance of control.

Initial therapy is based on prednisone, at dose of 1 mg / kg and the dose is maintained as long as the CPK remains elevated. Thereafter the doses can be slowly weaned to a lower and safer dose. Reduction in the dose requires monitoring of the patients muscle weakness and the blood levels of the CPK, as well as markers of inflammation, in particular the sedimentation rate, ESR and C- Reactive protein, CRP.

Physical therapy to maintain and strengthen the muscles is very important.

If the patient shows poor response to prednisone, or the doses required remain high, with risk of side effect, then steroid sparing agents, in particular, Methotrexate or azathioprine, can be added. In cases of therapeutic resistance, pulse steroids - medrol, 250mg pulses can be used, and if still resistant, cases of response to cyclosporin or even chlorambucil are well reported.

Inclusion Body Myositis

Inclusion body myositis is a variant of polymyositis which is diagnosed on biopsy. The proximal and distal muscles are involved and there is a tendency in some patients to be more resistant to standard therapy of myositis. The microscopic inclusions or vacuoles seen on biopsy are not invariably present on all biopsy specimens, and may be missed.

Differential diagnosis.

Many other factors or conditioners can appear similar to polymyositis.

Metabolic disorders: Such as hypothyroidism, or even hyperthyroidism.
Drugs: Including statin anti-cholesterol drugs, colchicine and penicillamine.
Blood electrolyte disorders of magnesium ar potassium.
Alcohol can cause a myopathy.
Corticosteroids can result in proximal weakness.
Post viral disease, especially coxsackie and hepatitis and lyme disease can induce a myopathy.
Neurological disease, especially myasthenia gravis, where there is increasing weakness depending on activity.
Myositis can also be seen associated with other rheumatic diseases, especially, scleroderma, mixed connective tissue disease, Rheumatoid arthritis, lupus, sjogrens, and behcets disease, where focal myositis may occur. 

Metabolic conditions including hypothyroidism, can be associated with weakness and raised muscle enzyme.

Prognosis has improved with corticosteroid therapy, with a survival of 90 percent at 5 years for inflammatory polymyositis, and is worse for cancer associated myositis at 50-60% and dermatomyositis (80%).
The disease is subject to episodes of remission and flare of activity. The chronic use of steroids is associated with potential complications of osteoporosis, and aggravation of diabetes and hypertension. These require monitoring and treatment as required.