Dermatomyositis is a rare autoimmune inflammatory myositis of unknown etiology affecting both children and adults. It involves striated muscles and skin. The juvenile form is associated with a multisystemic vasculitis and a high frequency of calcinosis. However, unlike the adult form, it does not have an increased risk of malignancy.
Dermatomyositis is an inflammatory myopathy distinguished by a mononuclear inflammatory infiltrate of the striated muscle. Genetic susceptibility is no longer in doubt since predisposing HLA systems have been identified. There is an activation of the complement leading to the deposition of the membrane attack complex in the wall of blood vessels, which causes microangiopathy and an inflammatory reaction.
The mean age at onset of the disease is around 7 years of age, with earlier onset in girls. The mean time between the onset of the first symptoms and confirming the diagnosis is 6 months, ranging from 5 weeks to 2 years.
In juvenile dermatomyositis (JDM), the clinical features are usually insidious. Muscle weakness is the main reason for consultation, usually accompanied by systemic signs like asthenia, anorexia, irritability, pain, fever, and deterioration of one's general condition. The dermatological manifestations are like those of adults. They are highly characteristic. Their recognition is essential because they can precede muscular signs. The periorbital violaceous patches like frames of glasses are almost pathognomonic as well as Gottron's papules. These latter are found in 30% to 70% of cases and are easy to recognize. Violaceous papules are usually seen over the metacarpophalangeal, the proximal and distal interphalangeal joints of hands, less commonly, in the extensor surfaces of elbows and knees. We can also observe painful periungual erythema with telangiectasia, cuticular hypertrophy, facial edema, erythematous patches involving the "V-neck," shoulders, and anterior chest wall and Raynaud's phenomenon. More rarely, there are skin lesions of cutaneous vasculitis and photosensitivity. The muscular deficit is usually progressive, symmetric, bilateral, proximal and non-selective, involving striated muscles of pelvic and shoulders. The intensity of the muscle weakness varies from one patient to another, ranging from simple myalgia to a severe muscle deficit. Children may report difficulties in dressing, combing hair, and climbing stairs. During evolution, calcinosis may occur more commonly in children than in adults. These calcifications arise from subcutaneous tissues or deeper locations such as musculoaponeurotic structures. The commonest sites of calcinosis include elbows, knees, buttocks and trauma areas. Its evolution is unpredictable. It can regress spontaneously, ulcerate with a risk of secondary infections causing major aesthetic and functional sequelae. It can mimic cold abscesses. No standardized approach to the management of calcinosis associated with JDM has been established. Its incidence may decrease with early treatment by systemic corticosteroid therapy at high doses. Experienced physicians are more likely to use bisphosphonate as a first-line treatment. The involvement of the pharyngeal and esophageal muscles is responsible for dysphonia, dysphagia, and dyspnea. The vital prognosis is affected. Transferring the patient for admission to the intensive care unit is necessary to initiate appropriate treatment. Amyopathic forms of JDM are rare. They are characterized by typical skin manifestations without muscle involvement. Serum muscle enzymes, electromyographic examination, and muscle biopsy are normal. There is usually no calcinosis or vasculopathy. The prognosis is excellent.
Myocardial involvement may cause ventricular arrhythmias. Lipodystrophy usually occurs after a long evolution. The fat loss is slow and progressive, typically affecting the upper part of the trunk. It can be associated with hirsutism, hepatomegaly and acanthosis nigricans lesions. JDM is associated with a high risk of cardiovascular and cerebrovascular comorbidities. Pulmonary manifestations include interstitial lung disease and respiratory muscles involvement.
Abnormal elevation of serum activities of muscle enzymes is the most common biological finding in JDM. Elevation of creatine kinase is expected in 75% to 85% of cases with average values around 2000 U/L plus or minus 1000. A normal rate can be observed at the beginning of the disease and in amyopathic forms. Regular creatine phosphokinase (CPK) monitoring is recommended during the evolution under treatment to evaluate its effectiveness. The electromyogram shows a myogenic pattern. Muscle biopsy confirms the diagnosis. Some histological abnormalities are common to other myositides: myofiber degeneration and an inflammatory mononuclear infiltrate. Other findings are more specific, for example, a highly characteristic perifascicular atrophy, perivascular inflammatory infiltrate, and sometimes intravascular microthrombi. Magnetic resonance imaging (MRI) is a non-invasive tool used to guide the site of the muscle biopsy to optimize its sensitivity. Muscle edema is the major abnormality observed on MRI, particularly in T2 sequence with fat suppression or STIR sequence. This edema is usually proximal and symmetrical but may be focal and distal. The signal intensity seems to be correlated with an aggressive chronic disease course.
Systemic corticosteroids are the established primary treatment for the management of JDM. As yet, there is no consensus regarding the dose regimen and the therapeutic modalities. It is recommended to initiate the treatment with 1 milligram per kilogram of daily equivalent prednisone. Intravenous (IV) treatment is preferable in the case of gastrointestinal (GI) involvement. This late is known to be associated with vasculitis causing malabsorption. Long-term steroid treatment is usually required to achieve complete remission. It should be gradually decreased. Adjunctive hygienic-dietary measures, as well as daily supplementation with calcium and vitamin D, must be systematic. However, these children are at high risk of osteoporosis.
In cases of therapeutic failure or significant side effects under steroid therapy, the use of subcutaneous methotrexate should be considered. Combining corticosteroids and methotrexate is a good alternative to control the disease activity without significant adverse reaction. Using hydroxychloroquine may be a new therapeutic approach. Targeted biotherapies (infliximab, rituximab) may be indicated in refractory forms of JDM. Only controlled and randomized clinical trials will be able to validate their safety in children. The therapeutic abstention in amyopathic JDM is the rule. In this case, patients generally remain asymptomatic even after several years of evolution. Physical therapy is an important part of the treatment. It should be started as soon as symptoms are under control. It aims to rebuild muscle strength and to prevent joint contraction.
The evolution is unpredictable. The disease may have a monocyclic, polycyclic or chronic course. The prognosis has significantly improved over recent years with the emergence of new therapeutic options. Early treatment and close monitoring may lead to better control of the disease. However, developing countries still have a high mortality rate. The major causes of death are usually severe muscle weakness, super-added infection, gastrointestinal vasculitis with a risk of bowel perforation, myocardial failure, and respiratory distress. JDM is usually not associated with the development of malignancies, unlike adult form.
Having a child with JDM may impact the family quality of life negatively. Psychological assistance is necessary for the child and his or her parents.