Juvenile Dermatomyositis

Abstract

The articles prior to January 2008 are part of the back file collection and are not available with a current paid subscription. To access the article, you may purchase it or purchase the complete back file collection here Robert Rennebohm, MD Iuvenile dermatomyositis (JDM) is a chronic multisystem immune-mediated disease that primarily affects the skin, muscles, and gasntestinal tract. It is the most common of the idiopathic inflammatory myopathies of childhood.1 The primary clinical features are proximal muscle weakness and characteristic cutaneous manifestations. JDM varies greatly regarding its presenting features, cumulative clinical manifestations, severity, pace, treatment needs, and longterm clinical course. The primary goals of this article are to help the practicing pediatrician to appreciate the diverse ways in which JDM can present, make a timely diagnosis, and understand the spectrum regarding clinical course and treatment needs. EPIDEMIOLOGY JDM is a rare disease. The best estimate of its incidence appears to be 3.1 new cases per 1 million children per year.23 So far, no ethnic predisposition has been demonstrated, but this has not been adequately studied. The mean age at disease onset is approximately 7 years, but the median age is 5 to 6 years. Of the 36 new patients with JDM who presented to our rheumatology clinic during the past 11 years (3.3 new cases per year), the median age at onset was 5.1 years. Only 8% were adolescents, and 72% were girls. Therefore, JDM disproportionately affects young children, particularly young girls. IMMUNE-MEDIATED EHDOTHELIOPATHY The pathophysiology of JDM is incompletely understood. Evidence strongly suggests that immune-mediated vasculopathy plays a central and critical role. This vasculopathy primarily consists of immune-mediated injury to the endothelial lining of small-caliber vessels (capillaries, venules, and small arteries) in muscles, skin, and the gastrointestinal tract.4 When endothelial cells are injured, they swell and may become necrotic. The swelling and cellular debris narrow the lumen of the vessel. Endothelial injury also favors thrombosis, which may further compromise the lumen (Fig. 1). Narrowing of the lumen results in diminished blood flow to the tissues normally perfused by the affected vasculature, resulting in varying degrees of ischemia. This "immunemediated endotheliopathy" is more common than is true vasculitis. A helpful way to appreciate these concepts is to examine the nailfold region (the area at the base of the fingernail, immediately proximal to the cuticle) (Fig. 2). In healthy children, the capillary loops are thin, wispy, delicate, and numerous and cannot be visualized without magnification (eg, using an otoscope to look through a droplet of water placed over the nailfold). The cuticles, which are perfused by the nailfold capillaries, are thin and translucent. In contrast, in a child with typical JDM, the nailfold capillary loops are likely to be dilated, enlarged, tortuous, and fewer in number (because some have been injured to the point of obliteration, resulting in "capillary drop-out").5 When severe, these abnormalities can be seen with the naked eye. Nailfold capillary endotheliopathy can cause ischemic injury to the cuticles, causing them to become thickened, dystrophic, opaque, yellowish, and hypertrophied ("overgrown"). In the past, electromyogram findings, muscle biopsy, or both, was routinely used to help confirm a diagnosis of JDM. In recent years, magnetic resonance imaging (MRI), typically of the hip girdle and thigh musculature, has proved helpful.9 The MRI abnormalities are due to increased water content (edema or inflammation) in these tissues (Fig. 7). At least mild abnormalities may be seen even in patients who appear to have normal strength. When a patient has classic cutaneous findings, muscle weakness, elevated muscle enzymes, and typical MRI abnormalities, it is be reasonable to forego an electromyogram and muscle biopsy. However, muscle biopsy is of value either to confirm a less obvious case or to help determine prognosis. CLINICAL COURSE SUBSETS Patients with JDM may be grouped into four basic clinical course subsets:4'8 monocyclic course; polycyclic course; prolonged, chronic, continuous, nonulcerative course;… 10.3928/0090-4481-20020701-07