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Long-term follow-up into adulthood among patients diagnosed with juvenile myositis showed generally favorable outcomes, researchers said, despite finding that almost all had developed disease-related damage.

Among 49 patients followed in U.S. tertiary referral centers, 95% had positive findings on the Myositis Damage Index (MDI), with a median score of 6 out of a possible 38, according to Lisa G. Rider, MD, of the NIH's National Institute of Environmental Health Sciences in Bethesda, Md., and colleagues.

But disability was mild, with median scores on the Health Assessment Questionnaire of 0.4 on a scale of 0 to 3, the researchers reported online in .

Juvenile idiopathic myositis comprises a group of heterogeneous autoimmune disorders in which patients exhibit inflammation and muscle weakness. These consist of juvenile dermatomyositis, a small-vessel vasculopathy characterized by rashes; juvenile polymyositis, primarily involving muscle inflammation; and juvenile connective tissue myositis, with patients having features of other types of connective tissue diseases.

The most common of these is juvenile dermatomyositis, and although the skeletal muscle is the primary organ of involvement, disease manifestations can extend to the skin, gastrointestinal tract, and lung parenchyma. The disease course is classified as monocyclic, polycyclic, or chronic continuous.

For decades the cornerstone of therapy has been corticosteroids, but in recent years the use of methotrexate and biologics has expanded and mortality is now low. from Europe have suggested that patients diagnosed since 1990 have experienced less damage, and that while some degree of damage remains common, serious disability is unusual.

Only limited data have been available for juvenile myositis patients in the U.S., however, with no information available about outcomes in adulthood.

Accordingly, Rider and colleagues conducted a cross-sectional analysis of patients who had been referred to the National Institutes of Health or the George Washington University Myositis Center in Washington, D.C., from 1994 to 2016. The analysis included patients who were diagnosed in childhood but were 18 or older at the time of the last visit.

The majority of patients were white females. Three-quarters had juvenile dermatomyositis, and two-thirds had the chronic continuous subtype. Age at the last evaluation was 24, and median disease duration was 11.5 years.

Patients typically had a history of poor global function, with the median worst recorded functional class according to the American College of Rheumatology (ACR) being 4 out of 5. This had improved, however, by the time of the cross-sectional visit, to a median ACR functional class of 2.

All patients had used prednisone at some time, 86% had received methotrexate, and 32% had been given biologics. At the time of the last visit, 57% were still on prednisone, 39% were taking methotrexate, 39% were receiving intravenous immune globulin, and 9% were on a biologic. The observation that the majority of patients continued to take prednisone at the last visit suggested that they still had active disease, the researchers pointed out. They also noted that steroid-related adverse events were fairly common, with 12% of patients having developed diabetes and 8% having osteoporosis or hypertension.

Disease activity was mild at the final cross-sectional visit, with a median physician global activity score of 1.7, and weakness was moderate, with a median muscle testing score of 229 out of a possible 260.

The most common organ systems involved were the muscle and skin, with disease manifestations including proximal weakness in 98%, fatigue in 90%, Gottron's papules in 88%, and periungual capillary changes in 84%. By the time of the final visit, 70% still had proximal muscle weakness.

Damage also was most common in the muscle and skin at the final visit, including joint contractures in 63%, scarring in 60%, atrophy of the muscle in 49%, and lipodystrophy in 33%.

A number of factors were associated with MDI scores at the cross-sectional visit. For instance, patients with erythroderma had an increase of nearly 4 points on that score (β=3.94, P=0.01), while each additional point on the worst ever ACR function score was associated with an increase of 1.31 points (P=0.001), and each additional year of disease duration was associated with an increase in the MDI score of 0.08 points (P=0.04).

A total of 55% of patients had calcinosis -- a proxy for overall disease damage -- at the final visit. A history of Gottron's papules was present in 63% of those with calcinosis, and the likelihood of having calcinosis was almost nine times higher among patients with a history of periungual capillary changes (OR 8.65, 95% CI 1.63-46.08).

The likelihood of calcinosis also increased by 12% for each year of disease duration (OR 1.12, 95% CI 1.03-1.21) but decreased by 19% for each additional year of age at the time of diagnosis (OR 0.81, 95% CI 0.70-0.95).

Disease features associated with calcinosis that have not been previously reported included:

• Documented episodes of falling, OR 5.89 (95% CI 1.38-25.23)
• Lipodystrophy, OR 5.07 (95% CI 1.21-21.28)
• Contractures, OR 3.98 (95% CI 1.02-15.51)

The study "revealed significant long-term damage, especially persistent muscle weakness, calcinosis and other skin damage, but without severe, clinically appreciable damage to other organ systems or significant functional limitations," the researchers concluded.

They noted that a limitation of their analysis was its severe, highly selected referral population.

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