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In adult dermatomyositis 10 muscle specimens with no or minimal histological alterations were compared with 7 that showed typical alterations. Five specimens from patients with inclusion body myositis, 5 from patients with polymyositis, and 8 from normal subjects served as controls. Vascular endothelium, visualized with the lectin Ulex europaeus agglutinin I, and complement membrane attack complex were demonstrated in the same cryostat sections by paired immunofluorescence. Large randomly selected fields were analyzed to determine the number of capillaries per square millimeter of fiber area (capillary density), per 1,000-microns 2 area of each muscle fiber (capillary index), and in 100 x 100-microns grid squares. In dermatomyositis specimens with minimal structural alterations there was focal capillary depletion, the capillary density was significantly reduced, and the frequency distributions of the capillary index and grid count were shifted to the left. In advanced dermatomyositis specimens, the findings were similar but more severe. In both kinds of specimens, clusters of capillaries reacted for complement membrane attack complex. The 2 patients with the highest proportion of vessels positive for membrane attack complex had a fulminant and fatal course. In polymyositis and inclusion body myositis specimens, the capillaries had a normal overall density and none reacted for membrane attack complex. The findings imply that the capillaries are an early and specific target of the disease process in dermatomyositis.

In 6 adults with dermatomyositis, minimally weak or nonweak muscles that showed inconclusive light-microscopic alterations were examined by electron microscopy. In all 6 specimens, this revealed pathologic changes in endomysial capillaries. The endothelial cells harbored microtubular inclusions and microvacuoles in all cases; pale swollen endothelial cells were observed in 3 specimens. There were no significant ultrastructural changes in the muscle fibers. In all cases, a small proportion of muscle capillaries were immunoreactive for complement membrane attack complex neoantigens. The findings imply that in adult dermatomyositis capillary injury precedes muscle fiber damage and infiltration by inflammatory cells, and that the microvasculature is an early and specific target of the disease process in muscle.

Objectives: To study muscle biopsy tissue from patients with juvenile dermatomyositis (JDM) in order to test the reliability of a score tool designed to quantify the severity of histological abnormalities when applied to biceps humeri in addition to quadriceps femoris. Additionally, to evaluate whether elements of the tool correlate with clinical measures of disease severity. Methods: 55 patients with JDM with muscle biopsy tissue and clinical data available were included. Biopsy samples (33 quadriceps, 22 biceps) were prepared and stained using standardised protocols. A Latin square design was used by the International Juvenile Dermatomyositis Biopsy Consensus Group to score cases using our previously published score tool. Reliability was assessed by intraclass correlation coefficient (ICC) and scorer agreement (α) by assessing variation in scorers’ ratings. Scores from the most reliable tool items correlated with clinical measures of disease activity at the time of biopsy. Results: Inter- and intraobserver agreement was good or high for many tool items, including overall assessment of severity using a Visual Analogue Scale. The tool functioned equally well on biceps and quadriceps samples. A modified tool using the most reliable score items showed good correlation with measures of disease activity. Conclusions: The JDM biopsy score tool has high inter- and intraobserver agreement and can be used on both biceps and quadriceps muscle tissue. Importantly, the modified tool correlates well with clinical measures of disease activity. We propose that standardised assessment of muscle biopsy tissue should be considered in diagnostic investigation and clinical trials in JDM.