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Table 1 Histological score tool for muscle biopsy pathology. TRI: tubuloreticular inclusions. EM: electron microscopy. MHC: major histocompatibility complex

From: Distinct tissue injury patterns in juvenile dermatomyositis auto-antibody subgroups

Domain Score Definitions and Instructions
INFLAMMATORY DOMAIN
 CD3+ endomysial infiltration 0,1,2 For each of endomysial, perimysial, perivascular distributions, score for CD3+ infiltrating cells as follows: if none, or < 4 cells in a × 20 field = score 0; if≥4 cells in a 20x field and/or1 cluster (cluster is 10 cells or more) = score of 1; if ≥2 clusters in whole biopsy and/or diffuse infiltrating cells (i.e. > 20 cells in a 20x field) = score of 2.
 CD3+ perimysial infiltration 0,1,2
 CD3+ perivasclar infiltration 0,1,2
 Macrophage endomysial infiltration 0,1,2 For each of endomysial, perimysial, perivascular distributions, score for macrophage, by acid phosphatase stain: if none, or < 4 cells in a ×20 field = score 0; if ≥4 cells in a 20x field and/or1 cluster (cluster is 10 cells or more) = score of 1; if ≥2 clusters in whole biopsy and/or diffuse infiltrating cells (i.e. > 20 cells in a 20x field) = score of 2.
 Macrophage perimysial infiltration 0,1,2
 Macrophage perivascular infiltration 0,1,2
VASCULAR DOMAIN
 Capillary endothelial TRI by EM 0,1,2 Score = 0 if no TRI; score = 1 if < 2 TRI/10 examined capillaries; score = 2 if ≥2TRI/10 examined capillaries.
 C5b-9 positive capillaries 0,1,2 scores = 0 if none, score = 1 if rare, score = 2 if multifocal and robust
 Arterial abnormality 0,1 Mural thickening and/or endothelial swelling and/or transmural inflammation in arteries/arterioles. Absence = 0, presence = 1.
 Infarction 0,1 Well demarcated regional loss of myofiber nuclei and loss of normal cytoarchitecture. Absence = 0, presence = 1.
MUSCLE FIBER DOMAIN
 MHC Class I over expression 0,1 Presence of MHC1 in myofibers. Absence = 0, presence = 1.
 Atrophy perifascicular 0,1,2 Affecting > 6 myofibers out of 10 along one edge of a fascicle, not exclusively type II fibers. 0 = absent, 1 = present in one or two fascicles, 2 = present in 3 or more fascicles.
 Atrophy nonperifascicular 0,1 Atrophy of nonperifascicular myofibers: 0 = absent, 1 = present.
 C5b-9 positive necrotic fiber 0,1,2 Myofibers with strong sarcoplasmic C5b-9 expression: 0 = absent, 1 if < 6 positive fibers in a 20x field, 2 if ≥6 positive fibers in a 20x field.
 Degenerating fibers perifascicular 0,1,2 Includes focal basophilia within a fiber, vacuolation, sarcoplasmic rarification, and/or pallor, myophagocytosis, acid phosphatase positive fibers, alkaline phosphatase positive fibers): 0 = absent, 1 = present in one or two fascicles, 2 = present in three or more fascicles
 Degenerating fibers non-perifascicular 0,1,2
 Internal nuclei 0,1 Internal nuclei in non-basophilic otherwise normal fibers: 0 if < 3% of fibers, 1 if > 3% of fibers.
CONNECTIVE TISSUE DOMAIN
 Endomysial fibrosis 0,1 Assessed on Masson’s trichrome stain: 0 = absent, 1 = present
 Perimysium alkaline phosphatase reactivity 0,1,2 Alkaline phosphatase connective tissue reactivity, excluding arteries/arterioles: 0 = absent, 1 = present, but weak and focal, 2 = strong and widespread.