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

 CD3+ endomysial infiltration

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

 CD3+ perivasclar infiltration

 Macrophage endomysial infiltration

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

 Macrophage perivascular infiltration

 Capillary endothelial TRI by EM

Score = 0 if no TRI; score = 1 if < 2 TRI/10 examined capillaries; score = 2 if ≥2TRI/10 examined capillaries.

 C5b-9 positive capillaries

scores = 0 if none, score = 1 if rare, score = 2 if multifocal and robust

 Arterial abnormality

Mural thickening and/or endothelial swelling and/or transmural inflammation in arteries/arterioles. Absence = 0, presence = 1.

 Infarction

Well demarcated regional loss of myofiber nuclei and loss of normal cytoarchitecture. Absence = 0, presence = 1.

 MHC Class I over expression

Presence of MHC1 in myofibers. Absence = 0, presence = 1.

 Atrophy perifascicular

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

Atrophy of nonperifascicular myofibers: 0 = absent, 1 = present.

 C5b-9 positive necrotic fiber

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

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

 Internal nuclei

Internal nuclei in non-basophilic otherwise normal fibers: 0 if < 3% of fibers, 1 if > 3% of fibers.

 Endomysial fibrosis

Assessed on Masson’s trichrome stain: 0 = absent, 1 = present

 Perimysium alkaline phosphatase reactivity

Alkaline phosphatase connective tissue reactivity, excluding arteries/arterioles: 0 = absent, 1 = present, but weak and focal, 2 = strong and widespread.