Comparative utility of LC3, p62 and TDP-43 immunohistochemistry in differentiation of inclusion body myositis from polymyositis and related inflammatory myopathies

Table 1 Summary of consensus diagnostic criteria used for case classification

Diagnostic criterion	PM	PM-COX	pIBM	IBM
Lymphocytic endomysial inflammation^a	Present	Present	Present	Present
Inflammatory infiltrate composition^b	T-cell rich, B-cell poor	T-cell rich, B-cell poor	T-cell rich, B-cell poor	T-cell rich, B-cell poor
Degenerating / regenerating fibers^a	Present (random distribution)	Present (random distribution)	Present (random distribution)	Present (random distribution)
Fiber invasion^b	Present	Present	Present	Present
Diffuse MHC-1 positivity^b	Present	Present	Present	Present
Endomysial fibrosis^a	None or mild	None or mild	Moderate to severe	Moderate to severe
Fiber size variation^a	None or mild	None or mild	Moderate to severe^c	Moderate to severe^c
Percentage of COX-negative fibers^a	<1%	≥1%	Any (generally >1%)	Any (generally >1%)
Ragged red fibers^b	Absent	Present	Either (generally present)	Either (generally present)
Classic rimmed vacuoles^a	Absent	Absent	Absent	Present
Rimmed cracks or basophilic granular debris^b	Absent	Absent	Present^d	Either (generally present)

^a Main criteria (required for diagnosis / classification).
^b Supporting criteria.
^c Specimens with “moderate to severe fiber size variation” included the entire range of muscle fiber sizes (from atrophic to hypertropic; <5 to >100 μm) rather than just two populations of normal-sized and atrophic fibers, as can be seen in PM.
^d For pIBM classification, presence of rimmed cracks (incomplete rimmed vacuoles; Figure 5E, white arrowhead) or basophilic granular debris (material reminiscent of basophilic RV rim) was required in cases with moderate fibrosis but optional in cases with severe fibrosis.

ISSN: 2051-5960