DERMATOMYOSITIS: Childhood type pathology

Capillary pathology   H&E stain   Alkaline phosphatase   Ulex stain Complement deposition Inflammation MHC Class I Perifascicular atrophy   GT stain   H&E stain   NADH stain   Mitochondrial stains   Alkaline phosphatase Muscle fiber pathology DM: Adult pattern

Muscle fiber atrophy: Perifascicular distribution Gomori trichrome stain H&E stain

Muscle fibers near the perimysium are small but not necrotic.   Necrotic fibers are more common in DM-like disorders in adults. Nuclei in small muscle fibers are enlarged and occasionally internal.

Batten FE Br J Child Dis 1912;9:247

Perifascicular pathology: Other stains

NADH NADH stain Muscle fibers near the perimysium are small & stain darkly

MITOCHONDRIAL The extent of the perifascicular muscle fiber pathology in active DM is often more prominent on cytochrome oxidase stain. SDH stain is not reduced. Reduced cytochrome oxidase staining is not typically present in IMPP syndromes which have a different pattern of perifascicular muscle fiber pathology. Cytochrome Oxidase COX stain Muscle fibers near the perimysium are small & have pale COX staining Succinate Dehydrogenase SDH stain Muscle fibers near the perimysium are small & have normal or dark SDH staining

ATPase pH 9.4 ATPase pH 9.4 stain Perifascicular atrophy is often more obvious on ATPase stains. Small fibers are both types I & II.

ATPase pH 4.3 ATPase pH 4.3 stain Type IIC muscle fibers (interrmediate staining) are present in regions of atrophy and large muscle fibers.

PAS PAS stain Increased glycogen staining in small perifascicular muscle fibers (Bottom).1

Alkaline Phosphatase Alkaline phosphatase staining of enlarged capillaries (Arrows) in regions near the smallest muscle fibers.   Fewer capillaries stain among larger fibers or amid the smallest fibers at the edge of fascicles. Alkaline phosphatase stain

Alkaline phosphatase may stain several different structures in patients with dermatomyositis.
Alkaline phosphatase stain	Alkaline phosphatase stain	Alkaline phosphatase stain
Perimysium Vascular staining. More common in children	Endomysium Capillary staining, especially in perifascicular regions.	Perimysium Diffuse staining. More common in adults Rare in children

Perifascicular atrophy: Muscle fiber pathology Muscle fibers in regions of perifascicular pathology show several abnormalities in addition to being small. Some muscle fibers have vacuolar or reticulated cytoplasm (Below). Muscle fiber nuclei are enlarged. Muscle fiber necrosis is rare.   H&E stain

Muscle fibers may also have   Coarse internal architecture with basophilia (Left, Arrow), or   Internal nuclei (Right). The abnormal internal architecture may be better visualized on Gomori trichrome   H&E stain

Endomysial capillaries near regions of perifascicular muscle fiber pathology are commonly enlarged (Arrows).   H&E stain

Abnormal internal architecture in dermatomyositis muscle fibers is more apparent on Gomori trichrome stains. The abnormalities may be present in both small and large muscle fibers. Rod-like structures may occur (Bottom).     GT stain

Dermatomyositis: Inflammation
Mononuclear cell inflammation: Mostly present in perimysium and associated with intermediate-sized vessels.
H&E stain	H&E stain

Mononuclear cell inflammation: Stains for CD4 and, in some regions, CD20 (B-cells) Commonly perivascular.
CD4 stain	CD20 stain	Congo red stain

Childhood dermatomyositis: Capillary pathology

Stain: Ulex europaeus agglutinin I Dermatomyositis: Capillaries   Capillary pathology is patchy and most prominent in perifascicular regions.   Capillary staing with Ulex lectin is often absent in regions with small perifascicular muscle fibers (Above left).   Capillaries in regions near perifascicular atrophy are often enlarged (Above right).

Stain: Ulex europaeus agglutinin I Control: Capillaries are abundant and small

Childhood dermatomyositis: Complement deposition in capillaries

Stain: C5b-9 components of complement (Membrane attack complex) Dermatomyositis: C5b-9 deposition in endomysial capillaries Changes are patchy and most prominent in perifascicular regions amid small muscle fibers.

Normal control Stain: C5b-9 components of complement (Membrane attack complex) No C5b-9 deposition in endomysial capillaries

Childhood dermatomyositis: MHC Class I upregulation

Stain: MHC Class I Dermatomyositis: MHC I upregulation On muscle fibers, especially near perimysium A similar pattern may occur in IMPP syndromes	Stain: MHC Class I Dermatomyositis: MHC I upregulation Diffusely on the surface and within muscle fibers
Stain: MHC Class I Normal: MHC I staining only on capillaries