1. Diseases of Muscle: Histopathologic Features
David Lacomis, MD

2. Organization of Skeletal Muscle Including Connective Tissue (CT) Compartments
EPIMYSIUM
PERIMYSIUM
Loose CT
Blood vessels
Septa
Nerve branches
Muscle spindles
Fat
Blood vessels
ENDOMYSIUM
Muscle fibers
Capillaries
Small nerve fibers

3. Normal H&E-Stained Frozen Cross-Section of Skeletal Muscle
Perimysial
connective tissue
Endomysial
connective tissue
Note uniform sizes, polygonal shapes, and eccentric nuclei.

4. Normal H&E-Stained Longitudinal Paraffin Section
Note the banding pattern.
Nuclei are eccentrically placed.

5. Normal Structures: Muscle Spindle
and Associated Nerve Fibers (Gomori trichrome)
Spindle
Nerve Twig

6. Neuromuscular Junctions
Can be identified by the esterase reaction due to the presence of acetylcholinesterase.

7. Neuromuscular Junction (Electron Microscopy)
presynaptic
postsynaptic

8. Histochemical Staining Intensity Based on Fiber Types
Type I
Type II
Type IIB
Slow twitch, oxidative; stain dark with Gomori trichrome, NADH, SDH, and ATPase at acidic pH; more lipid than type II
Fast twitch, glycolytic; stain dark with ATPase at alkaline pH and with PAS stains, as well as phosphorylase
Intermediate staining intensity with ATPase pH4.6
NADH = nicotinamide adenine dinucleotide
SDH = succinic dehydrogenase
ATPase = adenosine triphosphatase

9. Normal (ATPase pH 9.4)
Type I fibers are light
Type II fibers are dark (pattern reverses at ATPase pH 4.3)

10. Ultrastructure of a Sarcomere*
Actin
Myosin
I band
I band
H band Z M Z
A band
*Extends from Z-band to Z-band.
A band includes overlap of actin and myosin.
Note arrangement of thick and thin filaments.

11. Normal (Electron Microscopy)
Dark A-bands
Light I-bands
Z-band is present in the middle of the light band
Thin filaments are attached at the Z-band

12. Classification of Myopathies
ACQUIRED
INHERITED
Inflammatory Myopathies
Dystrophies
Polymyositis (PM)
Dystrophinopathies
Dermatomyositis (DM)
Limb-Girdle
Inclusion body myositis (IBM)
Myotonic
Granulomatous myositis
Facioscapulohumeral (FSHD)
Infectious myositis
Oculopharyngeal (OPD)
Toxic
Distal
Endocrine
Congenital
Metabolic
Mitochondrial
Glycogen & lipid storage

13. Muscle Biopsy
Often necessary for final diagnosis of myopathy
Choose site based on clinical, electrodiagnostic, or imaging features
Avoid “end-stage” fatty muscle
Frozen sections most useful
Routine stains
Histochemistry
Immunohistochemistry

14. Polymyositis (Longitudinal Paraffin-Embedded Section)
In all myopathies, degenerating fibers stain pale initially and then become digested by macrophages.
Mononuclear inflammatory cell infiltrates and many basophilic regenerating fibers (arrow)

15. Polymyositis (Longitudinal Paraffin-Embedded Section-Higher Power)
Regenerating fiber (non-specific)
Fiber is basophilic due to presence of increased RNA and DNA.
Activated plump nuclei and prominent nucleoli

16. Polymyositis (Longitudinal Paraffin-Embedded Section-Higher Power)
As regeneration advances, a myotube “bridge” is formed.

17. Invasion of a Non-necrotic Fiber by Inflammatory Cells
Seen in polymyositis, inclusion body myositis, and a few dystrophies.

18. Myophagocytosis (Esterase Stain)
Macrophages are ingesting the remnants of a degenerating fiber. This is a non-specific myopathic finding.

19. Dermatomyositis
Perifascicular atrophy & Degeneration
Perimysial nflammatory cells surround a blood vessel.
Inflammatory cells tend to be B-cells.
Vasculitis with bowel infarction and subcutaneous calcifications sometimes occur in the childhood form.

20. Perifascicular Atrophy (NADH-Reacted Section)

21. Membrane Attack Complex (MAC) (Immunohistochemical Stain)
MAC is the terminal component of the complement pathway.
It is often deposited in capillaries in dermatomyositis.

22. Inclusion Body Myositis (IBM)
Invaded fiber
Features of chronic myopathy with endomysial inflammation and rimmed vacuoles are characteristic.
Vacuole

23. Lymphocytic inflammation
“Rimmed vacuoles”

24. (Congo Red)
IBM: Vacuoles contain amyloid.

25. IBM Intracytoplasmic (within Vacuoles) or Intranuclear Filamentous Inclusions

26. Granulomatous Myositis in a Patient with Sarcoidosis
Giant cell
Granulomas tend not to cause significant damage to adjacent myofibers.

27. Endocrine Disturbance Type II Fiber Atrophy (ATPase pH9.4)
Characteristic of most endocrine myopathies and steroid myopathy

28. Inherited Polyneuropathy Chronic Neurogenic Atrophy
Groups of angulated atrophic fibers
Marked variation in myofiber size

29. Acute Denervation (NADH Reaction)
Manifested by small, darkly staining angulated fibers.

30. Denervation (Esterase Stain)
Denervated fibers also stain darkly with non-specific esterase.

31. Chronic Neurogenic Processes (NADH Reaction)
Target fibers noted.
Light center surrounded by a darker rim.
Generally only seen in type I fibers.

32. Chronic Neurogenic Atrophy (ATPase Reaction)
Fiber type grouping

33. Frozen Section from a Patient with Duchenne Muscular Dystrophy
Group of basophilic regenerating fibers
Opaque or hyaline fibers (arrows)
Increase in endomysial connective tissue

34. Normal Immunohistochemical Stain for Dystrophin (Subsarcolemmal Staining)

35. Duchenne Muscular Dystrophy (Absent Staining for Dystrophin)

36. Becker Muscular Dystrophy (Reduced but Present Staining)
split fiber
(non-specific chronic change)

37. Female Carrier of Duchenne Muscular Dystrophy (A Mosaic Staining Pattern)

38. Mutations in “Limb-Girdle” and Other Dystrophies
INHERITANCE
GENETIC
ABNORMALITY
DISORDER
X-linked
Dystrophin
Emerin
Duchenne, Becker MD
Emery-Dreifuss MD AD
Myotilin
Lamin A/C
Caveolin – 3
PABP2
-crystallin/Desmin
Limb-Girdle MD (LGMD 1A)
LGMD 1B
LGMD 1C
Oculopharyngeal
Myofibrillar Myopathy AR
Calpain – 3
Dysferlin
g Sarcoglycan
a Sarcoglycan
 Sarcoglycan
Δ Sarcoglycan
Telethonin
LGMD 2A
LGMD 2B
LGMD 2C
LGMD 2D
LGMD 2E
LGMD 2F
LGMD 2G

39. Locations of Affected Proteins in Muscular Dystrophies
Extracellular Matrix
Laminin-2
Dystroglycan
complex a g b a  b
sarcoglycans
Sarcolemma
Lamin A/C
(emerin)
Caveolin 3
Dysferlin
Dystrophin
nucleus
Actin

40. Emery-Dreifuss Muscular Dystrophy (Gomori Trichrome-Stained Frozen Section)
Necrotic fiber
Variation in fiber size with many hypertrophic fibers
Increase in endomysial connective tissue
Nonspecific so-called dystrophic changes seen in many of the muscular dystrophies.
Can also be seen in any chronic myopathic disorder.
This disorder is due to loss of the protein emerin.

41. Myotonic Dystrophy
Chronic changes
Marked excess in internalized nuclei
Variation in fiber sizes
Nuclear clumps (not shown)

42. (H & E, Paraffin)
The excess of internalized nuclei can lead to nuclear chains.

43. Myotonic Dystrophy (NADH-Reacted Section)
Ring fibers in which myofilaments are organized in different directions

44. Fascioscapulohumeral Dystrophy (FSHD)
The majority of dystrophies do not have a specific histopathologic appearance.
Clinical features are also very important.
For example, winging of the scapula is characteristic of FSHD.

45. FSH Dystrophy
Variable non-specific changes
Range from scattered atrophy to “dystrophic” features.
Inflammation can be present (arrow).

46. Congenital Myopathies: Central Core Myopathy (NADH)
Central areas of absent staining in the dark type I fibers
Mitochondria absent

47. Congenital Myopathies: Central Core Myopathy (NADH)
The core consists of disorganized myofibrils and the area is devoid of mitochondria.

48. Congenital Fiber Type Disproportion (H&E)
Bimodal size population

49. Congenital Fiber Type Disproportion (ATPase pH 4.3)
Smaller fibers are type I
More numerous
Stain lightly
Larger or normal fibers are type II

50. Nemaline Myopathy
Eosinophilic inclusions present.

51. Nemaline Myopathy (Gomori Trichrome)
Eosinophilic inclusions stain darkly.

52. Nemaline Myopathy (Electron Microscopy)
Named for thread-like appearance
Inclusions extend from Z-band to Z-band

53. Muscle Biopsy from an Infant
Internalized nuclei predominant.
Consistent with centronuclear myopathy.
Can be seen in other disorders such as myotonic dystrophy with congenital onset.

54. Muscle Biopsy from an Infant: Centronuclear Myopathy
Central position of the nucleus resembling an embryonic myotube

55. Metabolic: Inherited – Mitochondrial Myopathy
Ragged red fiber present (Gomori trichrome)
Due to proliferation of abnormal mitochondria

56. Mitochondrial Myopathy (Succinic Dehydrogenase Reaction)
SDH-rich fibers are seen with mitochondrial proliferation.
SDH is a respiratory chain enzyme encoded by nuclear DNA.

57. Cytochrome Oxidase (COX) Respiratory Chain Enzyme
Normal Fibers

58. Many COX-Negative Fibers
COX-negative fibers are usually seen with mtDNA mutations.

59. Mitochondrial Disorders (Electron Microscopy)
Aggregates of mitochondria containing paracrystalline inclusions are frequent.
Non-specific

60. Mitochondrial Disorders (Electron Microscopy)
Higher power view of paracrystalline inclusion

61. (Oil-Red-O Stain)
Increased lipid storage
Seen in carnitine deficiency states (primary or secondary)
Sometimes as a consequence of certain toxins
Focal increases can be non-specific.

62. Lipid Storage Myopathy (Electron Microscopy)

63. Glycogen Storage Myopathies
Some glycogen storage myopathies, such as myophosphorylase deficiency (McArdle’s Disease), cause subsarcolemmal blebs.
PAS-positive due to the presence of glycogen.
Only with acid maltase deficiency is glycogen deposited in lysomsomes.

64. McArdle’s Disease (Electron Microscopy)
Subsarcolemmal collection of glycogen is shown.

65. Acid Maltase Deficiency (Acid Phosphatase)
Vacuolar myopathy noted.
Due to the intralysosomal activity of this enzyme
Prominent staining with acid phosphatase in vacuoles

66. Normal Glycogen (PAS Stain) Control

67. Increased Glycogen
Acid maltase deficiency
Increased glycogen (diffusely and in vacuoles)