This document summarizes skeletal muscle disorders and the histopathological findings seen in muscle biopsy specimens. It describes the normal muscle histology and classifications of acquired and inherited myopathies. Key points include the inflammatory features of polymyositis and dermatomyositis, rimmed vacuoles and inclusions in inclusion body myositis, and the absence of dystrophin staining in Duchenne muscular dystrophy. Overall, the document provides an overview of muscle biopsy findings and their significance in diagnosing different skeletal muscle disorders.

1. Skeletal muscle disorder
Dr Tushar

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

3. Perimyseal
connective tissue
Endomyseal
connective tissue
Normal H&E-Stained Frozen Cross-Section of Skeletal Muscle
 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.  Can be identified by the esterase
reaction due to the presence of
acetylcholinesterase.
Neuromuscular
Junctions
Normal Structures: Muscle Spindle
and Associated Nerve Fibers
(Gomori trichrome)

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

7. Skeletal Muscle atrophy
• common features of many
disorders
• causes:- loss of innervation ,
disuse, cachexia, old age, and
primary myopathies
• patterns:-
– clusters or groups of atrophic
fibers are seen in neurogenic
disease
– perifascicular atrophy is seen
in dermatomyositis
– type ii fiber atrophy with
sparing of type i fibers is seen
with prolonged corticosteroid
therapy or disuse.

8. 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

9. 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

10. ACQUIRED

11. Inflammatory Myopathies

12. Polymyositis
• Adult-onset inflammatory myopathy that shares myalgia and weakness
with dermatomyositis but lacks its distinctive cutaneous features.
• Pathogenesis:
– Believed to have an immunologic basis.
– CD8-positve cytotoxic T cells are a prominent part of the
inflammatory infiltrate in affected muscle (mediators of tissue
damage)
– Vascular injury does not play major role (unlike dermatomyositis)
• Morphology:
– Endomysial mononuclear inflammatory cell infiltrates
– Degenerating necrotic, regenerating and atrophic myofibers are
typically found in a random or patchy distribution
– Absent perifascicular pattern of atrophy (characteristic of

13. 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)

14. Polymyositis
(Longitudinal Paraffin-Embedded Section-Higher Power)
• regenerating fiber (non-specific)
• fiber is basophilic due to presence of increased
RNA and RNA.
• activated plump nuclei and prominent nucleoli

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

16. Myophagocytosis
(Esterase Stain)
• macrophages are ingesting the remnants of a degenerating
fiber. this is a non-specific myopathic finding.

17. Dermatomyositis
• Immunologic disease in which damage to small blood vessels contributes to muscle
injury.
• Vasculopathic changes – Telangiectasias
• Pathogenesis :
– Inflammatory signature enriched for genes that are unregulated by type I
interferons is seen in muscle and in leukocytes (prominence – disease activity)
– Autoantibodies:
• Anti-Mi2 antibodies – Directed against a helicase implicated in nucleosome
remodeling. Strong association with prominent Gottron papules and heliotrope
rash.
• Anti-Jo1 antibodies – Directed against the enzyme histidyl t-RNA synthetase,
associated with interstitial lung disease, nonerosive arthritis and a skin rash
(Mechanic’s hand)
• Anti-P155/P140 antibodies – Directed against several transcriptional

18. • Morphology:
– Perimysial mononuclear inflammatory infiltrates in connective
tissue and around blood vessels.
– Myofiber atrophy is accentuated at the edges of the fascicles –
Perifascicular atrophy
– Segmental fiber necrosis and regeneration.
– Deposition of CD4+ T-helper cells and C5b-9 (MAC) in capillary
vessels.
– EM: tubuloreticular endothelial cell inclusion

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)

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

23. INCLUSION BODY MYOSITIS
• Disease of late adulthood that typically affects patients older than 50
years and is the most common inflammatory myopathy in patients older
than age 65 years.
• Slowly progressive muscle weakness – m/c feature
– Most severe in quadriceps and distal upper extremity muscles.
– Dysphasia from esophageal and pharyngeal muscle involvement
• Lab investigation:
– S. creatine kinase level increased
– Myositis associated autoantibodies are absent.

24. • Morphology:
– Patchy often endomysial mononuclear inflammatory cell infiltrate rich in
CD8+ T- cells
– Increased sarcolemmal expression of MHC class I antigens
– Focal invasion of normal appearing myofibers by inflammatory cells
– Admixed degenerating and regenrating myofibers
– Abnormal cytoplasmic inclusions described as “rimmed vacuoles”
– Tubolofilamentous inclusions in myofibers – EM
– Cytoplasmic inclusions containing proteins typically associated with
neurodegenerative disease, like beta-amyloid, TDP-43, and ubiquintin
– Endomysial fibrosis and fatty replacement, reflective of a chronic disease
course.

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

26. Lymphocytic inflammation
“Rimmed vacuoles”

27. • IBM: Vacuoles contain amyloid.
(Congo Red)

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

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

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

32. Toxic myopathies
• Statin induced
• Chloroquine & hydroxychloroquine (Drug induced lysosomal storage
myopathy)
• ICU myopathy (corticosteroid therapy)
– Degradation of sarcomeric myosin thick filaments leading
to profound weakness
• Thyrotoxic myopathy
– Proximal muscle weakness, exophthalamic
ophthalmoplegia
• Alcohol

33. INHERITED

34. Congenital Myopathies

36. Central Core Myopathy
(NADH)
• Central areas of absent staining in the dark type I fibers
• Mitochondria absent

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

38.  Eosinophilic inclusions present.
Nemaline Myopathy

39. Nemaline Myopathy
(Gomori Trichrome)
• Eosinophilic inclusions stain
darkly.

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

41. Centronuclear myopathy
 Internalized nuclei predominant.
 Consistent with centronuclear myopathy.
 Can be seen in other disorders such as myotonic dystrophy with
congenital onset.

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

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

44. Muscular
Dystrophies

45. X linked muscular dystrophy
with dystrophin mutation

46. Duchenne and Becker musclar
dystrophy
• Most common muscular dystrophies x-linked and stem from mutations that disrupt the
function of a large structural protein called dystrophin.
• Early onset form – Duchenne muscular dystrophy
– Severe progressive phenotype
• Late onset form - Becker muscular dystrophy
– Isolated cardiomyopathy, asymptomatic elevation of creatine kinase, exercise
intolerance
• Pathogenesis:
– Loss of function mutations in the dystrophin gene on X- chormosome
– Dystrophin provide mechanical stability to the myofiber and its cell complex
• Morphology:
– Chnages in Duchenne and Becker muscular dystrophy are similar, but differ in
degree.
– Chronic muscle damage that outpaces the capacity for repair.
– Segmental myofiber degeneration and regeneration with an admixture of atrophic
myofirbers.
– Fatty replacement as disease progress
– IHC studies show absence of the normal sarcolemmal staining pattern in
Duchenne muscular dystrophin and reduced stationing in Becker muscular

47. • Clinical Feature
– Duchenne muscular dystrphy
• Normal at birth
• Early motor milestones
• Walking is delayed
• Clumsiness & inability to keep up with peers
• Pseudohyperthrophy of muscle
• Mean age of wheel chair dependence around 9.5 years
• Cardiomyopathy & arrhythmias
• Frank mental retardation
• Mean age of death 25 to 30 years
– Becker muscular dystrophy
• Later onset and slowly progressive

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

49. Normal Immunohistochemical Stain for
Dystrophin
(Subsarcolemmal Staining)

50. Duchenne Muscular Dystrophy
(Absent Staining for Dystrophin)

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

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

53. Myotonic Dystrophy
• Autosomal dominant multisystem disorder associated with skeletal
muscle weakness, cataracts, endocrinopathy, and cardiomyopathy.
• Myotonia key feature
• Pathogenesis
– Expansions of CTG triplet repeats in 3’-noncoding region of
myotonic dystrophy protein kinase (DMPK)
– Toxic gain of function
– CUG-repeats in the DMPK mRNA transcript appear to bind and
sequester a protein called muscleblind-like1 – Important role in
RNA splicing.
– This inhibits muscleblind-like-1function leading to missplicing of
other RNA transcripts including transcript for a chloride channel
called CLC1and is responsible for characteristic myotonia.

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

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

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

57. Emery-Dreifuss Muscular
Dystrophy
• Caused by mutation in genes that encode nuclear lamina proteins.
• Triad
– Slowly progressive humeroperoneal weakness
– Cardiomyopathy
– Early contractures of the Achilles tendon, spine & elbow
• X-linked form [EMD1]– mutation in genes encoding emerin
• Autosomal form [EMD2] – mutation in genes encoding lamin
• These protein helps in maintaining the shape and mechanical stability of the
nucleus during muscle contraction.

58. 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.

59. Fascioscapulohumeral
Dystrophy
• Characteristic pattern of muscle involvement that includes prominent
weakness of facial muscles and muscles of the shoulder girdle.
• Autosomal Dominant
• Pathogenesis:
– Overexpression of a gene called DUX4, located in a region of
subtelomeric repeats on the long arm of chromosome 4.
– Deletion in flanking repeats causes changes in chromatin that derepress
the remaining copies of DUX4 thus leading to overexpression.

60. 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.

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

62. Limb-Girdle Muscular Dystrophy
• Heterogeneous group
– 6 AD
– 15 AT
• Characterized by muscle weakness that preferentially involves proximal
muscle groups.
• Pathogenesis
– Genes encoding structural components (sarcoglycans) of the
dystrophin glycoprotein complex
– Genes encoding enzymes that are responsible for glycosylation of a-
dystroglycan, a component of the dystrophin glycoprotein complex
– Genes encoding proteins that associate with the Z-disks of sarcomeres
– Genes encoding proteins involved in vesicle trafficking and cell
signaling
– Genes that seemingly stand alone, such as those encoding the
protease calpain 3 and laminA/C (which is also mutated in some
patients with Emery-Dreifuss muscular dystrophy)

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

64. Mitochondrial Myopathies
• Complex systemic conditions that can involve many organ systems,
including skeletal muscle.
• Manifest as weakness, elevations in serum creatine kinase levels, or
rhabdomyolysis.
• Morphology:
– Most consistent pathologic change in skeletal muscle is abnormal
aggregates of mitochondria that are seen preferentially in the
subsarcolemmal area.
– Ragged red fibers appearance.
– Morphologically abnormal mitochondria – EM
• Clinical feature
– Chronic progressive external ophthalmoplegia
– Mitochondrial encephalomyopathy with lactic acidosis and strokelike
episodes
– Kearns-Sayre syndrome
– Myoclonic epilepsy with ragged red fibers
– Leber hereditary optic neuropathy

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

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

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

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

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

70. Disease of Lipid or Glycogen
Metabolism
• Inborn errors of lipid or glycogen metabolism
• Severe muscle cramping and pain
• Extensive muscle necrosis (rhabdomyolysis)
• Example
– Carnitine palmitoyltransferace II deficiency (m/c)
– Myophosphorylase deficiency (McArdle ds)
– Acid maltase deficiency

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

72. Lipid Storage Myopathy
(Electron Microscopy)

73. 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.

74. Acid Maltase Deficiency
(Acid Phosphatase)
• Due to the intralysosomal activity of this enzyme
• Prominent staining with acid phosphatase in vacuoles
Vacuolarmyopathy noted.

75. Normal Glycogen
(PAS Stain) Control

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

77. Ion Channel Myopathies
(Channelopathies)
• KCNJ2 :
– mutations affecting this potassium channel cause Andersen-Twail
syndrome
• AD, Periodic paralysis, Heart arrhythmias, skeletal abnormalities
• SCN4A :
– Mutations affecting this sodium channel cause several AD with
presentations ranging from myotonia to periodic paralysis.
• CACNA1S :
– Missense mutations in this protein, a subunit of a muscle calcium
channel, are the most common cause of hypokalemic paralysis.
• CLC1:
– Mutations affecting this chloride channel cuases myotonia congenita
– Expression decreased in myotonic dystrophy
• RYR1:
– Mutation in the RYR1 gene disrupt the funciton of the ryanodine
receptor , which regulated calcium release from the sarcoplasmic
reticulum
– Central core myopathy

78. Thank you for listening.
Have a Good Day.