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Disorders of Muscles

Muscular dystrophies

  1. Types of muscular dystrophies
    1. Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder.
      1. Although distal muscles are eventually affected as well, patients experience initial pro­gressive proximal muscular weakness.
      2. Prednisolone treatment slows progression of weakness.
      3. Death usually occurs in the third decade of life, often as a result of pneumonia.


Figure - Diagram showing the relationship between the cell membrane (sarcolemma) and the sarcolemmal associated proteins. Dystrophin, an intracellular protein, forms an interface between the cytoskeletal proteins and a group of transmembrane proteins, the dystroglycans and the sarcoglycans. These transmembrane proteins have interactions with the extracellular matrix, including the laminin proteins. Dystrophin also interacts with dystrobrevin and the syntrophins, which form a link with neuronal-type nitric oxide synthetase (nNOS) and caveolin. Mutations in dystrophin are associated with the X-linked muscular dystrophies, mutations in caveolin and the sarcoglycan proteins with the autosomal limb girdle muscular dystrophies, and mutations in the α2-laminin (merosin) with a form of congenital muscular dystrophy.

Extra Edge: In Duchenne muscular dystrophy (DMD)  (AIIMS Nov 2010)

  1. Pseudohypertrophy of the calf muscles occurs.
  2. Dystrophin gene mutation occurs.
  3. Gower sign is positive.
  4. Sarcolemmal protein defect occurs.
  5. Cardiomyopathy can occurs.
  1. Becker's muscular dystrophy (BMD) is an X-linked recessive disorder of muscle. Patients with BMD have a more benign course than those with DMD, With a 50% survival rate beyond age 50.
  2. Myotonic muscular dystrophy (DM1) is an autosomal dominant disorder.
    The clinical expression of DM1 varies widely and involves many systems other than muscle. 

Extra Edge: (Ref. Hari. 18th ed., Pg- 3496) Myotomic Muscular Dystrophy

  1. Affected patients have a typical "hatchet-faced" appearance due to temporalis, masseter, and facial muscle atrophy and weakness.
  2. Characteristic muscle myotonia (i.e., persistent muscle activity in response to contraction or percussion).
  3. Distal weakness, cataracts, impaired intellect, hypersomnia, testicular atrophy, cardiomyopathy, mitral valve pro­lapse, and cardiac conduction defects.
  4. Frontal baldness is also characteristic of the disease.
  5. Death occurs in the fifth or sixth decade and typically is attributable to respiratory com­promise or cardiac arrhythmia.
  1. Facio- scapulohumeral muscular dystrophy is an autosomal dominant disorder character­ized by progressive weakness about the face, neck, upper torso, and proximal arms.
  2. Limb-girdle muscular dystrophy is actually a group of autosomal dominant and recessive dis­orders characterized by progressive loss of motor strength of the trunk and proximal limbs.


Extra Edge: Gene or locus in limb-girdle muscular dystrophy – (Ref. Hari. 18th ed., pg-2683)

  1. Calpain – 3
  2. Sarcoglycan
  3. Dysferlin
  4. Titin
  1. Diagnosis. Patients typically have an elevated serum CK level. EMG demonstrates a "myopathic" pattern (i.e., brief, small amplitude muscle potentials). A muscle biopsy is often informative. Genetic studies are increasingly pursued.

Table: The muscular dystrophies Table 387-5 (Harrison ed.18th Pg, 3492)



Age at onset (yrs)



Duchenne type

X-linked recessive


Pelvic, then shoulder girdle; later, limb and respiratory muscles.

Rapid progression. Death within about 15 years after onset


X-linked recessive


Pelvic, then shoulder girdle

Slow progression. May have normal life span

Limb-girdle (Erb)

Autosomal recessive, dominant or sporadic

10 – 30

Pelvic or shoulder girdle initially, with later spread to the other

Variable severity and rate of progression. Possible severe disability in middle life


Autosomal dominant

Any age

Face and shoulder girdle initially; later, pelvic girdle and legs

Slow progression. Minor disability. Usually normal life span

Deafness coat’s eye disease (H-2682)


Congenital myopathy AIIMS May 2011 (Ref. Hari. 18th ed., Pg- 3496)

  1. Central core         
  2. Nemalin               
  3. Central nuclear

Acquired Myopathy


Table: Selected Causes of Acquired Myopathy

A. Polymyositis

1. Idiopathic

2. Associated with other connective

 tissue diseases, infectious agents (including HIV), and drugs

3. Associated with malignancy

B. Electrolyte disorders
(LQ 2012)

1. Hypokalemia

2. Hyperkalemia

3. Hypercalcemia

C. Endocrine disorders

1. Diabetes

2. Hypothyroidism

3. Hyperthyroidism

4. Cushing's disease

5. Acromegaly

6. Hyperparathyroid

D. Drugs

1. Statins,

2. Chloroquine

3. Zidovudine

4. Steroids


Extra Edge:

  1. CPK level is normal in all endocrine myopathy except hypothyroid where it is increased. (Ref. Hari. 18th ed., page -2693)
  2. Inverted champagne bottle appearance is seen in peroneal muscle dystrophy.

Idiopathic inflammatory myopathies (Polymyositis & Dermatomyositis)
(Ref. Hari. 18th ed., Pg- 3510) 

  1. Principal manifestation is muscle weakness
  2. They affect persons of any age group, but the peak incidence is in the fifth and sixth decades of life.
  3. Women are affected twice as commonly as men.
  4. There is an increased risk of malignancy, (cancers of ovary, breast, melanoma, colon, non Hodgkin lymphoma).
  5. Rare patients with dermatomyositis have skin disease without overt muscle involvement, a condition termed “dermatomyositis sine myositis.”

Clinical Findings

  1. Symptoms And Signs
    1. Gradual and progressive muscle weakness. The weakness chiefly involves proximal muscle groups of the upper and lower extremities as well as the neck.
    2. Dermatomyositis do not cause facial or ocular muscle weakness. (AIPG 2007)
    3. In all forms of inflammatory myopathy, pharyngeal and neck-flexor muscles are often involved, causing dysphagia or difficulty in holding up the head (head drop). In advanced and rarely in acute cases, respiratory muscles may also be affected. (H- 18th Pg- 3509)
    4. Dermatomyositis involves the striated muscles of the upper pharynx and can make initiation of deglutition difficult.
    5. In dermatomyositis skin signs as (AIIMS Nov 09)
      1. Erythema also occurs over other areas of the face, neck, shoulders, and upper chest and back (“shawl sign”).
      2. Periorbital edema and a purplish (heliotrope) suffusion over the eyelids are typical signs.
      3. Scaly patches over the dorsum of PIP and MCP joints (Gottron's sign).
      4. Mechanic’s hand
      5. V sign – Rash over the neck area.
  2. Laboratory Findings
    1. Raised muscle enzymes, creatine kinase and aldolase
    2. Antinuclear antibodies are present in many patients,
    3. Anti-Jo-1 antibodies, Anti synthetase antibody, Anti-signal recognition particles antibody.

Extra Edge: Paraneoplastic syndrome & Ophthalmoplegia are not the feature of polymyositis. (AIIMS Nov 2012)

  1. Muscle Biopsy
    Biopsy of clinically involved muscle is the only specific diagnostic test.
    1. Most patients respond to corticosteroids.
    2. Immunosuppressive Mycophenolate mofetil.
    3. Since the rash of dermatomyositis is often photosensitive, patients should limit sun exposure.
    4. Hydroxychloroquine.
Table 388-2 (Harrison ed.18th, Pg 3515)) Criteria for Diagnosis of Inflammatory Myopathies
Criterion Polymyositis Dermatomyositis Inclusion Body Myositis
Myopathic muscle weaknessa 
Yes Yesb  Yes; slow onset, early involvement of distal muscles, frequent falls
Electromyographic findings Myopathic Myopathic Myopathic with mixed potentials
Muscle enzymes Elevated (up to 50-fold) Elevated (up to 50-fold) or normal Elevated (up to 10-fold) or normal
Muscle biopsy findings
"Primary" inflammation with the CD8/MHC-I complex and no vacuoles Perifascicular, perimysial, or perivascular infiltrates, perifascicular atrophy Primary inflammation with CD8/MHC-I complex; vacuolated fibers with -amyloid deposits; cytochrome oxygenase–negative fibers; signs of chronic myopathy
Rash or calcinosis Absent Present Absent

  1. ESR & RA factor would not directly help in establishing the diagnosis of inflammatory myopathy
  2. EMG shows myopathic potentials characterized by short-duration, low-amplitude polyphasic units on voluntary activation and increased spontaneous activity with fibrillations, complex repetitive discharges, and positive sharp waves.
  3. These EMG findings are not diagnostic of an inflammatory myopathy but are useful to identify the presence of active or chronic myopathy and to exclude neurogenic disorders.
  4. Muscle biopsy is the definitive test for establishing the diagnosis of inflammatory myopathy and for excluding other neuromuscular diseases.


Extra Edge

Critical illness myopathy (Ref. Hari. 18th ed.,  Pg- 2261&3507) (AIIMS May 2014)

  1. Critically ill patients, especially those with sepsis, frequently develop muscle wasting, often in the face of seemingly adequate nutritional support.
  2. It is a catabolic myopathy brought about as a result of multiple factors, including elevated cortisol and catecholamine release and other circulating factors induced by the SIRS.
  3. In this syndrome, known as cachectic myopathy, serum creatine kinase levels and electromyography (EMG) are normal. Muscle biopsy shows type II fiber atrophy.
  4. Panfascicular muscle fiber necrosis may also occur in the setting of profound sepsis.
  5. This so-called septic myopathy is characterized clinically by weakness progressing to a profound level over just a few days. 
  6. There may be associated elevations in serum creatine kinase and urine myoglobin. Both EMG and muscle biopsy may be normal initially but eventually show abnormal spontaneous activity and panfascicular necrosis with an accompanying inflammatory reaction.
  7. Both of these myopathic syndromes may be considered under the broader heading of critical illness myopathy.

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