CONGENITAL MUSCLE FIBRE-TYPE DISPROPORTION

 

CONGENITAL MUSCLE FIBRE-TYPE DISPROPORTION

 

DEFINITION:

A congenital myopathy characterized by generalized hypotonia, muscle weakness, with small and numerous type 1 fibres on muscle biopsy.

EPIDEMIOLOGY:

  • incidence: rare
  • age of onset:
    • newborn
  • risk factors:
    • familial - autosomal recessive
      • chrom.#: ?
      • gene: ?
      • may also be sporadic and autosomal dominant forms
    • M = F
    • may be associated with:
      • cerebellar hypoplasia
      • Fetal Alcohol Syndrome
      • Krabbe's Disease
      • Myotonic Muscular Dystrophy
      • Nemaline Rod Myopathy
      • Rigid Spine Syndrome

PATHOGENESIS:

1. Genetic Defect

  • genetic defect -> ? neurogenic etiology -> abnormal neurological influence affects normal development of the motor unit -> maturational arrest of fetal muscle at myotubular stage (20-28 weeks gestational age) so that muscle biopsy has the appearance of an embryonic distribution of fibre type differentiation and growth

PATHOLOGY:

1. Muscle Biopsy

  • 1. Type 1 Fibres
    • predominant and uniformly small in size
    • normal mature staining pattern
  • 2. Type 2 Fibres
    • normal or large in size
    • normal mature staining pattern
  • CLINICAL FEATURES:

    1. Neurological Manifestations

    • generalized mild infantile hypotonia and diffuse muscle weakness
      • proximal > distal muscle groups
        • tends to be benign and nonprogressive
        • gross motor developmental delay
        • thin, wasted body habitus with decreased muscle bulk
      • respiratory distress and dysphagia rare
      • facial muscles -> myopathic facies
      • ocular muscles -> ophthalmoplegia
      • sternocleidomastoid muscle -> poor head control
    • deep tendon reflexes absent or depressed
    • normal intelligence

    2. Musculoskeletal Manifestations

    1. Dysmorphic Features

    • dolichocephalic head, high-arched palate

    2. Skeletal

    • foot deformities
    • hip subluxation
    • kyphoscoliosis
    • mild congenital contractures of multiple joints
    • short stature

    INVESTIGATIONS:

    1. Serum

    • normal or slightly elevated CPK

    2. EMG

    • normal

    3. Nerve Conduction Studies

    • normal; ? denervation pattern

    MANAGEMENT:

    1. Supportive

    • multidisciplinary approach
      • Paediatrics - promote ambulation and physiotherapy, moniter deformities
      • Surgery - moniter and correct deformities
      • Genetics - genetic counselling, ? prenatal diagnosis

     

     

     

    Pediatric Database - CONGENITAL MUSCLE FIBRE-TYPE DISPROPORTION

    Pediatric Organization - Pedbase [at] Gmail.com