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Fibrodysplasia (Myositis) Ossificans Progressiva

  1. It is a rare autosomal dominant (AD) disorder of connective tissue differentiation.
  2. Main pathogenic mechanism is defective regulation of the induction of endochondral ossification
  3. Intense perivascular lymphocytic infiltration into normal skeletal muscles is the earliest feature.
  4. The proliferating loose myxoid fibrous tissue infiltrates and replaces normally formed fibrous connective tissue and striated muscle.
  5. Numerous small blood vessels, cells immunoreactive for S - 100 protein and cartilagenous foci are scattered among proliferating lesion.
  6. Endochondral ossification is a feature of maturing lesion. Only the absence of normal anatomical orientation differentiates this hetrotropic bone from normal.
  7. Bone morphogenetic protein - 4 (BMP - 4) is over expressed and Basic fibroblast growth factor which is an extremely potent stimulator of angiogenesis (in vivo) is elevated in urine during acute flare up stage.
Clinical features
  1. Begins in childhood usually before age of 6 years and becomes evident with in first 10 years of life.
  2. Primary congenital skeletal abnormality is deformity of great toe. The toes may be short, in valgus, monophalangic or have abnormal shaped proximal phalanx.
  3. Begins as painful erythematous subfascial nodule mostly located on posterior aspect of neck & back which gradually calcify & eventually ossify (hetrotropic ossification).
  4. The hetrotropic ossification progress in a axial to appendicular, cranial to caudal and proximal to distal direction. So the most commonly involved site is neck followed by spine & shoulder girdle. And same is the order for limitation of motion.
  5. Trauma inform of tripping & falling, immunization, injection or biopsy stimulates new bouts of ossification and can be catastrophic.
  6. The ossification is irreversible, unlike other forms of hetrotropic ossification. This condition is quite different from myositis ossificans traumatica.
  7. Diaphragm, extraocular muscles & smooth muscles are characteristically spared.
  8. Limitations of jaw mobility, extremely limited chest expansion, reduced lung volumes (~ 44% of normal) but relatively preserved flow rates and scoliosis / hypokyphosis are other feature.
  9. Life expectancy is decreased and premature death usually result from respiratory failure restrictive lung disease and their complications or from inanimation caused by jaw ankylosis.
  10. In myositis ossificans progressiva ,the life expectancy is decreased and the commonest causes of death is lung disease.

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