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Achondroplasia

(see also: Pseudoachondroplasia)

Discussion

  • autosomal dominant disorder;
  • arises due to a point mutation for the gene encoding a fibroblast growth factor (Fibroblast growth factor receptor 3)
  • over 80% of all persons with this disorder are born to parents who are not achondroplastic, & such parents rarely have second achondroplastic child;
    • it is presumed in these instances that disorder is consequence of mutation and may be related to a mutation in a fibroblast growth factor gene;
  • growth of endochondral bones is abnormal in achondroplasia and causes disproportionate and rhizomelic shortening of limbs;
    • since intramembranous ossification is unaffected head is relatively large and the forehead is prominent;
    • long bones are wide with flaring of the metaphyses;
  • characteristics in infancy:
    • rhizomelic short limbs and normal trunk height,
    • prominent frontal bossing & depressed nasal bridge;
    • midface hypoplasia, and relative mandibular prominence;
    • maxillary hypoplasia may lead to dental crowding and malocclusion;
    • recurrent otitis media will lead to higher incidence of conductive hearing loss in adults;

Development & Motor Milestones

  • patients often have a delay in reaching milestones;
  • many children do not begin to walk independently until 2-3 years;
  • hypotonic ligamentous laxity, relatively large head, & disproportionate trunk and limbs may contribute to problem;
  • obesity is common;
  • cognition:
    • intelligence is normal;
    • early speech may be difficult because of tongue thrust, but this usually resolves by school age;

PreOperative Assessment

  • careful preoperative assessment and postoperative monitoring of pulmonary function are necessary;
  • respiratory:
    • significant respiratory problems develop in 10% of affected individuals because of an abnormal thoracic cage configuration, midfacial hypoplasia, upper airway obstruction, or spinal cord compression at the foramen magnum;
  • hydrocephalus:
    • 3% of affected individuals have hydrocephalus; detection is difficult because head size may run above 97th percentile;

Spine in Achondroplasty »

Upper Extremity

  • elbow:
    • elbow extension is often diminished & posterolateral radial head dislocation may occur;
  • hands:
    • hands are short and broad;
    • trident hands (increased space between third and fourth fingers);
    • fingers are three-pronged with a space between the third and fourth digits, producing characteristic trident hand, a feature that is lost late in childhood;

Pelvis

  • pelvis is broad and short, w/ relatively wide iliac wings & horizontal acetabular margins.

Lower Extremity

  • flexion contractures of the hip, and genu varum;
  • knee:
    • genu varum and varus ankle deformities are found;
    • unlike dysplasias that involve the epiphysis, DJD is rare, and prophylactic surgical correction of deformities is probably not indicated;
    • bracing is not beneficial because orthosis often opens the joint medially, secondary to ligamentous laxity;
    • fibular epiphysiodesis and/or tibial osteotomies may be required for severe genum varum;

Misc