Heterozygous Achondroplasia  

• Prototype of rhizomelic dwarfism!
  = autosomal dominant / sporadic (80%) disease with quantitatively defective endochondral bone formation; related to advanced paternal age; epiphyseal maturation + ossification unaffected
• Prevalence: 1÷26,000–66,000 births; M <F ◊ Most common of lethal bone dysplasias!

• normal intelligence + motor function
• neurologic defects
• classically circus dwarfs

• Skull
  • flat nasal bridge ( = hypoplastic base of skull)
  • macrocephaly + brachycephaly with enlarged bulging forehead (= nonprogressive hydrocephalus)
  • relative prognathism
  • large calvarium with frontal bossing
  • depression of nasion
  • broad mandible
  • constricted basicranium + small foramen magnum:
    • communicating hydrocephalus caused by obstruction of basal cisterns + aqueduct

Cx: apnea + sudden death ← compression of spinal cord and lower brain stem

• Chest
  • anteroposterior narrowing of chest
  • short anteriorly flared concave ribs
  • squaring of inferior scapular margin
• Spine
  • hypoplastic bullet- / wedge-shaped vertebra:
    • rounded anterior beaking of vertebra in upper lumbar spine (DDx: Hurler disease)
    • decreased vertebral height
  • scalloped posteriorly concave vertebral margin
  • scoliosis:
    • thoracolumbar angular kyphosis (gibbus)
    • exaggerated sacral lordosis
  • stenosis of lumbar spine:
    • narrowing of interpedicular space ← laminar thickening
    • ventrodorsal narrowing of spine ← short pedicles
    • bulging / herniation of intervertebral disks
  • wide intervertebral foramina
• Pelvis
  • protuberant abdomen; prominent buttocks
  • rolling gait from backward tilt of pelvis and hip joints
  • square flattened iliac bones = tombstone configuration
  • “champagne glass”-shaped pelvic inlet
  • lack of flaring of iliac wings
  • horizontal acetabula (= flat acetabular angle)
  • small sacrosciatic notch
• Extremities
  • trident hand = separation of 2^nd + 3^rd digit and inability to approximate 3^rd + 4^th finger; short stubby limbs + fingers
  • limited range of motion of elbow
  • predominantly rhizomelic micromelia of long bones (ie, femur, humerus):
    • “trumpet” appearance of long bones = shortening with disproportionate metaphyseal flaring (= actually normal width of metaphysis)
    • short femoral necks
    • limb bowing
  • “ball-in-socket” epiphysis = broad V-shaped distal femoral metaphysis in which epiphysis is incorporated
  • high position of fibular head (= disproportionately long fibula)
  • short ulna with thick proximal + slender distal end
  • brachydactyly (short tubular bones of hand + feet), especially short proximal + middle phalanges

OB-US (diagnosable >21^st–27^th week GA):

• shortening of proximal long bones: femur length <99^th percentile between 21 and 27 weeks MA
• increased BPD, HC, HC÷AC ratio
• decreased FL÷BPD ratio
• normal mineralization, no fractures
• normal thorax + normal cardiothoracic ratio
• three-pronged (= trident) hand = 2^nd + 3^rd + 4^th finger of similarly short length without completely approximating each other (= PATHOGNOMONIC)

Cx:

1. Hydrocephalus + syringomyelia (small foramen magnum)
2. Recurrent ear infection (poorly developed facial bones)
3. Neurologic complications (compression of spinal cord, lower brainstem, cauda equina, nerve roots): → apnea and sudden death
4. Crowded dentition + malocclusion

Prognosis: long life

DDx: various mucopolysaccharidoses