Heterozygous Achondroplasia

  • Prototypical rhizomelic dwarfism
  • Autosomal dominant / Sporadic (80%)
    • Defective endochondrial bone formation
    • Epipyseal maturation & ossification unaffected
    • Related to advanced paternal age
  • Classical circus dwarfs
  • Normal intelligence, motor function and long life

Imaging Features


  • Skull base hypoplasia [1, 2, 3, 4] and constricted foramen magnum [4]
    • causing non-progressive communicating hydrocephalus (obstruction of basal cisterns and aqueduct) leading to
  • Midfacial hypoplasia [1, 3]
    • flat nasal bridge [1, 3] and
    • depressed nasion [3]


Apnea & sudden death (compression at craniovcervical junction)


Lateral skull radiograph showing

  • frontal bossing,
  • flat nasal bridge,
  • midfacial hypoplasia and
  • constricted basicaranium.

Sagittal T1W MRI image showing

  • frontal bossing,
  • large calvarium,
  • hypoplastic base of skull.

Lateral skull radiographs showing

  • pronounced midfacial hypoplasia,
  • frontal bossing,
  • flat nasal bridge and
  • constricted basicaranium.
  • with relative prognathism, in the later image

Sagittal T2W MRI image of the cervical spine showing

  • significant narrowing of the foramen magnum

    causing compression of the craniovcervical junction of the spinal cord

  • basicranial hypoplasia


  • AP narrowing
  • Anteriorly flared short concave ribs
  • Square inferior scapular margin


  • Hypoplastic bullet/wedge shaped vertebrae

    • Round anterior beaking upper lumbar region, DDx: Hurler disease
    • Decreased body height
    • Scalloped posterior concave vertebral margin
  • Lumbar spinal stenosis

    • progessive craniocaudal narrowing interpedicular distance

      due to laminar thickening

    • ventrodorsal narrowing

      due to short stubby pedicles

    • bulging / herniation of intervertebral discs

  • Scoliosis

    • thoracolumbar angular kyphosis (gibbus)
    • exaggerated sacral lordosis


  • Tombstone configuration, flat sqaure shaped, iliac bones
  • Champagne glass-shaped pelvic inlet
  • Horizontal actebulla = flat acetabular angle
  • Small scaroiliac notch


  • Predominatly rhizomelic micomelia (femur + humerus) [1, 2, 3]

    • trumpet appearance of the long bones

      shortening with disproportionate metaphyseal flaring but preserved metaphyseal width

    • short femoral neck

    • limb bowing [2, 7]

  • ball-in-socket epiphysis

    inverted-V shaped distal femoral epiphysis with incorporated epiphysis

  • trident hand [4, 5, 6]

    • separation of 2nd & 3rd digits
    • inability to approximate 3rd and 4th digits
  • brachydactyly

  • High fibular head = disproportionately long fibula

  • Short ulna with thick proximal and slender distal end

  • Limited elbow movement [7]



  • Increased

    • BPD
    • HC
    • HC:AC ratio
  • Decreased

    • FL:BPD ratio
  • Shortening of proximal long bones

    FL < 99th percentile at 21-27 weeks MA

  • trident hand = three pronged hand

    2nd 3rd and 4th fingers of similar short lengths without completely approximating each other.


  • Hydrocephalus

Homozygous Achondroplasia

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is a clinical and radiological term that refers to a generalised increase in size of the cranial vault.
head shapes
refers to the condition when the head is disproportionately wide compared to its height. The bi-parietal diameter to fronto-occipital diameter approaches the 95th percentile.
short tubular bones of hand & feet.

a brass musical instrument with a flared bell and a bright, penetrating tone.


is a three-pronged spear.