Achondroplasia

  • Etiology:
    — Autosomal dominant but 80% are spontaneous mutations
    — Defect of enchondral bone formation (bone lengthening)
  • Imaging Skull:
    — Small face bones due to dysplastic skull bone with normal cranium gives the appearance of enlarged cranium compared with size of face
    — Frontal bossing
    — Brachiocephaly
    — Defective growth of skull base results in pinched appearance with small (keyhole) foramen magnum
  • Imaging Spine:
    — Narrowed spinal canal
    — Progressive narrowing of interpediculate distances in lower lumbar vertebrae
    — Posterior vertebral body scalloping
    — Decreased vertebral body height due to bullet shaped
    — Short pedicles leading to spinal stenosis
    — Infant thoracic-lumbar kyphosis reverses to lordosis with weight
  • Imaging Pelvis:
    — Hypoplastic and square iliac bones
    — Small sacroiliac notches leads to champagne glass pelvic inlet
    — Flat acetabula
    — Trident pelvis is small bony spurs at the medial and lateral acetabular margins with a more subtle central spur
    — Proximal femora rounded with radiolucent appearance is an ice cream scoop
  • Imaging Tubular Bones:
    — Shortened especially proximally
    — Metaphyseal flaring
    — Fibular overgrowth causes genu varum
    — Inverted V shaped growth plate of distal femur
    — Shortened ribs
    — Decreased AP diameter of chest
    — Short metacarpals and phalanges leads to trident hand
  • DDX:
  • Complications:
    — Spinal stenosis
    — Constricted jugular foramina causing intracranial venous hypertension and communicating hydrocephalus
  • Treatment:
  • Clinical:
    — Rhizomelic skeletal dysplasia
    — Most common nonlethal skeletal dysplasia
    — Spectrum: Thanatophoric dysplasia (fatal), homozygous achondroplasia (fatal), heterozygous achondroplasia, hypochondroplasia
    — Shortened and bowed extremities
    — Enlarged cranium with frontal bossing and sunken nasal root
    — Normal trunk
    — Kyphosis at thoracic-lumbar junction in infancy
    — Prominent buttocks and abdomen due to increased pelvic tilt
    — Genu varum
    — Trident hand

Radiology Cases of Achondroplasia

Radiology Cases of Achondroplasia Causing Macrocrania

Radiograph of skull in achondroplasia
Lateral radiograph of the skull shows small facial bones and a relatively large cranium.

Radiology Cases of Achondroplasia Causing Trident Hand

Radiograph of trident hand in achondroplasia
AP radiograph of the hand shows diffuse shortening of the phalanges and metacarpals, giving the hand the appearance of a trident. There is also metaphyseal flaring of the distal radius and ulna.

Radiology Cases of Achondroplasia Causing Rhizomelic Shortening

Radiograph of rhizomelic shortening in achondroplasia
AP radiograph of the upper extremity shows proximal (rhizomelic) shortening of the humerus along with flaring of the distal metaphysis of the radius and ulna.

Radiology Cases of Achondroplasia Causing Champagne Pelvis

Radiograph of champagne glass pelvis of achrondroplasia
AP radiograph of the pelvis shows squaring of the iliac bones, flattening of the acetabulae, and small sacroiliac notches bilaterally, leading to a champagne glass appearance to the pelvic inlet.
Radiograph of rhizomelic shortening of femurs in achondroplasia
AP radiograph of the lower extremities shows bilateral rhizomelic shortening of the femora and flaring of the distal femora and proximal and distal tibiae and fibulae.

Radiology Cases of Achondroplasia Causing Genu Varum

Radiograph of achondroplasia with shortened femurs
AP radiograph of the lower extremities shows a genu varum deformity and bilateral shortening of the femora and flaring of the proximal tibiae and fibulae.

Radiology Cases of Achondroplasia Causing Spinal Stenosis

Spine radiograph of achondroplasia
Lateral radiograph of the spine shows kyphosis of the thoracolumbar spine and shortened pedicles, posterior scalloping of vertebral bodies and anterior beaking of vertebral bodies.
Radiograph of interpedicular narrowing and spinal stenosis in achondroplasia
AP radiograph of the lumbar spine (left) shows progressive narrowing of the distance between the pedicles descending through the lumbar spine (interpediculate narrowing). The lateral radiograph of the lumbar spine (right) shows shortened pedicles leading to narrowing of the anterior-posterior diameter of the spinal canal (spinal stenosis) and that the vertebral bodies have some anterior wedging and posterior scalloping.
MRI of spinal stenosis in achondroplasia
Sagittal T2 MRI of the spine shows a very narrow spinal canal with only a small amount of cerebrospinal fluid present anteriorly and posteriorly to the spinal cord, especially at the skull base. There is mild scalloping of the posterior aspect of the vertebral bodies.