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Achondroplasia


KEY FACTS

Terminology

  • Autosomal-dominant dwarfism affecting spine and extremities

  • a.k.a. achondroplastic dwarfism

Imaging

  • Shortened vertebral pedicles

    • Decreasing interpediculate distance toward lower levels of lumbar spine

  • Mildly flattened &/or anteriorly wedged vertebral bodies

  • Thoracolumbar kyphosis

  • Lumbar hyperlordosis

  • Small foramen magnum

  • Other

    • Growth disturbance more obvious in proximal limbs (rhizomelic dwarfism)

    • “Champagne glass” pelvis: Pelvic inlet is flat and broad

    • Squared iliac wings

    • Short ribs

    • Enlarged skull with constricted skull base

Top Differential Diagnoses

  • Pseudoachondroplasia

  • Hypochondroplasia

  • Diastrophic dysplasia

  • Spondyloepiphyseal dysplasia

  • Thanatophoric dysplasia

  • Osteogenesis imperfecta

Pathology

  • Defect in FGFR3 , responsible gene mapped to 4p16.3

  • Usually spontaneous mutation (80%)

  • Results in defective enchondral bone formation

  • Autosomal dominant transmission

Clinical Issues

  • Most common nonlethal skeletal dysplasia

  • High morbidity from spinal stenosis

  • Surgical correction of progressive/unresolving kyphosis

  • Surgical decompression of foramen magnum in severe cases

Graphic shows progressive narrowing of interpediculate distance in caudad direction. Axial insert image shows spinal stenosis related to short pedicles and decreased interpediculate distance.

Sagittal T2WI MR shows constricted skull base relative to the visualized cranial vault. Stenosis of the foramen magnum
compresses the cervicomedullary junction, with mildly increased signal in the upper cervical cord due to myelopathic changes.

AP radiograph of the lumbar spine shows narrowing of the interpediculate distance and progressive interpedicular narrowing between L1
and L4
, causing narrowing of the lumbar canal in the transverse dimension.

Sagittal T2WI MR shows a diffusely narrowed AP dimension of the lumbar spinal canal, reflecting shortened vertebral pedicles. Congenital canal stenosis is further narrowed by a small disc protrusion
at L2-L3.

TERMINOLOGY

Synonyms

  • Achondroplastic dwarfism

Definitions

  • Autosomal dominant dwarfism affecting spine and extremities

  • OMIM #100800

IMAGING

General Features

  • Best diagnostic clue

    • Flattened vertebral bodies with short pedicles

    • Interpediculate distance of lumbar spine decreases in caudal direction (reversal of normal relationship)

  • Location

    • Characteristic skeletal changes in spine, skull, pelvis, extremities

  • Morphology

    • Severe dwarfism involves trunk and extremities

      • –

        Growth disturbance more obvious in proximal limbs (rhizomelic dwarfism)

    • Vertebral bodies mildly flattened

    • Shortened pedicles

    • Lumbar hyperlordosis

    • Thoracolumbar kyphosis

Radiographic Findings

  • Radiography

    • Thickened, short lumbar pedicles

      • –

        Decreasing interpediculate distance toward lower levels of lumbar spine

    • Dysmorphic vertebral bodies

      • –

        Bullet-shaped (anteriorly blunted) in childhood

      • –

        Mildly flattened &/or mildly anteriorly wedged

    • Thoracolumbar kyphosis

      • –

        Initially flexible deformity, located T10 through L4

      • –

        Resolves as child begins to walk independently

    • Lumbar hyperlordosis

    • Mild scoliosis may be present

    • C1-C2 instability rare

    • Extraspinal findings

      • –

        “Champagne glass” pelvis: Pelvic inlet is flat and broad

      • –

        Squared iliac wings

      • –

        Shortened long bones, most prominent in proximal limbs (“rhizomelic”)

      • –

        “Trident hand”: 2nd, 3rd, and 4th digits equal in length

      • –

        Enlarged skull with constricted skull base

    • Bone mineral density normal

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