It occurs due to sporadic mutations in the majority of cases but can be inherited as an autosomal dominant condition. Homozygous achondroplasia is lethal.
There is a prevalence of approximately 1 in 25,000-50,000 births with males affected more frequently than females.
Achondroplasia is the most common cause of short limb dwarfism. Patients are of normal intelligence with normal motor function. However, they may have specific neurologic deficits.
The disease results from a mutation in the fibroblast growth factor gene 3 (FGFR3) located on chromosome 4p16.3 which causes abnormal cartilage formation. All bones that form by endochondral ossification are affected. Bones that form by membranous ossification are not affected, thus allowing the skull vault to develop normally.
- SADDAN syndrome: severe achondroplasia with developmental delay and acanthosis nigricans
Almost all the bones of the skeleton are affected, and hence all parts of the body have bony changes with secondary soft tissue changes. Antenatally it is difficult to diagnose achondroplastic features until the 3rd trimester 13.
Antenatally detectable sonographic features include:
- short femur length measurement: often well below the 5th centile
- the femur length (FL) to biparietal diameter (BPD) is taken as a useful measurement
- trident hand 11: 2,3 and 4 fingers appearing separated and similar in length
- separation of 1st, 2nd, 3rd and 4th fingers
- protruding forehead: frontal bossing
Features on radiographs, CT, and MRI are similar and discussed together here.
- relatively large cranial vault with small skull base
- prominent forehead with depressed nasal bridge
- narrowed foramen magnum
- cervico medullary kink
- relative elevation of the brainstem resulting in a large suprasellar cistern and vertically-oriented straight sinus
- communicating hydrocephalus (due to venous obstruction at sigmoid sinus)
Also see achondroplastic base of skull abnormalities for further discussion.
- posterior vertebral scalloping
- progressive decrease in interpedicular distance in lumbar spine
- gibbus: thoracolumbar kyphosis with bullet-shaped/hypoplastic vertebra (not to be confused with Hurler syndrome)
- short pedicle canal stenosis
- laminar thickening
- widening of intervertebral discs 8
- increased angle between sacrum and lumbar spine
- anterior flaring of ribs
- anteroposterior narrowing of ribs
Pelvis and hips
- horizontal acetabular roof (decreased acetabular angle)
- small squared (tombstone) iliac wings
- small trident pelvis
- champagne glass type pelvic inlet
- short sacroiliac notches
- metaphyseal flaring 8: can give a trumpet bone type appearance
- the femora and humeri are particularly shortened (rhizomelic shortening)
- long fibula: fibular head is at the same level of tibial plateau (case 3)
- they may also appear thickened but in fact normal in absolute terms compared to the normal adult diameter (thickening is perceived due to reduced length)
- trident hand
- metacarpal and metatarsal bones and sometimes proximal phalanges are short and are of similar length
Treatment and prognosis
There is often a danger of cervical cord compression from due to narrowing of the foramen magnum.
Treatment varies and is usually orthopaedic, particularly to correct kyphoscolioses as well as neurosurgical to decompress the foramen magnum or shunt hydrocephalus 6-7.
Overall prognosis is good, with near normal life expectancy in heterozygous individuals. When homozygous, the condition is usually fatal due to respiratory compromise 7.
The differential diagnosis is that of other (less common) skeletal dysplasias including 6:
- camptomelic dysplasia
- thanatophoric dysplasia
- Ellis-van Creveld syndrome - chondroectodermal dysplasia
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