Achondroplasia

Achondroplasia is a congenital genetic disorder and the most common the skeletal dysplasia. It has numerous distinctive radiographic features.

Epidemiology

Achondroplasia is the most common cause of short limb dwarfism.  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.

Patients are of normal intelligence with normal motor function. However, they may have specific neurologic deficits.

Pathology

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 enchondral ossification are affected. Bones that form by membranous ossification are not affected, thus allowing the skull vault to develop normally.

Associations

• SADDAN syndrome: severe achondroplasia with developmental delay and acanthosis nigricans

Radiographic features

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 3^rd trimester¹³.

Antenatal ultrasound 

Antenatally detectable sonographic features include

• short femur length measurement : often well below the 5^th 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
• seperation of 1^st and 2^nd, 3^rd and 4^th fingers
• protruding forehead: frontal bossing

Plain film / MRI 

Features on plain film and MRI are similar and discussed together here.

Cranial

• 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.

Spinal 

• 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

Chest

• 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

Limbs

• metaphyseal flaring^8 8: can give a trumpet bone type appearance
• the femora and humeri are particularly shortened (rhizomelic shortening)
• long fibula
• 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).
• V shaped growth plates
• trident hand

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.  

Differential diagnosis

The differential diagnosis is that of other (less common) skeletal dysplasias including ⁶:

• achondrogenesis
• camptomelic dysplasia
• thanatophoric dysplasia
• Ellis-van Creveld syndrome- chondroectodermal dysplasia

See also

• causes of limb bowing: general differential for (lower) limb bowing 
• rhizomelic dwarfism: general differential for rhizomelic limb shortening