Achondrogenesis

Achondrogenesis refers to a group of rare disorders which are extreme skeletal dysplasias.

Epidemiology

The estimated incidence is at ~ 1 : 40,000. There is no recognised gender predilection.

Pathology

It is classified under a osteochondrodysplasias meaning deficiency of both bone and cartilage development.

Sub types

There are several recognised sub types

• type I : 
  • type Ia : Houston-Harris sub type
  • type Ib : Parenti-Fraccaro sub type
• type II : Langer-Saldino achondrogenesis

Genetics

There are various patterns of inheritance ranging from autosomal dominant to autosomal recessive forms. Type II achondrogenesis is thought to be sporadic.

Radiographic features

Antenatal ultrasound

Sonographic diagnosis may be possible after ~ 13 weeks where nuchal oedema may be evident as an early (although non specific) sign. 

The fetal bony structures are often unable to be identified. There may also be extreme micromelia ⁵. Calvarial ossification can be preserved with the type II sub type which can in turn give a floating head appearence.

Additional sonographic findings include

• micrognathia
• narrow thorax - narrow fetal thorax
• rib fractures : can be present in type Ia
• disproportionately small body compared with head

Other ancilliary sonographic features that may be present include

• polyhydramnios
• development of hydrops fetalis

Prognosis

The prognosis is generally poor with most infants being stillborn or dying soon after birth. 

Differential diagnosis

General considerations for mild cases include

• hypochondrogenesis

For an absent calvarial bone(s) (type Ia / Ib) on antenatal ultrasound consider

• acrania

See also

• achondroplasia