Last revised by Joshua Yap on 8 May 2023

Achondrogenesis refers to a group of rare and extreme skeletal dysplasias.

The estimated incidence is 1:40,000, with no recognized gender predilection.

It is classified as an osteochondrodysplasia, meaning deficiency of both bone and cartilage development. The condition has several recognized subtypes:

Mutations in the SLC26A2 and COL2A1 genes cause achondrogenesis types 1B and 2, respectively. The genetic cause of achondrogenesis type 1A is still unknown. The patterns of inheritance are autosomal recessive in types 1A and 1B and autosomal dominant with de-novo mutations in type 2.

Sonographic diagnosis may be possible after 13 weeks of gestation, where nuchal edema may be evident as an early (though non-specific) sign.

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

Additional sonographic findings include:

Other ancillary sonographic features that may be present include:

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

The term "achondrogenesis" was first used in 1952 by Marco Fraccaro (1926-2008 12), a Professor of cytogenetics, who noted that a similar case had been published by G C Parenti in 1936 2. The 1B subtype is therefore also known as Parenti-Fraccaro syndrome.

General considerations for mild cases include hypochondrogenesis. If there is calvarial bone absence (type 1A/1B) on antenatal ultrasound, consider acrania.

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