Genetic anomalies: Type IA: unknown. Type IB: mutation in the gene for diastrophic dysplasia sulfate transporter gene (DTDST) on the long arm (locus 32-33) of chromosome 5,,. The diastrophic dysplasia sulfate transporter gene is recessively transmitted and is an allele of the diastrophic dysplasia gene. This is important since the Type II (see below) is an autosomal dominant mutation (and thus involves a new mutation for each case since the disease is lethal) and therefore has a much lower likelihood of recurrence than the 25% risk of the type IB. The diagnosis can be made by CVS in at-risk couples. Type II achondrogenesis is the Langer-Saldino type and is caused by new dominant mutation in the type II collagen gene (COL2A1 gene) on chromosome 12. It is therefore more related to hypochondroplasia, spondyloepiphyseal dysplasia and the Kniest-Stickler syndrome. These may be allelic variants with hypochondrogenesis related to achondrogenesis as hypochondroplasia is related to achondroplasia.
Differential diagnosis: Osteogenesis Imperfecta (type II and occasionally IIIc) and hypophosphatasia also present with demineralization but the limb shortening is not usually as severe.
Management: Termination of pregnancy can be offered before viability. Standard prenatal care is not altered when continuation the pregnancy is opted for. Confirmation of diagnosis after birth is important for genetic counseling.
Figures (all figures reprinted with permission from The Fetus 2:3 7564-11, 1992)
Fig 1: Almost absent mineralization of the spine (not the spinal cord)
Fig 2: The appearance of transparent bones in which both cortical can be seen.
Fig 3: Micromelia with the arms not joining in front of the chest.
Fig 4: The appearance of the fetus at 19 weeks
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