HYPOPHOSPHATASIA

• Rare short-limbed dwarfism that occurs in approximately 1 in 100,000 births.
• Autosomal recessive pattern of inheritance.
• Low serum alkaline phosphatase (amniotic fluid levels variable and can't be used to diagnose).

High urine phosphoethanolamine.

• Alkaline phosphatase normally acts on pyrophosphatase and other phosphate esters leading to the accumulation of inorganic phosphates, which are critical for the formation of bone crystals.
• The homozygote has a severe deficiency of tissue and serum alkaline phosphatase and is uniformly fatal
• The heterozygote is generally normal but may be mildly affected.

ULTRASOUND

• Poor ossification of all fetal bones especially skull.
• May be diagnosed as early as 14 weeks GA.
• Increased echogenicity of the falx (increased beam transmission due to the poorly mineralized cranium).
• Limb shortening.
• Lack of ossification of groups of vertebral bodies.
• Lack of ossification of neural arches of the spine.
• Lack of ossification of the hands.
• Occasionally irregularly poorly ossified metaphyseal areas in long bones.
• Polyhydramnios.

TYPES

• Group I = Neonatal congenital form.
• Group II = Juvenile severe form (onset within weeks to months).
• Group III = Adult mild form (recognized in childhood, adolescence or adulthood.
• Group IV = Latent form (borderline levels of alkaline phosphatase).

DIFFERENTIAL DIAGNOSIS

• Osteogenesis Imperfecta Type ll
• Achondrogenesis - no ossification of vertebral bodies but better ossification of neural arches.

REFERENCES

1. DeLange M & Rouse GA. Prenatal diagnosis of hypophosphatasia. J Ultrasound Med 1990: 9; 115-117.
2. Tongsong T, Sirichotiyakul S, Siriangkul S. Prenatal diagnosis of congenital hypophosphatasia. J Clin Ultrasound 1995;23:52-55.