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Hypophosphatasia is a genetic disorder that is characterised by deficient activity of the enzyme alkaline phosphatase. Clinically, it can present as a lethal infantile form, an intermediate childhood form with rachitic changes and premature deciduous tooth loss or a mild adult form with a tendency for fractures and bone pain. Both autosomal-dominant and recessive inheritance has been reported for the mild forms, but only autosomal-recessive inheritance has been implicated in the lethal infantile form. Our case presented as a severe skeletal dysplasia antenatally and was carried to term. The baby succumbed shortly after delivery. Initially, the radiographic findings were thought to be consistent with the severe lethal form of osteogenesis imperfecta (OI) type IIA. Investigations performed at birth including genetic testing, conclusively determined that the correct diagnosis was the perinatal lethal form of hypophosphatasia. Differentiating these two entities can be difficult given the variability of findings and the wide spectrum of severity (Tables I and II). Cases of perinatal lethal hypophosphatasia commonly have markedly reduced ossification and may have complete absence of ossification of some bones. The type of ossification defect in OI type IIA is slightly different in that there is usually severe diffuse demineralisation of the skull (Fig. 1) instead of the demineralisation with ‘island-like’ centres of ossification in the frontal, parietal and occipital bones which is seen in hypophosphatasia (Fig. 2). Also, in OI type IIA there is overall diffuse osteopenia (demineralised bones) and usually not the near complete lack of mineralisation in individual bones with adjacent more densely mineralised bones. The latter is a feature of hypophosphatasia (Figs 3 and 4). One specific prenatal sonographic area to help differentiate between the two is the infant’s hands. The bones of the hands are sonolucent in patients with hypophosphatasia but are echogenic in patients with osteogenesis imperfecta.

Radiographic and prenatal ultrasound features of perinatal lethal hypophosphatasia - differentiation from osteogenesis imperfecta type II