Hypophosphatasia

Hypophosphatasia is a disease of low or absent serum alkaline phosphatase activity that results in incomplete ossification of cartilage and metaphyses and accumulation of unmineralized osteoid.

At least three types have been described:

• Neonatal: Diagnosed during the first week of life. Uniformly fatal.
• Infantile: Manifests by 6 months. Variable survival.
• Adult: Autosomal recessive.

Radiographically, there is decreased ossification of the skull and vertebrae. The decreased ossification of the skull may appear as isolated segments of thin bone or as a generalized boneless skull (caput membranaceum). Wormian bones may also be seen.

There are also poorly and irregularly ossified short tubular bones that are prone to bowing and fracture and may mimic osteogenesis imperfecta or other causes of florid rickets. Frayed and cupped metaphyses can also be seen.

The accumulation of unmineralized osteoid at the metaphyses is fairly specific and can help differentiate hypophosphatasia from osteogenesis imperfecta and other forms rickets.

References

• Glass RB, Fernbach SK, Norton KI, Choi PS, Naidich TP. The infant skull: a vault of information. Radiographics. 2004 Mar-Apr;24(2):507-22.
• States LJ. Imaging of metabolic bone disease and marrow disorders in children. Radiol Clin North Am. 2001 Jul;39(4):749-72. Review.