Hypophosphatasia
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A neonate presents with severe skeletal undermineralization, respiratory distress, and hypercalcemia. Alkaline phosphatase level is very low. The baby has "punched out" lesions on skull X-ray.
AR (severe) or AD (mild); ALPL (tissue non-specific alkaline phosphatase)
- Defective bone mineralization
- LOW alkaline phosphatase (opposite of most bone disorders)
- Elevated phosphoethanolamine and pyridoxal-5'-phosphate (PLP) in blood/urine
- Spectrum from lethal perinatal to mild adult (dental only)
- Premature loss of deciduous teeth (roots intact)
- Craniosynostosis, rachitic changes
- LOW serum alkaline phosphatase for age is the key biochemical clue
- Elevated substrates: pyridoxal-5'-phosphate (PLP) in blood, phosphoethanolamine (PEA) in urine
- Radiographs: undermineralization, rachitic-appearing metaphyses, "punched out" skull lucencies in severe forms
- ALPL sequencing confirms and clarifies inheritance (AR in severe, AD in milder forms)
- Asfotase alfa (enzyme replacement therapy)
This comparison table differentiates X-linked hypophosphatemia (PHEX) from hypophosphatasia (ALPL) across genetics, mechanism, lab findings, clinical features, and treatment.
