High-yield overview
Low Alkaline Phosphatase and Impaired Mineralization
ALPL / TNSALPLoss-of-function -> enzyme deficiency
LOW ALPThe paradoxical diagnostic clue
No bisphosphonatesContraindicated (PPi analogues)
Asfotase alfaEnzyme replacement therapy
Clinical spectrum (earlier onset = more severe)
Perinatal
PatternSevere hypomineralization, often lethal (respiratory failure); B6-dependent seizures.
Treatment
Infantile
PatternFailure to thrive, rickets, HYPERCALCAEMIA/hypercalciuria, craniosynostosis, respiratory compromise, seizures.
Treatment
Childhood
PatternRickets, bowing, short stature, delayed walking/waddling gait, and PREMATURE LOSS OF PRIMARY (deciduous) TEETH with the root intact.
Treatment
Adult
PatternOsteomalacia, recurrent poorly-healing stress/pseudofractures (metatarsals, subtrochanteric femur), chondrocalcinosis/pseudogout, early tooth loss, musculoskeletal pain.
Treatment
Odonto-HPP
PatternDental disease only (premature tooth loss) without skeletal disease.
Treatment
Critical Must-Knows
- Hypophosphatasia (HPP) is an inherited metabolic bone disease caused by LOSS-OF-FUNCTION mutations in the ALPL gene, which encodes TISSUE-NONSPECIFIC ALKALINE PHOSPHATASE (TNSALP); the enzyme deficiency impairs skeletal and dental mineralization.
- The DIAGNOSTIC CLUE is a paradoxically LOW serum ALKALINE PHOSPHATASE - the opposite of most bone diseases (rickets, osteomalacia, Paget's, fracture healing) which RAISE ALP - so a persistently LOW ALP in a patient with fractures or bone disease should prompt the diagnosis.
- Deficient TNSALP causes accumulation of its substrates: INORGANIC PYROPHOSPHATE (PPi - a potent inhibitor of mineralization, so it builds up and blocks bone formation and causes chondrocalcinosis/pseudogout), PYRIDOXAL-5'-PHOSPHATE (PLP/vitamin B6 - hence B6-dependent seizures in severe infantile disease), and PHOSPHOETHANOLAMINE (PEA, raised in urine).
- The clinical SPECTRUM is broad and severity is inversely related to age of onset: perinatal (often lethal) and infantile (failure to thrive, rickets, HYPERCALCAEMIA, craniosynostosis, seizures), childhood (rickets, bowing, short stature, and the characteristic PREMATURE LOSS OF PRIMARY TEETH with intact roots), adult (osteomalacia, recurrent poorly-healing stress/pseudofractures of the metatarsals and subtrochanteric femur, chondrocalcinosis/pseudogout), and odonto-HPP (dental only).
- The cardinal ORTHOPAEDIC PITFALL: BISPHOSPHONATES are CONTRAINDICATED in HPP - they are pyrophosphate analogues that further inhibit mineralization and can precipitate ATYPICAL FEMORAL FRACTURES; a low ALP must be recognised BEFORE prescribing them for a 'fragility' fracture.
- DIAGNOSIS combines a low ALP with raised substrates (serum PLP/B6, urinary PEA), characteristic radiographs (rickets with 'tongue-like' metaphyseal lucencies in children; osteomalacia/pseudofractures in adults) and ALPL genetic testing; the key TREATMENT advance is ASFOTASE ALFA, a bone-targeted recombinant TNSALP enzyme-replacement therapy that transforms outcomes in perinatal/infantile/childhood HPP, with supportive care, B6 for seizures, and avoidance of bisphosphonates and excess vitamin D.
Clinical Pearls
- “HPP = ALPL/TNSALP deficiency; the clue is a LOW serum alkaline phosphatase (opposite of other bone diseases).
- “Substrate build-up: PPi (inhibits mineralization, chondrocalcinosis), PLP/B6 (seizures), urinary PEA. Childhood hallmark = premature loss of PRIMARY teeth.
- “Do NOT give bisphosphonates (PPi analogues - worsen it / atypical femoral fractures). Treat with asfotase alfa (enzyme replacement).
The Trap: A LOW Alkaline Phosphatase
Recognise it
A persistently LOW serum ALP with fractures/bone disease, premature tooth loss, or chondrocalcinosis = think hypophosphatasia (most bone diseases RAISE ALP).
Don't worsen it
Do NOT give bisphosphonates (pyrophosphate analogues) - they further inhibit mineralization and risk atypical femoral fractures. Treat with asfotase alfa.
Biochemistry & Pathophysiology
No Alkaline Phosphatase -> Pyrophosphate Wins
TNSALP normally hydrolyses inorganic pyrophosphate (PPi) at the mineralizing surface; PPi is a potent inhibitor of hydroxyapatite formation, so removing it allows mineralization. In HPP, deficient TNSALP lets PPi accumulate, which blocks mineralization (rickets/osteomalacia) and deposits as calcium pyrophosphate (chondrocalcinosis/pseudogout). The enzyme's other substrates also build up: pyridoxal- 5'-phosphate (PLP, vitamin B6) - its failure to enter the brain causes the B6-dependent seizures of severe infantile disease - and phosphoethanolamine (PEA), raised in the urine. Crucially, because the enzyme itself is deficient, serum alkaline phosphatase is LOW - the diagnostic signature.
Clinical Spectrum & Diagnosis
From Lethal Perinatal Disease to Adult-Only Tooth Loss
Severity is inversely related to age of onset:
- Perinatal: profound hypomineralization, frequently lethal (respiratory failure).
- Infantile: failure to thrive, rickets, hypercalcaemia/hypercalciuria, craniosynostosis, seizures, respiratory problems.
- Childhood: rickets, bowing, short stature, delayed/waddling gait, and the characteristic premature loss of primary teeth with the root intact.
- Adult: osteomalacia, recurrent, poorly-healing stress/pseudofractures (classically metatarsals and the subtrochanteric femur), chondrocalcinosis/pseudogout, early tooth loss and chronic musculoskeletal pain.
- Odonto-HPP: dental disease only. Diagnosis: a LOW serum ALP (age/sex-adjusted) plus raised substrates (serum PLP/B6, urinary PEA), characteristic radiographs ('tongue-like' metaphyseal radiolucencies in children; osteomalacia/ Looser-zone pseudofractures in adults), and ALPL genetic testing.
Management
Enzyme Replacement - and What to Avoid
- Asfotase alfa: a bone-targeted recombinant TNSALP enzyme-replacement therapy that markedly improves survival, skeletal mineralization and growth in perinatal/infantile/childhood HPP - early diagnosis is essential to start it in time.
- Supportive/multidisciplinary: vitamin B6 for the B6-dependent seizures of severe infantile disease; respiratory support; orthopaedic care of fractures and deformity; dental care; physiotherapy; and management of hypercalcaemia in infants.
- AVOID: bisphosphonates (pyrophosphate analogues - worsen the defect and risk atypical femoral fractures) and excessive vitamin D/calcium loading (hypercalcaemia); teriparatide has been used off-label in some adults.
- Fracture care: fractures heal poorly, so optimise fixation, anticipate delayed/non-union, and treat the underlying disease (asfotase alfa where indicated).
A Low ALP Should Stop Your Pen
The single most important practical message is that a patient presenting with fragility or stress fractures and a persistently LOW alkaline phosphatase may have hypophosphatasia, in which bisphosphonates are contraindicated - giving them is a recognised error that can worsen mineralization and precipitate atypical femoral fractures. Check the ALP before treating a 'fragility fracture' as routine osteoporosis, and refer suspected HPP to metabolic-bone specialists for confirmation and enzyme-replacement consideration.
Evidence & Key Studies
Evidence
Hypophosphatasia in childhood: diagnosis to management
Im M, Cho SY • Osteoporosis and Sarcopenia (2025)
Key Findings:
- HPP results from loss-of-function ALPL mutations causing deficient TNSALP activity and is diagnosed from combined clinical, laboratory (low ALP), radiographic and genetic findings.
- Clinical heterogeneity makes diagnosis challenging, so early recognition is essential.
- Asfotase alfa, a bone-targeted recombinant TNSALP enzyme-replacement therapy, has improved prognosis.
Evidence
Phenotypic and genotypic spectrum of Indian patients with hypophosphatasia
Dhananjaya MS, Thrupti S, Reddy HV, et al. • Indian Journal of Endocrinology and Metabolism (2025)
Key Findings:
- Five genetically proven HPP patients ranged from an infant (failure to thrive, hypercalcaemia, low ALP) and an adolescent (genu valgum, delayed dentition, 'tongue-like' metaphyseal translucencies) to adults with vertebral fractures and musculoskeletal pain.
- Compound heterozygous ALPL variants caused severe early disease; heterozygous variants caused milder adult disease; an infant inappropriately received bisphosphonate before HPP was recognised.
- Asfotase alfa produced remarkable growth improvement, underscoring the need to recognise a low ALP and avoid harmful treatment.
Evidence Attribution
According to PubMed, the ALPL/TNSALP basis, the diagnostic combination including a low ALP, and the role of asfotase alfa come from the cited Im & Cho review, and the clinical spectrum (infantile hypercalcaemia, 'tongue-like' metaphyseal translucencies, adult fractures), the harm of inappropriate bisphosphonate use and the response to asfotase alfa from the cited Dhananjaya case series. The substrate biochemistry (PPi, PLP/B6, PEA) and the bisphosphonate contraindication are standard, well-established teaching. (See also our Rickets/ Osteomalacia and Calcium/Metabolic Bone topics.)
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
Viva scenarioStandard
Clinical prompt
“An adult presents with recurrent metatarsal stress fractures and a persistently low alkaline phosphatase. What diagnosis must you consider, and why does it change management?”
Practical approach
A persistently low alkaline phosphatase in a patient with recurrent, poorly-healing stress or pseudofractures should make me strongly consider hypophosphatasia, because almost every other metabolic bone disease - rickets, osteomalacia, Paget's disease and fracture healing - raises rather than lowers the alkaline phosphatase, so a low value is a specific clue. Hypophosphatasia is caused by loss-of-function mutations in the ALPL gene encoding tissue-nonspecific alkaline phosphatase; the enzyme deficiency lets its substrate inorganic pyrophosphate accumulate, which inhibits mineralization and causes osteomalacia, pseudofractures and chondrocalcinosis, while pyridoxal-5-phosphate and phosphoethanolamine also build up. I would confirm the diagnosis with the low ALP plus elevated substrates - serum vitamin B6 as pyridoxal-5-phosphate and urinary phosphoethanolamine - characteristic radiographs and ALPL genetic testing. This changes management critically because bisphosphonates are contraindicated: they are pyrophosphate analogues that further impair mineralization and can precipitate atypical femoral fractures, so I must not treat this as routine osteoporosis. Instead I would refer to a metabolic-bone service, optimise fracture care anticipating poor healing, avoid bisphosphonates and excess vitamin D, and consider enzyme-replacement therapy with asfotase alfa, with teriparatide used off-label in some adults.
Key clinical points
Low ALP + recurrent poorly-healing fractures = consider hypophosphatasia (other bone diseases RAISE ALP)
ALPL/TNSALP deficiency -> PPi accumulation -> impaired mineralization/pseudofractures/chondrocalcinosis
Confirm: low ALP + raised PLP(B6)/urinary PEA + radiographs + ALPL genetics
Bisphosphonates CONTRAINDICATED (atypical femoral fractures); asfotase alfa is the ERT
Common pitfalls
Treating as routine osteoporosis with bisphosphonates
Overlooking the low ALP
Forgetting the substrate tests (PLP/PEA) and genetics
Further questions
“Why are bisphosphonates harmful? - PPi analogues; worsen mineralization, risk atypical femoral fractures”
“What is the enzyme-replacement therapy? - Asfotase alfa (recombinant TNSALP)”
Viva scenarioStandard
Clinical prompt
“How does hypophosphatasia present across the age spectrum, and what is the diagnostic biochemistry?”
Practical approach
Hypophosphatasia has a broad spectrum whose severity is inversely related to the age of onset. The perinatal form has profound hypomineralization and is often lethal from respiratory failure. The infantile form presents with failure to thrive, rickets, hypercalcaemia and hypercalciuria, craniosynostosis, respiratory problems and B6-dependent seizures. The childhood form causes rickets, bowing, short stature, a waddling gait and, characteristically, premature loss of the primary teeth with the roots intact. The adult form causes osteomalacia, recurrent poorly-healing stress and pseudofractures - classically of the metatarsals and the subtrochanteric femur - chondrocalcinosis and pseudogout, early tooth loss and chronic musculoskeletal pain. There is also an odonto form with dental disease only. The diagnostic biochemistry centres on a persistently low serum alkaline phosphatase, which is the key clue, together with accumulation of the enzyme's substrates: a raised serum pyridoxal-5-phosphate, which is vitamin B6, raised urinary phosphoethanolamine, and elevated inorganic pyrophosphate. This is supported by characteristic radiographs - tongue-like metaphyseal radiolucencies in children and osteomalacia with Looser-zone pseudofractures in adults - and confirmed by ALPL genetic testing.
Key clinical points
Severity inversely related to onset: perinatal (lethal) -> infantile -> childhood -> adult -> odonto
Childhood hallmark: premature loss of PRIMARY teeth (intact roots); adult: metatarsal/subtrochanteric pseudofractures
Diagnostic biochemistry: LOW ALP + raised PLP(B6) + urinary PEA + elevated PPi
Radiographs (tongue-like metaphyseal lucencies / Looser zones) + ALPL genetics
Common pitfalls
Not knowing the premature primary-tooth-loss hallmark
Confusing it with vitamin D-deficiency rickets (which raises ALP)
Missing infantile hypercalcaemia
Further questions
“Which teeth are lost prematurely in childhood HPP? - Primary (deciduous) teeth, roots intact”
“Why do severe infants seize? - Low brain pyridoxal-5-phosphate (B6-dependent seizures)”
Mnemonics & Memory Aids
Mnemonic
LOW ALP
L
LOW alkaline phosphatase (the clue)
O
Odonto - premature loss of primary teeth
W
Weak bone - osteomalacia/pseudofractures (metatarsal, subtrochanteric)
A
ALPL/TNSALP deficiency; Avoid bisphosphonates
L
Leftover substrates: PPi, PLP (B6), PEA
P
Pseudogout/chondrocalcinosis; asfotase alfa = enzyme rePlacement
Hook:Everything turns on a LOW ALP.
Mnemonic
DON'T GIVE
D
Do NOT give bisphosphonates (PPi analogues)
G
Give asfotase alfa (enzyme replacement) instead
V
Vitamin B6 for seizures (infantile)
E
Excess vitamin D/calcium - avoid (hypercalcaemia)
Hook:In HPP, DON'T GIVE bisphosphonates - GIVE asfotase alfa.
Exam day cheat sheet
Hypophosphatasia - Exam Day Cheat Sheet
Cause & biochemistry
- ALPL loss-of-function -> deficient TNSALP
- LOW serum alkaline phosphatase (the clue; others RAISE ALP)
- Substrate build-up: PPi (inhibits mineralization), PLP/B6 (seizures), urinary PEA
Spectrum
- Perinatal (lethal) / infantile (FTT, hypercalcaemia, seizures)
- Childhood: rickets + premature loss of PRIMARY teeth (intact roots)
- Adult: osteomalacia, metatarsal/subtrochanteric pseudofractures, chondrocalcinosis; odonto = dental only
Diagnosis
- Low ALP + raised serum PLP (B6) + urinary PEA
- Radiographs: tongue-like metaphyseal lucencies (child) / Looser zones (adult)
- ALPL genetic testing
Management
- Asfotase alfa (recombinant TNSALP ERT) - early diagnosis essential
- B6 for seizures; treat fractures (poor healing); MDT/dental care
- AVOID bisphosphonates (atypical femoral fractures) and excess vitamin D