Hypophosphatasia: Clinical manifestations and diagnosis

INTRODUCTION — 

Hypophosphatasia (HPP) is an inherited metabolic disease with a broad spectrum of clinical manifestations. It is caused by loss-of-function variants in the ALPL gene, which encodes the tissue nonspecific alkaline phosphatase (TNSALP) enzyme [1]. This enzyme is widely expressed throughout the body and is essential for numerous physiologic processes including bone mineralization and the transport of vitamin B6 into cells [2]. In HPP, deficient TNSALP activity leads to low levels of serum alkaline phosphatase (ALP) activity with accumulation of natural substrates like inorganic pyrophosphate (PPi) [3] and pyridoxal 5'-phosphate (PLP) [4], and gives rise to mineralization defects, neuronal dysfunction, and systemic complications [5]. The clinical severity of HPP roughly correlates with residual TNSALP function and age of onset, resulting in a nosology based on the age at which symptoms first occurred [6,7]. However, disease severity can vary dramatically even among family members with the same ALPL variant, highlighting an incomplete understanding of disease physiology [8].

This topic will review the clinical manifestations and diagnosis of HPP. The management of HPP is reviewed separately, as are the diagnostic and etiologic evaluations for other forms of osteomalacia in adults. (See "Hypophosphatasia: Management" and "Clinical manifestations, diagnosis, and treatment of osteomalacia in adults".)

The evaluation of skeletal dysplasias in children is also reviewed separately.

●(See "Skeletal dysplasias: Approach to evaluation".)

●(See "Approach to the child with bow-legs".)

●(See "Overview of rickets in children".)

PATHOPHYSIOLOGY AND EPIDEMIOLOGY

Genetics and pathophysiology

●Biological roles of tissue nonspecific alkaline phosphatase (TNSALP) – The TNSALP enzyme is widely expressed throughout the body with different functions in various tissues [9]. Functionally, TNSALP exists as a homodimer and requires zinc and magnesium as cofactors for its enzymatic activity. It plays a critical role in skeletal mineralization by degrading powerful mineralization inhibitors (eg, inorganic pyrophosphate [PPi] and phosphorylated osteopontin) and increasing local availability of PPi for hydroxyapatite formation [3]. TNSALP also regulates the intracellular entry of vitamin B6 in the central nervous system by converting pyridoxal 5'-phosphate (PLP) to pyridoxal [4] and plays a role in mitochondrial function and purinergic receptor activity [3,10-12].

●Genetic basis of hypophosphatasia (HPP) – Pathogenic variants in the ALPL gene, located on chromosome 1p36.1-p34, can cause HPP and may be inherited in either autosomal recessive or autosomal dominant manner. These variants result in reduced activity or stability of the TNSALP enzyme. Over 450 different variants in the ALPL gene have been identified in all ethnic backgrounds, contributing to the wide phenotypic variability observed in HPP patients [7]. A curated database of ALPL variants is maintained by the Johannes Kepler University in Linz, Austria.

●Pathophysiology – The pathophysiology of HPP involves the accumulation of substrates typically hydrolyzed by TNSALP, such as PPi, phosphorylated osteopontin, PLP, and nucleotides like adenosine triphosphate (ATP) [3,5]. Elevated levels of PPi and phosphorylated osteopontin inhibit the formation and growth of hydroxyapatite crystals, leading to defective bone mineralization and the development of rickets or osteomalacia [3]. Impaired vitamin B6 processing by defective TNSALP results in elevated PLP, the primary circulating B6 vitamer, and can lead to neurologic symptoms due to impaired neurotransmitter metabolism [3,4].

Epidemiology — Severe forms of HPP, such as the perinatal and infantile forms, are rare, whereas the less severe forms often detected in adulthood are much more common and underdiagnosed. The prevalence of HPP is approximately 1 in 300,000 live births for the severe autosomal recessive forms and up to 1 in 500 for the milder adult form [7]. However, this may be an underestimation due to the variable expression and phenotypic diversity of the disease.

CLINICAL MANIFESTATIONS

Subtype classification — Historically, hypophosphatasia (HPP) has been classified into the following subtypes based on the age of onset of clinical manifestations:

●Perinatal severe (perinatal onset)

●Perinatal benign (perinatal onset)

●Infantile (onset before 6 months of age)

●Childhood (onset between ages 6 months and 18 years)

●Adult (onset after 18 years of age)

Odontohypophosphatasia is a sixth subtype that may occur in children or adults. Its only clinical manifestations are dental defects, such as early tooth loss [6].

Clinical findings — HPP has widely variable clinical manifestations including impaired bone mineralization, dental defects, rheumatologic disorders, muscle weakness, fatigue, and pain [5,13-15]. Symptom severity roughly correlates with age of onset and ranges from life-threatening manifestations with perinatal onset to isolated dental abnormalities to completely asymptomatic adults. However, neither the age of symptom onset nor the specific genotype fully predicts HPP severity. (See "Hypophosphatasia: Management", section on 'Distinguishing adult- from childhood-onset HPP'.)

Pediatric forms of HPP — Pediatric forms of hypophosphatasia (HPP) include perinatal, infantile, and childhood HPP.

●Perinatal HPP – Severe and benign forms of perinatal HPP are typically identified on prenatal ultrasound demonstrating defective skeletal mineralization. The bones may appear short, bowed, and hypomineralized. In perinatal benign HPP, skeletal mineralization slowly improves spontaneously during late pregnancy or after birth, but in perinatal severe HPP, profound skeletal abnormalities and muscle weakness can lead to clinical consequences including stillbirth, postnatal death, and respiratory failure.

Perinatal severe HPP is typically lethal in the neonatal period. Survivors often experience poor functional status and exhibit the following clinical findings:

•Severe skeletal hypomineralization – Newborns exhibit extreme undermineralization of bones on prenatal ultrasound or postnatal radiographs. Associated skeletal abnormalities include rachitic changes, accordion limbs, fractures, and craniosynostosis, which can lead to intracranial hypertension. (See 'Imaging studies' below.)

•Respiratory insufficiency – A poorly developed thoracic cage leads to flail chest, hypoplastic lungs, and respiratory distress, which is a primary cause of mortality. Tracheomalacia can also compromise respiratory function.

•Hypercalcemia, hyperphosphatemia, and nephrocalcinosis – High circulating levels of calcium and phosphate can cause calcification in the kidneys, leading to nephrocalcinosis and kidney damage.

•Vitamin B6-dependent seizures – Impaired metabolism of pyridoxal 5'-phosphate (PLP) can result in seizures that may respond to vitamin B6 supplementation.

The natural history of perinatal benign HPP is variable. It can be a transient prenatal radiographic finding that manifests postnatally as odontohypophosphatasia, or it may evolve into a phenotype of intermediate severity. (See 'Odontohypophosphatasia' below.)

●Infantile HPP – Infantile HPP presents within the first six months of life in patients who may not have had any clinical features apparent at birth. The clinical manifestations are severe and can be life-threatening but typically are not as critical as the perinatal form. Infantile HPP can be associated with increased mortality and may require extensive medical support. Key findings include:

•Failure to thrive – Infants may exhibit poor weight gain and growth.

•Irritability and pain – Persistent crying and discomfort are common due to bone pain.

•Premature craniosynostosis – Early fusion of skull sutures can lead to abnormal head shape and increased intracranial pressure.

•Rickets and fractures – Infants can develop rickets, characterized by bone softening and deformities, along with an increased risk of fractures.

•Respiratory complications – Respiratory issues can occur due to chest deformities, although these are usually less severe than in perinatal HPP.

•Nephrocalcinosis and nephrolithiasis – Impaired bone mineralization can result in hypercalcemia and hyperphosphatemia with calcification in the kidneys, leading to nephrocalcinosis, kidney stones, and kidney injury.

•Vitamin B6-dependent seizures – Impaired PLP metabolism occasionally can give rise to seizures that may respond to vitamin B6 supplementation.

•Muscle weakness – Infants may have severe muscle weakness and delayed motor development.

●Childhood or juvenile HPP – Childhood HPP usually manifests after six months of age and can vary widely in severity based on the degree of mineralization defects. The clinical findings in this age group may resemble rickets, and the severe end of the spectrum overlaps with infantile HPP. Additional findings include:

•Premature loss of primary teeth – A hallmark of childhood HPP is primary tooth loss before age 5 years. This occurs in approximately one-half of affected children. Typically, deciduous tooth loss occurs after age 5 with resorption of the tooth root. Impaired mineralization of cementum in HPP accelerates tooth loss, often with intact roots in the shed teeth.

•Short stature and skeletal deformities – Impaired growth and skeletal deformities such as bowed legs or knock-knees are common.

•Fractures – Increased susceptibility to fractures, particularly of the metatarsals and long bones, is a common feature of childhood HPP.

•Delayed motor development – Children often experience delays in walking and other motor milestones due to muscle weakness and bone pain.

•Painful myopathies – Muscle pain and weakness are frequently reported.

•Psychological disorders – Childhood HPP may be associated with increased risk of attention deficit/hyperactivity disorder, behavioral issues, and sleep disturbances.

The Global Hypophosphatasia Registry, an observational, prospective, multinational study, has been collecting clinical information on patients of all ages diagnosed with HPP since 2015 to improve understanding of the disease. In children, the most commonly reported HPP-related findings included premature loss of deciduous teeth (48 percent), bone deformity (33 percent), and failure to thrive (27 percent) [13,14]. Compared with children diagnosed at <6 months of age (ie, infantile HPP), those with childhood HPP were less likely to have respiratory failure, seizures, hypercalcemia/hyperphosphatemia, and skeletal abnormalities [14].

Adult HPP — Adult-onset hypophosphatasia (HPP) typically presents in middle age and may have a wide range of clinical manifestations. Although the adult-onset HPP phenotype is generally milder than that of pediatric forms, adults with childhood-onset and adult-onset HPP have significant clinical overlap [13,16]. (See "Hypophosphatasia: Management", section on 'Distinguishing adult- from childhood-onset HPP'.)

●Signs and symptoms – In adults with HPP, signs and symptoms may be nonspecific. In its milder forms, HPP may be mistaken for osteoporosis. (See 'Differential diagnosis' below.)

Key clinical findings include:

•Osteomalacia – Defective bone mineralization can result in scoliosis, bone pain, and increased fracture risk. However, bone mineral density (BMD) is quite variable when assessed by dual-energy x-ray absorptiometry (DXA), which frequently shows normal to elevated lumbar spine density in adults with HPP [17,18]. The biologic basis for this preserved BMD is not well understood.

•Fractures and pseudofractures – Metatarsal fractures and pseudofractures are common. Pseudofractures in HPP tend to occur in the lateral femoral cortex and may be bilateral [19]. Patients with HPP have increased risk of atypical femoral fractures and can develop lateral subtrochanteric pseudofractures [16,19]. (See 'Imaging studies' below.)

•Rheumatologic disorders – Adult patients with HPP have increased susceptibility to osteoarthritis and joint injury. Elevated calcium and pyrophosphate concentrations can result in calcium pyrophosphate crystal deposition in and around joints and give rise to chondrocalcinosis and pseudogout [5,20].

•Nephrocalcinosis – Elevated circulating calcium and phosphate levels can lead to ectopic calcium deposition, kidney stones, and nephrocalcinosis.

•Muscle weakness and fatigue – Muscle weakness with fatigability and lack of endurance are common symptoms of HPP and may be evident on the six-minute walk test, the timed up-and-go tests, repeated chair-rise test, and lower extremity functional scale tests [19,21,22]. These tests can be used clinically to assess the presence and degree of weakness.

•Chronic pain – Persistent bone and joint pain, particularly in the feet, hips, and lower back, are common manifestations of HPP in adults [16]. Approximately 30 percent of adults with HPP reported receiving pain medication, with 12 percent taking opioids. Self-reported pain scores were similar for adults with pediatric-onset (ie, infantile or childhood) and adult-onset HPP [16]. Many symptoms of HPP overlap with fibromyalgia, including myalgia, arthralgia, brain fog, depression, and bone pain, and several case reports describe initial misdiagnosis as fibromyalgia [23].

•Dental issues – Premature tooth loss and periodontal disease remain significant issues in adults [6].

●Symptom prevalence and impact on daily life – Based on the Global Hypophosphatasia registry, which includes over 500 adults with HPP, the most common findings were pain (67 percent), dental problems (54 percent), skeletal or orthopedic problems (43 percent), constitutional/metabolic manifestations (33 percent), and muscular deficits (29 percent) [16]. The prevalence of respiratory, neurologic, and rheumatologic findings ranged between 4 to 13 percent in adults [16].

Adults with HPP reported that symptoms impaired their physical function and mobility, limited their activities of daily living, and negatively impacted their quality of life. Almost two-thirds of patients experienced manifestations in ≥3 body systems, indicating the systemic nature of the disease and highlighting the burden of pain and disability even in "mildly" affected patients [16]. Over one-half of adult patients reported that their health negatively affected their physical and mental functioning, and 17 percent required some form of assistive device or home modification for a physical disability, most often the use of crutches or a cane [16].

Odontohypophosphatasia — Patients with HPP can present with isolated dental manifestations consisting primarily of tooth loss and periodontal disease without significant skeletal involvement. Key findings include:

●Premature exfoliation of deciduous teeth – Loss of primary teeth before age 5 years is the principal manifestation, and teeth often are shed with intact roots due to defective mineralization of dental cementum. Imaging studies can show loss of alveolar bone and enlarged pulp chambers. Impaired dentinogenesis and enamel hypoplasia can also be seen.

●Dental caries and periodontal disease – Odontohypophosphatasia can cause increased susceptibility to cavities and gum disease [1].

Laboratory findings — Key laboratory features include:

●Low serum alkaline phosphatase (ALP) – The hallmark biochemical finding in HPP is a serum level of ALP activity below the age- and sex-specific reference range. Measuring serum ALP is a key step in the diagnostic evaluation of HPP and is reviewed in detail below. (See 'Laboratory testing' below.)

●Hypercalcemia, hypercalciuria, and hyperphosphatemia – Impaired mineralization can result in elevated serum calcium and phosphate levels and hypercalciuria, particularly in severe forms of HPP. Phosphate elevation appears to be caused by decreased phosphate clearance due to suppressed phosphatonins and is more common than hypercalcemia in milder adult forms of HPP [5,24].

●Other – Other characteristic laboratory findings in HPP, including elevated levels of tissue nonspecific alkaline phosphatase (TNSALP) substrates, are helpful for diagnosis and reviewed below. (See 'Additional testing' below.)

Imaging findings — The radiographic manifestations of HPP vary widely, ranging from profound skeletal hypomineralization on prenatal ultrasound in perinatal forms to normal skeletal imaging in milder, adult-onset cases. Radiographic imaging is essential for evaluating the skeletal sequelae of HPP and is reviewed in detail below. (See 'Imaging studies' below.)

BMD is quite variable in adults with HPP when assessed by DXA. Multiple studies have found elevated lumbar spine BMD in adults with HPP who have sustained fractures, indicating that BMD is insensitive for assessing fracture risk in this population [17,18].

DIAGNOSIS

Whom to evaluate — In general, the diagnosis of hypophosphatasia (HPP) should be considered in patients with known family history of HPP, those who have persistently low alkaline phosphatase (ALP) measurements with no other clear secondary cause, and those with suggestive clinical findings. (See 'Alkaline phosphatase' below and 'Clinical findings' above.)

A few specific clinical scenarios are particularly suggestive of HPP:

●A child who experiences loss of primary teeth prior to age 5 years, particularly if the shed teeth have intact roots

●An adolescent with mild muscle weakness and fatigue who experiences a low trauma fracture

●A healthy young adult with persistently low ALP activity and any first-degree relatives with clinical findings that suggest HPP

●A middle-aged adult with nonspecific musculoskeletal symptoms or a diagnosis of fibromyalgia who has experienced prior metatarsal fracture(s) and/or kidney stone(s)

●An older adult with chondrocalcinosis who sustains a fragility fracture

●A middle-aged adult who has a fragility fracture and subnormal ALP activity

Approach to diagnosis — The diagnosis of HPP is based on a combination of clinical, biochemical, and genetic findings, as no single clinical finding or test result is sufficient. The utility of HPP diagnosis is not exclusively for identifying candidates for enzyme replacement therapy. Diagnosis may spare the patient further evaluation for other disorders, prevent exposure to potentially harmful therapies, guide future disease surveillance, and help optimize calcium and vitamin D intake [5].

Initial assessment — Patients may vary widely in clinical presentation and available laboratory studies, both of which guide the approach to subsequent evaluation.

●Severe presentation – Severely affected patients with classic rachitic changes, vitamin B6-dependent seizures, and profound weakness are likely to be diagnosed rather quickly. If not yet measured, a serum ALP level should be obtained and may be all that is needed for an initial diagnosis. (See 'Alkaline phosphatase' below.)

Confirmatory testing, which usually entails genetic testing and measuring pyridoxal 5'-phosphate (PLP) and urine phosphoethanolamine (PEA), can then be completed under specialist care to help direct management. (See 'Genetic testing' below and 'Additional testing' below.)

●Milder presentation – At the milder end of the phenotypic spectrum, the diagnosis of HPP can be challenging given the heterogeneity of clinical presentations, frequent presence of nonspecific symptoms, lower sensitivity and specificity of the diagnostic tests, and difficulty of obtaining specialized tests in general practice. In such patients, serum ALP should be measured. More specific but potentially remote or obscure medical history (eg, age of primary tooth loss, developmental milestones) should be elicited. Extensive time and effort are frequently required to obtain such history and integrate findings that support the diagnosis and justify more specialized testing. Such presentations include mild musculoskeletal, neuromuscular, kidney, or dental phenotypes. (See 'Adult HPP' above and 'Odontohypophosphatasia' above.)

●Low serum ALP – A diagnostic evaluation is often initiated after an unexplained low ALP activity is found on a routine blood test. In such cases, ALP measurement should be repeated, and alternative causes of low serum ALP should be considered. (See 'Alkaline phosphatase' below.)

If persistently low ALP is confirmed and alternative causes are excluded, extensive retrospective and prospective evaluation may be needed to identify clinical findings that may support an HPP diagnosis. Patients should be monitored for the development of characteristic findings, as they may evolve over time or become apparent with greater retrospective information [5,20,25]. (See 'Clinical findings' above.)

Laboratory testing

Alkaline phosphatase — In patients with suspected HPP based on initial clinical assessment, the next step is measuring serum ALP. Although most patients with HPP have low ALP activity, most patients with low serum ALP activity do not have HPP. Further, although serum ALP is generally low in patients with HPP, the distribution of values can overlap with the lower limits of the reference range [26]. In some laboratory assays, the lower end of the reference range may extend to zero; therefore, the ALP value should be considered even when the result is not flagged as "low."

●Caveats for test interpretation – Serum ALP activity must be assessed using age- and sex-specific reference ranges, as serum ALP varies by age and sex. For example, because of bone growth, serum ALP activity in children and young adults can be several-fold higher than in middle-aged adults [27].

Measurements of serum ALP activity reflect the activity of all four ALP isoenzymes. These include tissue nonspecific alkaline phosphatase (TNSALP), which is expressed in liver, bone, and kidney [1,2], as well as isoenzymes with tissue-specific expression in the intestines, placenta, and germ cells. TNSALP is the most widely expressed ALP isoenzyme, and its isoforms usually contribute the greatest share of total serum ALP activity.

●Result interpretation

•ALP activity normal or elevated – Decreased ALP activity from HPP can be masked by conditions that typically increase serum ALP levels, the most common of which is cholestatic liver disease [28,29]. Conditions that increase bone-specific ALP activity similarly can mask low TNSALP activity. These include hyperparathyroidism, hyperthyroidism, neoplasms in bone, anabolic bone therapy (eg, teriparatide, romosozumab), healing fracture, or delayed bone age relative to chronologic age in adolescents and young adults. Intestinal ALP can contribute up to 20 percent of the total serum ALP activity [28], with further increases seen with some gastrointestinal conditions [30]. Total serum ALP activity can also increase due to placental ALP during pregnancy or germ cell ALP in the presence of certain malignancies [31].

•ALP activity low – A low ALP value may have numerous causes other than HPP. In the absence of a compelling clinical presentation, these alternative causes should be considered prior to diagnosing HPP. Such causes include malnutrition, hypothyroidism, and Wilson disease. However, these conditions usually present with additional clinical features that are not characteristic of HPP. (See "Clinical manifestations of hypothyroidism" and "Wilson disease: Clinical manifestations, diagnosis, and natural history".)

-Low bone formation – Any cause of low bone formation can lead to low ALP. Common causes include glucocorticoid use and antiresorptive pharmacotherapy for osteoporosis [32]. Lower age-specific ALP values than expected also may be evident in adolescents or young adults with accelerated bone age or in young, healthy women taking oral contraceptives.

-Assay interference – In other settings, low ALP values may result from conditions that cause assay interference. ALP activity requires zinc and magnesium as cofactors; thus, conditions that decrease the availability of these cofactors (eg, malnutrition, gastrointestinal malabsorption, zinc deficiency) can lower measured ALP values.

-Other causes – Other causes of low ALP activity include vitamin C deficiency, profound anemia, massive transfusion, cardiac surgery, critical illness, or improper sample collection with the use of ethylenediaminetetraacetic acid (EDTA). Most of these cause only transient decreases in ALP; thus, repeat testing or reviewing past values can help identify a temporary decrease in ALP and thereby rule out HPP.

Additional testing — Additional laboratory testing may be necessary to help support or exclude the diagnosis of HPP. For example, additional testing is warranted if ALP activity is not overtly low but suspicion for HPP remains high based on clinical presentation and/or family history. In adults who initially present with low ALP activity and/or a pathogenic ALPL variant but no clinical diagnostic features, natural substrates of TNSALP should be measured before excluding the clinical diagnosis of HPP [20]. The presence of a pathogenic ALPL variant alone is not sufficient for diagnosis; pathogenic variants may be present in a heterozygous state without leading to significant enzymatic dysfunction. Elevated PLP and urinary PEA levels provide evidence of enzymatic dysfunction and are considered major criteria for diagnosing HPP; consequently, these measurements are increasingly obtained in the diagnostic evaluation. (See 'Genetic testing' below and 'Diagnostic criteria' below.)

●Fractionated or bone-specific alkaline phosphatase (ALP) – If the ALP value is unexpectedly normal, fractionated ALP measurements may help determine whether the normal value reflects ALP isoenzymes other than TNSALP. In patients with HPP, fractionated ALP measurements typically show decreased levels of both the liver and bone isoforms of TNSALP, whereas the intestinal ALP isoenzyme has normal total activity.

Some groups have advocated for measuring bone-specific ALP in all patients with suspected HPP. This preference is based on findings that bone-specific ALP values were lower in HPP compared with other causes of low bone mineral density (BMD) and correlated better with other bone turnover markers than total ALP [21,26,29]. However, the sensitivity and specificity of bone-specific ALP have not been extensively studied or validated in HPP.

●Natural substrates of tissue nonspecific alkaline phosphatase (TNSALP) – Elevated levels of natural substrates of TNSALP can support the diagnosis of HPP and constitute major diagnostic criteria for childhood and adult HPP. (See 'Childhood and adult HPP' below.)

•Elevated serum pyridoxal 5'-phosphate (PLP) – High serum levels of PLP, the circulating form of vitamin B6, suggest HPP and result from impaired dephosphorylation of PLP to pyridoxal. The degree of PLP elevation roughly correlates with the degree of ALP deficiency [33]. Patients should stop all vitamin B6-containing supplements, including energy drinks and electrolyte solutions, approximately one week prior to PLP measurement [4]. Use of such vitamin B6-containing products may occasionally cause elevated PLP levels in the absence of HPP.

Individuals with HPP and vitamin B6 deficiency, evident with low pyridoxic acid measurements, may have PLP levels in the normal range despite impaired metabolism. Expected PLP elevations become apparent after vitamin B6 repletion [34]. In adults with HPP, normal PLP levels also may result from either biologic or assay-specific factors [32]. PLP levels may be affected by age, sex, and ethnicity and were lower in patients with inflammatory disorders or chronic kidney disease [35]. When exposed to light, PLP is inactivated, which can result in artificially lower levels if samples are not handled properly. Samples must be drawn in EDTA tubes in the absence of direct sunlight and should be frozen or refrigerated prior to processing. Ideally, amber-colored glassware and plastic ware should be used in sample collection and processing.

•Elevated urinary phosphoethanolamine (PEA) – Elevated concentrations of urinary PEA may be helpful to support the diagnosis and to distinguish HPP from other causes of low ALP activity [3]. Elevated levels of PEA are a biochemical marker of HPP [6], as TNSALP regulates the degradation of PEA to ethanolamine and inorganic phosphate. Urinary PEA levels are generally higher in pediatric patients with biallelic disease than in mildly affected adults and tend to correlate inversely with circulating liver, but not bone, TNSALP activity [6]. Urine PEA measurements are not readily available in all laboratories and may need to be done as part of a specialized urine amino acid panel, which suffers from a scarcity of published reference ranges.

Measuring urinary PEA levels may also be useful for monitoring enzyme replacement therapy [32]. (See "Hypophosphatasia: Management", section on 'Monitoring'.)

•Elevated inorganic pyrophosphate (PPi; rarely measured) – Deficient TNSALP activity results in the accumulation of PPi and an altered ratio of phosphate to PPi, which interferes with skeletal and dental mineralization. In HPP, PPi can be elevated in both plasma and urine; however, measurements are costly and generally confined to a research setting [25,27].

Imaging studies — Imaging findings are important for both diagnosis and establishing the severity of disease and thus the approach to treatment.

●Pediatric imaging – Perinatal HPP is often detected incidentally on routine prenatal sonogram. In contrast, infantile and childhood HPP typically present with clinical findings, and plain radiographs are obtained during diagnostic evaluation.

•Prenatal sonogram – In severe perinatal HPP, radiographic findings are dramatic and include extreme skeletal hypomineralization with distinct features such as shortening, bowing, and angulation of long bones, a small and narrow thorax, and osteochondral spurs (Bowdler spurs). Fractures and metaphyseal irregularities, including radiolucent "tongues" projecting from growth plates into metaphyses, are common. Additionally, ribs may appear short and beaded in the second trimester, becoming thin in full-term neonates. The lack of ossification in bones such as the tubular bones, skull vault, ribs, and vertebrae are often observed, along with abnormal sonolucency of bony structures and a hypoechogenic skull. Increased nuchal translucency, wide sutures, fontanelles, and polyhydramnios can also be evident [36].

•Postnatal plain radiographs – In infants and children with suspected HPP, imaging sites are selected based on clinical signs and symptoms. Plain radiographs have diagnostic value, as detection of skeletal demineralization with rachitic changes is a key criterion for diagnosis. (See 'Diagnostic criteria' below.)

Some associated findings include metaphyseal fraying, metaphyseal sclerosis, and beading of costochondral junctions. Radiographs typically reveal the characteristic radiolucent "tongues" of bone projecting from growth plates into metaphyses. Additional findings can include bowed legs or knock-knees, enlarged joints due to metaphyseal flaring, and a misshapen skull with a "beaten-copper" appearance, indicating craniosynostosis. In severe cases, premature closure of cranial sutures and progressive thoracic deformity may be seen, predisposing patients to increased intracranial pressure and respiratory complications [6]. Kidney complications (eg, nephrocalcinosis, nephrolithiasis) may be detected in severe forms of HPP in children.

●Adult imaging – In adults with suspected HPP, plain radiographs should be obtained as guided by clinical signs and symptoms. Radiographic findings frequently reflect a combination of persistent skeletal changes from childhood and new skeletal and extraskeletal manifestations unique to adulthood. Both calcific phenomena and recurrent fractures are common radiographic findings of adult HPP [6]. Dedicated studies to look for chondrocalcinosis and nephrocalcinosis can have diagnostic utility as these findings are included as minor diagnostic criteria for HPP in adults [20]. (See 'Childhood and adult HPP' below.)

•Features of osteomalacia – Skeletal abnormalities that persist from childhood may include generalized osteopenia and evidence of chronic osteomalacia, characterized by cortical thinning and the presence of pseudofractures, particularly in the lateral subtrochanteric region of the femur. These pseudofractures, known as Looser zones, differ in HPP from other forms of osteomalacia, as they tend to occur on the lateral femoral cortex rather than on the medial surface. Additionally, recurrent metatarsal stress fractures are common and may fail to heal properly [6].

•Calcific changes – Kidney complications such as nephrocalcinosis and nephrolithiasis can develop in adults even with mild disease. Radiographic features predominantly seen in adults include calcific periarthritis, characterized by hydroxyapatite crystal deposition near joints, and ossification of ligaments, which can resemble spinal hyperostosis (Forestier disease). The extracellular accumulation of PPi can result in calcium pyrophosphate dihydrate (CPPD) deposition and consequent chondrocalcinosis or pseudogout.

Measurement of BMD by dual-energy x-ray absorptiometry (DXA) is not part of the standard diagnostic evaluation for HPP in adults. However, it may be obtained in selected patients (eg, postmenopausal people) for additional assessment of fracture risk and to establish a pretreatment baseline.

Genetic testing — Genetic testing for ALPL variants is warranted in patients with suspected HPP to confirm the diagnosis. The finding of pathogenic or likely pathogenic variants in ALPL is a major criterion in the diagnosis of HPP in both children [25] and adults [20]. However, genetic testing is not required to make the diagnosis, and not all patients with HPP are found to have ALPL variants [5]; for example, ALP enzymatic dysfunction could be caused by deep intronic or regulatory variants missed by sequencing or by other variants outside the ALPL gene [7]. Commercially available genetic testing for ALPL identifies variants in approximately 95 percent of patients with clinical HPP. The sensitivity and specificity of ALPL genetic testing depend on the clinical criteria used to select patients for testing. Thus, genetic testing is much more likely to identify a pathogenic ALPL variant in pediatric patients with classic HPP manifestations than in adults with nonspecific symptoms and borderline ALP measurements. The timing of genetic testing should be guided by provider expertise and patient-specific considerations. For experienced providers or those with the necessary infrastructure to properly consent and counsel patients based on genetic results, earlier genetic testing may be reasonable.

In perinatal and infantile HPP, genetic testing for ALPL variants is often unnecessary for diagnosis; however, testing may uncover biallelic pathogenic variants that may guide the evaluation of parents or other first-degree relatives [1,25].

●Other indications for testing – Relatives of a proband diagnosed with HPP may undergo genetic testing to identify those who may benefit from early monitoring or therapeutic interventions. Genetic testing also informs counseling for individuals and families by providing insights into inheritance patterns, recurrence risks, and reproductive options.

●Considerations for preconception counseling – Identifying ALPL variants through preconception genetic screening presents challenges, especially when individuals lack clinical features of HPP. Given the variable expressivity, incomplete penetrance, and potential recessive inheritance of ALP variants, counseling in these situations requires a nuanced and individualized approach. The pathogenicity of the ALPL variant must be confirmed and its inheritance pattern assessed; autosomal recessive variants typically indicate carrier status, whereas dominant variants may have unpredictable clinical significance [8]. A thorough evaluation should include biochemical markers (eg, ALP, PLP, PEA), clinical assessments, and family history to identify subclinical disease or risk factors. Genetic counseling should address potential transmission, reproductive options (eg, partner screening, preimplantation genetic testing), and the need for long-term monitoring for emerging symptoms.

Diagnostic criteria

Perinatal and infantile HPP — The clinical presentation of perinatal and infantile hypophosphatasia (HPP) is typically unequivocal, and the combination of severe clinical findings with low serum ALP activity is usually sufficient for diagnosis. (See 'Pediatric forms of HPP' above and 'Alkaline phosphatase' above.)

Childhood and adult HPP — As the diagnosis of childhood or adult hypophosphatasia (HPP) is often more challenging, an international working group of experts developed formal diagnostic criteria based on a systematic review [5]. In milder forms of HPP, most clinical manifestations have low diagnostic sensitivity, and few clinical findings may be evident at initial evaluation.

Low serum ALP (essential criterion) — For either childhood or adult HPP, an essential component of diagnosis is the presence of low serum alkaline phosphatase (ALP) activity. Low serum ALP must be demonstrated using age- and sex-specific reference ranges and must be persistent (ie, present in at least two measurements) [5,25,27]. Other conditions that may influence serum ALP must be excluded. (See 'Alkaline phosphatase' above.)

Childhood HPP — In addition to the presence of low serum ALP, the proposed diagnostic framework for childhood hypophosphatasia (HPP) includes 13 other diagnostic criteria. These are organized into four major and nine minor criteria based on their pooled prevalence. In the setting of persistently low serum ALP, two major criteria or one major and two minor criteria are sufficient for HPP diagnosis, provided at least one clinical diagnostic feature is present. This provision is to prevent genetic and/or biochemical diagnosis in asymptomatic individuals. The presence of HPP in a first-degree relative is not a diagnostic criterion, given the variable and incomplete penetrance of HPP [25].

●Major criteria – The major diagnostic criteria include:

•Presence of a pathogenic or likely pathogenic ALPL variant.

•Elevation of the natural substrates of TNSALP (PLP, PEA, PPi).

•Nontraumatic loss of primary teeth before age 5 years (particularly with intact root).

•Radiographic evidence of rickets. Although this had a pooled prevalence <50 percent, the combination of rachitic changes and low ALP activity was deemed a strong indicator of HPP, as other hereditary or nutritional causes of rickets tend to be associated with increased ALP activity [25].

●Minor criteria – Minor criteria include features that are more specific but less prevalent or those that are less specific but more prevalent [25]. These include:

•Nephrocalcinosis

•Vitamin B6-responsive seizures

•Craniosynostosis

•Delayed motor milestones

•Impaired mobility

•Knee abnormalities

•Short stature

•Low muscle tone

•Chronic musculoskeletal pain

Low BMD and osteomalacia on histomorphometry were not included as diagnostic criteria. DXA has limited utility in children with HPP, and histomorphometry is rarely used to diagnose HPP [25,37].

Adult HPP — In addition to the presence of persistently low serum ALP, nine criteria have been proposed for the diagnosis of hypophosphatasia (HPP) in adults. These are organized into four major and five minor criteria based on their pooled prevalence and relative specificity for HPP. In the setting of a persistently low serum ALP, the presence of two major criteria or one major and two minor criteria are sufficient for diagnosis.

●Major criteria – The major criteria include:

•Elevation of the natural substrates of TNSALP (PLP, PEA, PPi)

•Presence of a pathogenic or likely pathogenic ALPL variant

•Atypical femur fractures

•Recurrent metatarsal stress fractures

Although the last two features each had a pooled prevalence <50 percent, these findings appeared much more specific for HPP than the other clinical features [20].

●Minor criteria – Minor criteria include features that are more specific but less prevalent or those that are less specific but more prevalent [20]. These include:

•Nephrocalcinosis

•Chondrocalcinosis

•Poorly healing fractures

•Chronic musculoskeletal pain

•History of early nontraumatic loss of either primary or secondary teeth

Low BMD is not a diagnostic criterion, as DXA findings can be variable and have limited utility in HPP diagnosis [20]. Although BMD can overlap between adults with HPP and those with osteoporosis, BMD is not generally low in HPP and may be increased at the lumbar spine as assessed by DXA [17,37].

Differential diagnosis — Differentiating HPP from other conditions with similar clinical and biochemical presentations is essential to ensure accurate diagnosis and appropriate management.

●Other causes of low bone mass and/or fracture

•Osteoporosis – Osteoporosis is a key condition to consider, as it also presents with low bone mass and an increased risk of fractures. However, osteoporosis typically features normal or elevated serum ALP levels, in contrast to the low ALP levels characteristic of HPP. Bone biopsy findings in osteoporosis usually show normal or high bone turnover, whereas HPP is marked by defective mineralization. Distinguishing between HPP and osteoporosis is essential because antiresorptive therapy is often used for treating osteoporosis but should generally be avoided in patients with HPP due to risk of worsening the underlying defect in bone mineralization. Additionally, HPP itself is associated with an increased risk of atypical femur fractures, and antiresorptive therapy may further heighten this risk. (See "Clinical manifestations, diagnosis, and evaluation of osteoporosis in men" and "Clinical manifestations, diagnosis, and evaluation of osteoporosis in postmenopausal women" and "Evaluation and treatment of premenopausal osteoporosis".)

•Other forms of rickets or osteomalacia – Various forms of rickets and osteomalacia, such as those caused by vitamin D deficiency, chronic kidney disease, and genetic disorders like X-linked hypophosphatemia, must also be considered in the differential diagnosis of HPP. These conditions typically present with elevated serum ALP activity, unlike the low measurements seen in HPP. Typical laboratory characteristics of different osteomalacia etiologies are shown in the table (table 1). (See "Clinical manifestations, diagnosis, and treatment of osteomalacia in adults" and "Overview of rickets in children".)

●Other causes of low ALP values – Other causes of low ALP measurements should always be considered. Alternative causes of low ALP values are reviewed above. (See 'Alkaline phosphatase' above.)

SUMMARY AND RECOMMENDATIONS

●Pathophysiology – Pathogenic variants in the ALPL gene can cause hypophosphatasia (HPP) and may be inherited in either autosomal recessive or autosomal dominant manner. These variants result in reduced activity or stability of the tissue nonspecific alkaline phosphatase (TNSALP) enzyme. The pathophysiology of HPP involves the accumulation of substrates typically hydrolyzed by TNSALP. (See 'Pathophysiology and epidemiology' above.)

●Subtype classification – HPP has been classified into subtypes based on the age of onset of clinical manifestations. (See 'Subtype classification' above.)

●Clinical findings – HPP has widely variable clinical manifestations including impaired bone mineralization, dental defects, rheumatologic disorders, muscle weakness, fatigue, and pain. Symptom severity roughly correlates with age of onset and ranges from life-threatening manifestations with perinatal onset to isolated dental abnormalities to completely asymptomatic adults. (See 'Clinical findings' above.)

●Laboratory and imaging findings – The hallmark biochemical finding in HPP is a serum level of alkaline phosphatase (ALP) activity below the age- and sex-specific reference range (table 1). Impaired mineralization can result in elevated serum calcium and phosphate levels and hypercalciuria, particularly in severe forms of HPP. (See 'Laboratory findings' above.)

The radiographic manifestations of HPP vary widely, ranging from profound skeletal hypomineralization on prenatal ultrasound in perinatal forms to normal skeletal imaging in milder, adult-onset cases. Bone mineral density (BMD) is quite variable in adults with HPP when assessed by dual-energy x-ray absorptiometry (DXA). (See 'Imaging findings' above.)

●Diagnostic evaluation – The diagnosis of HPP should be considered in patients with known family history of HPP, those who have persistently low ALP levels with no other clear secondary cause, and those with suggestive clinical findings. Patients may vary widely in clinical presentation and available laboratory studies, both of which guide the approach to subsequent evaluation. (See 'Whom to evaluate' above and 'Initial assessment' above.)

•Laboratory testing – In all patients with suspected HPP, serum ALP should be measured. Serum ALP activity must be assessed using age- and sex-specific reference ranges, and other causes of low ALP should be excluded. Elevated levels of natural substrates of TNSALP can support the diagnosis of HPP. (See 'Laboratory testing' above.)

•Imaging – Imaging findings are important for both diagnosis and establishing the severity of disease. (See 'Imaging studies' above.)

-Pediatric HPP – Perinatal HPP is often detected incidentally on routine prenatal sonogram. In infants and children with suspected HPP, plain radiographs have diagnostic value, as detection of skeletal demineralization with rachitic changes is a key criterion for diagnosis.

-Adult HPP – In adults with suspected HPP, plain radiographs should be obtained as guided by clinical signs and symptoms. Radiographic findings frequently reflect a combination of persistent skeletal changes from childhood and new skeletal and extraskeletal manifestations unique to adulthood.

•Genetic testing – Genetic testing for ALPL variants is warranted in patients with suspected HPP to help confirm the diagnosis. (See 'Genetic testing' above.)

●Diagnostic criteria – The clinical presentation of perinatal and infantile HPP is typically unequivocal, and the combination of severe clinical findings with low serum ALP activity is usually sufficient for diagnosis. As the diagnosis of childhood or adult HPP is often more challenging, an international working group of experts developed formal minor and major criteria for diagnosis. (See 'Diagnostic criteria' above.)