Clinical and Radiographic Findings in Adults With Persistent Hypophosphatasemia

Authors


ABSTRACT

A serum alkaline phosphatase value below the age-adjusted lower limits of normal (hypophosphatasemia) is uncommonly encountered in clinical practice. The electronic and paper medical records of 885,165 patients treated between 2002 and 2012 at a large, rural, multispecialty health clinic were interrogated to estimate the prevalence and characterize the clinical and radiographic findings of adults whose serum alkaline phosphatase was almost always low (persistent hypophosphatasemia). We hypothesized that some of these patients might harbor previously unrecognized hypophosphatasia, a rare, inherited condition of impaired mineralization of bones and teeth. Persistent hypophosphatasemia (serum alkaline phosphatase ≤30 IU/L) was found in 1 of 1544 adult patients. These adult patients had more crystalline arthritis, orthopedic surgery, chondrocalcinosis, calcific periarthritis, enthesopathy, and diffuse idiopathic skeletal hyperostosis than a general adult patient population. A gender effect was observed. The clinical and radiographic findings of adult patients with persistent hypophosphatasemia resemble those of the adult form of hypophosphatasia. Clinicians should take notice of persistent hypophosphatasemia, consider the diagnosis of hypophosphatasia, and be cautious when considering potent anti-remodeling therapy in these adults. This population warrants further evaluation. © 2014 American Society for Bone and Mineral Research.

Introduction

Commercially available chemistry auto-analyzers measure serum alkaline phosphatase (ALP) activity, which includes the enzymatic activity of ALP isoenzymes and ALP isoforms. Isoenzymes are the products of different genes, whereas isoforms are posttranslational modifications of a single gene product. Tissue-specific ALP isoenzymes arise from placental, intestinal, and germ line cells and are encoded by genes on chromosome 2q34-37.[1, 2] Tissue-nonspecific ALP (TNSALP: orthophosphoric-monoester phosphohydrolase) is a highly conserved, ubiquitous cell surface enzyme encoded by a separate gene (ALPL) located on chromosome 1p36.1-p34.[1, 2] Human TNSALP isoforms, the result of posttranslational modification, are primarily hepatic, renal, or skeletal in origin. In health and dependent on blood type and feeding status, TNSALP constitutes between 70% and 95% of serum ALP.[1] Serum ALP values vary by age and, in children, by gender, but normal adult values for serum ALP typically range between 40 and 125 IU/L. Elevated serum ALP values are frequently encountered in clinical practice and usually indicate hepatobiliary or skeletal disease.[3] Conversely, a low serum ALP value (hypophosphatasemia) is uncommon in clinical practice and, in the absence of analytic error, the reported causes are wide ranging, sometimes anecdotal and often unfamiliar[3-7] (Table 1). Therefore, the etiology and clinical significance of hypophosphatasemia might be easily overlooked.

Table 1. Differential Diagnosis of Hypophosphatasemia Based on the Temporal Evolution of the Low Serum ALP Value[6]
Temporal evolution of the low serum ALP value
Persistently lowTransiently lowPrecipitously low
• Hypophosphatasia (HPP: OMIM 146300,241500,241510)• Osteogenesis imperfecta type II (OMIM 166210)• Cardiac bypass surgery
• Cleidocranial dysplasia (OMIM166210)• Profound hypothyroidism• Major trauma
• Mseleni joint disease (OMIM 613342)• Cushing's disease• Major surgery
• Benign familial hypophosphatasemia (?)• Bisphosphonate therapy• Cancers and chemotherapy
 • Adynamic renal osteodystrophy• Multiple myeloma
 • Milk-alkali syndrome• Transfusion (often massive)
 • Vitamin D intoxication• Starvation/acute caloric restriction
 • Wilson's disease• Sepsis/multi-organ/hepatic failure
 • Nutritional deficiencies (Vit C)• Analytic error
 • Hypomagnesemia• Improperly collected specimen (eg, EDTA, citrate, oxalate)
 • Hypozincemia 
 • Celiac disease 
 • Pernicious anemia 
 • Radioactive heavy metal contamination 

Reduced or absent TNSALP activity results in extracellular accumulation of inorganic pyrophosphate (PPi).[1, 2, 7] Because PPi inhibits normal mineralization, its accumulation within bone leads to rickets, osteomalacia, and altered bone quality.[1, 2, 7] Hypophosphatasia (HPP; OMIM 146300, 241500, 241510), perhaps the most common cause of persistent hypophosphatasemia, is a heritable inborn error of metabolism resulting from deficient TNSALP activity.[2, 7, 8] Thus far, more than 280 distinct ALPL mutations have been recognized of which 80% are missense mutations.[9] The number and diversity of these mutations, as well as the effects of gene dose and penetrance, accounts for the highly variable phenotypic expression of HPP. Six phenotypes have been described that include benign-prenatal, perinatal, infantile, childhood, adult, and, when disease expression is limited to abnormal dentition, odontohypophosphatasia.[2, 7, 8] Low age-adjusted serum ALP values are expected in all forms of HPP.[1, 2] Low or low-normal serum ALP values are reported in HPP carriers.[2, 10]

In general, morbidity associated with HPP is inversely proportional to the age at presentation. Perinatal HPP, the most severe and recognizable phenotype, presents in utero or at birth with profound skeletal hypomineralization, progressive respiratory failure, and neonatal death.[2, 7, 8] The combined incidence of infantile and perinatal HPP has been estimated to be between 1 in 100,000 and 1 in 300,000 births.[2, 7, 8, 11] Adult HPP presents less dramatically, and its prevalence is less certain. By estimating disease penetrance in heterozygotes for dominant mutations, Mornet and colleagues recently suggested a prevalence of 1 in 6370 for moderate adult forms of HPP in Europeans.[11] Moderate adult forms in that study included adult survivors of childhood HPP, those diagnosed in adulthood, and those with odontohypophosphatasia only. Nevertheless, the prevalence and clinical spectrum of adult HPP is likely still not fully appreciated because the condition may be asymptomatic or be misdiagnosed as idiopathic pseudogout or more likely, postmenopausal osteoporosis.[12-29] Classic adult HPP is characterized by premature dental loss, extraskeletal calcification, chondrocalcinosis, and slowly healing metatarsal and subtrochanteric femur stress fractures.[2, 7, 28] A recent 32-year retrospective review of 22 unrelated subjects diagnosed with HPP in adulthood at a single institution suggested an adverse female gender effect on the musculoskeletal phenotype.[28] In the appropriate clinical setting, HPP can be diagnosed by the presence of a persistently low serum ALP and elevated ALP substrates in blood (pyridoxal-5-phosphate [PLP] or phosphoethanolamine [PEA]) or in urine (PEA or inorganic pyrophosphate [PPi]), although urine PPi testing remains investigational.[1, 2]

Based on observations made in clinical practice, we wondered whether some adults with persistent hypophosphatasemia could be harboring previously unrecognized HPP and that the adult form of HPP may be more common than previously appreciated. In a previous feasibility study, we identified 21 adults with persistent hypophosphatasemia in whom the diagnosis of HPP was considered possible based on review of their clinical, laboratory, and radiographic records.[30] Subsequently, 5 of these 21 adults were further evaluated and the diagnosis of adult HPP established biochemically in 3. The primary objectives of our retrospective study are to estimate the prevalence of persistent hypophosphatasemia in a large, rural, multispecialty clinic population; to characterize the clinical and radiograph findings of adult patients with persistent hypophosphatasemia; and to compare these findings with those expected to be present in the adult form of HPP.

Materials and Methods

The Marshfield Clinic (MC) Institutional Review Board approved use of its medical records to characterize the prevalence and clinical and radiograph findings of adults with persistent hypophosphatasemia. Patients who had previously denied or restricted access to their MC electronic medical record (EMR) for research purposes were excluded from the study. The MC EMR is a state-of-the-art, comprehensive record that includes ICD-9 diagnosis codes from 1960, CPT codes from 1985, laboratory data from 1980, and clinical and radiographic data from approximately 1990. When necessary and available, additional information was retrieved from the MC paper medical record. Previous research projects have demonstrated that diagnosis coding at MC is sufficiently accurate to successfully address a variety of research questions.[31-34] The study population was created by first interrogating the MC EMR for all patients with at least two serum ALP values ≤30 IU/L (normal 40 to 125 IU/L) recorded between 2002 and 2012 (Fig. 1). The lower cut-point was chosen to increase the likelihood that hypophosphatasemia was physiologically significant and to keep the task of manual chart abstraction feasible; only 0.2% of serum ALP values in adults met this threshold. A requirement of 2 low-serum ALP values was implemented to reduce the likelihood of analytic error. Next, all electronically available serum ALP values were plotted against time for each subject, and each individual plot was visually inspected (by author FEM) to determine the temporal pattern of the qualifying serum ALP values (Fig. 2). Those patients with persistent hypophosphatasemia (serum ALP values persistently ≤30 IU/L) constituted the study population, whereas patients whose serum ALP values were largely normal and only occasionally ≤30 IU/L were excluded. Two control populations were then constructed; the first, larger control population was used for the electronic interrogation of the EMR and the second, smaller control population for the manual chart abstraction. The larger control population consisted of all MC primary care patients seen over the defined study period who had at least 2 serum ALP values of record, none of which were <40 IU/L. The smaller control population consisted of a random extraction from the larger control population selected 2:1 with respect to the study population.

Figure 1.

Explanation of study design, definition of larger and smaller control populations, and definition of study population of adults with persistent hypophosphatasemia.

Figure 2.

Illustration of serum ALP values of two patients, each having at least two values ≤30 IU/L, plotted against 27 years of observation. Patient 1 (closed circles) had serum ALP values persistently ≤30 IU/L, was deemed to have persistent hypophosphatasemia, and qualified as a study subject. Patient 2 (open circles) had serum ALP values only occasionally ≤30 IU/L and did not qualify as a study subject.

Between 1988 and 2004, the MC Joint Venture Laboratory employed a Hitachi 747 (Indianapolis, IN, USA) analyzer to measure serum ALP activity. In 2004, the JVL switched platforms to the Coulter-Beckman (Brea, CA, USA) DxC chemistry system[35] and clinically acceptable correlation and comparison was demonstrated between the two instruments (personal communication, Dr Annu Khajuria, Marshfield Clinic Joint Venture Laboratory, January 2014). Both instruments measure ALP activity by a kinetic rate method in which a colorless organic phosphate ester substrate (p-nitrophenyl phosphate) is hydrolyzed by ALP to the yellow-colored product p-nitrophenol and phosphate at pH 10.3, thus the term “alkaline.” Changes in absorbance at 410 nanometers are directly proportional to the enzymatic activity of ALP.[1, 35] The normal range for serum ALP in adults is 40 to 125 IU/L.[35]

Clusters of ICD-9 diagnosis, CPT procedure, and radiographic codes that were considered likely to be present in an adult HPP population were constructed (Supplemental Tables S1–S3).{TBL S1}{TBL S2}{TBL S3} The EMR was then interrogated for these code clusters and the prevalence of matching clusters compared between the study and larger control populations. The ICD-9 code assigned to HPP is 275.3, which also codes for familial hypophosphatemia, vitamin D-resistant rickets, and “disorders of phosphate metabolism.” In clinical practice, “disorders of phosphate metabolism” account for the overwhelming use of ICD-9 code 275.3 and almost invariably indicates either hyperphosphatemia or hypophosphatemia. Therefore, when interrogating the EMR for HPP with ICD-9 code 275.3, we censored those with simultaneous evidence of hyperphosphatemia or hypophosphatemia. Medical Encounter Coding by Clinical Application (MECCA) is a proprietary MC diagnosis retrieval system available from the early 1990s that maps clinical diagnosis terms to appropriate ICD-9 codes. The MECCA terms assigned to HPP are “hypophosphatasia” and “hypophosphatasemia.” MECCA was searched for HPP using these terms.

Next, the EMR (and paper medical record when indicated and available) of patients with persistent hypophosphatasemia was manually abstracted to document clinical, laboratory, densitometric, and radiographic findings that might suggest the presence of adult HPP and results compared with the smaller control population. The radiologist's interpretive narrative of record was considered complete and final when images were unavailable for review; these results were therefore blinded. When archived images were available for review on the Picture Archiving and Communication System, they were reexamined for evidence of chondrocalcinosis, calcific periarthritis, enthesopathy, diffuse idiopathic skeletal hyperostosis (DISH), and exostosis (by principal investigator [PI] FM); these results were not blind to the subject's serum ALP status. Indeterminate radiographic findings on PI reexamination were adjudicated by an MC board-certified musculoskeletal radiologist blind to the original radiologist's interpretation, to results of the PI's reexamination, and to the subject's serum ALP status. Bone mineral density (BMD) testing at all MC facilities is performed using dual-energy X-ray absorptiometry (DXA) on GE/Lunar Prodigy (Madison, WI, USA) densitometers. The earliest available, technically valid, site-specific BMD Z-score was assigned to each individual patient. Z-scores were chosen because they represent the number of standard deviations a measured BMD result varies from a manufacture control population matched for age, race, gender, and body mass index.

The primary statistical analyses were conducted using SAS (SAS Institute, Inc., Cary NC, USA) version 9.2 statistical software and based on logistic regression, with stratification by gender and adjustment for age with covariates (using restricted cubic splines to allow for very general age relationships). Firth's procedure was used for estimates from the logistic models to reduce the possible bias resulting from relatively small event numbers.[36, 37] Model results are presented as odds ratios (OR) with 95% confidence intervals based on the profile likelihood. Results are deemed statistically significant at the 5% level (p < 0.05) without adjustment for multiple comparisons.

Results

There were 885,165 individual patients seen at MC between January 1, 2002, and December 31, 2011. Of these, 458,767 patients (51.8%) had at least one serum ALP value recorded, and 413,761 of those were adults at the time of their first serum ALP measurement. During the same period, 2,584,051 serum ALP measurements were made of which 31,830 (1.2%) fell at or below the normal adult reference range (≤40 IU/L). To illustrate the consistency of these results, among 3836 patients retested within 90 days of a low result, the second result was also ≤40 for 1586 (41%). In total, of those patients with at least one serum ALP measurement, 5190 (1.1%) had at least two values ≤40 IU/L and 809 (0.2%) had at least two values ≤30 IU/L. Among those 809 patients, 269 (171 female; 98 male) had persistent hypophosphatasemia (serum ALP values persistently ≤30 IU/L), and all but 1 of these were adults. The sole pediatric patient had an established diagnosis of perinatal HPP. Thus, persistent hypophosphatasemia was found in approximately 1 of every 1544 adults (0.06%) in this patient population.

The average ages at the time of the first and last qualifying serum ALP measurements were 46 and 55 years, respectively, and the majority (64%) was female (Table 2). A higher percentage of control subjects were male, and controls were on average a few years older at first ALP measurement, but all groups showed broad distributions in date of birth, and the groups were similar with respect to total numbers of ALP measurements.

Table 2. Descriptive Characteristics of Study Subjects and Controls by Group ([n] for Subjects)
 Controls
Persistent hypophosphatasemia (n = 269)Electronic interrogation (n = 270,353)Manual abstraction (n = 548)
Male gender (%)36.446.252.2
Birth cohort (%)% (n)% (n)% (n)
Before 1929 (26.3)17.5 (47)25.1 (67,911)26.3 (144)
1930–1939 (16.6)12.3 (33)14.3 (38,643)16.6 (91)
1940–1949 (14.1)17.2 (46)15.7 (42,443)14.1 (77)
1950–1959 (15.9)19.3 (52)15.8 (42,743)15.9 (87)
After 1960 (27.2)33.8 (91)29.1 (78,613)27.2 (149)
1st ALP46.6 (<193)50.2 (<1105)50.7 (<195)
Last ALP55.7 (2100)57.8 (<1108)58.3 (<1101)
No. of ALP values% (n)% (n)% (n)
220.4 (55)25.5 (69,016)23.9 (131)
315.6 (42)14.7 (39,614)15.0 (82)
49.3 (25)9.9 (26,653)10.8 (59)
≥554.6 (147)50.0 (135,070)50.4 (276)

Electronic interrogation of the EMR showed ICD-9 diagnosis clusters for CPDD and disorders of phosphorus metabolism more frequently in both males and females with persistent hypophosphatasemia than in control subjects but results for other diagnosis clusters varied by gender (Table 3). Female patients with persistent hypophosphatasemia showed more CPT codes for surgical repair of femur and metatarsal fractures than female controls (OR = 4.6, p < 0.001) but this was not found for male patients (OR = 2.7, p = 0.227). When genders were combined, this difference remained statistically significant (p = 0.001; data not shown). Male patients with persistent hypophosphatasemia showed a higher prevalence of hypermobility (OR = 2.3, p = 0.037) and “other myopathy” (OR = 4.8, p < 0.001) than male controls. The clinical term matching the ICD-9 code for “other myopathy” revealed a variety of conditions including “eye,” “ICU,” “statin,” and “other” myopathies as well as “generalized weakness.” Female patients with persistent hypophosphatasemia showed code clusters for metatarsal/femur fractures more often than female controls (OR = 2.5, p < 0.001). ICD-9 code clusters for rickets, osteomalacia, and fracture malunion and nonunion were rarely encountered (≤1% each) and no more frequently in subjects than in controls for either gender (Table 4). In general, CPT codes for musculoskeletal radiology procedures were not more frequent in study subjects than controls but when analyzed by gender and anatomic site, male patients with persistent hypophosphatasemia showed more frequent radiographic procedures of wrists (OR = 2.3, p = 0.001) and knees (OR = 1.6, p < 0.03) than male controls.

Table 3. Odds Ratios for Likelihood of a Matching ICD-9 Diagnostic Code Cluster in Study Subjects Compared With Controls Reported by Gender ([n] for Subjects)a
 Odds ratio (95% CI)p Value
  1. aStudy subjects: 171 females, 98 males. Controls: 145,450 females, 124,903 males.
Calcium pyrophosphate deposition disease
Female (7)4.5 (2.0, 9.0)<0.001
Male (5)3.8 (1.4, 8.4)0.003
Disorders of phosphorus metabolism
Female (11)4.9 (2.6, 8.6)<0.001
Male (5)3.0 (1.1, 6.6)0.011
Metatarsal, femur, and stress fractures
Female (13)2.5 (1.4, 4.3)<0.001
Male (3)2.0 (0.6, 5.1)0.191
Hypermobility
Female (6)0.9 (0.4, 1.8)0.743
Male (6)2.3 (0.9, 4.8)0.037
Other myopathy
Female (3)1.1 (0.3, 2.7)0.887
Male (8)4.8 (2.2, 9.2)<0.001
Table 4. Statistically Nonsignificant Odds Ratios for Likelihood of a Matching ICD-9 Diagnostic Code Cluster in Study Subjects Compared With Controls by Gender ([n] for Subjects)a
 Odds ratio (95% CI)p Value
  1. aStudy subjects: 171 females, 98 males. Controls: 145,450 females, 124,903 males.
Fracture malunion and nonunion
Female (2)1.6 (0.3, 4.6)0.444
Male (0)0.6 (0.0, 4.1)0.718
Rickets and osteomalacia
Female (0)0.3 (0.0, 2.3)0.447
Male (1)1.5 (0.2,5.5)0.615
Edentulation
Female (16)1.3 (0.7,2.1)0.365
Male (6)1.8 (0.7, 3.8)0.167
Developmental delay
Female (17)1.3 (0.7, 2.1)0.361
Male (11)1.2 (0.6, 2.3)0.495
Kyphoscoliosis
Female (1)2.5 (0.34, 9.0)0.265
Male (0)3.0 (0.0, 20.6)0.439
Other osteoporosis
Female (1)2.3 (0.3,8.3)0.311
Male (0)3.1 (0.0, 21.7)0.418
Periodontal disease
Female (8)1.2 (0.5, 2.2)0.666
Male (4)1.1 (0.4, 2.5)0.868

Manual abstraction of the medical record indicated that 154 of 269 subjects (43%) with persistent hypophosphatasemia had at least one musculoskeletal report or image available for review and most had fewer than five. Similar rates were observed in the control group. Radiographic reports and images showed that patients with persistent hypophosphatasemia showed significantly higher prevalence of chondrocalcinosis, calcific periarthritis, enthesopathy, and DISH than in controls (Table 5). Sites most affected by chondrocalcinosis were pubic symphysis, hips, knees, wrists, and less frequently shoulders (Fig. 3A–F). Proven or suspected crystalline arthritis occurred in 13% of patients with persistent hypophosphatasemia, with a greater likelihood in female patients (OR = 6.8) than in male patients (OR = 1.8) compared with controls (Table 6). “Suspected” crystalline arthritis was a rheumatologist-confirmed diagnosis characterized by episodic, sterile, inflammatory mono- or oligo-arthritis in which no crystal was identified by polarization microscopy of synovial fluid. Of the confirmed cases of crystalline arthritis, half were owing to monosodium urate monohydrate (gout) and half owing to calcium pyrophosphate (pseudogout). Enthesopathy was more frequently documented at the knees, shoulder, and pelvis in subjects with persistent hypophosphatasemia than in controls (p < 0.001). Calcific periarthritis was most frequently documented at the hips, knees, pelvis, and shoulders in subjects than in controls (p < 0.001). Odds ratios for DISH in female and male patients with persistent hypophosphatasemia were 19.3 and 8.4 (p < 0.001), respectively, compared with controls (Table 5). “Other musculoskeletal,” defined as an established diagnosis of a primary rheumatic disease or sufficient musculoskeletal complaints to warrant specialty consultation, additional imaging, or treatment intervention, was recorded in about one-third of patients with persistent hypophosphatasemia and was significantly greater (p < 0.001) than controls (Table 6). Male subjects with persistent hypophosphatasemia showed a higher prevalence of (any) fractures compared with male controls (OR = 2.3, p = 0.001), but this was not true of females (OR = 1.1, p = 0.628). Most common recorded fractures were radius, tibia, femur, and metatarsal. Fracture healing, when reported, was uncomplicated. Orthopedic surgery was common in subjects of both genders (22% female; 32% male) but only male subjects showed significantly higher prevalence than male controls (OR = 2.1, p = 0.006). Most frequently reported orthopedic surgeries were fracture repair, total joint arthroplasty, and “other.” DXA had been performed in 58 (22%) of study subjects with persistent hypophosphatasemia. Average BMD Z-scores were close to zero or marginally positive, indicating near normal BMD when compared with manufacturer database matched for age, race, gender, and body weight (Table 7). Kidney stones, fibromyalgia, and dystocia showed similar prevalence between subjects and controls (data not shown). Early edentulation, although common (9.4% female; 6.1% male), was not significantly different from controls (Table 4). Hyperphosphatemia (serum phosphate >4.5 mg/dL) was recorded at least once in 169 in 269 (62.8%) patients with persistent hypophosphatasemia but was usually mild (= 5.2 mg/mL, range 4.6 to 7.9 mg/mL), intermittent (= 3.8 events/patient), and not sustained. Serum 25-hydroxyvitamin D (25-OHD) values were recorded in 33 study subjects with persistent hypophosphatasemia, and the average first recorded 25-OHD value was 39 ng/mL (range 12 to 87 ng/mL). Only 2 patients had initial serum 25-OHD values below 20 ng/mL (12 and 13 ng/mL).

Table 5. Odds Ratio of Radiographic Findings Based on Review of Radiographic Reports and Images in Study Subjects (n = 171 Females, 98 Males) Compared With Controls (n = 262 Females, 286 Males) ([n] for Subjects)
 Odds ratiop Value
(95% CI)
Chondrocalcinosis
Female (23)7.2 (3.2, 17.8)<0.001
Male (20)8.3 (3.6, 20.4)<0.001
Enthesopathy
Female (38)2.7 (1.6, 4.7)<0.001
Male (28)3.1 (1.7, 5.6)<0.001
Calcific periarthritis
Female (35)12.2 (5.5, 29.7)<0.001
Male (22)32.7 (10.2, 165.6)<0.001
Diffuse idiopathic skeletal hyperostosis
Female (16)19.3 (5.5, 102.9)<0.001
Male (19)8.4 (3.6, 21.4)<0.001
Exostosis
Female (4)2.5 (0.5, 14.9)0.287
Male (3)6.2 (1.0, 64.5)0.098
Figure 3.

Radiographs of subjects with persistent hypophosphatasemia demonstrating (A) periarticular mineralization at the distal interphalyngeal joint of an index finger, (B) diffuse chondrocalcinosis of meniscal cartilage of the knee, (C) prominent mineralized enthesopathy at the tibia, (D) prominent calcaneal and retrocalcaneal mineralized spurs and vigorous dorsal foot mineralization, (E) chondrocalcinosis of the pubis symphysis, and (F) prominent mineralized enthesopathy at the iliac crest.

Table 6. Clinical Features of Study Subjects (n = 171 Females, 98 Males) Compared With Controls (n = 262 Females, 286 Males) Based on Manual Review of the Medical Record ([n] for Subjects)
 Subjects n (%)Controls n (%)Adjusted odds ratio (95% CI)p Value
  1. aMusculoskeletal complaints sufficient to warrant specialty consultation, additional imaging or treatment intervention, and diagnosis of significant rheumatic disease.
Other musculoskeletala
Female64 (37.4)18 (6.9)7.8 (4.5, 14.3)<0.001
Male31 (31.6)31 (10.8)3.6 (2.0, 6.3)<0.001
Fracture
Female50 (29.2)79 (30.2)(0.7, 1.7)0.628
Male37 (37.8)60 (21.0)2.3 (1.4, 3.7)0.001
Orthopedic surgery
Female38 (22.2)49 (18.7)1.4 (0.9, 2.3)0.156
Male31 (31.6)49 (17.1)2.1 (1.2, 3.6)0.006
Crystalline arthritis
Female16 (9.4)6 (2.3)6.8 (2.6, 20.1)<0.001
Male18 (18.4)30 (10.5)1.8 (0.9, 3.4)0.074
Edentulous
Female19 (11.1)29 (11.1)1.5 (0.8, 2.9)0.248
Male18 (18.4)27 (9.4)2.2 (1.1, 4.2)0.024
Table 7. Initial BMD Z-Scores for Lumbar Spine and Proximal Femur in Study Subjects ([n] for Subjects)
 nZ-score (mean)Z-score (range)
Lumbar spine58+0.6–4.0 to +5.4
Femoral neck55  
Left520.00–1.5 to +3.4
Right510.00–2.0 to +3.4
Mean550.00–1.7 to +4.2
Total hip43  
Left43+0.21–2.4 to +4.2
Right41+0.13–4.0 to +4.3
Mean43+0.15–2.4 to +4.3

Discussion

An elevated serum ALP value is a common indicator of skeletal or hepatobiliary disease for which the differential diagnosis is familiar to most clinicians.[3] In contrast, a low serum ALP value is uncommon in clinical practice and, in the absence of analytic error, the reported causes are wide ranging, sometimes anecdotal and often unfamiliar.[1-7] Our finding that 1.2% of all serum ALP values fell below the lower range of normal for adults is somewhat higher than previous reports[4, 5] and may reflect a more liberal test-requesting culture amongst MC providers or a somewhat sicker population. Far fewer adult patients in this population (0.06%) had persistent hypophosphatasemia, but our results indicate that the clinical and radiograph findings of these patients resemble those of the adult form of HPP.[2, 7, 26, 28, 29] Both populations show more frequent radiographic extraskeletal calcification, DISH, and crystalline arthritis. We also found gender differences similar to those recently reported in adults with HPP by others.[28] Our results showed more stress fractures (females), (any) fracture or orthopedic surgery (males), and more orthopedic repair of metatarsal and femur fractures (females) than the general adult patient population. If some adult patients with persistent hypophosphatasemia are harboring HPP, it may be that their disease expression is comparatively mild because we did not detect an increased prevalence of periodontal disease, edentulation, pseudofracture, or fracture nonunion. These results support recent suggestions that moderate adult forms of HPP may be substantially more common than previously appreciated[11] and that low serum ALP values in ostensibly normal men may have physiologic consequences for skeletal health.[38] Consistent with our observations, calcific periarthritis was recently described as the only manifestation of adult HPP in three middle-aged sisters.[29]

Bone anti-remodeling agents (estrogen, bisphosphonates, denosumab) are associated with modest reductions of bone-specific ALP isoforms (BSAP), but serum ALP values rarely fall below the lower range of normal[39] because hepatic ALP isoforms, which account for approximately half of serum ALP in adults, are unaffected by bisphosphonates. For similar reasons, adynamic renal osteodystrophy may be associated with a relatively low, but rarely an absolutely low, serum ALP.[40] Starvation and severe uncorrected nutrient or micronutrient deficiencies (protein, zinc, vitamins B12, and C) and electrolyte abnormalities have been associated with transiently low serum ALP levels.[3] Untreated profound hypothyroidism, Celiac disease, Cushing's disease, and obstructive uropathy have rarely been associated with transiently low serum ALP, but more often the value is normal or even elevated, although comprehensive studies have not been reported.[1, 3] In these instances, serum ALP normalizes with treatment of the underlying condition.[3] Precipitous lowering of serum ALP may indicate profound physiological stress and has been associated with increased short-term mortality.[6]

Calcium pyrophosphate and hydroxyapatite crystal deposition are controlled by a complex interplay of ALP with other proteins such as ENPP-1 (ectonucleotide pyrophosphatase/phosphodiesterase 1), ANKH (human analogue of progressive ankylosis protein), and PiT-1, which determines the ratio of phosphate (Pi) to PPi in bone.[41-46] PPi is a potent inhibitor of mineralization, and excess extracellular PPi is associated with osteomalacia. Conversely, Pi promotes extraskeletal mineralization. Calcium pyrophosphate crystal formation may be favored when the ambient extracellular Pi:PPi ratio is < 3:1 as in HPP, whereas hydroxyapatite crystal formation favored when this ratio exceeds 100:1.[45] Thus, persistently low extracellular ALP activity would favor calcium pyrophosphate crystal deposition, and this is consistent with our findings. Nevertheless, polarization microscopy only identified a pathogenic crystal in the synovial fluid of half our presumptive cases of acute crystalline arthritis. Other basic calcium crystals, including hydrogen phosphate dihydrate, hydroxyapatite, and even brushite, have been reported to cause acute crystalline arthritis but cannot be visualized using polarization microscopy, which could explain these results.[41, 45] Mseleni joint disease, a rare and crippling osteoarthropathy confined to a remote area in the Maputaland region in northern Kwazulu Natal, South Africa, is marked by a persistently low serum ALP and prolonged mineralization lag time.[47, 48] The condition shows a female predominance and is characterized by protrusio acetabula and exuberant osteophytosis, but there have been no reports of basic calcium crystal deposition, ALPL analysis, or of ALP substrate accumulation in this disease.[47]

Bone mineral density in our adult patient population with persistent hypophosphatasemia was similar on average to an age-, gender-, race-, and BMI-matched control population, although the range was wide. In spite of an increased risk of fracture in patients with HPP and other disorders of mineralization, BMD is not usually low and may even be elevated.[26, 28, 38, 49-51]

Perhaps the most clinically important reason to be vigilant for persistent hypophosphatasemia is the admonition against using potent anti-remodeling agents (particularly bisphosphonates and possibly denosumab) in patients with HPP.[21, 23, 25, 27-29] Atypical subtrochanteric femur (AFF) and some metatarsal fractures reported after prolonged bisphosphonate exposure bear radiographic and clinical resemblance to fractures seen in the adult form of HPP.[23] In at least one case, an adult carrier of previously unrecognized HPP developed an AFF while taking bisphosphonates.[27] Bisphosphonates are structural analogues of PPi, and both have been associated with impaired mineralization.[51-54] Furthermore, amino-bisphosphonates have been shown to inhibit TNSALP activity by binding Zn++ and Mg++.[55] These concerns alone should prompt clinicians to take notice of serum ALP values in all patients before prescribing bisphosphonates. Teriparatide (PTH1-34) stimulates BSAP and has been used off-label in a few patients with HPP to promote fracture healing, although reports of benefit have been inconsistent.[22, 24, 25] Teriparatide should be safe for patients with persistent hypophosphatasemia when its use is otherwise indicated. Clinicians should consider the diagnosis of adult HPP when encountering adults with recurring or poorly healing metatarsal or femur shaft fractures but also in the differential diagnosis of CPDD, DISH, calcific tendonopathy, and crystalline arthritis, particularly when the latter occur in younger women.[2, 7, 26, 28, 29] In the presence of persistent hypophosphatasemia and in the appropriate clinical context, a diagnosis of HPP can be made by finding an elevated serum PLP, provided exogenous vitamin B6 has not been taken within 1 and preferably 2 weeks. When serum ALP is only minimally low or a carrier state is suspected, an abnormal vitamin B6 challenge test has been reported to support the diagnosis.[10] In the rare and still puzzling case, molecular diagnosis is available through several commercial laboratories.

This retrospective study has both limitations and strengths. The MC population is predominantly of northern European descent and is not ethnically diverse.[33] Inherent in our research method are the possibilities of coding inaccuracies and diagnosis retrieval errors as well as an ascertainment bias because only half the population ever had serum ALP measured. The estimated prevalence of persistent hypophosphatasemia could be overstated because of reliance on patient contact and diagnostic testing because a larger population denominator that included persons never seeking medical care would presumably decrease the prevalence estimate. Determination of a more complete phenotype and a more accurate estimate of incidence and prevalence of adults with persistent hypophosphatasemia would require the prospective diagnostic evaluation of an extremely large random population, which would be challenging given the infrequency of the condition. The similarities we note between the clinical and radiographic finding of adults with persistent hypophosphatasemia and the adult form of HPP are not supported by systematic measurements of elevated ALP substrates, TNSALP, or by ALPL gene analysis as, with the few noted exceptions, our subjects with persistent hypophosphatasemia have not been evaluated biochemically for HPP. Finally, some radiographic interpretations were not blind to the subject's serum ALP status. Conversely, the demographically stable nature of this population, its high utilization of MC services, the robustness of the MC EMR, and the very large number of surveyed patients constitute study strengths.

In conclusion, we found that approximately 1 in 1544 adult patients in this large, rural, multispecialty health clinic have persistent hypophosphatasemia. As a group, these patients have more radiographic evidence of chondrocalcinosis, calcific periarthritis, enthesopathy, and DISH than the general adult patient population (p < 0.001). The EMR of male and female adult patients with persistent hypophosphatasemia showed more frequent ICD-9 codes for CPDD and disorders of phosphorus metabolism (p ≤ 0.011) than the general adult patient population and in females more frequent ICD-9 codes for metatarsal/femur stress fractures (p < 0.001). Thus, the clinical and radiologic findings of adult patients with persistent hypophosphatasemia resemble those of the adult form of HPP. These reported associations do not prove causality, but we hope that living patients from this cohort will allow further clinical, biochemical, and radiographic characterization and gene analysis. Clinicians should take notice of persistent hypophosphatasemia and consider a carrier state or a possible diagnosis of or carrier state for HPP in these adults. Caution should be exercised when prescribing potent anti-remodeling therapy, particularly bisphosphonates, in this population. Further evaluation of adult patients with persistent hypophosphatasemia is warranted.

Disclosures

All authors state that they have no conflicts of interest.

Acknowledgments

This research was funded by a grant from Alexion, Incorporated.

Drs Jose Luis Millan, Agnes Champigneulle, and Nerissa Kreher provided helpful commentary in preparing this manuscript.

Authors' roles: Study design: FM, RB, and JF. Study conduct: FM. Data collection: FM and JF. Data analysis: FM, RB, and JF. Data interpretation: FM and RB. Drafting manuscript: FM. Revising manuscript content: FM and RB. Approving final version of manuscript: FM and RB. FM takes responsibility for the integrity of the data analysis.

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