Correspondence to Dr Guillaume Larid; [email protected]
WHAT IS ALREADY KNOWN ON THIS TOPIC
Hypophosphatasia is a rare and often undiagnosed disorder.
Low alkaline phosphatase (ALP) values are overlooked by a majority of clinicians.
WHAT THIS STUDY ADDS
In this multicentre study, low ALPs are poorly recognised by clinicians.
70.8% of patients treated with bisphosphonates never underwent ALP measurement before treatment initiation.
Using a combination of multiple evocative symptoms to select patients for genetic testing seems interesting as a means of increasing the diagnosis rate and control healthcare costs.
Mild to moderate adult hypophosphatasia may be more frequent than previously thought.
HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY
Sensitisation of clinicians to ALP values is needed.
ALP measurement should be mandatory in the secondary osteoporosis investigations before bisphosphonate treatment initiation.
Introduction
Hypophosphatasia (HPP) is a rare genetic skeletal disease due to an inherited metabolic disorder caused by mutations of the ALPL gene coding for tissue non-specific alkaline phosphatase (TNSALP).1 Prevalence of severe forms is estimated as ranging from 1/100 000 to 1/300 00,2 while prevalence of mild HPP was estimated at 1/6370 in Europe.3
Six forms of the disease have been defined: perinatal severe HPP, perinatal benign HPP, infantile HPP, childhood HPP, adult HPP and odontohypophosphatasia.2 4
In adults, clinical manifestations are dominated by fractures and joint disease. The most evocative fractures are localised at the metatarsals. These fractures are usually recurrent, with delayed consolidation potentially leading to pseudarthrosis. Other typical fractures affect the femoral diaphysis and occur mainly in the lateral cortex of the subtrochanteric region. Joint disease is represented mainly by calcium pyrophosphate deposition disease.5
While elevated ALP is usually taken into account by clinicians, low ALP levels are easily overlooked. A monocentre study in a tertiary care hospital in France found that notification was given in only 3% of cases.6
The aetiologies of low ALP are multiple and differ according to hypophosphatasaemia temporality.7 8 Furthermore, these causes are often unknown by clinicians.
The aims of this study were to estimate the recognition of hypophosphatasaemia in rheumatology and internal medicine departments, to analyse the characteristics of the population presenting persistently low ALP measurements and to estimate the number of patients highly suspected of adult HPP. Secondary analyses were performed to compare patients with persistently low ALP measurements while using or not using bisphosphonates, and those with or without an identified cause of persistently low ALP levels.
Materials and methods
Study design
This retrospective, descriptive and multicentre study included patients from the University hospitals of Poitiers, Nantes, Rennes, Brest, Angers and Tours. It consisted of the detection of low ALP measurement at least twice among patients hospitalised in the departments of Rheumatology and Internal Medicine between 1 July 2007 and 1 July 2017. No limit in duration between two measurements was necessary. We followed STrengthening the Reporting of OBservational studies in Epidemiology (STROBE) instructions throughout this work.
In France, internal medicine departments are departments encompassing a combination of geriatrics, clinical immunology, infectious diseases, oncohaematological diseases and rheumatology subspecialties dealing with systemic autoimmune and autoinflammatory disorders.
Patients
The listing of patients was established from records of the Biochemistry Department of several French university hospitals by laboratory database request on the criteria of low ALP values ≤35 U/L (normal range: 40–120 IU/L). Low cut-off values were identical for men and women in the laboratories that performed the analysis. A minimum of 2 low ALP values (≤35 IU/L) was required to minimise the likelihood of an analytical error; 35 U/L was defined as it is an average between the lower bounds of adult normal values and less exclusive than previous studies have set the limit at 30 UI/L.6 9 Patients who had previously denied or restricted access to their record for research purposes and aged less than 18 years were excluded from the study.
Once authorisations were given, paper and electronic medical records were used to search patient history, symptoms, laboratory results (basic tests, calcium phosphate metabolism and specialised blood tests (bone ALP)), bone densitometry, X-rays, CT- scan and MRI results. If done, the genetic test was notified.
Bone demineralisation was defined as a T-score <−2.5 SD.
Chondrocalcinosis and hydroxyapatite deposition disease were diagnosed based on the aspect of the calcifications visualised on X-rays.
Only non-traumatic fractures were considered in the analyses.
Scoliosis was considered as present if described in the radiologist reports of spine imaging.
Aetiologies for low ALP were defined as follows:
Corticosteroids were considered as a possible cause of hypophosphatasaemia if patients received at least a very high dose of corticosteroids (>100 mg per day) using the standardised nomenclature for glucocorticoid dosages by Buttgereit et al.10
Severe anaemia was defined as haemoglobin (Hb) <60 g/L.
Pernicious anaemia was considered if patients were currently not substituted.
Hypothyroidism was considered if patients were not substituted and thyroid-stimulating hormone (TSH) was higher than 4 mUI/L.
Hepatic insufficiency was considered if prothrombin time was lower than 50%.
Hypervitaminosis D was considered if 25-OH vitamin D was higher than 150 ng/L.
Hypomagnesaemia was considered if magnesium level was lower than 0.7 mmol/L.
Vitamin C insufficiency was considered for levels lower than 2.5 mg/L.
Zinc insufficiency was considered for levels lower than 9 µmol/L.
Cushing disease, coeliac disease and Wilson disease were considered only if they were not currently being treated or at equilibrium.
Intensive care stay was considered if it was an actual stay or less than 1 month before.
Ongoing oncohaematological disease, bisphosphonate treatment, denosumab treatment, septicaemia, inflammatory disease flare and intravenous immunoglobulins were considered as potential causes of low ALP.
To determine the number of patients for whom low ALP ≤35 U/L was recognised and noted in their records, the discharge summary, the diagnosis written in the letter and/or the ICD-10 (International Classification of Diseases 10th Revision) code were used.
Patients were defined as possible HPP if they exhibited at least three symptoms evocative of HPP in addition to persistent low ALP (arthralgia, fractures, stress fractures, low bone density, dental abnormalities, chondrocalcinosis, scoliosis, high B6 levels or high urinary phosphoethanolamine).
Biochemical assays
Both of the instruments measure ALP activity by a kinetic rate method in which a colourless organic phosphate ester substrate (nitrophenylphosphate) is hydrolysed by ALP to the yellow-coloured product p-nitrophenol and phosphate at pH of 10.3, thereby explaining the term ‘alkaline’. Changes in absorbance at 410 nm are directly proportional to the enzymatic activity of ALP. A requirement of two low ALP values (≤35 U/L) was set so as to minimise the likelihood of low ALP results due to analytic error. For each selected patient, all previous ALP values against time were visually examined to determine the temporal pattern of the qualifying serum ALP values and to separate two groups of patients. When the temporal pattern of ALP values indicated a precipitous fall from usually normal values, the patient was considered to have acute hypophosphatasaemia. Diagnostic conditions and circumstances associated with acute hypophosphatasaemia were analysed. Laboratory used Glims, JMP or DXLab software.
When the temporal pattern of ALP values indicated a persistently low ALP or only 2 values, both of them under 35 U/L, the patient was considered to have persistent hypophosphatasaemia. More precise analysis was carried out to identify detailed patient history, symptoms, laboratory results (basic tests including calcium phosphate metabolism exploration and specialised blood tests such as bone ALP), bone densitometry, X-rays, CT scan and MRI results and genetic test when they had been made. To determine the number of patients in whom persistently low ALP ≤35 U/L was recognised, the discharge summary, the written diagnosis and/or the ICD-10 code (E833) were used.
Statistical methodology
Qualitative data were expressed as percentages and quantitative data as means±SD. Analysis was conducted using the Student’s t-test (or Wilcoxon, as appropriate) for quantitative data and χ² (or Fisher’s exact test, as appropriate) for qualitative data. A p value of 0.05 was considered as significant. Statistical analysis was performed by using SAS software, V.9.1 (SAS Institute) and GraphPad Prism (GraphPad Software, California).
Patient and public involvement
Patients or members of the public were not involved in the design, or conduct, or reporting, or dissemination plans of the research.
Results
Population characteristics
Between 1 July 2007 and 1 July 2017, 144 242 ALP measurements were performed; 56 321 hospitalised patients had at least 2 serum ALP measurements. Inclusion period differed according to the centres, with mean inclusion time of 8.42 years (±2.478). A total of 664 patients hospitalised in the rheumatology and internal medicine departments of the University Hospitals of Poitiers, Nantes, Rennes, Brest, Angers and Tours had at least two ALP values below or equal to 35 IU/L (figure 1 and table 1). There was a difference in the sex ratio with 57.8% of female patients (208/360) in internal medicine departments vs 66.8% in rheumatology departments (203/304) (p=0.017). Prevalence of all-cause hypophosphatasaemia was 1.18%. Among the patients, 182 (27.4 %) had persistently low serum ALP levels, representing a general prevalence of 0.32% for persistent hypophosphatasaemia (182/56321), while 482 patients (72,6%) had fluctuating serum ALP values, at least two of which were below or equal 35 IU/L, which representing prevalence of 0.86%.
Table 1
Population characteristics (n=664)
| Age (years; mean±SD) (n=660) | 60.3 (±19.4) |
| Male sex | 253 (38.1%) |
| Weight (n=446) (kg; mean±SD) | 67.4 (±17.4) |
| Height (n=338) (cm; mean±SD) | 164.4 (±10.7) |
| Mean value of ALP (IU; mean±SD) | 29.5 (±4.0) |
| Transient hypophosphatasaemia | 482 (72.6%) |
| Persistent hypophosphatasaemia | 182 (27.4%) |
| Inclusions by departments | |
| 360 (54.2%) | |
| 304 (45.8%) | |
| Notification | |
| 24 (3.61%) | |
| 3 (0.8% of patients) | |
| 21 (6.9% of patients) | |
| Inclusions by centre (n; inclusion period) | |
| 71 (8.5 years) | |
| 35 (9 years) | |
| 149 (9.5 years) | |
| 34 (3.5 years) | |
| 226 (10 years) | |
| 149 (10 years) | |
ALP, alkaline phosphatase; n, number of observations.
All in all, 38.1% of patients were male. In only 24 cases (3.61%) was hypophosphatasaemia reported in the patient’s records.
Reasons for hospitalisations were various. In rheumatology departments, the top 10 reasons for hospitalisations were lumbosacral radiculopathy, haemopathy, osteoporosis, arthritis, fractures, rheumatoid arthritis, polyarthralgia, low back pain, spondyloarthritis and suspicion of rheumatic disease. In internal medicine departments, the top 10 reasons for hospitalisations were infectious disease, chronic myeloid leukaemia or lymphoma or multiple myeloma, polyarthralgia, severe anaemia/haemorrhage, autoimmune cytopenia, vasculitis, inflammatory myositis, intravenous immunoglobulin infusions, systemic lupus erythematosus, undernutrition or severe anorexia or hydro electrolytic disorders.
Initial comparisons of characteristics of patients with transient versus persistent hypophosphatasaemia
Clinical characteristics of the patients with transient and persistent hypophosphatasaemia were compared (table 2). Patients with persistent hypophosphatasaemia were younger (53.36 vs 62.93 years/old), were less heavy (64.01 vs 68.82 kg) and were more frequently treated in the rheumatology department (74.2% vs 35.1%). Their mean ALP values were significantly lower than in the transient group (28.0 vs 30.1 UI/L respectively).
Table 2Clinical and radiological features of patients with transient hypophosphatasaemia and persistent hypophosphatasaemia
| Transient (N=482) | Persistent (N=182) | P value | |
| Age | 62.9 (±19.3) N=478 | 53.4 (±18.0) | <0.0001 |
| Female sex | 289 (60%) | 122 (67%) | 0.094 |
| Weight (kgs) | 68.8 (±18.4) N=314 | 64.0 (±14.5) N=132 | 0.0122 |
| Height (cm) | 164.4 (±11.3) N=218 | 164.4 (±9.6) N=120 | 0.8045 |
| Department of inclusion | |||
| 313 | 47 | <0.0001 | |
| 169 | 135 | ||
| Alkaline phosphatase measurement | |||
| 30.1 (±3.5) | 28.0 (±4.9) | <0.0001 | |
| Detection | |||
| 1 (0.2%) | 23 (12.6%) | <0.0001* | |
| Pain | |||
| 110 (22.8%) | 164 (90.1%) | <0.0001 | |
| 54 (49.1%) | 94 (57.3%) | 0.180 | |
| 54 (49.1%) | 38 (23.2%) | <0.0001 | |
| 2 (1.8%) | 32 (19.5%) | <0.0001 | |
| Pain location | |||
| 66 (60%) | 70 (42.7%) | 0.005 | |
| 40 (36.4%) | 61 (37.2%) | 0.889 | |
| 4 (3.6%) | 33 (20.1%) | <0.0001 | |
| Fractures history | |||
| 3 (27.3%) N=11 | 43 (23.8%) N=181 | – | |
| 2 (66.7%) | 19 (44.2%) | – | |
| 0 | 4 (9.3%) | – | |
| 0 | 2 (4.7%) | – | |
| 1 (33.3%) | 6 (14.0%) | – | |
| 0 | 4 (9.3%) | – | |
| 0 | 8 (18.6%) | – | |
| 1±0 | 2.4±2.2 | – | |
| Stress fractures | |||
| 0 N=10 | 14 (7.7%) N=182 | – | |
| – | 2 (14.3%) | – | |
| – | 3 (21.4%) | – | |
| – | 1 (7.1%) | – | |
| – | 8 (57.1%) | – | |
| 0 | 1.6±0.7 | – | |
| Chondrocalcinosis history | |||
| 0 N=9 | 18 (9.9%) N=182 | – | |
| – | 5 (27.8%) | – | |
| – | 2 (11.1%) | – | |
| – | 7 (38.9%) | – | |
| – | 4 (22.2%) | – | |
| – | 5 (27.8%) | – | |
| Medical history | |||
| 0 N=9 | 24 (13.2%) N=182 | – | |
| 0 N=6 | 10 (5.5%) N=182 | – | |
| 0 N=3 | 4 (28.6%) N=14 | – | |
| 3 (23.1%) N=13 | 45 (24.9%) N=181 | – | |
| 0 N=7 | 5 (7.9%) N=63 | – | |
| 0 N=4 | 1 (2.5%) N=40 | – | |
| 0 N=4 | 4 (10.5%) N=38 | – | |
| 0 N=5 | 6 (15%) N=40 | – | |
| 0 N=4 | 0 N=38 | – | |
| Radiological findings | |||
| 1 (11.1%) N=9 | 51 (38.3%) N=133 | – | |
| 1 (11.1%) N=9 | 30 (22.6%) N=133 | – | |
| 1 | 1.9±1.6 | – | |
| 5 (41.7%) N=12 | 62 (55.4%) N=112 | – | |
| 3 (60%) | 12 (19.4%) | – | |
| 1 (20%) | 9 (14.5%) | – | |
| 2 (40%) | 17 (27.4%) | – | |
| 1 (20%) | 3 (4.8%) | – | |
| 2 (40%) | 4 (6.4%) | – | |
| 1 (20%) | 13 (21.0%) | – | |
| 1 (20%) | 21 (33.9%) | – | |
| 0 | 13 (21.0%) | – | |
| 4 (80%) | 21 (33.9%) | – | |
| 3 (33.3%) N=9 | 16 (12.0%) N=133 | – | |
| 4 (44.4%) N=9 | 16 (12.0%) N=133 | – | |
Significant values are in bold.
*Fisher’s exact test.
In terms of recognition, persistent hypophosphatasaemia was more frequently identified than transient hypophosphatasaemia (12.6% vs 0.2%).
Among the 182 patients with persistent hypophosphatasaemia, 70 patients (38.4%) had no joint imaging and 49 patients (26.9%) had no spinal imaging. Only 12 patients in the transient hypophosphatasaemia group had peripheral joint X-rays and nine had spinal X-rays (table 2).
Patients with persistent hypophosphatasaemia experienced pain more frequently (90.1% vs 22.8%). Stress fractures were present only in patients with persistent hypophosphatasaemia.
As regards medical history, chondrocalcinosis, hydroxyapatite deposition disease, dental abnormalities, early tooth loss, childhood rickets, family HPP and convulsion were found only in patients with persistent hypophosphatasaemia.
A total of 48 patients with persistent hypophosphatasaemia had a bone mineral density (BMD) measurement. However, values were not always available and sometimes only the conclusion appeared. Osteopenia was diagnosed in 20 patients, and osteoporosis in 15, while 13 patients presented with normal BMD. Details are found in online supplemental table 1. Only four patients in the transient hypophosphatasaemia group had a BMD measurement, with three normal BMD and one osteopenia.
Further comparisons of aetiologies in patients with transient versus persistent hypophosphatasaemia
Potential aetiologies of hypophosphatasaemia were compared (table 3).
Table 3Comparisons between patients with transient or persistent hypophosphatasaemia
| Transient (N=482) | Persistent (N=182) | P value | |
| Possible cause of hypophosphatasaemia | |||
| 0 (0%) N=387 | 1 (0.6%) N=160 | 0.292* | |
| 4 (1.0%) N=419 | 0 (0%) N=146 | 0.577* | |
| 21 (5.1%) N=411 | 9 (7.0%) N=129 | 0.419 | |
| 14 (3.1%) N=458 | 3 (1.7%) N=179 | 0.421* | |
| 110 (23.3%) N=473 | 21 (11.8%) N=178 | 0.001 | |
| 4 (1.37%) N=291 | 0 (0%) N=116 | 0.581* | |
| 20 (15.5%) N=129 | 4 (6.0%) N=67 | 0.066* | |
| 1 (0.34%) N=294 | 1 (1.0%) N=98 | 0.438* | |
| 18 (14.8%) N=122 | 5 (8.2%) N=61 | 0.207 | |
| 13 (27.1%) N=48 | 1 (5%) N=20 | 0.050* | |
| 15 (14.7%) N=102 | 1 (3.4%) N=29 | 0.121* | |
| 42 (9.3%) N=451 | 16 (8.9%) N=179 | 0.884 | |
| 92 (19.1%) N=481 | 3 (1.6%) N=182 | <0.0001* | |
| 116 (24.1%) N=481 | 23 (12.6%) N=182 | 0.001 | |
| 132 (27.5%) N=480 | 24 (13.2%) N=182 | 0.0001 | |
| 4 (0.8%) N=480 | 1 (0.5%) N=182 | >0.999* | |
| 70 (14.5%) N=481 | 6 (3.3%) N=182 | <0.0001 | |
| 141 (29.3%) N=481 | 17 (9.3%) N=182 | <0.0001 | |
| 64 (13.6%) N=469 | 37 (20.4%) N=181 | 0.032 | |
| 41 (8.5%) N=480 | 6 (3.3%) N=182 | 0.019 | |
| Identification of the cause of hypophosphatasaemia | |||
| 445 (92.3%) | 98 (53.8%) | <0.0001 | |
| 37 (7.7%) | 84 (46.2%) | ||
| Possible hypophosphatasia (at least 2 signs of hypophosphatasia+persistent low ALP values) | |||
| – | 69 (37.9%) | – | |
| – | 32 (46.4%) | – | |
| – | 37 (63.6%) | – | |
Significant values are in bold.
*Fisher’s exact test.
ALP, alkaline phosphatase.
Considering possible causes of hypophosphatasaemia, severe anaemia, intensive care unit stay, active oncohaematological disease, ongoing bisphosphonate treatment, sepsis, inflammatory disease flare and intravenous immunoglobulin treatment were more frequently found in transient hypophosphatasaemia, while corticosteroid intake was more frequent in persistent hypophosphatasaemia. In those patients, HPP was possible in 69 patients, in 37 of whom there was no identified cause.
Documentation of low ALP values in patients with persistent hypophosphatasaemia with bisphosphonate treatment
Since bisphosphonate treatment is contraindicated in HPP, we analysed whether patients with low ALP measurements were tested before bisphosphonate initiation. Among the 24 patients treated with bisphosphonates, 19 (79.2%) had never undergone ALP measurement before treatment, while in 5 patients (20.8%), this treatment had been initiated despite an abnormal decrease of ALP. Details for those patients are in online supplemental table 2.
Comparisons of clinical and radiological features of patients with persistent hypophosphatasaemia with and without identified cause
Out of the 182 ‘persistent’ patients, 84 cases had an identified cause and 98 did not (table 4). There were no differences in ALP measurements between groups.
Table 4Clinical and radiological features of persistent hypophosphatasaemia with and without identified cause
| Identified cause (N=98) | No identified cause (N=84) | P value | |
| Alkaline phosphatase measurement | |||
| Mean (±SD) | 28.3 (±4.3) | 27.6 (±5.5) | 0.827 |
| Pain | |||
| 85 (86.7%) | 78 (92.9%) | 0.178 | |
| 38 (44.7%) | 55 (70.5%) | 0.0003 | |
| 23 (27.1%) | 15 (19.2%) | 0.353 | |
| 24 (28.2%) | 8 (10.3%) | 0.008 | |
| Pain location | |||
| 32 (37.6%) | 35 (44.9%) | 0.208 | |
| 40 (47.1%) | 22 (28.2%) | 0.038 | |
| 13 (15.3%) | 21 (26.9%) | 0.043 | |
| Fractures history | |||
| 29 (29.9%) N=97 | 14 (16.7%) N=84 | 0.037 | |
| 15 (51.7%) | 4 (28.6%) | 0.199* | |
| 3 (10.3%) | 1 (7.1%) | 1* | |
| 0 | 2 (14.3%) | 0.101* | |
| 5 (17.2%) | 1 (7.1%) | 0.645* | |
| 1 (3.4%) | 3 (21.4%) | 0.094* | |
| 5 (17.2%) | 3 (21.4%) | 1* | |
| 2.3±2.2 | 2.5±2.3 | 0.908 | |
| Stress fractures | |||
| 7 (7.1%) N=98 | 7 (8.3%) N=84 | 0.764 | |
| 1 (14.3%) | 1 (14.3%) | 1* | |
| – | 3 (42.9%) | 0.192* | |
| – | 1 (14.3%) | 1* | |
| 6 (85.7%) | 2 (28.6%) | 0.103* | |
| 1.8±1.0 | 1.4±0.5 | 0.762 | |
| Chondrocalcinosis history | |||
| 9 (9.2%) N=98 | 9 (10.7%) N=84 | 0.730 | |
| 4 (44.4%) | 1 (11.1%) | 0.294* | |
| 2 (22.2%) | 0 | 0.470* | |
| 1 (11.1%) | 6 (66.7%) | 0.050* | |
| – | 3 (33.3%) | 0.206* | |
| 3 (33.3%) | 2 (22.2%) | 1* | |
| Medical history | |||
| 12 (12.2%) N=98 | 12 (14.3%) N=84 | 0.685 | |
| 2 (2.0%) N=98 | 8 (9.5%) N=84 | 0.046* | |
| 1 (14.3%) N=7 | 3 (42.9%) N=7 | – | |
| 36 (37.1%) N=97 | 9 (10.7%) N=84 | <0.0001 | |
| 2 (5.6%) N=36 | 3 (8.1%) N=27 | – | |
| 1 (4.2%) N=24 | 0 N=16 | – | |
| 2 (10%) N=20 | 2 (11.1%) N=18 | – | |
| 3 (13.0%) N=23 | 3 (17.6%) N=17 | – | |
| 0 N=20 | 0 N=18 | – | |
| Radiological findings | |||
| 30 (39.0%) N=77 | 21 (37.5%) N=56 | 0.864 | |
| 24 (31.2%) N=77 | 6 (10.7%) N=56 | 0.005 | |
| 2.0±1.8 | 1.5±0.8 | 0.520 | |
| 33 (50.8%) N=65 | 29 (61.7%) N=47 | 0.251 | |
| 9 (27.3%) | 3 (10.3%) | 0.116* | |
| 3 (9.1%) | 6 (20.7%) | 0.283* | |
| 9 (27.3%) | 8 (27.6%) | 1* | |
| 2 (6.1%) | 1 (3.4%) | 1* | |
| 3 (9.1%) | 2 (6.9%) | 1* | |
| 5 (15.2%) | 7 (24.1%) | 0.521* | |
| 10 (30.3%) | 11 (37.9%) | 0.596* | |
| 5 (15.2%) | 6 (20.7%) | 0.741* | |
| 11 (33.3%) | 10 (34.5%) | 0.923 | |
| 11 (14.3%) N=77 | 11 (19.6%) N=56 | 0.411 | |
| 10 (13.0%) N=77 | 12 (21.4%) N=56 | 0.196 | |
Significant values are in bold.
*Fisher’s exact test.
Patients with unidentified cause of hypophosphatasaemia were more likely to have mechanical pain (70.5% vs 44.7%), diffuse pain (26.9% vs 15.3%) and knee chondrocalcinosis history (66.7% vs 11.1%), while they less frequently had pain in the limbs (28.2% vs 47.1%), fracture history (16.7% vs 29.9%), mixed pattern pain (10.3% vs 28.2%), low BMD (10.7% vs 37.1%) and radiographic vertebral fractures (10.7% vs 31.2%).
HPP among patients with persistent hypophosphatasaemia
Among all patients with persistent hypophosphatasaemia, 69 presented at least three symptoms evocative of HPP in addition to persistent low ALP and were classified as possible HPP (table 3). Among them, 18 underwent genetic analysis in search of ALPL gene mutation, and 11 patients presented with genetically proven HPP (61.1%). The diagnosis of genetic HPP was thereby confirmed in at least 1.7% of our total population (11/664). Among those 11 patients, 3 had another potential cause of low ALP (2 had taken corticosteroids, and 1 had a vitamin C deficiency). Selection of patients with persistently decreased ALP rendered genetic analysis more cost-effective, with a positive diagnosis ranging from at least 1.7% (11/664) to at least 6% (11/182), and even higher than 15.9% (11/69) if they were classified as possible HPP. Pyridoxal phosphate (PLP) measurements had been performed in only 6 patients out of the 664 patients included (mean±SD: 58.33±18.26 nmol/L (normal range: 30–100 nmol/L)). All of them had persistent low ALP: five were genetically tested, among whom three were positive
Discussion
In our study, the prevalence of all-cause hypophosphatasaemia among patients hospitalised in the internal medicine and rheumatology departments was 1.18% while that of persistent hypophosphatasaemia was 0.32%. This proportion was higher than in the study by Maman et al in which 0.13% of hospitalised patients (every department except the emergency department) had persistently low values with a less stringent threshold of 40 UI/L,6 as in the study of Hepp et al, in which prevalence of 0.20% was found in adults admitted to an endocrinological outpatient clinic in Denmark,11 or in the study by García-Fontana et al with prevalence of 0.12% in a Spanish university hospital.12 A German study retrospectively analysing 6 918 126 subjects with a measurement of ALP between 2011 and 2016 in a single laboratory identified prevalence of ALP values below 30 of 8.46% and 9.47% between 30 and 40 UI/L, respectively, thereby underscoring the need to focus on persistent low ALP levels since transient hypophosphatasaemia is quite common.9 McKiernan et al identified 1.1% of patients with at least two values under 40 UI/L among consultants in a multidisciplinary centre, and 0.06% of ALP level persistently below 30 UI/L.8 This is concordant with a German study, which found 1.31% of patients treated in rheumatology at the University Hospital of Bonn from 2017 to 2019 showed persistently low serum ALP levels (<35 UI/L).13 As regards the proportion of patients with persistent hypophosphatasaemia, it was 33.3% in the study by McKiernan et al,8 and 39% in a study by Vieira et al,14 which is concordant with our result of 27.6%. Similarly, Feurstein et al found 5.5% of patients with at least one low ALP value under 40 UI/L, with only 13.9% of patients presenting persistent low ALP levels and musculoskeletal symptoms; they represented 0.8% of the whole population from a rheumatology outpatient clinic in Vienna specialised in rheumatology and rare bone diseases.15
In terms of notification, reporting of low ALP values was found in 3.61% in our population, which is close to the 3% noted by Maman et al.6 Low ALP is clearly not sufficiently recognised, even if rheumatologists seem to better identify this abnormality with a reported 6.91% vs 0.83% in internal medicine. As a result, adult HPP is highly underdiagnosed. A few years ago, some laboratories only indicated the ‘high’ cut-off and, in the absence of personal knowledge of the lower normal cut-off, the ALP drop was not always noticed,16 and therefore, easily overlooked.17 In our study, many patients had not been explored, and the final report never mentioned low ALP. Indeed, it ‘normal liver test’ was often noted without details, even though the ALP levels were lower than 35 IU/L. That is why hypophosphatasaemia was not coded and did not result in further explorations.
The difficulty of diagnosing HPP led several teams to propose algorithms to enhance the rate of diagnosis. The first strategy is based on the adjunction of PLP measurement to ALP so as to better stratify the likelihood of HPP diagnosis with high PLP and low ALP as features of HPP.18 19 Another team added BMD measurement by Dual-energy X-ray absorptiometry to generate a strategy of rationalised mutational analysis in resource-limiting conditions.20 While these approaches are interesting, PLP is not performed in daily practice, thereby limiting its usefulness if low ALP has not been previously identified. In our study, PLP measurements had been performed in only 6 out of the 664 patients included. All of them had persistent low ALP; five were genetically tested, among whom three were positive. This lack of data is not surprising since low ALP is poorly recognised in daily practice. Therefore, physicians would not dose PLP since they did not take low ALP into account. Moreover, the algorithm mentioning PLP measurement in order to better screen patients was published after our inclusion period, which may be another, though less important, explanation.
Another approach is to focus on populations with highly suggestive features of HPP. Tsiantouli et al analysed ALP values in a population of 72 patients with atypical femur fractures (AFF) with at least 1 ALP value available. There was no difference in the median value of ALP compared with the control group with hip fracture, and no difference in the titre of ALP if they were treated with antiresorptive agent. Moreover, none of the patients with AFF without antiresorptive drugs in this single-centre study presented with low ALP levels.21 Similarly, Marini et al performed ALPL genotyping in patients with AFF or other biochemical or clinical signs of adult HPP.22 This led them to identify three rare variants of ALPL (2.8%) in this population. Monozygotic ALPL common variants were found in 11.3% of the patients, with a higher proportion of 22% of patients with normal ALP values, 30.8% of patients with AFF, 16.7% of patients with normal ALP and high PLP levels and also, unfortunately, in 13.5% of non-HPP controls. Those results should draw the attention of clinicians to the need to carefully consider the possibility that some variants have no detrimental effect on the ALP protein and that different kinds of disease severity or carrying a nonpathogenic mutation can be encountered.23 Since metatarsal fractures are suggestive of HPP, Koehler et al focused on this population and found 0.12% prevalence of pathogenic ALPL variants in a population of 1611 metatarsal fractures, a proportion that rose to 15% when low ALP measurement was associated.24 In our study, the same approach using clinical, biological and imaging features identified 69 patients with at least three evocative symptoms of HPP in addition to persistently low ALP values (possible HPP) among which 11 were found to have genetically proven HPP, representing a diagnosis rate of at least 15.9%. This value is probably underestimated since only 18 patients benefited from genetic testing, corresponding to a diagnosis rate of 61.1% of the tested patients. The combination of at least three signs in addition to persistent hypophosphatasaemia should, therefore, be tested in a larger population to evaluate its cost-effectiveness.
As expected, possible causes of hypophosphatasaemia were more frequently found in transient cases. Interestingly, corticosteroids were more frequently found in persistent hypophosphatasaemia. Since patients with persistent hypophosphatasaemia more frequently had crystal arthropathy history as well as pain, we may hypothesise that this difference is the result of its use to treat arthritis flares or pain as well as long-term treatments in systemic inflammatory disease. Indeed, patients with chronic low dose corticosteroid use were frequently treated with bisphosphonates in order to prevent corticosteroid-induced osteoporosis.
Pain in itself is also an important point to consider insofar as more than 90% of patients with persistent hypophosphatasaemia in our study reported pain. Pain also represents the greatest burden in HPP patients, as shown in the global HPP registry.25 Moreover, there is multiple evidence in the literature that TNSALP exerts a role in the biosynthesis of adenosine, a key molecule with antinociceptive effect with TNSALP, prostatic acid phosphatase and ecto-5’-nucleotidase playing crucial roles in determining the overall sensitivity of the nociceptive circuits, as reviewed extensively by Street and Sowa.26
The relationship between tissue-nonspecific ALP and inflammation is an increasing source of interest. A recent review article by Graser et al27 affirms that TNSALP deficiency contributes to inflammatory reactions. TNSALP is implicated in the balance between proinflammatory ATP effects and anti-inflammatory effects of adenosine. Moreover, TNSALP’s ectophosphatase activity is involved in the modulation of TLR ligands like LPS and double-stranded RNA mimic poly-inosine:cytosine. TNSALP is also a T-cell activity modulator. In synthesis, TNSALP is now known to exert an anti-inflammatory effect. Furthermore, ALP levels are higher in case of systemic inflammation.28 29 In our study, the two main reasons in which inflammatory disease flare were associated with low ALP were corticosteroids, intravenous immunoglobulin treatments or bisphosphonate therapy for prevention of corticosteroid-induced osteoporosis. In Internal Medicine departments, patients were often hospitalised for sepsis or active oncohaematological disease, which can also explain low ALP.
Concerning the patients with persistent hypophosphatasaemia, patients under bisphosphonate treatment were analysed separately to identify differentiating features. The observed differences all seem to be related to osteoporosis with frequent history of fractures, bone deformities, bone demineralisation and vertebral fractures.
In terms of bone frailty, vertebral fractures were numerically less frequent in patients with persistent hypophosphatasaemia without identified cause. In the study by Hepp et al,30 none of the HPP patients had vertebral fractures. The study by Genest et al found a significant correlation between low ALP levels and high spine BMD in a cohort of HPP patients.31 In the literature review by Sadhukhan et al, vertebral fractures were not observed in HPP patients and high lumbar spine BMD was more likely.20
This study has some limitations. First of all, this study has a retrospective design which induced differences in the number of observations of the different parameters, and therefore, may wane some of our conclusions. There was a bias regarding the variability of the number of patients included per centre, and it did not allow us to be completely exhaustive. Indeed, 10 years of inclusion was not possible in all centres. The listing established through the cooperation with the laboratory technicians of each centre was not always complete and the extraction of data by the resident at each site was time-consuming. This difference can come from the diversity of software laboratories, which did not always lead to online results for a number of years. This required a longer duration of analysis with a risk of error and limitation of the data to a more restricted period. In addition, some centres underwent software changes during the inclusion period, generating difficulty in returning to previous data. Moreover, the time interval between the two measurements was variable, depending on each patient. The need for two measurements to limit the risk of analytical error may also be at the origin of a bias since body weight could be different at each time point. Since this condition is poorly recognised, genetic testing was performed in only a few cases in which physicians suspected HPP. In this study, long-term low-dose glucocorticoid treatment was not taken into account as a cause of low ALP levels which is another limitation. However, literature is scarce and conflicting about this point which led us not to consider it as a possible cause of low ALP levels. Indeed, in a study by LoCascio et al about 23 patients treated with 10–25 mg a day of glucocorticoid for various immune diseases, no significant decline of ALP levels were found after 1–2 months, 5–7 months or 12 months glucocorticoid treatment.32 Moreover, in 13 patients treated for chronic glomerulonephritis with a mean dose of 43.8 mg a day of GC progressively tapered, Sasaki et al demonstrated that ALP levels decreased significantly at 1, 3 and 6 months endpoints compared with baseline and bone-specific ALP levels decreased significantly only at 3 and 6 months of follow-up but each measurement remained in the normal range.33 Pearce et al showed that GC doses of 10 mg and less for polymyalgia rheumatica resulted in higher bone specific ALP levels during a 27-month follow-up.34 Finally, Korczowska et al showed that ALP levels increased significantly at 12 months after glucocorticoid initiation in patients with rheumatoid arthritis, while there was no difference in patients already treated before the beginning of the follow-up.35
Although this study could not be exhaustive (missing data, impossibility to carry out genetic research or to perform PLP measurements in all the patients suspected of HPP in a retrospective study), it has the advantage of inventorying our practice in view of improvement, and the multicentre character over 10 years reinforces our conclusions. The results suggest that moderate forms of genetic HPP in adults are certainly more frequent than previously thought and highlight the need for special attention to the value of ALP.
The situation is further complicated by the hypothesis that some variants could lead to low density osteopathy, without HPP-disease criteria.36 Indeed, the presence of heterozygosity in some patients with suggestive symptoms suggests that other mechanisms are involved in the phenotypic expression of adult HPP.37 For very mild adult forms and exclusive dental forms, mutations may be heterozygous.38 In their proposal of genetic-based nosology of HPP, Mornet et al described a mild HPP form with adult onset of unspecific symptoms caused by an autosomal dominant haploinsufficiency with prevalence of 1/508.23 However, the existence of such an entity is still controversial.
As a conclusion, hypophosphatasaemia was recognised only in 3.61% of the patients presenting this biological abnormality and hospitalised in rheumatology and internal medicine departments. At least 15.9% of patients with three or more evocative symptoms of HPP in addition to persistent hypophosphatasaemia had HPP. This multicentre retrospective study shows that adult HPP remains underdiagnosed. The prevalence of moderate forms of adult HPP appears to be higher than previously thought and highlights the need for according special attention to ALP values.
Data availability statement
Data are available on reasonable request.
Ethics statements
Patient consent for publication
Not applicable.
Ethics approval
The study was approved by the local institutional ethics committee and conducted in accordance with the Declaration of Helsinki. It was likewise approved by the CNIL (French Data Protection Commission). Written consent was not required according to the MR-004 French legislation.
GL and JV contributed equally.
Contributors The authors confirm contribution to the paper as follows: study conception, design, and guarantor: FD; data collection: JV, PP, FR, AT, CD, FK, BB, DCML, PG, YM and AS; analysis and interpretation of results: GL, JV and FD; draft manuscript preparation: GL, JV and FD. All authors reviewed the results and approved the final version of the manuscript. GL and JV had equal contribution.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.
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Details
; Vix, Justine 2 ; Preuss, Pauline 3 ; François, Robin 4
; Tison, Alice 5 ; Delaveau, Clémentine 6 ; Krajewski, Faustine 7 ; Bouvard, Béatrice 6 ; Delphine Chu Miow Lin 8 ; Guggenbuhl, Pascal 4 ; Maugars, Yves 9 ; Saraux, Alain 5 ; Debiais, Francoise 2 1 Department of Rheumatology, CHU Poitiers, Poitiers, France; LITEC, Poitiers University, Poitiers, France
2 Department of Rheumatology, CHU Poitiers, Poitiers, France
3 CHU Nantes, Nantes, France
4 Department of Rheumatology, CHU Rennes, Rennes, France
5 CHRU de Brest, Brest, France
6 Rhumatologie, Centre Hospitalier Universitaire, Angers, France
7 Department of Rheumatology, CHU de Tours, Tours, France
8 Department of Rheumatology, CHU de Tours, Tours, France; Université de Tours, Tours, France
9 Clinique Nantes Atlantique, Nantes, France