Characterization of Oral Health Status in Chilean Patients with X-Linked Hypophosphatemia

Abstract

X-Linked Hypophosphatemia (XLH) is the most common cause of inherited hypophosphatemic rickets. Dental involvement, including spontaneous abscesses and/or fistulae, is an important part of the disease and has not been completely defined, especially in cohorts from developing countries. To describe oral health status in a cohort of Chilean patients with XLH and explore its correlation with biochemical presentation and treatment, we conducted a cross-sectional observational study of patients with PHEX mutation-confirmed XLH. All patients had an oral clinical exam, radiographic evaluation; clinical and biochemical data were obtained to determine their association with oral features. Twenty-six patients were included, 77% adults and 23% children. Most adults (89%) had past or current dental pulp pathology (abscesses and/or fistulae). Pulpal chamber enlargement and radiolucent apical lesions were common radiological features (94 and 74%, respectively). In children, abscess and/or fistulae were also common (33%). Caries index, which was determined by dmft/DMFT, was higher than the Chilean national average. Early and long-term therapy with phosphate and activated vitamin D was associated with lower carious index and attachment loss. XLH patients frequently present with high pulpal involvement and carious index. Conventional therapy was associated with lower carious index and attachment loss. These data highlight the importance of early and periodical dental care in order to prevent dental damage and assure a good quality of oral health for XLH patients.

Introduction

Hereditary hypophosphatemic rickets comprises a group of diseases characterized by an alteration in phosphate metabolism that leads to diminished renal phosphate reabsorption and hypophosphatemia, with the consequent development of impaired skeletal and dental mineralization. In children, the disease is characterized by rickets, impaired mineralization of the growth plate, which leads to skeletal deformity and reduced growth rates. In adults, decreased bone mineralization manifests as osteomalacia with musculoskeletal pain, weakness, and fractures [1].

The most common cause of inherited hypophosphatemic rickets is dominant X-linked hypophosphatemia (XLH) in which inactivation of the phosphate-regulating endopeptidase homolog X-linked (PHEX) gene leads to an increase in fibroblastic growth factor 23 (FGF23) levels. FGF23 inhibits renal expression of phosphate transporters responsible for the renal reabsorption of phosphate, causing increased phosphate urinary excretion. In addition, increased FGF23 levels suppress calcitriol synthesis and promote its catabolism, which leads to low 1,25 dihydroxyvitamin D (1,25 (OH)₂) and thus, reduced calcium and phosphate intestinal absorption. Therefore, FGF23 excess results in hypophosphatemia with impaired mineralization of bones and teeth [2, 3].

XLH, which affects approximately 1 in 20,000 individuals, can usually be identified within the first years of life [1]. Primary clinical symptoms in childhood include pain, skeletal deformity (especially of the lower extremities), altered gait, growth retardation, muscle weakness, pseudofractures, craniosynostosis of sagittal sutures, and dental abscesses [2, 4, 5]. Conventional treatment currently consists of vitamin D analogs (calcitriol or alfacalcidol) and phosphate supplements. This treatment has been shown to partially improve rickets and dental manifestations. Specifically, phosphate and vitamin D analogs have been shown to prevent or effectively treat dental abnormalities in many patients [5-7].

The main oral findings of XLH are recurrent spontaneous abscesses and/or fistulas without previous caries or trauma in deciduous and permanent teeth, due to a deterioration of dental mineralization [4, 5, 7-9]. Despite medical therapy, dental pathology remains prevalent throughout life even during appropriate conventional treatment. Other symptoms include delayed eruption of primary and secondary dentition. In X-rays, large pulp chambers with an abnormally high ratio can be visualized [5, 6, 10]. In addition, other common manifestations in primary dentition are short roots and rhizolysis (progressive loss of dentin and cementum). Young adults have a higher frequency of periodontitis and alveolar bone disorders, such as hypoplasia of the alveolar rim and poorly defined cortical bone. Furthermore, enamel hypoplasia in incisors, canines, and permanent first molars are common [4, 5, 10, 11].

Most dental phenotype characterizations of XLH patients were conducted in cohorts from developed countries. Little information has been published regarding oral manifestations of patients with hypophosphatemic rickets in South America. The lack of research in the region may leave the medical and dental community less informed and unable to provide high-quality care to their patients, thus identifying a significant unmet need. The aim of our study is to describe the oral findings in XLH Chilean population in order to promote the importance of a comprehensive management of the disease, in which dental care is of the utmost importance.

Methods

Patients

An open call was made to Chilean endocrinologists and bone experts through the Chilean Society of Endocrinology and Diabetes to refer patients with hereditary hypophosphatemic rickets. Diagnosis of hypophosphatemic rickets was based on clinical, radiological, and biochemical findings that indicated the occurrence of congenital hypophosphatemia and isolated renal phosphate wasting. The diagnosis of XLH was later confirmed by genetic testing. Patients who were under pharmacological treatment with phosphorus and/or analogs of vitamin D had to discontinue their use 7 days prior to the medical enrollment.

Patients were recruited between May and November 2019 at the Center for Translational Research in Endocrinology of the Endocrinology Department, Pontificia Universidad Católica de Chile. Detailed clinical evaluation was performed including medical history and a physical examination focused on family history, history of fractures, limb deformities, and height Z-scores. In addition, treatment exposure was assessed, recording whether the patients had received conventional therapy before 18 years of age, as well as the proportion of total life with treatment calculated by dividing the total years receiving phosphate and/or calcitriol by total year of life (range from 0 to 1).

The clinical study was conducted with the approval of the Ethics Committee, Faculty of Medicine, Pontificia Universidad Católica de Chile. All adults and parents of pediatric patients signed the written informed consent forms before entering the study.

Biochemical and Molecular Evaluation

After an overnight fast, blood samples and second void morning urine were obtained. Serum analyses included phosphorus, calcium, albumin, bicarbonate, creatinine, and total alkaline phosphatase (ALP). Phosphorous and ALP were interpreted using age-specific normal value ranges. Fasting morning urine analyses included phosphorus and creatinine. Tubular reabsorption of phosphate (TRP) was calculated from fasting blood and urine measurements. 25(OH) vitamin D was measured by chemiluminescent immunometric assay (CMIA) (Architecti–Abbott). Intact PTH was measured by electrochemiluminescence immunoassay (Cobas, Roche Diagnostics, Basel, Switzerland; normal range 16–65 pg/ml). Intact and Carboxyterminal FGF23 (iFGF23 and cFGF23, respectively) were measured in EDTA-plasma using an enzyme-linked immunoabsorbent assay (ELISA) kit (Immutopics, Quidel, San Diego, CA, USA) (iFGF23 above 30 pg/ml in patients with hypophosphatemia indicates excess of FGF23 [12], and cFGF23 age-specific normal ranges for patients with normophosphatemia provided by Mayo Medical Laboratories is as follows: 3 months-17 years: ≤ 230 RU/mL, ≥ 18 years: ≤ 180 RU/mL).

Dental Examination Protocol

Patients were evaluated at Clínica Odontológica Docente Universidad Católica (CODUC) as part of their complete medical examination. Two dentists (AM or PM) performed a comprehensive oral examination including assessment for dental decay, endodontic compromise, and periodontal evaluation (where periodontal charting was only performed in adults in order to evaluate attachment loss). To determine carious lesions in each patient, DMFT and dmft indexes were assessed [13]. DMFT is applied to permanent dentition and is expressed as the total number of teeth that are decayed (D), missing (M), or filled (F). The lower case dmft is applied to the primary dentition. All patients underwent a panoramic radiography, in order to evaluate pulp chambers, periapical lesions, and endodontic treatments.

Statistical Analysis

Normally distributed continuous variables were presented as mean ± standard deviation (SD) and compared with student’s t -test. Non-normally distributed continuous variables were expressed as median and interquartile range (IQR) and compared with Mann–Whitney U test. Frequencies for categorical variables were compared with the Fisher’s exact test. Linear correlations between continuous variables were assessed using Pearson’s correlation coefficient if the distribution was normal, or Spearman correlation test if the variables were not normally distributed. In addition, using a univariate linear regression model, we assessed the association between age, proportion of time with treatment, ALP and Phosphate Z-score, PTH, and iFGF23 with DMFT and attachment loss, respectively.

A p -value < 0.05 was considered statistically significant. Analyses were performed with GraphPad Prism (version 7.01; GraphPad Software, Inc., La Jolla, CA, USA).

Results

General Characteristics

A total of 26 Chilean patients from 17 unrelated families were included. Twenty (77%) were adults and 6 (23%) were children. Of the adults, 15 (75%) were female and the median age was 34 years (IQR 27–38 years, range 20–64 years). Of the six pediatric patients, 3 (50%) were female and the median age was 8 years (IQR 6–12, range 5–16 years). The median age at diagnosis was 2 years (IQR 1–11.5 years).

At the time of evaluation, 8 (40%) of the adults were receiving therapy with calcitriol and/or phosphate. Of note, three (15%) adults had never received therapy, and 9 (45%) had previously received therapy but were no longer on it. In contrast, all pediatric patients were under conventional pharmacological treatment. No patient had previously received or was using Burosumab at the time of evaluation for this study.

Biochemical results are listed in Table 1 all but one patient had hypophosphatemia, all had reduced TRP, and 65% had elevated age-specific alkaline phosphatase. In addition, all patients had iFGF above 30 pg/ml.

Table 1.

Biochemical results of XLH studied patients

Biochemical results	All (n = 26)	Adults (n = 20)	Children (n = 6)
Serum phosphate (mg/dl)a	2.0 ± 0.5	1.8 ± 0.4	2.6 ± 0.2
Z-score phosphate	− 3.8 ± 1.0	− 3.7 ± 0.9	− 4.1 ± 1.5
Albumin-adjusted calcium (mg/dl)	9.6 ± 0.4	9.5 ± 0.4	9.8 ± 0.3
Alkaline phosphatase (U/L)b	133.0 (84.5–300.3)	117.5 (79.5–153.5)	346.0 (305.0–455.0)
Z-score ALP	3.0 (1.1–5.0)	3.0 (0.6–4.6)	3.8 (2.0–5.8)
Parathyroid hormone (pg/ml)	58(44–80)	61 (51–83)	43 (35–63)
25 (OH)2D (ng/ml)	20.6 ± 7.5	19.6 ± 8.0	23.6 ± 4.1
TRP (%)	84.6 (76.0–89.6)	84.1 (73.8–88.1)	88.8 (81.8.3–91.9)
Intact FGF23 (pg/ml)	96.0 (66.7–155.8)	92.9 (51.0–119.5)	141.5 (91.0–200.8)
cFGF23 (RU/ml)c	105.7 (76.8–150.3)	102.2 (73.5–146.5)	130.0 (90.5–163.5)

Values are presented as median with interquartile range (IQR) or media ± standard deviation (SD) as appropriate

^aPhosphate age-specific normal range (mg/dL) 6 days–3 years: 3.9–6.5; 4–6 years: 4.0–5.4; 7–11 years: 3.7–5.6; 12–13 years: 3.3–5.4; 14–15 years: 2.9–5.4; 16–19 years: 2.8–4.7, > 19 years: 2.6–4.5. Normal values of Universidad Catolica Clinic Laboratory, Chile

^bAlkaline phosphatase age-specific normal range, males (U/L) 1–3 years: 104–305; 4–6 years: 93–309; 7–9 years: 86–315; 10–12 years: 42–362; 13–15 years: 74–390; 16–18 years: 52–171; > 18 years: 45–115. Age-specific normal range, females (U/L) 1–3 years: 108–317; 4–6 years: 96–297; 7–9 years: 69–325; 10–12 years: 51–332; 13–15 years: 50–162; 16–18 years: 47–119; > 18 years: 35–100. Normal values of Universidad Catolica Clinic Laboratory, Chile

^ccFGF age-specific normal range for normophosphatemic patients: 3 months-17 years: ≤ 230 RU/mL, ≥ 18 years: ≤ 180 RU/mL

Clinical examination revealed that short stature was common in adults and in the pediatric subjects (75 and 33%, respectively). Out of the total studied subjects, additional skeletal findings included coxa and/or genu varo/valgum (92%), history of orthopedic surgery (50%) and radiographic skeletal fracture and/or delayed healing (34%).

Dental Features

Most adults examined (89%) had past or current dental pulp pathology including abscesses and/or fistulae. At the time of dental evaluation, 7 adults (35%) and 2 pediatric subjects (33%) presented clinical evidence of active disease. A history of root canal treatment was reported in 74% of adult patients. In addition, 25% of the patients had gingivitis and 70% had periodontitis of varying degrees at the time of evaluation.

The average DMFT of the adults examined was 17.5 ± 9.3. Panoramic radiographs revealed pulp chamber enlargement in 94% of the adult patients, and radiolucent apical lesions in 74%, with a mean of − 3.3 mm ± 4.2 mm of attachment loss (Fig. 1). Of note, one patient was edentulous and therefore was not included in this analysis (Table 2).

Fig. 1.

a Panoramic radiograph of an adult subject with XLH with enlarged pulp chamber of tooth 17 and endodontic treatment of teeth 11 and 21. b Panoramic radiograph of an infant with enlarged pulp chamber on tooth 46 and endodontic treatment of tooth 85. c Panoramic radiograph of an adult with XLH with extensive tooth loss and bone resorption. d Panoramic radiograph of an adult with extensive endodontic, prosthodontic, and implant treatments

Table 2.

Results of the oral evaluation of the 20 examined XLH adults

Cases	Age	DMFT	Attachment loss (mm)	Fistula/ abscess	Radio graphic lesion	Endodontic treatments	Pulp chamber enlargement	Age of treatment begin
1	28	11	− 2.1	+	−	2	+	13 years
2	61	32	− 2.8	+	+	14	−	18 years
3	34	32	− 3.9	+	+	0	+	Never
4	30	12	− 1.9	+	+	3	+	11 years
5	33	19	− 1.5	+	+	3	+	2 years
6	64	23	− 5.7	+	+	2	+	31 years
7	37	14	− 2.9	+	−	3	+	5 years
8	44	30	− 2.4	+	+	14	+	32 years
9	36	15	− 2.5	+	+	1	+	18 months
10	35	23	− 1.4	+	+	6	+	3 years
11	38	25	− 1.8	+	+	6	+	2 years
12	31	16	− 1,9	+	+	4	+	2 years
13	32	21	− 2.7	+	+	1	+	Never
14	22	13	− 0.8	+	+	2	+	18 months
15	21	4	− 0.2	−	−	0	+	1 month
16	20	1	− 2.8	+	+	0	+	2 years
17	20	6	0	+	−	0	+	1 year
18a	63	32	−	−-	−	−	−	2 years
19	38	10	− 1.3	−	−	0	+	2 years
20	26	10	0	+	+	2	+	9 months

DMFT total number of teeth that are decayed (D), missing (M), or filled (F) (+) indicates present and (−) absent

^aPatient n°18 is edentulous

Mean DMFT and attachment loss were significantly higher in the group that started therapy after age 18 or never received therapy (N = 5), compared to those who started earlier (N = 15) (27.6 vs. 14.1 and − 3.5 mm vs. − 1.5 mm, respectively). In addition, the proportion of time exposed to treatment had a significant negative correlation with DMFT (r = − 0.67, p < 0.05), which persisted after adjusting by age. In contrast, attachment loss showed a positive correlation with the proportion of time with treatment (r = 0.64, p < 0.05), but lost significance after adjusting by age.

In addition, height Z-score had a significant positive correlation with attachment loss (r = 0.5, p < 0.05), but had no correlation with DMFT index or number of endodontic treatments. No biochemical parameters had a significant correlation with DMFT or attachment loss. These results are shown in Table 3.

Table 3.

Univariate regression models of the relationship of DMFT index and attachment loss, respectively

	DMFT		Attachment loss
	β (SE)	p value	β (SE)	p value
Age, years	1.05 (0.22)	< 0.01	− 6.04 (1.5)	< 0.01
Proportion of time with treatment	− 0.02 (0.01)	< 0.01	0.11 (0.05)	0.04
Phosphate Z-score	0.03(0.02)	0.16	− 0.19 (0.15)	0.2
ALP Z-score	0.04 (0.08)	0.61	− 0.25 (0.4)	0.55
PTH (pg/ml)	0.28 (1.0)	0.78	− 1.27 (6.9)	0.86
iFGF23 (pg/ml)	− 0.14 (2.3)	0.95	11.53 (16.6)	0.5

DMFT total number of teeth that are decayed (D), missing (M) or filled (F)

In the pediatric group, 2 (33%) patients had dental abscesses and/or fistulas, including decay associated and spontaneous lesions. Two children (33%) had a history of endodontic treatments. The dmft index found in the examined group reached a mean value of 2.8 ± 3.0 and the DMFT index reached a mean value of 2.6 ± 2.5. Results are reported in Table 4

Table 4.

Dental characteristics of children

Cases	Age in yearsa	d/D	m/M	f/F	Spontaneous fistula/abscess	Carious fistula/ abscess	Endodontic treatments	Treatment start age
1	6	6/–	–	1/–	2	–	–	1 year
2	11	0/0	0/0	1/0	–	–	1	18 months
3	6	0/–	0/-	0/–	–	–	–	3 years
4	5	0/–	0/-	1/–	–	–	–	6 months
5	10	3/4	0/0	2/1	–	2	–	2 years
6	16	–/0	–/0	–/3	–	–	3	5 years

DMFT total number of teeth that are decayed (D), missing (M) or filled (F). dmft is applied to the primary dentition

^aCurrent age at assessment

Discussion

These data represent the first dental characterization of XLH patients in our region, and to our knowledge, one of the most complete studies of the correlation between dental health features and other relevant clinical variables of the disease. The cohort demonstrated typical dental findings associated with XLH, including spontaneous abscesses or fistulae, endodontic treatment, radiographic evidence of pulp chamber widening, periodontal involvement, and caries severity indexes that involve decayed, filled, and missing teeth. The findings are generally similar to what has been described in the literature, including recurrent spontaneous abscesses or fistulas, delayed eruption of primary and secondary dentition, large pulp chambers (which reduces dentin and enamel thickness but does not constitute a pathology per se), short roots and rhizolysis in primary dentition, higher frequency of periodontitis, and alveolar bone disorders [4, 5, 7, 8, 10].

The prevalence of dental involvement described in the literature in XLH patients include a 77.5% of oral symptoms [4], 13.3–30.8% endodontic treatment [11, 16], and near 80% of periodontitis and alveolar bone loss [16]. It is important to highlight that methodologies, inclusion criteria, and age ranges of included subjects in these studies are diverse. This undermines the possibility to appropriately compare our population to those previously reported.

In our cohort, most adults (89%) had at least one abscess or dental fistula, 74% demonstrated osteolytic periapical lesions, and 74% had a history of endodontic treatment, as well as a high rate of pulp chamber involvement. Findings were different in children, where a smaller proportion had pulp pathology. This could possibly be explained by less time to accumulate damage and/or a better oral health, possibly due to better dental hygiene. Thirty-three percent of children had dental abscess or fistula including decay associated and spontaneous lesions, and 33% had a history of endodontic treatment. These numbers remain high compared to the national prevalence of endodontic treatment needs (4.3% at age 12) [14].

Our data are similar to previous studies, where elevated pulp involvement in XLH patients is explained by impaired dentin mineralization and dentin thinness, making dental tissue prone to fissures and bacterial infiltration [4, 5, 7, 8, 10]. From a histological point of view, dentin dysplasia with poor mineralization has been found in deciduous and permanent teeth [15]. Enamel appears to form normally in XLH, but mineralization defects are common in dentin, even though odontoblast function appears to be normal. This is presumed to be because hypophosphatemia does not allow for calcospherites fusion in the circum-pulpal region during the mineralization process, causing a dental pulp dysplasia characterized by large areas of interglobular dentin, leading to the formation of gaps within this tissue. In addition, pulp horns are prominent (frequently extending beyond the dental junction) and pulp chambers are large, which reduces dentin and enamel thickness. Consequently, the absence of adequate dentin mineralization added to thin dental hard tissues generates an enamel and dentine prone to fissures and large cracks. Due to these factors, since the teeth are constantly subjected to masticatory forces, the defective dentin can be exposed, generating bacterial contamination of the dentinal tubules, reaching the pulp, determining endodontic involvement [4, 5, 8, 10]. Radiographic evaluation showed 90% of the adults with pulp chamber enlargement and prominent pulp horns, which is in line with the main dental manifestations of the disease [5, 10].

The need for endodontic treatment increases with age in XLH, given the relationship between attrition/abrasion and periapical periodontitis. This emphasizes the importance of good oral hygiene. Patients with XLH should have periodic dental evaluation for the early detection of dentine exposures, as well as prophylactic measures such as furrow sealing, fluoride varnish application, and cleaning. These procedures seal fissures and dentin, and are the most effective measures to prevent bacterial penetration from the tubules into the pulp [4, 5, 7].

In the older group, there was a lower prevalence of gingivitis (25%), while most patients had periodontitis (70%). This is similar to what has been previously reported (78.1%) [16], but it is still below the national periodontitis prevalence (93.5%)[17, 18]. The prevalence of mean attachment loss in the Chilean adult population ages 35–44 years is − 2.7 mm ± 0.9 mm and ages 65–74 years is − 3.8 mm ± 1.4 mm [17].

A relevant finding is that patients who started their therapy after age 18 had a significantly higher degree of attachment loss (− 3.5 mm) compared to those starting it earlier (− 1.5 mm). This observation suggests that the early initiation of conventional treatment may be helpful in preserving periodontal tissues, as suggested by Biosse et al., who described that patients that received early and continuous activated vitamin D and phosphorus supplementation had less severe attachment loss versus patients with late and discontinuous supplementation. In addition to pulp injury, the nature of this disease might also alter periodontal stability, however, studies with longer follow-up are required to obtain more conclusive evidence [16].

Adult mean DMFT (17.5 ± 9.3) was higher than the Chilean mean in adult population (15.1) [18]. Furthermore, DMFT was considerably higher for the group starting therapy after age 18 (27.6), compared to those starting earlier (14.1). Additionally, time of conventional treatment exposure was significantly correlated to a lower DMFT. The latter proposes that, despite that caries severity index in XLH patients is higher than in general population, early and long-term conventional treatment may decrease the dental decay risk. In children, the dmft index found in the examined group reached a mean value of 2.8 ± 3.0 and the DMFT index reached a mean of 2.6 ± 2.5, both values are higher than national average between 6 and 12 years old, corresponding to dmft 1.67 ± 2.4 and DMFT 0.48 ± 1.0 [19].

In conclusion, XLH patients frequently present with pulp involvement associated with larger pulp horns and poor dentin mineralization, as well as a high caries index. Conventional therapy was associated with lower carious index and attachment loss. These findings highlight the fact that dental care should begin at the same time of diagnosis of XLH, in order to prevent dental damage and assure a good quality of oral health.

Overall, these data inform our understanding of the current dental status of XLH patients in Chile, and provide a baseline evaluation that can inform public health officials in decisions on the need of optimizing access to conventional treatment. These data will also likely have implications for similar socioeconomically defined countries.

Data Availability

All data and materials as well as custom code support our published claims.