Bone mineral density and explanatory factors in children and adults with juvenile dermatomyositis at long term follow-up; a cross sectional study

This study is part of a larger controlled, cross-sectional study in Norway. Patients were included based on a probable or definitive diagnosis of JDM according to the Bohan and Peter criteria [20]. Sixty-seven patients diagnosed between January 1970 and June 2006, fullfilled the inclusion criteria: age < 18 years at disease onset, disease duration 24 month and age ≥ 6y at inclusion. These patients were tracked through the National Population Register, and of them, 59 (95%) participated in the overall study [21]. Age- and sex-matched controls were drawn from the same register. Data on controls, including demographics, have previously been published [21].

During a one- to two-day follow-up programme (September 2005–May 2009), study participants were examined at Oslo University Hospital (OUS). Clinical examination was performed by a single physician (HS), and DXA scans and non-fasting blood samples (serum) were taken and frozen in smaller batches at − 80 °C for later analyses [21]. Disease activity was measured by the JDM disease activity score (DAS)(0–20) [21, 22] and cumulative organ damage by MDI(0–40) [17, 22]. MDI osteoporosis was defined as occurrences of low-energy fracture or vertebral collapse (excluding avascular necrosis). Disease duration was defined as the time from the first muscle or skin symptom associated with JDM to the follow-up examination. The patients’ medication histories, including prednisolone doses, were obtained from the medical records and cumulative doses were calculated by chart review [17]. 87% of patients were treated with oral prednisolone and 15% with intravenous methylprednisolone during disease course; at follow-up 17% were on oral prednisolone [17] .

Participants ≥20y reported average weekly physical activity the last year by self-reporting questionnaire [23]. Due to a low number of replies from participants < 20y they were not included. We categorised sweat-inducing or breathless activities as number of hours of exercise: < or ≥ 2 h/week; and to exercise frequency: < or ≥ 2 activities/week.

In study participants ≥20y, a physical component summary scale (PCS) was measured through use of the Norwegian version of the Short Form 36 health survey (SF-36), version 1.0 [17]. Low scores indicated poor physical status.

Bone mineral content was determined using DXA. A narrow fan beam densitometer (GE Healthcare Lunar Prodigy, Madison, WI, USA) was used and all the scans were reanalysed in the same software version 14.10 according to a standard protocol. No hardware was changed during the study period. We analysed the anterior-posterior lumbar spine L2-L4 (spine), and whole body (WB) in study participants < 20y using pediatric software and calculated BMD (bone mineral content in g/cm²) for these regions. For study participants ≥20y, additionally BMD for ultra-distal and distal 33% radius (forearm), and bilateral proximal femur, dual total hip were analysed using adult software. Hence the participants were divided into two age-groups: younger than 20 years (< 20y), and older or equal to 20 years (≥20y). BMD Z-scores were estimated by comparison with the Lunar reference database incorporated in the software and provided by the manufacturer. The database includes BMD data from healthy subjects in the general population of the United States, which has been validated as applicable for clinical use in the adult [24] as well as the pediatric Norwegian population [25]. Z-scores <−1SD below the age-specific mean for healthy individuals were defined as reduced [12, 26, 27].

The biomarkers of bone resorption (CTX) and formation (P1NP) were measured by electroluminescence technology using a Cobas e601 (Roche). The bone metabolism marker 25(OH) VitD was measured using liquid chromatography/tandem mass spectrometry. Insufficient amounts of 25(OH) VitD were defined as levels ≤20 nmol/l according to expert consensus [28] and low as ≤37 nmol/l (corresponding to the lower level of the reference value for the general population in Norway). All analyses were performed at the Hormone Laboratory, Department of Medical Biochemistry, OUS, Oslo in 2019. Levels of IP-10 (CXCL10), a known marker of disease activity in JDM [29], was measured as part of a 27-plex cytokine panel (Bio-Plex immunoassay systems, #m500kcaf0y, Bio-Rad, Hercules, CA), based on xMAP technology (Luminex, Austin, TX) in 2012. All samples were handled and analysed according to manufacturer protocol without freeze/thaw cycles. Measurements of erythrocyte sedimentation rate (ESR) and serum levels of high-sensitivity C-reactive protein (hsCRP), and of the bone minerals albumin, phosphate, PTH, alkaline phosphatase and ionised calcium, were performed consecutively at the Department of Medical Biochemistry, OUS or at local hospitals when appropriate. All analyses were performed in non-fasting serum samples according to standard protocols for the analytical methods. We are aware that non-fasting samples contribute to some uncertainty in the absolute measures reported, as feeding affects concentrations of most biological serum factors.

In total 28/59 (47.5%) of both patients and controls were under the age of 20; 20 girls and eight boys (Table 1). Median disease duration was 16.8y (Table 1); 6.5y (IQR 4.9–8.7y) and 26.3y (IQR 18.9–31.0y) for patients < 20y and > 20y respectively. There was no significant difference in body mass index between patients and controls; however, patients were on average 2.4 cm shorter than controls (p-value =0.05). Time from diagnosis to immunosuppressive medication was comparable for both age groups. The occurrence of fractures during life was comparable between patients and controls (Table 1). As previously described, the SF-36 physical component score chart revealed a significantly reduced score for physical activity among the patients compared with controls [23]. The SF-36 scores were 4.3 units lower in patients ≥20y compared with controls. There were no significant differences in neither hours spent nor frequency of physical activity per week in patients vs. controls ≥20y.

BMD and BMD Z-scores for WB and spine in all patients tended to be lower than in controls (Table 2). Patients < 20y had lower BMD WB and spine (Δ = 0.04 g/cm² and 0.08 g/cm², p-values = 0.02 and 0.04) and BMD Z-scores for WB and spine (Δ = 0.04 g/cm² and 0.08 g/cm², p-values = 0.02 and 0.04) compared with controls <20y. However, these differences in BMD and BMD Z-scores for WB and spine were not found between patients and controls ≥20y. For DXA variables that were only assessed in study participants ≥20y, patients had lower BMD and BMD Z-scores for the forearm compared with controls (Δ = 0.06 g/cm² and 0.71 both p-values =0.01), whereas no significant differences were found between groups for ultra-distal radius and hip. In study participants ≥20y, there were no differences in T-scores for any of the acquisition regions.

12% more patient had reduced BMD Z-scores for WB and 20% more had reduced BMD Z-scores for spine compared to controls (both p-values < 0.01) (Fig. 1a). Also, in the participants <20y, 26% more patients than controls had reduced Z-values for the spine (Fig. 1b). In the participants ≥20y, reduced Z-scores in the forearm were found in 26% more patients than in controls; for the other regions examined, there were non-significant trends of more frequent reduced BMD Z-scores for spine and total hip in patients vs. controls (Fig. 1c).

At follow-up, 10 patients (16.9%) used prednisolone; of those, seven were < 20y (Table 3). Two patients <20y and six ≥20y had not been medicated with prednisolone during the disease course. Not surprisingly, patients ≥20y had used prednisolone for twice as long as patients <20y, and cumulatively, their doses were 1.5 times greater than the doses given to patients <20y (both p-values =0.02). However, months from last prednisolone dose to FU were 3.4 times longer in patients ≥20y compared to patients <20y. Cumulative prednisolone doses after 6 months and 2 years were comparable between patients <20y and ≥ 20y. However, when comparing cumulative prednisolone doses at 6.5y, (which was the mean time of disease duration for patients <20y), patients ≥20y were medicated with prednisolone doses that on average were 1.5 times as high as those given to patients <20y.

In all patients, reduced levels of 25(OH)VitD (Δ = 11 nmol/L) and higher levels of phosphate (Δ = 0.10 nmol/L) were found compared with controls (p-values =0.001 and 0.005) (Table 4). Also, total calcium and albumin levels were lower in patients than in controls (Δ = 0.04 nmol/L and 1.5 nmol/L, p-values = 0.004 and 0.002); however, insufficient levels of 25(OH)VitD and ionised calcium were comparable between patients and controls.

In study participants <20y, phosphate levels were higher in patients compared with the respective controls (Δ = 0.12 nmol/L, p-value =0.005). There were no significant differences in levels of any bone remodelling factors including CTX, P1NP and PTH between patients and controls nor between the age groups </≥20y (Table 4).

For all patients, we found a negative correlation between prednisolone use at follow-up (yes/no) and the Z-score of spine (Table 5). In patients <20y, all prednisolone variables (use at follow-up, use in months and cumulative dosage) correlated negatively with the BMD Z-score for spine (Table 5). In patients ≥20y, there were positive correlations between prednisolone use in months and the Z-score for WB (Table 5).

In all patients, we found levels of PTH and IP-10 to correlate negatively with the BMD Z-score in WB (p-values =0.01 and 0.03) (Table 5). In patients <20y levels of IP-10 correlated negatively with the BMD Z-score for spine. We also found a positive correlation between levels of hsCRP and the BMD Z-score in WB in controls <20y. In patients ≥20y: levels of IP-10 were negatively associated with the BMD Z-score for WB. Further, the ESR and hsCRP were negatively associated with the BMD Z-score for spine in patients ≥20y (Table 5).

In multivariate linear regression models, the BMD Z-scores for the WB and spine were used as dependent variables. The age at diagnosis, age ≥ 20y at follow-up, prednisolone use at follow-up (yes/no), and low levels of 25(OH)VitD, IP-10 and hsCRP were treated as independent variables. We found age ≥ 20y to be correlated to the BMD Z-score in WB (p-value =0.05), and prednisolone use at follow-up (yes/no) was associated with the BMD Z-score in spine (p-value =0.007) (Supplementary Table 1).