SAT0364 High prevalence of enthesopathies in patients with x-linked hypophosphatemia
Authors: A. Salcion Picaud, L. Lassalle, V. Merzoug, A. Usardi, A. Rothenbuhler, P. Kamenicky, C. Roux, A. Linglart, K. Briot
Journal: Annals of the Rheumatic Diseases (2018)

<h3>Background</h3> X-linked hypophosphatemia (XLH) is a rare genetic disease (incidence 1/20 000) due to mutations in the <i>PHEX</i> gene. The genetic defects is associated with high level of FGF23 leading to impaired renal phosphate reabsorption and 1,25(OH)vitaminD synthesis. Most patients are diagnosed at walking age because of lower limb deformities and rickets. In adulthood, patients complain of musculoskeletal pain in relation to bone lesions (pseudofractures, osteomalacia) and joint involvement but also ossification of entheses. The latter is a strong determinant of the altered quality of life in this population. <h3>Objectives</h3> The aim of this work was to describe the structural bone and joint involvement of the entire skeleton in a large cohort of adult patients with XLH. <h3>Methods</h3> The study was prospectively conducted in symptomatic adult XLH patients in the context of the systematic evaluation at the French reference centre for rare diseases of calcium and phosphate metabolism. 81 of the 136 patients included between June 2011 and December 2017 had full body x-rays performed with the EOS low-radiation system. Two trained readers performed a standardised analysis of radiographs with a reading grid including the collection of ossifications of the anterior and lateral vertebral ligament, iliac crests, ischial sites, cotyles and achilles tendons; osteophytes on the spine and hip osteoarthritis; and sacroiliac joint aspect. <h3>Results</h3> Of the 81 patients, 55 are women (68%), mean age 42.4±13.6 years, diagnosed on average at the age of 11 years. 63 (78%) patients were treated during childhood with phosphate and/or vitamin D supplementation. At the time of study, 41 (50.6%) were still upon phosphate supplements and/or vitamin D analogues, 32 (40%) were taking analgesics, although 65 (80%) suffered from pain (71% articular pain, 22% bone pain and 20% enthesitic pain). Of the 81 patients, 71 (88%) presented with structural damage including 44 (63%) with syndesmophytes or ossifications. The average number of syndesmophytes/ossifications was 4,1±2,6 per patient in this subgroup. Most of them were localised at the cervical spine (82%) and the lumbar spine (62%). In addition, these ossifications were fine in 59% (syndesmophytes-like) and coarse in 41%. The other localizations were the ischial region (40%), iliac crests (37%) and the cotyles (90%). Calcanei were visible on 52 radiographs and showed coarse ossifications for 22 of them (42%). 46 patients (65%) had already hip osteoarthritis and 26 (37%) had at least one osteophyte on the spine mainly on the thoracic spine (58%). We found complete ankylosis of sacro-iliac joints in 12 patients (17%). <h3>Conclusions</h3> This observational study shows that 88% of adult XLH patients have structural damages and at least one ossification on the spine and/or on peri-articular joint (specially on the cotyles). Of importance, these ossifications were as frequent and as severe in patients not treated with phosphate and vitamin D analogues than in untreated patients. Knowing that this disease can be misdiagnosed and eventually diagnosed only at the adult age, attention must be paid to ossification of the entheses, in particular of the common anterior vertebral ligament and Achilles tendons. The mechanisms of formation of these enthesopathies remain to be determined. <h3>Disclosure of Interest</h3> None declared