Sclerotic enthesopathy in X-linked hypophosphatemia: An atypical radiological mimic of sclerosing bone disorders

ABSTRACT

A 26-year-old woman presented with progressive bilateral hip and leg pain and worsening gait difficulty over 5 years. She had lower-limb bowing since childhood, without a family history of skeletal deformities. Examination revealed genu varum and poor dentition. Biochemical evaluation showed hypophosphatemia (1.92 mg/dL), normal corrected calcium (9.7 mg/dL), elevated alkaline phosphatase (212 IU/L), vitamin D insufficiency (25-hydroxyvitamin D: 22 ng/mL), and normal parathyroid hormone (48 pg/mL). Vitamin D deficiency was corrected before assessment of tubular phosphate handling. Tubular maximum for phosphate reabsorption per glomerular filtration rate was reduced (0.6 mmol/L), confirming renal phosphate wasting. Serum intact fibroblast growth Factor (FGF) 23 was inappropriately elevated (108 pg/mL; reference range: 23.2–95.4 pg/mL; electrochemiluminescence immunoassay), supporting an FGF23-mediated process. Radiographs demonstrated interosseous membrane calcification, femoral pseudofracture, diffuse skeletal sclerosis, and enthesopathic changes of the femur and pelvis. There was no history of excess fluoride exposure, renal function was normal, and imaging showed no evidence of malignancy, excluding fluorosis, chronic kidney disease, and tumor-induced osteomalacia. Although genetic confirmation was not available, the clinical, biochemical, and radiological features strongly supported X-linked hypophosphatemia (XLH). Treatment with phosphate and calcitriol improved symptoms. This case highlights an atypical sclerotic phenotype of XLH mimicking sclerosing bone disorders.