Bone Abstracts

Background: Hypophosphatemic rickets commonly presents in early childhood as inherited disorders. Acquired hypophosphatemic rickets or osteomalacia is a rare condition in children caused by paraneoplastic production of phosphaturic factor or FGF23 and is called tumor-induced rickets or osteomalacia. Localization of tumor is important as hypophosphatemic rickets completely resolves after resection of tumor.

Presenting problem: We present a challenging case of a 13-year-old male with acquired hypophosphatemic rickets, the etiology of which has not yet been identified.

Clinical management: The patient presented to orthopaedic clinic with bilateral knee pain and genu valgum at age 10.5 years. Knee X-ray showed widened physes in distal femur and proximal tibia with mild metaphyseal fraying of distal femur, suggesting remote rickets. Additional X-ray revealed mild widening and irregularity of distal radius and ulnar physes. Lab showed normal calcium, normal 25-OH vitamin D (28 ng/dl; normal 32–100), normal 1,25 dihydroxy vitamin D (45 pg/ml; normal 15–90). He continued to have worsening knee pain with difficulty walking. He was referred to the Metabolic Bone Clinic at age 13 years, and was noted to have valgus angulation of 24 degree with persistent mildly widened physes on his knee X-ray. Biochemical studies showed normal calcium (9.1), low phosphorus (2.8 mg/dl; normal 3.3–5.4), elevated alkaline phosphatase (488 U/l; normal 100–400), normal 25-OH vitamin (28), normal 1,25 dihydroxy vitamin D (51 pg/ml), and normal PTH (51 pg/ml; normal 10–65). Hypophosphatemic rickets was suspected as he also had low tubular resorption of phosphorus. He had normal knee X-ray at age 1 year with no signs of rickets. Serum FGF23 was 220 RU/ml (normal < or =230; Mayo Laboratory). His bone mineral density (BMD) showed normal lumbar BMD Z-score at −0.3, while his total body BMD Z-score was low at −2.3. Patient was started on calcitriol and phosphate therapy. A month later, he had medial distal femur hemiepiphysiodesis performed to correct his deformity.

Discussion: Hypophosphatemic rickets in the case is acquired as he was well until age 10 years and had normal joint X-ray during early childhood. Lab investigation is ongoing.