A rare case of concomitant hypophosphatemic osteomalacia and neurofibromatosis type 1

Abstract Introduction Neurofibromatosis type 1 is a rare genetic disorder with an increased susceptibility to develop skeletal abnormalities. However, the association of hypophosphatemic osteomalacia and neurofibromatosis type 1 is extremely rare. Herein, we report a case of hypophosphatemic osteomalacia in concomitant with neurofibromatosis type 1. Clinical Case A 45-year-old Turkish female patient, who was followed up in another hospital due to osteoporosis and kyphoscoliosis, applied to us with the complaints of persistent bone pain and inability to walk. She had no chronic diseases other than osteoporosis. The physical examination revealed widespread freckling over her trunk and proximal parts of upper and lower extremities and multiple neurofibromas and cafe au lait spots all over her body (figure 1). It was learned that her 2 brothers, mother and daughter also had the widespread freckling. They have no known chronic diseases. She had a severe degree of kyphoscoliosis. She had not have any complaints up to 24 years of age, and low back pain started after delivery. She was diagnosed with osteoporosis at the age of 29. In about 15 years, her height decreased from 167 cm to 135 cm. She had a severe degree of kyphoscoliosis and severe bone pain and was unable to walk on her own because of pain. Bilateral ophthalmic examination revealed no Lisch nodules. Audiometer revealed no abnormalities. Laboratory findings were as follows: Ca: 9.4 (8.4–10.6 mg/dl), phosphorus: 1.8 (2.3–4.7 mg/dl), albumin: 4.2 g/dl, parathormone: 88.3 (11–80 pg/ml), 25 hydroxy vitamin D: 26.1 ug/L, creatinine: 0.4 mg/dl, alkaline phosphatase: 79 (35–105 U/L), the 24-hour urinary excretion of calcium: 187.5 mg/24 h, the 24-hour urinary excretion of phosphorus: 253 mg/24 h, fractional excretion of phosphate: %8.5 and maximum transport of phosphorus in the renal proximal tubules (TmP/GFR): 0.74 (0.88–1.42 mmol/L). According to laboratory data there was a hyperphosphaturia. On the basis of the physical examination, laboratory and radiological findings, she was diagnosed with neurofibromatosis type 1 and hypophosphatemic osteomalacia. Oral phosphate solution (1.5 gram as elemantary phosphorus per day in 3 divided doses) and calcitriol (1 microgram per day in 2 divided doses) were started. After 6 months of therapy, bone pain was significantly decreased and her serum phosphorus level increased. The results of genetic analysis are awaited. Conclusion The coexistence of hypophosphatemic osteomalacia and neurofibromatosis type 1 have been reported in less than 50 cases in the literature. The underlying mechanism is not clarified. The diagnosis of hypophosphatemic osteomalacia should be kept in mind in patients with neurofibromatosis type 1, as they will benefit greatly from oral phosphorus solution and active vitamin D.