Introduction: Vitamin D-dependent rickets type I (OMIM # 264700) is a rare autosomical recessive disorder that affects vitamin-D metabolism characterized by severe hypocalcaemia with moderate hypophosphatemia. The most common form is type IA, caused by the mutation of gene coding for 1-alpha-hydroxylase (CYP27B1OMIM * 609506. 12q14) that converts the 25-OH-vitaminD in 1,25-OH-vitaminD.
Case report: A 2.3 years female with an isolated and unvestigated history of severe symptomatic neonatal hypocalcaemia come to our attention for suspected hypophosphatemic rickets and a regression of psychomotor development with difficulties in keeping orthostatic position because of a severe hypotonia. At physical examination the child presented rachitic rosary, widening of wrist, deformity of legs with varus knees and an accentuated kyphosis in sitting position. X-ray showed typical alterations of rickets. Blood tests showed moderate hypocalcaemia (8.7 mg/dl), severe hypophosphatemia (1.8 mg/dl), unexpected and very high parathyroid hormone (924 pg/ml) and alkaline phosphatase (3936 IU/L) levels, normal 25-hydroxyvitaminD (48 ng/mL) and inappropriately normal 1,25-dihydroxyvitamin D (31.8 pg/mL). Because the severe hypophosphatemia, a possible vitamin D resistant rickets was hypothesized and FGF23 was requested, as well as 24-hour urinary calcium and phosphorus. So, we started a treatment with Joulie’s oral solution, calcium and calcitriol, discovering a significant reduction in serum phosphate (1.2 mg/dl), and calcium (6.9 mg/dl) levels and a significant increase of parathyroid hormone level (1028 pg/ml) and unmodified alkaline phosphatase level (3599 IU/L). Meanwhile, FGF23, 24-hour urinary calcium and phosphorus were normal. So, in relation to the clinical and laboratoristic course, a Vitamin D dependent rickets type I was hypothesized; we stopped Joulie’s oral solution adjusting appropriately the dosage of calcium and calcitriol, with a progressive normalization of the bone metabolism parameters and significative amelioration of clinical features.
Conclusion: Severe Hypophosphatemia, associated to mild hypocalcaemia are typical of hypophospatemic rickets but could be present also in other forms of rickets, as well as vitamin D-dependent type I rickets. For this reason, waiting for the dosage of FGF23, some parameters of the bone metabolism, such as unexpected parathyroid hormone, alkaline phosphatase and 1,25-dihydroxyvitamin D levels, may be useful markers in suspecting a form of calcipenic rather than phosphopenic rickets.
15 Sep 2022 - 17 Sep 2022