Bone Abstracts

Introduction: Pseudovitamin D deficiency rickets type I is inherited in an autosomal recessive pattern and forms usually as a result of mutation of CYP27B1 gene localised at chromosome 12. It leads to the deficiency of L 1-hydroxylase and abnormal hydroxylation of 25-hydroxycholecalciferol (25OHD) at C 1 which constitutes the last (renal) stage of transformation of vitamin D to 1,25dihydroxycholecalciferol (1,25(OH)₂D). The clinical picture includes features characteristic for deficiency rickets such as: prominent frontal eminences, costochondral swelling, chest abnormalities, widening of epiphyses of long bones, varus knees. Other symptoms involve hypotonia, muscle weakness, growth retardation and in severe cases hypocalcaemia-induced seizures, in particular in infancy. The diagnosis is made on the basis of history, physical examination and laboratory tests, with hyperphosphatasia, hypophosphatemia, and hypocalcaemia and secondary hyperparathyroidism, the serum concentration of 1,25 (OH)₂D is undetectable or decreased, while 25OHD is normal or above the norm.

Aim of study: We present a case of an almost 2-year-old boy with type 1 vitamin D-dependent rickets (VDDR1), which is also known as pseudovitamin D deficiency rickets or vitamin D 1α-hydroxylase deficiency vitamin D-dependent rickets type I (VDDR I). The clinical course, imaging and laboratory tests results, applied treatment and differentiation with other calcium and phosphate metabolism disorders have been discussed.

Conclusions:
1. Due to non-specific clinical picture, the diagnosis of vitamin D resistant rickets forms is complicated and often time consuming, and patients with the diagnosis of vitamin D-dependent rickets type 1 require constant multispecialistic medical care.

2. Optimal treatment is difficult to establish and should be based on supplementation of 1,25-dihydroxycholecalciferol or alphadiol in individually adjusted doses, as treatment with cholecalciferol is not effective.

3. After normalisation of biochemical tests results patients require systematic follow-up (physical examination and laboratory tests) as they may require modification of treatment and determination of the lowest effective dose of drugs.

Disclosure: The authors declared no competing interests.