Vitamin D Dependent Rickets – Type I (VDDR-I)
VDDR-I is an autosomal recessive disorder.
It is characterized by the defective conversion of 25OHD-1,25(OH) 2 D.
It is also called pseudovitamin D deficiency rickets
VDDR-I is caused due to 1-alpha-hydroxylase deficiency.
Patients with VDDR-I have inactivating mutations in the CYP27B1 gene which encodes the enzyme 1-alpha hydroxylase responsible for the conversion of 25OHD to 1,25 (OH) 2 D.
VDDR-I is characterized by early onset of rickets (within the 1 st year of life) and severe hypocalcemia (sometimes with tetany), with accompanying moderate hypophosphatemia.
Enamel hypoplasia may occur.
Serum calcium – Low
Serum phosphate – Low
Serum alkaline phosphatase – Increased
Parathyroid hormone assay – increased
25(OH) vitamin D – Normal
1,25(OH)2 vitamin D – Severely decreased
The treatment for VDDR-I is 1,25 (OH) 2 D (calcitriol).
The initial dose for rickets is 1 μg/day till bone healing occurs. Later the dose is reduced to 0.25 and 1 μg/day.
Adequate intake of dietary calcium (30-75 mg/kg/day of elemental calcium) should be maintained.
Possible side effects of 1,25(OH) 2 D therapy include hypercalcemia, hypercalciuria, nephrocalcinosis, and intraocular calcifications. Therefore, it is important to check the urinary calcium/creatinine ratio and kidney function (e.g. serum creatinine) at each visit.
Renal ultrasound and ophthalmologic consultation (slit-lamp examination) should be performed once per year.