Rare, genetically conditioned forms of rickets: Differential diagnosis and advances in diagnostics and treatment.

Apart from the classic forms of rickets, there are rare genetic disorders from the group of vitamin D-resistant rickets where the clinical picture is very similar to the classic forms. Diagnosis of genetically conditioned rickets is often delayed. It is very important to know that a disorder of genetic background may be the cause of the failure of classic treatment in patients with rachitic symptoms. In the group of genetically conditioned rickets there are, among others, congenital hypophosphatemic rickets and vitamin D-dependent rickets type I and II. Congenital hypophosphatemic rickets is characterised by bone mineralisation disturbances related to hypophosphatemia secondary to renal loss of phosphates. The term "hypophosphatemic rickets" covers a group of diseases with similar phenotype but with different genotypes, inheritance models and etiopathogeneses. Mutation of at least 10 genes underlying this disease entity has been described. Vitamin D-dependent rickets are caused by defects of vitamin D metabolism. There are 4 forms described in literature that are distinguished by their genetic causes: type 1A (vitamin D-dependent rickets type IA), type 1B (vitamin D-dependent rickets type IB) and type 2A (vitamin D-dependent rickets type 2A), type 2B (vitamin D-dependent rickets type 2B). A detailed family history in combination with a physical examination, biochemistry and X-ray imaging helps in differential diagnostics of rare forms of rickets.