(P30-017-25) Achondrogenesis Type 2 Masquerading as Vitamin D Resistant Rickets

Objectives: Vitamin D dependent rickets is rare in developed countries due to fortification of milk and direct supplementation but remains common in developing nations like India. Resistant rickets that is unresponsive to vitamin D therapy, diagnosed by the absence of radiological healing following the therapy, is classified as calcipenic (vitamin D metabolism defects) and phosphopenic (low phosphate disorders). Genetic disorders like achondrogenesis and metaphyseal dysplasia can mimic rickets but lack typical biochemical markers.

Methods: We report a 43 month old boy with significant height faltering for 2 years and features of rickets unresponsive to 3 months of vitamin D therapy. The mother’s bow legs and short stature suggested a familial link. Laboratory tests showed normal blood gas (pH 7.3, bicarbonate 23.3mmol/L, base deficit 1.8mmol/L), serum PTH (16pg/ml), calcium (9.5 mg/dl) thus ruling out primary renal rickets. Skeletal imaging showed classic rickets features like metaphyseal changes, poor epiphyseal mineralization, bowing, osteopenia (lower limb X-ray), delayed bone age (wrist X-ray) and upper to lower segment ratio of 1.2:1 indicating skeletal dysplasia.

Results: The child’s height faltering, initially attributed to nutritional rickets, did not improve with vitamin D supplementation is thus suspicious for a genetic condition rather than a deficiency disorder. Genetic testing confirmed a heterozygous missense variant in exon 16 of the COL2A1 gene, diagnosing Achondrogenesis Type 2. 

Conclusions: Despite the clinical presentation of skeletal deformities and the biochemical profile indicating hypophosphatemic rickets, mutation in COL2A1 gene confirmed the diagnosis of ACH2,  characterised by skeletal abnormalities and younger bone age. When rickets is unresponsive to vitamin D supplementation, we should consider skeletal dysplasia like ACH2. Early genetic testing is vital for accurate diagnosis, genetic counselling, and appropriate management. While nutrition is key in preventing rickets, this case provokes the limitations of supplements in genetic disorders.

Funding Sources: None