Hypochondroplasia due to FGFR3 gene mutation (N540K) and mosaic form of Down syndrome in the same patient.

The simultaneous presence of Down syndrome and achondroplasia has rarely been reported in the literature, and our search revealed only six patients with such an association. We are reporting the first case of a patient with Down syndrome and hypochondroplasia. In this patient, Down syndrome was clinically recognised and confirmed by the cytogenetic finding of mosaic karyotype (47,XX,+21/46,XX) shortly after birth. She was subsequently diagnosed with hypochondroplasia at the age of 6 years when disproportional short stature, stocky habitus and macrocephaly were observed. These phenotypic findings were later confirmed by the presence of fibroblast growth factor receptor 3 (FGFR3) gene mutation N540K. The overlapping common clinical features of Down syndrome and hypochondroplasia resulted in delayed diagnosis of hypochondroplasia in our patient and the associated deleterious effect on her linear growth. Her final height is 126.5 cm, which is -3.76 standard deviations (SD) lower than the median height in patients with Down syndrome, and is under the lower borderline of the adult height range for women with hypochondroplasia.