The ocular problem consists of congenital cataract (100% of cases), bilateral, usually severe, dense and most often total, associated with microcornea (96%), or even microphthalmia. In 93% of the cases, it is responsible for severe visual impairment evidenced by nystagmus (93%), sometimes associated with strabismus (43%). Dental abnormalities, although easily overlooked, are nearly constant, involve permanent and deciduous teeth and are of high diagnostic value. The most frequent are diastema, supernumerary incisors or posterior teeth, which are often impacted, and shape abnormalities (the most typical being screwdriver-shaped teeth). Facial dysmorphism is frequent and characterized by a long face, prognathism, a large nose, with a high nasal bridge, and large often protruding ears. Intellectual impairment is observed in about 30% of cases with inter- and intrafamilial variability. It is usually mild or moderate (80%), without motor delay, but in 20% of cases it is severe/profound and associated with autistic features. In heterozygote females, clinical manifestations are identical to those of affected males but they are attenuated and often limited to infraclinical findings.
Causative mutations have been identified in the NHS gene (localized to Xp22.2) and typically result in a truncated protein. The differential expression of two NHS isoforms, NHS-A and NHS-1A, and differences in the subcellular localization of these isoforms may partly explain the various clinical manifestations.
