In 1974 Margaret B. Horan, an Australian pediatrician, and Walter E. Nance, an American internist and geneticist independently described patients with this syndrome in which dental anomalies and cataracts are associated.
Males are nearly always born with cataracts. These may be dense enough to severely impair vision and require early removal. Females frequently have some opacities in the lenses (cataracts) as well but these are usually mild and do not require surgery. The corneas (windshield of the eye) may be small. The facial morphology is unusual with prominent noses and large ears but this is less pronounced in females. The teeth are almost always malformed and anomalous in both sexes. They may be ‘screwdriver shaped’ or small and pointed. Extra teeth are often present but sometimes several are missing. The front teeth often have wide spaces between them (diastema).
About 30% of males have some intellectual impairment but this is not seen in females.
This is an X-linked disorder in which males are the most severely affected but almost all female carriers have some signs upon careful examination. Thus, this disorder can be described as either an X-linked recessive or X-linked dominant disease. Affected males have a single X chromosome which is obligatorily transmitted to all of their daughters so that they become carriers if it contains the mutation. The other X chromosome in females usually does not have the mutation so they are usually more mildly affected. However, such carrier (heterozygous) females can expect that half of their sons will have the full syndrome while half their daughters will be carriers. Affected males cannot have affected sons.
The diagnosis is usually made by an ophthalmologist when cataracts are found in infant boys but the full syndrome will often require collaboration with dental experts and medical geneticists. It is important that visually significant cataracts are removed early to allow full maturation of the visual system. All offspring in families with evidence of the Nance-Horan disorder should have complete eye examinations right after birth to detect these cataracts. Cataract surgery in young children is more difficult than in adults and complications such as glaucoma and retinal detachments may occur.
It is also useful to have full dental X-rays in children to look for dental anomalies so that corrective surgery is undertaken in a timely manner. Children with significant intellectual challenges can benefit from special education.