Norrie disease is characterized by the blindness of male infants evident at birth or can occur shortly after birth. In addition, they can also experience hearing impairment, mental retardation and behavioral abnormalities. Although the disease does not affect the lifespan of the individual, the associated conditions such as blindness, deafness and mental retardation can reduce their lifespan. This condition affects each case differently as it presents a spectrum of symptoms and severity.
Causes
This rare disorder develops because of the mutations in the gene NDP which is essential for the production of a protein called norrin. This protein is involved in the way the cells and tissues develop. The norrin plays an important role in the specialization of retinal cells for their sensory abilities. It is also essential for the establishment of the blood supply to tissues of the retina and the inner ear along with the development of other systems of the body. The genetic mutation affects the ability of norrin to bind to another protein called frizzled-4 which causes the development of the associated symptoms. This disease is inherited in an X-linked genetic manner meaning the defective gene is located on one of the sex chromosomes. As males only have a single X chromosome, a single altered copy can cause the disorder. As females are the carrier of a single defective gene, they don’t generally experience the disorder but can pass it to their sons.
Symptoms
This disease can affect the vision in different ways based on the degree the eyes are affected. If the individuals are affected mildly, they vision may be good enough to see large print. However, in most of the cases, the back of the eye is impaired to a severe level causing poor vision. The affected child can experience reduced sharpness of vision with the ability to see only bright lights and large shapes. After a few years, their eye lens can become hazy which can further affect the vision. On occasion, the pressure in the eye can also become increased causing problems. The children could develop a condition called nystagmus; to-and-fro movements of the eyes or a squint. In around half of the cases, they can also present learning difficulties continuing to develop poor hearing and epilepsy.
Diagnosis
The diagnosis is possible with the examination from an ophthalmologist. Clinical genetic testing can help identify the genetic mutation responsible for the development of the disease.
Treatment
The treatment is based on the specific symptoms evident in each case. The treatment may require the coordinated efforts of a team of specialists consisting of pediatricians, audiologist and ophthalmologist to devise an effective treatment plan. Early intervention and providing special educational services can help the children reach their full potential. Those who have not lost their vision completely may benefit from a surgery or laser therapy in infancy. Hearing problems can be treated with hearing aids or with cochlear implants.
References
http://www.ssc.education.ed.ac.uk/resources/vi&multi/eyeconds/Norr.html
https://ghr.nlm.nih.gov/condition/norrie-disease#inheritance
https://rarediseases.info.nih.gov/diseases/7224/norrie-disease
https://www.norriedisease.org/node/3
