Norrie disease is a rare X-linked recessive genetic disorder that primarily affects the eyes and almost always leads to blindness. It is caused by mutations in the Norrin cystine knot growth factor gene, also referred to as Norrie Disease Pseudoglioma (NDP) gene. Norrie disease manifests with vision impairment either at birth, or within a few weeks of life, following an ocular event like retinal detachment and is progressive through childhood and adolescence. It generally begins with retinal degeneration, which occurs before birth and results in blindness at birth (congenital) or early infancy, usually by 3 months of age. Patients with Norrie disease may develop cataracts, leukocoria (where the pupils appear white when light is shone on them), along with other developmental issues in the eye, such as shrinking of the globe and the wasting away of the iris. In addition to the congenital ocular symptoms, the majority of individuals afflicted by this disease develop progressive hearing loss caused by vascular abnormalities in the cochlea. Hearing loss usually begins in early childhood and may be mild at first before becoming more progressive by the third or fourth decade of life. Roughly 30–50% of those affected by the disease might encounter cognitive challenges, learning difficulties, incoordination of movements or behavioral abnormalities. These developmental delays often surpass those expected from their visual impairment alone. Additionally, behavioral issues such as psychosis, aggression, and cognitive decline may manifest in patients. Intellectual disabilities have been observed in 20–30% of cases, while dementia, though uncommon, can emerge in late adulthood. About 15% of patients are estimated to develop all the features of the disease. Due to the X-linked recessive pattern of inheritance, Norrie disease affects almost entirely males. Only in very rare cases, females have been diagnosed with Norrie disease; cases of symptomatic female carriers have been reported. It is a very rare disorder that is not associated with any specific ethnic or racial groups, with cases reported worldwide (including cases in North America, South America, Europe, Asia and Australasia). While more than 400 cases have been described, the prevalence and incidence of the disease still remains unknown.
Presentation The most prominent symptoms initially observed in Norrie disease are ocular. Initial characteristics are usually identified at birth or in early infancy, with parents often noticing abnormal eye features or that their child fails to show a response to light. The first visible finding is leukocoria, a grayish-yellow pupillary reflection that originates from a mass of disorganized tissue behind the lens. This material, which possibly includes an already detached retina, may be confused with a tumor and thus is termed pseudoglioma. However, an affected baby may have a normally sized eye globe and unremarkable iris, anterior chamber, cornea and intraocular pressure. Over the first few months of life, complete or partial retinal detachment evolves. From infancy through childhood, the patient may undergo progressive changes in the disease. Disease progression often includes vitreoretinal hemorrhages, the formation of cataracts, deterioration of the iris with adhesions forming between the iris and the lens or the cornea, and shallowing of the anterior chamber which may increase intraocular pressure, causing eye pain. As the situation worsens, there is corneal opacification, where the cornea becomes opaque, and band keratopathy. Intraocular pressure is lost and the globe shrinks. In the last stage of Norrie disease, the globes appear small and sunken in (phthisis bulbi) and the cornea appears to be milky.Auditory symptoms are common with Norrie disease. Progressive hearing loss has been reported to occur in 85–90% of patients and onset is generally in childhood and before the patient reaches their mid-20s. Early hearing loss is sensorineural, mild and asymmetric. By adolescence, high-frequency hearing loss begins to appear. Hearing loss is severe, symmetric, and broad-spectrum by the age of 35 years. However, studies show that while hearing deteriorates, the ability to speak well is highly preserved. The slowly progressing hearing loss is more problematic to adjust to than the congenital blindness for most people with Norrie disease.
Additional characteristics Individuals with Norrie disease can also have cognitive and behavioral symptoms. Developmental delay or learning difficulties are present in about 30 to 50% of males who have Norrie disease. Psycho-social disturbances and poorly characterized behavior abnormalities may also be present. In a study reporting extraocular manifestations in 56 patients with Norrie disease, conditions reported included cognitive impairment (28% of patients), behavioral issues, for example autism spectrum disorder (27% of patients presented with autism or autism-like disorders), neurological features, including seizure disorders and epilepsy (16% of patients reported seizures or seizure history), and peripheral vascular disease (38% of patients).Additionally, children with visual impairment have been shown to struggle establishing regular sleep/wake cycles due to reduced light perception impacting on their understanding of night and day; this can impact on the individual's behavior, mood and cognitive ability. Consistent with this, some case reports of Norrie disease patients have reported the presence of sleep disorders. Peripheral vascular disease (PVD) has also been associated with Norrie disease. In a study of 56 patients with Norrie disease, 21 patients (38%) reported PVD (including varicose veins, peripheral venous stasis ulcers and erectile dysfunction). Due to the known role of the protein norrin in the vascular development of the eye and inner ear, as well as the association with PVD, norrin is thought to have an important angiogenic role in the body.
Genetics
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