Norrie disease is a rare genetic disorder that primarily affects the retina, usually leading to blindness. In addition to the congenital ocular symptoms, patients may suffer from other systemic symptoms, including childhood-onset progressive hearing loss, abnormal sleep wake cycles, peripheral vascular disease and learning or behavioral problems.
Patients with Norrie disease may develop retinal detachment, cataracts, leukocoria, iris atrophy as well as other developmental issues in the eye. Patients may experience psychotic-like features and poor coordination of movements.
Most patients are born with normal hearing with onset of hearing loss commonly seen in the second decade of life. Apart from retinal disease, there is significant variance in additional symptoms with only 15% of patients estimated to develop all the features of the disease.
The disease affects almost only male infants, owing to its X-linked recessive inheritance, although females have been reported to show some milder features of the disease.
Norrie disease (ND; OMIM 310600) is a rare X-linked recessive disorder resulting from a mutation in the the Norrin cystine knot growth factor gene also referred to as Norrie Disease Pseudoglioma (NDP) gene that is involved in retinal cell development and blood flow to the inner ear and retina.. It is characterized by retinopathy leading to blindness in male infants at birth or soon afterwards . Variable systemic features include growth and developmental delays with cognitive impairment and/or behavioral disorders, peripheral vascular disease as well as sensorineural hearing loss.
Norrie disease is the most severe phenotype in the spectrum of NDP-Related Retinopathies , a group that includes: Persistent fetal vasculature (PFV), X-linked familial exudative vitreoretinopathy (X-linked FEVR), NDP-related retinopathy of prematurity (ROP), and Coats disease.
NDP-related retinopathies are genetically determined diseases inherited in an X-linked manner and caused by mutations in the NDP gene. They are characterized by fibrous and vascular changes of the retina at birth that progress through childhood and adolescence and cause different degrees of visual impairment. Most of these also include diverse systemic features that can vary throughout and within families.
Other names for this disease include: Anderson-Warburg syndrome, Norrie-Warburg syndrome, Whitnall-Norman syndrome, atrophia bulborum hereditaria, Episkopi blindness, fetal iritis syndrome, congenital progressive oculo-acoustico-cerebral degeneration, oligophrenia microphthalmos, and pseudoglioma congenita.
Norrie disease is a genetically inherited, X-linked recessive disorder that results from mutations in the NDP gene (Xp11.4-p11.3) (GeneID: 4693) encoding the Norrin protein, involved in the vascular development of the retina and inner ear.  Pathogenic variants that disrupt normal protein formation cause retinal dysgenesis that in turn results in disorganized retinal tissue development with fibrovascular changes. Since Norrin is also expressed in other systems of the body, the effects of the disorder are pleiotropic.
There are more than 75 pathogenic variants of the NDP gene (including deletions) that cause Norrie disease with many novel mutations as might be expected for a disorder that leads to a reduction in reproductive fitness.   While penetrance is complete (100%) in affected males, carrier females are usually unaffected, except in rare cases with partial hearing loss or mild ocular phenotype. These cases are secondary to non-random X-chromosome inactivation or a mutation on both X chromosomes seen in consanguineous families or spontaneous somatic mutations. 
Norrie disease is a rare disorder; its exact incidence and prevalence are unknown, but more than 400 cases have been described. Affected patients are almost always male, while females are carriers.  It is not associated with any specific racial or ethnic group, and has been reported in the following groups: northern and central European, American of European descent, African American, French-Canadian, Hispanic, Chinese, and Japanese. Although no ethnic group appears to predominate, most of the individuals reported in the original descriptions of Norrie disease were from Scandinavia (Denmark).  
Currently there are no known risk factors or racial predisposition for this disease apart from genetic inheritance.
A retinal vasculopathy appears to be the primary pathologic ocular change underlying the secondary, fibrotic reaction and associated vitreous hemorrhage. Retinal ganglion cell loss may also occur.  Histology shows hemorrhagic necrosis of an undifferentiated glial mass. The primary defect seems to lie in the neurosensory retina with absence of the ganglion cells and dysplasia of the remaining layers. If allowed to progress, many eyes become phthisical. 
The NDP gene is a 28 kb gene used to synthesize the 133 amino acid protein Norrin, which plays a role in molecular signaling pathways that affect tissue development. Studies suggest that Norrin may play a role in Wnt signaling, which is important for cell division, adhesion, migration and many other cellular activities.  Norrin is one of many ligands that can bind to cell membrane receptors referred to as frizzled receptors. Norrin binds with the receptor frizzled-4 (produced from the FZD4 gene), and initiates a genetic regulatory process.  This pathway is believed to affect molecular processes that are crucial for normal development of the eye and other body systems. In particular, Norrin seems to play critical roles in the specialization of cells in the retina, with high expression of NDP in Muller cells, and in the establishment of blood supply to the retina and the inner ear.  
As a result of these mutations, masses of immature retinal cells accumulate in the vitreous chamber. Disruption in the establishment of blood vessels supplying the eye also causes retinal tissue breakdown leading to a retrolental mass of disorganized tissue. 
Dysfunction of Norrin protein depends on the type and location of the NDP gene mutation. Nonsense mutations that delete portions of the NDP gene prevent full production of Norrin and result in more severe disease. Missense mutations that delete or change single amino acids but do no cut short full protein production usually result in less widespread effects. This leads to a gradient of severity in systemic issues including intellectual disability, seizures, behavioral problems, peripheral vascular disease and delayed development. 
Norrie disease and other NDP related diseases are diagnosed with the combination of the characteristic clinical ocular findings and confirmation by molecular genetic testing.  These tests can identify pathogenic variants in approximately 95% of affected males. No biochemical or functional assays are available for diagnosis.
Molecular genetic testing can be used for more than a confirmatory diagnosis. Currently it is also used for testing possible carrier females, for prenatal diagnosis, and preimplantation genetic diagnosis. There are three types of clinical molecular genetic tests. In approximately 85% of males, missense and splice mutations of the NDP gene and partial or whole deletions are detected using sequence analysis. In addition, deletion/duplication analysis can be used to detect the 15% submicroscopic deletions that cannot be tested with sequence analysis. This method is also used for testing carrier females. The last test used is linkage analysis, which is done when the first two options are unavailable. Linkage analysis is also recommended for those families who have more than one family member affected by the disease.  Prenatal testing for at-risk pregnancies is available if the disease-causing mutation has been identified in the family.  Prenatal ultrasound has also been used to diagnose Norrie disease, albeit infrequently.
Norrie disease was first described by Mette Warburg, a Danish ophthalmologist, as a defined hereditary syndrome in Acta Ophtalmologica in 1961. She reported the disease in a 12 month old boy from a family with seven cases of a hereditary retinal degenerative in which five of the seven cases developed deafness later in life and four of the seven had abnormal mental capacity. In the literature, Warburg discovered 48 similar cases which she believed were caused by this disease as well. She named the disease after a fellow Danish ophthalmologist, Gordon Norrie (1855–1941), who was a highly recognized surgeon in the Danish Institute for the Blind for 35 years and a mentor.
Norrie had in fact described several familial cases of congenital blindness in 1927, although he didn’t recognize them as a clinical syndrome. Other physicians before Warburg, including Taylor (1959 in Episkopi, Greece),  Roberts (1937),  Whitnall, Norman (1940)  and Stephens (1947)  reported families with similar symptoms that were later assumed to be cases of Norrie disease. Warburg was credited with expanding the clinical description to include hearing loss and intellectual disability, noting the X-linked recessive heritability of the disease and further defining the characteristics of the disease.
On external examination, Norrie disease often presents at birth or soon after with leukocoria from the abnormal retrolental tissue. Microphthalmia is another possible initial presentation.  On slit lamp biomicroscopy the irises, anterior chambers, cornea, intraocular pressure, and size of the globe may be normal at birth, though iris atrophy, cataracts, a shallow anterior chamber and synechiae can be noted as well. . An important distinction of this disease is its predisposition for bilateral and symmetric manifestations. 
On fundoscopy the classic finding is a greyish-yellow, glistening, elevated mass that replaces the retina and is visible through a clear lens. These masses have been referred to as "pseudogliomas" due to their appearance.  Partial or complete retinal detachment evolves over the first few months. The mass consists of immature retinal cells and may be apparent a few days after birth but may not be noted until weeks or months later. Norrie disease is an important consideration in the possible differential diagnosis for any case of leukocoria (especially bilateral cases).
Signs and Symptoms
The ocular findings in males with Norrie disease are varied, but usually bilateral and symmetric. They are often present at birth and are mostly progressive, from infancy throughout childhood. 
The main disorder is retinal degeneration which occurs in utero and results in blindness at birth or early infancy.  This visual failure is caused by the abnormal development of the retina. As retinal detachment develops, a grayish-yellow mass can be seen in the back of eye. Other ocular signs include opacification of the lens (cataract), atrophy of the iris, anterior and/or posterior synechiae, and development of a shallow anterior chamber with occlusion of the outflow tracts which may result in increased intraocular pressure and glaucoma.  As the tissue dysplasia progresses, these changes can be followed by corneal opacification, calcific band keratopathy, loss of intraocular pressure, and shrinkage of the globe (phthisis bulbi). In the end stages of ND, the corneas appear milky and opacified; the globes appear small and sunken in the orbital cavity. 
Microphthalmia may be present at birth, and the pupils may be dilated with hypoplastic irides. Most patients are either blind from birth or develop poor vision during the course of the disease Cognitive/behavioral findings: Approximately 30%-50% of males with the ND phenotype have poorly characterized behavioral disturbances or developmental delay/intellectual disability and may show psychotic-like features. Abnormalities include autism or autism-like behavior (27%), depression, and labile affect (25%).  Intra- and interfamilial variability in the expression of the cognitive and behavioral difficulties is common. A severe neurologic phenotype including infantile spasms and chronic seizures has been reported in up to 9% of patients Dementia is rare but may occur in late adulthood.
Auditory findings: Most males with ND develop insidious and progressive sensorineural hearing loss starting during adolescence with a median age of 12 years, although the range of presentation of this loss can be between 5–48 years.  Audiologic data suggest the abnormality resides in the cochlea (specifically, the stria vascularis) and that retrocochlear and brain auditory system function is normal.  Early hearing loss is sensorineural, mild, and asymmetric. The description of hearing loss is homogenous, with most patients describing episodic hearing loss, initially of high frequencies, of variable severity, with a slow deterioration over time and tinnitus present for much of that duration. Many describe long plateaus and at times partial recovery of some hearing. By age 35 years, hearing loss is severe, symmetric, and broad-spectrum frequency. Speech discrimination is relatively well preserved even when the threshold loss is severe . 
Vascular findings: Peripheral vascular disease appears to be an associated clinical finding in a subset of affected males. Patients have been reported with venous stasis ulcers, varicose veins, and erectile dysfunction (ED).  These findings are present in nearly all male cases over the age of 50 years, perhaps the result of small vessel angiopathy. Of note, a significant amount of ND patients experience ED at an early age (between ages 16 and 30).
In more complex molecular genetic cases (NDP deletion), other clinical features may include severe growth failure, endocrine abnormalities or severe mental retardation.
The classic presentation of an ND patient includes congenital blindness, progressive hearing loss and cognitive-psychosocial disturbances.  Norrie disease (or any NDP-related retinopathy) should be suspected in individuals with the following ocular findings:
- Congenital visual failure/blindness
- Bilateral, often symmetric involvement of the eyes
- Persistent fetal vasculature, hyaloid vessels, shallow anterior chamber, and vitreoretinal hemorrhages
- Microphthalmia and cataracts
- Presence of retrolental fibrous and vascular retinal changes at birth (leukocoria) with progressive changes through childhood or adolescence
A significant amount of clinical variability exists. Other clinical manifestations can be progressive, post-lingual, sensorineural hearing loss and cognitive impairments or behavioral disturbance.
Pathogenic variants in NDP-Related Retinopathies, range from classic Norrie disease (ND) to X-linked familial exudative vitreoretinopathy (FEVR), some cases of persistent fetal vasculature (PFV), Coats disease, and advanced retinopathy of prematurity (ROP). These phenotypes appear to be a continuum of retinal findings with considerable overlap.
Once the clinical suspicion is established, to further characterize this disease the following evaluations are recommended: Complete ophthalmologic examination Baseline audiologic evaluation Neurodevelopmental assessment (if developmental milestones are not met) Behavioral evaluation (as needed) Clinical genetics consultation
The definitive diagnosis of an NDP-related retinopathy (including ND) is established in a proband with the identification of a pathogenic variant in the NDP gene by genetic testing. 
One genetic testing strategy is molecular testing of NDP. Sequence analysis is performed first,  followed by deletion/duplication analysis if no pathogenic variant is found. An alternative genetic testing strategy is the use of a multi-gene panel that includes NDP and other genes of interest. Specific genes included and the methods used in multi-gene panels vary by laboratory and over time.
In the very infrequent cases where a known NDP pathogenic variant is not identified, linkage analysis can be considered in families with more than one affected family member. Linkage studies are based on accurate clinical diagnosis of NDP-related retinopathies in the affected family members and accurate understanding of the genetic relationships in the family. Importantly linkage analysis is dependent on the availability and willingness of family members to be tested. The markers used for NDP linkage are highly informative and very tightly linked to the NDP locus; thus, they can be used in many families with NDP-related retinopathies with greater than 95% accuracy. In informative families, linkage analysis can also be used to determine the carrier status of an at-risk female. 
No biochemical or functional assays are available.
- Retinopathy of prematurity
- Persistent fetal vasculature
- Familial exudative vitreoretinopathy (X-linked or autosomal dominant)
- Coats Disease
- Walker-Warburg Syndrome
- Trisomy 13
Differential diagnosis may include retinoblastoma (RB), which is considered in any case of leukocoria.  Fundoscopic examination can often distinguish between the two disorders and ultrasonography to detect calcium in cases of RB. Other disorders related to NDP mutations such as retinopathy of prematurity in later stages, persistent fetal vasculature, X-linked familial exudative vitreoretinopathy and Coats disease can also be considered. The retinal dysplasia that occurs in ND can be clinically indistinguishable from the dysplasia found in trisomy 13 and Walker–Warburg syndrome (in the latter associated with lissencephaly) and must be distinguished through genetic testing.
Norrie disease is not considered in the differential diagnosis of intellectual disability and/or progressive sensorineural hearing loss in the absence of the characteristic ocular features. 
The treatment of Norrie disease requires the coordinated efforts of a team of specialists. Ophthalmologists, pediatricians, audiologists, and other healthcare professionals may need to systematically and comprehensively plan an affected child's treatment.
The treatment of individuals with Norrie disease is directed toward the specific symptoms that are apparent in each patient. Surgery may be necessary to remove cataracts and treat retinal detachment. These efforts may prevent phthisis bulbi, but often will not correct the underlying cause of poor retinal development. Early laser photocoagulation, as well as early vitrectomy have been reported as possible treatments to preserve vision, but are not yet standardized. Hearing assistive devices may be of benefit for individuals with hearing loss and can provide benefit into middle or late adulthood. When hearing becomes significantly impaired, a cochlear implant can be of aid. Other treatment is symptomatic and supportive. 
Early intervention and appropriate specialized education are important in ensuring that children with Norrie disease reach their highest potential. Services that may be beneficial include special remedial or personalized education in conjunction with adaptive medical, social, and/or vocational services. Genetic counseling is important for genetic risk assessment in family members. 
At present there is no medical therapy useful for the ocular manifestations of the disease. Behavioral issues are a lifelong challenge to many individuals with Norrie disease and to their guardians/caretakers, in the presence or absence of intellectual disability or cognitive impairment. Intervention and therapy are supportive and aimed at maximizing educational opportunities. An empiric trial of psychotropic medications may be warranted, although no studies have addressed or supported the use of specific medications for treatment of behavioral aspects of Norrie disease. 
Medical follow up
Routine monitoring of vision and hearing are recommended, including:
- Routine follow up with an ophthalmologist in all individuals with an NDP-related retinopathy, even when vision is severely reduced.
- Routine monitoring of hearing to detect changes early and manage them appropriately
- Observation for clinical evidence of venous stasis or other signs of vascular disease.
- Follow up on developmental and behavioral abnormalities by appropriate behavioral specialists.
Follow up timing is decided on an individual basis, depending on the severity and manifestations of each patient.
The majority of males with the classic Norrie disease (ND) phenotype have complete retinal detachment at birth; therefore, surgery is often targeted toward prevention of further complications. Treatment for partial retinal detachment includes pars plicata/plana vitrectomy or limbal lensectomy/vitrectomy, laser therapy and/or cryotherapy with the potential for improved outcomes if done at an early stage.
Some reports have described successful laser photocoagulation at birth, helping preserve some degree of visual function.  Other reports mention that early vitrectomy (done by age 12 months) can help with maintenance of light perception vision in at least one eye. There have been no systematic clinical studies indicating a preferred method of surgical treatment that can be used on all patients, especially regarding the variability of the disease. Each patient should be studied individually to determine if surgery is a possibility, and to find the best treatment options.
In the progressive stages of ND, development of increased intraocular pressure may also require glaucoma surgery. Rarely, in the most severe cases, enucleation of the eye is required to control ocular pain.
Genetic counseling should be offered to all affected families.
Norrie disease and other NDP-related retinopathies are inherited in an X-linked recessive manner (though in rare instances de novo mutations can occur). Affected males transmit the pathogenic variant to all their daughters (who will be carriers) and none of their sons. Carrier females have a 50% chance of transmitting the pathogenic variant to each child; males who inherit the pathogenic variant will be affected, and females who inherit the pathogenic variant will be carriers and will generally not be affected. Carrier testing for at-risk female relatives and prenatal testing for pregnancies at increased risk are possible if the pathogenic variant has been identified in the family. 
Risk to Family Members 
Parents of a male proband:
- The father of a male proband is neither affected nor is he a carrier.
- Many mothers of a male proband are carriers of an NDP pathogenic variant, even when the family history is negative.
- Rarely, affected males have a de novo pathogenic variant. Women who are carriers may have an inherited or de novo pathogenic variant.
- Women who have an affected child and one other affected relative are obligate heterozygotes (carriers).
Siblings of a male proband (The risk depends on the carrier status of the mother):
- If the mother of the proband has an NDP pathogenic variant, the chance of transmitting it in each pregnancy is 50%. Male siblings who inherit the pathogenic variant will be affected; female siblings who inherit the pathogenic variant will be carriers and will generally not be affected.
- If the pathogenic variant has not been identified in DNA extracted from the mother's leukocytes, the risk to siblings is low but greater than that of the general population because of the possibility of germline mosaicism.
Offspring of a male proband:
- Males with an NDP-related retinopathy will pass the NDP pathogenic variant to all their daughters, who will be carriers, and to none of their sons.
Other family members of a male proband:
- The proband's maternal aunts may be at risk of being carriers of an NDP pathogenic variant and the aunts’ offspring, depending on their gender, may be at risk of being carriers or of being affected.
Carrier Detection 
Identification of female carriers requires:
- Prior identification of the NDP pathogenic variant in the family;
- If an affected male is not available for testing, molecular genetic testing first by sequence analysis, and if no NDP pathogenic variant is identified, then by deletion/duplication analysis.
- Linkage analysis if sequence analysis and deletion analysis do not identify a pathogenic variant, the family structure is appropriate for linkage studies and the necessary family members are available for testing.
Family planning 
- The optimal time for determination of genetic risk, clarification of carrier status, and discussion of the availability of prenatal testing is before pregnancy.
- It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected, are carriers, or are at risk of being carriers.
Ocular prognosis is poor, and most patients develop blindness in the course of the disease. Overall health in systems not mentioned previously is normal. Life span may be shortened by general risks associated with peripheral vascular disease, intellectual disability, blindness, and/or hearing loss, such as increased risk of trauma, aspiration pneumonia, and complications of seizure disorder. 
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