Rare eye diseases

According to the World Health Organisation (WHO) definition, a disease can be defined as rare if it occurs with a frequency of less than 6.5 to 10 people per 10,000, although this number varies in different countries, depending on the organisation of the national health system and population. In general, there are three elements that allow a particular disease to be defined as 'rare' for a given country: 

• the total number of people affected by the disease
• the prevalence of pathology 
• the lack of treatment for that specific disorder

So far, between 5000 and 8000 rare diseases have been recognised, most of which have a genetic origin. The prevalence of rare diseases is estimated at between 6% and 8%.

Some rare diseases affect vision, either directly or as a consequence of pathologies that, for example, affect the nervous system. Again, these are often genetic disorders, many times due to abnormalities in the development of the neural crest in the embryo, which gives rise to the eye structures, but not only. Let us look at some of them. 

Rare eye diseases 

• Retinitis pigmentosaPeople born with a genetically based disease slowly lose their sight as they get older. The disease begins with difficulty seeing at night or difficulty with peripheral vision. Gradually, colour vision fades and patients also have difficulty with central vision. Although tools and devices for the visually impaired can be helpful, there is still no specific cure for this disease. Retinitis pigmentosa affects about 25 out of every 100,000 people.

• RetinoblastomaThis is an eye tumour that predominantly affects children under the age of five. Retinal cells undergo changes that trigger uncontrolled cell growth, causing a tumour to form in one or both eyes. As this tumour is hereditary, siblings and children of these patients may also be at risk of developing the disease. Early diagnosis and treatment are essential to preserve sight and save the child's life. Treatments include chemotherapy, laser therapy, cold therapy (cryotherapy) and surgery. It affects about 6 out of 100,000 people.

• Acanthamoeba keratitis: is an eye infection that is rare but has potentially very serious consequences for vision and occurs mainly in contact lens wearers. Additional risk factors are damage to the corneal surface and exposure to contaminated water. Currently, there is no authorised drug for the treatment of Acanthamoeba keratitis in any country, but a polyhexanide (PHMB) drug for ophthalmic use, indicated for the treatment of Acanthamoeba keratitis, has recently been developed and is expected to be approved in Europe by 2023. The approval of polyhexanide therapy would finally make a specific treatment available for this disease, which affects about 2 in 100,000 people. 

• Optic neuritisIt is an inflammation of the nerve that can blur vision and make eye movement painful. Optic neuritis can result from an infection or an autoimmune disease such as neuromyelitis optica (also a rare disease). It can also occur as a side effect of certain medications. Optic neuritis is often treated with steroids and affects about 115 out of 100,000 people.

• Stargardt's disease (juvenile macular degeneration): this condition may go unnoticed until adolescence or adulthood, when people develop blurred or distorted vision. Vision loss is progressive and can reach the level of legal blindness. This occurs when light-sensitive cells in the macula die. It affects up to 12 out of every 100,000 people.

• Anophthalmia or microphthalmiaInfants with these conditions are born without an eye (anophthalmia) or with very small eyes (microphthalmia). There is no cure, but ophthalmologists can protect the child's residual vision and prosthetic eyes can be designed to improve appearance. It affects about 10 out of every 100,000 children.

• Axenfeld-Rieger syndromePeople with this syndrome may present with various types of eye problems, from a thin iris to an off-centre pupil. In some cases, the disease also affects the cornea. About half of all patients develop glaucoma and are at risk of severe vision loss. Glaucoma can be treated with eye drops or surgery. Some patients experience glare or sensitivity to light and find relief with lenses or special contacts. It affects less than 1 in 100,000 people.

The importance of rare disease registries 

The need to identify and monitor rare eye diseases is growing because of the impact these diseases have in terms of serious public health problems.

In several countries registers of rare diseases that include lists of diseases, their characteristics, and recommendations for patient care. The establishment of specific registries for rare eye diseases, which are emerging in some countries, would have a positive impact on both patients' lives and scientific research opportunities.

1. Mohan R. Eye of the beholder: Rare eye diseases in focus. J Neurosci Res. 2019 Jan;97(1):3-6. 
2. Sharma M, Jain N, Singh V, Singla S, Aftab I. Ophthalmic registries for rare eye diseases. Indian J Ophthalmol. 2022 Jul;70(7):2225-2230. 
3. 20 Rare Eye Conditions That Ophthalmologists Treat, American Acadmey of Ophtalmology https://www.aao.org/eye-health/tips-prevention/20-rare-eye-conditions-that-ophthalmologists-treat 
4. Nora Szentmary et al, Acanthamoeba keratitis and Clinical signs, differential diagnosis and treatment, J Curr Ophthalmol. 2018 Oct 19;31(1):16-23. doi: 10.1016/j.joco.2018.09.008. eCollection 2019 Mar.