Glasses and contact lenses are a very normal part of life today. At least 6 out of 10 Americans use them daily.
In fact, corrective lenses (i.e., glasses and contacts) treat the most common eye issue there is—refractive error. This includes nearsightedness, shortsightedness, astigmatism, and presbyopia. The first eyeglasses we know of date back to the year 1290, so it’s easy to take for granted how simple it is to correct the most widespread vision issue in the world.
Yet what about other, less common vision issues? A disease is classified as “rare” if it affects fewer than 200,000 people in the United States. Many rare diseases, unlike near- and farsightedness, have no treatment or cure.
Whereas eyeglasses have been around for longer than 700 years, there are eye diseases just being identified by today’s scientists. There are many reasons why such rare diseases are only now being understood.
For one, our understanding of genetics has advanced by leaps and bounds in the last two decades. Like many other diseases in the rest of the body, eye problems often have a genetic component. Rare eye conditions are getting more attention today as new genetic techniques give us windows into their causes—and sometimes, their cures as well. Below are five such conditions we are just beginning to understand thanks to advances in genetics.
1. Leber Hereditary Optic Neuropathy
Unlike most familial diseases, it is inherited through mitochondrial genes.
Most of our genome—the set of instructions that determines what physical characteristics individuals will have—is located in the nucleus, or “headquarters,” of each cell.
But mitochondrial DNA is not located in the nucleus, and it is inherited only through the mother. Although both men and women can get Leber hereditary optic neuropathy, only women can pass on the genes for this disease to their children.
In the case of this condition, at least four distinct genes from mitochondria are thought to be involved.
Features of this disease that impact vision include:
- Progressive blurred vision
- Optic atrophy
- Vascular tortuosity of central retinal vessels
- Circumpapillary telangiectatic macroangiopathy
- Retinal nerve fibers swelling
There is also a rare genetic eye condition called Leber congenital amaurosis, which involves even more genes.
2. Ectopia Lentis
This disorder shares many of the same features as Marfan’s syndrome. Both are inherited through genes. Both involve connective tissue abnormalities. Both cause patients to have longer than average bones. And in terms of vision, both include dislocation of the lens of the eye.
The lens is the round-ish crystalline structure that focuses incoming light rays that pass through it. When the lens is dislocated, it cannot do its job of properly focusing incoming light, and serious vision problems occur.
In the eye, small amounts of dislocation lead to significant refractive error.
The specific vision problems caused by a lens that is out of place are myopia (nearsightedness), astigmatism, and fluctuating or blurred vision, all of which can be severe in patients with ectopia lentis.
3. Usher Syndrome
Many diseases that cause blindness have to do with the functioning of the retina, the surface in the back of the eyeball that focuses incoming light rays.
These are classified as IRDs—inherited retinal diseases. Often, retinal disease research overlaps because different disorders share many of the same features.
Usher syndrome is one such inherited retinal disease that impairs vision. However, Usher syndrome has another component: hearing loss. Hearing and vision loss occur together because these patients carry mutations for both the photoreceptors of the eye and the hair cells of the inner ear (cochlea). It is these specialized hair cells that pick up sound waves and transmit their signals to the brain.
Together, as many as 11 and possibly more genes are malfunctioning in the case of Usher syndrome.
While there is no cure for this disease, there is promising research on how to slow down the progression of vision loss.
4. Sickle Cell Anemia (SCA)
While sickle cell anemia is not usually considered an eye disease per se, the genetic condition has several harmful effects on vision. Because SCA involves red blood cells that feed tissues all over the body with oxygen, it can cause extensive damage to eyes in the following ways:
- Retinal and vitreous hemorrhages
- Tractional retinal detachments
- Retinal vascular occlusions
- Comma-shaped vessels in the conjunctiva
- Angioid streaks
Patients with SCA inherited a copy of the defective gene from both parents. This gene for hemoglobin—the part of a red blood cell that carries oxygen—is located on chromosome 11.
Involving the mutation of only one single nucleotide, SCA’s cause is very simple, but the consequences are very serious, affecting many body systems and shortening life expectancy by decades.
Many eye diseases are progressive, getting gradually worse over time and some leading to total blindness.
Fortunately, this is not the case in the rare genetic disorder of achromatopsia.
The main symptom of this disease, as the name suggests, is the inability to see color.
The retina is home to two types of photoreceptor cells that detect incoming light. Cone cells are responsible for the detection of color, while we use rod cells at night and in low light. They have no ability to detect color. This means that if cone cells are damaged, a person’s ability to sense color information from incoming light rays will be impacted.
Besides complete or near-complete color blindness, this condition also affects visual acuity, with patients experiencing blurry vision. Other common features are photophobia (avoiding strong daylight) and nystagmus (“dancing” eyes that move involuntarily). There are five genes known to cause achromatopsia, but 75 percent of cases are due to just two of those genes. Because of this, gene therapy is a promising avenue of research and potential treatment.