Cataracts have been around since the Axial age, before 500 B.C. The first documented case was a surgery by Indian surgeon Sushruta and have been prevalent in medicine ever since. Cataracts are the number one cause of blindness in the world, so it is obvious why studying them is extremely important1. Cataracts are defined as lens opacities, derived from the fact that they look like a white spot has covered part of the eye, similar to what an egg white might look like. It could be anywhere from a small speck, to covering the whole pupil.
Cataracts affect the lens of the eye. The lens helps focus light onto the retina, which converts light signals to electrical signals so the brain can interpret them. The lens is located directly behind the pupil, so the cataract is visible through the pupil as demonstrated in Figure 1.
I’m sure most of you readers have heard of cataracts before, known someone with them, or may have had them yourselves! You might even be aware of some of their symptoms. Cataracts are most primarily marked by the presence of the white film as I stated, which can impair the vision in a multitude of ways. They are able to lead to blurry vision and even the complete loss of sight if severe enough2. I’m sure you can imagine that if you have a huge white blob covering your eye that it could heavily impact your ability to see. It is comparable to looking through a fogged up window or dashboard. Cataracts can cause objects to seem dimmer, affect night vision, a sensitivity to light, and double vision. Changing glasses and prescriptions can help some of these symptoms, but once cataracts begin to disturb your daily activities it is usually time to take care of the issue through surgery. Cataracts can be harmless, but typically the problem worsens over time and can cause complete blindness if not treated. Clinicians can identify cataracts through the physical appearance by a routine eye exam where they examine the eye’s structures with a slit lamp2. If you have been to the eye doctor, you may have been in this position where a strong light is shined in your eye and your are told to look at the doctors’ ear/earring to keep your eye focused and open (usually causing you to get a little teary eyed, in my personal experience). This allows them to better see the severity of the white blob rather than just by the naked eye.
The lens is made up of mostly proteins, which help focus the light onto the retina. The most important structural proteins in the eye are called crystallin proteins, which make up 90% of the proteins in the eye4. They have to be tightly packed in order to correctly refract the light onto the retina. Ironically, these proteins do not even form crystals as this would create alternate paths for the light, in fact they do the exact opposite. There are three different types of crystallins with two being purely structural, while the other has the ability to prevent the formation of large clumps of proteins. These clusters of proteins are called aggregates and are very dangerous to the health of the eye. α-, ꞵ-, and 𝛾-crystallins all have the structural component to direct light, while only α-crystallins protect the structure of other proteins in the cell, including the ꞵ- and 𝛾-crystallins5. Crystallin proteins are made at birth, and do not undergo any regeneration so they have to last your whole lifetime! It is easy to see how this can lead to issues with your eyes if these proteins are altered in some way and stop working.
Cataracts can either occur at birth or a young age, called congenital cataracts, or later in life due to aging. Each type of cataract can have various causes. Congenital cataracts may occur due to medications of infections the mother experienced or be inherited. Inherited cases account for 10-25% of congenital cataracts and cause changes to the genes for crystallin proteins4. This causes them to be unable to direct light correctly or the breakdown of proteins by α-crystallin and cause the buildup of crystallins into the white blob you may see. In cataracts due to age, modifications can be made to the proteins over time which alters their structure and prevents them from performing their job1. Additionally, microstructures of the cell such as vacuoles can change the refraction of light and change the ability to see. Vacuoles are structures of cells which hold liquid, so an increased number of them can alter light scattering and cause visual impairment. This in turn can compromise the health of the cell and lead to precipitation of proteins and worsen the issue.
Different causes of cataracts additionally form in different areas of the eye – nuclear, cortical, and posterior. Cortical cataracts most often are associated with the cell membrane health, nuclear with the breakdown of proteins, and posterior with medication side effects, trauma, and other eye issues6.
Thank you so much for reading and I encourage you to check out the rest of my blog for further in-depth content about cataracts!
Table of Contents
- Bloemendal 2004
- Hejtmancik 2008
- Hejtmancik 1998
- Chylack 1993