What is a Cataract?

The Structure of the Lens and How It Affects Vision

To understand cataracts, it’s helpful to know a bit about the structure and function of the eye’s lens. The lens is made up of water and proteins arranged in a way that keeps it transparent. This transparency is essential because it allows light to pass through without obstruction. The lens is also highly flexible and can change shape, allowing us to focus on objects at varying distances — a process known as accommodation.

Over time, changes in the structure of lens proteins can cause parts of the lens to become cloudy. This cloudiness scatters and blocks light, impairing the eye’s ability to produce clear images on the retina. When cataracts progress, they create a visual experience similar to looking through a frosted or foggy window. The extent of clouding, the cataract’s density, and its location in the lens all influence how much and what type of vision impairment occurs.

How Cataracts Develop: Pathology and Process

Cataracts develop as the proteins and fibers in the lens break down or clump together, forming a cloudy or opaque area. This process is usually gradual and can occur in one or both eyes. Here are the main stages and types of changes within the lens that contribute to cataract formation:

1. Protein Aggregation and Oxidative Stress
   Over time, the lens cells experience cumulative oxidative stress due to exposure to sunlight, environmental factors, and natural metabolic processes. Oxidative stress damages proteins and cells, causing proteins within the lens to clump together. These clumps scatter light, reducing the clarity of vision. Oxidative stress is a major factor in cataract formation, as it alters the structural integrity of the lens.

2. Water Content and Fluid Imbalance
   As the lens ages, its cellular components lose their ability to maintain water balance effectively. This imbalance can lead to increased water uptake in the lens, causing it to swell and disrupt the orderly arrangement of lens fibers. This structural change contributes to the opacity that characterizes cataracts.

3. Formation of Pigments
   Over time, certain molecules within the lens may undergo chemical reactions that produce yellow or brown pigments. These pigments accumulate in the lens, further blocking and distorting the light that enters the eye. As these pigments accumulate, they change the color of the lens, giving it a yellowish or brownish hue. This discoloration causes difficulties with color perception, making it challenging to distinguish between certain hues, particularly blue and purple.

Types of Cataracts Based on Their Location in the Lens

Cataracts can form in different parts of the lens, each impacting vision in unique ways. The three most common types include:

1. Nuclear Sclerotic Cataract
   The most common type of age-related cataract, nuclear sclerotic cataracts, form in the center, or nucleus, of the lens. They cause the nucleus to harden and become yellow or brown over time. Vision with nuclear sclerotic cataracts typically becomes blurred, especially for objects at a distance. Interestingly, in the early stages, people with this type of cataract may temporarily experience improved near vision, a phenomenon known as "second sight." However, as the cataract progresses, overall vision becomes impaired.

2. Cortical Cataract
   Cortical cataracts develop in the cortex, or outer edges, of the lens. They appear as wedge-shaped or streak-like opacities that start at the periphery and move inward toward the center. These types of cataracts often cause issues with glare and contrast sensitivity. People with cortical cataracts might experience difficulty with night driving or in brightly lit environments, as light entering the eye is scattered by the clouded lens edges.

3. Posterior Subcapsular Cataract
   Posterior subcapsular cataracts occur at the back of the lens, just in front of the capsule that holds the lens in place. This type of cataract often develops more rapidly than nuclear or cortical cataracts and is more common in younger individuals, especially those who have taken high doses of steroids or have diabetes. Posterior subcapsular cataracts often cause glare, light sensitivity, and difficulties with reading, as they interfere with light directly in the path it takes to reach the retina.

How Cataracts Impact Vision

Cataracts impact vision in several ways, varying based on the type, location, and severity of the clouding. Common visual symptoms include:

1. Blurred Vision
   As the lens becomes increasingly opaque, light is scattered or blocked, making vision appear hazy or blurred. This blurriness can affect vision at all distances, making activities like reading, driving, or recognizing faces difficult.

2. Increased Glare and Light Sensitivity
   Cataracts scatter incoming light, often causing people to experience glare or halos around light sources. This issue is especially noticeable at night or in bright sunlight, making driving after dark or in bright conditions challenging and uncomfortable.

3. Difficulty with Color Perception
   Cataracts, particularly nuclear sclerotic ones, can cause colors to appear dull or yellowed. Blue and purple hues may be harder to distinguish, and overall color contrast may seem muted. This effect is due to the accumulation of pigment within the lens, which blocks certain wavelengths of light and alters color perception.

4. Double Vision or Multiple Images in One Eye
   Some people with cataracts experience double vision (diplopia) or the perception of multiple images in a single eye, known as monocular diplopia. This symptom can be unsettling and makes focusing difficult, as the brain struggles to merge the conflicting information it receives from each eye.

Conclusion

A cataract is more than just a clouded lens; it’s a gradual, degenerative process that affects how light enters and is processed by the eye. Over time, cataracts can significantly impair vision, altering clarity, color perception, and the ability to see in various lighting conditions. Understanding what a cataract is and how it develops can help patients recognize early signs and work closely with their ophthalmologist to monitor changes in vision. By knowing the impact cataracts have on vision, individuals can make informed choices about their eye health and preserve their quality of life.