A cataract is the clouding of the eye’s lens, which results in decreased or obscured vision. This change causes light entering the eye to scatter rather than focusing sharply on the retina, making the world appear hazy, blurry, or less colorful. Cataracts develop slowly over time and are the single most common cause of vision impairment globally, making it a highly prevalent, age-related condition.
The Anatomy of Clouding
The eye’s lens, often called the crystalline lens, is a transparent structure positioned just behind the iris and the pupil. Its primary function is to bend and focus light onto the retina at the back of the eye, a process similar to a camera lens. This delicate structure is composed primarily of specialized water-soluble proteins called crystallins, which are precisely aligned to maintain clarity.
As the lens ages, new layers of cells are continuously added to the outside, compacting the older cells toward the center, much like the rings of a tree. Over many years, the proteins within these older cells begin to break down, denature, and clump together. This aggregation causes the lens to lose its transparency and scatter light, leading to the formation of an opaque area known as a cataract.
Categorizing Cataracts by Location
Cataracts are classified based on where the clouding first appears within the lens structure, with each location causing distinct visual disturbances. Understanding this “geography” of lens clouding helps explain the variety of symptoms patients experience.
Nuclear cataracts form deep in the central zone of the lens, known as the nucleus. As this type progresses, the lens nucleus often hardens and may turn yellow or brown, a process called sclerosis. This hardening initially changes the eye’s refractive power, sometimes temporarily improving near vision, a phenomenon known as “second sight,” before distance vision worsens.
Cortical cataracts affect the outer layer of the lens, called the cortex. They appear as white, wedge-shaped opacities or streaks that begin at the edge and move inward toward the center, resembling the spokes of a wheel. Because these streaks cause light to scatter as it passes through the periphery, people with cortical cataracts frequently report problems with glare and halos around lights, especially at night.
Posterior subcapsular cataracts develop as a small, opaque area on the back surface of the lens, directly beneath the lens capsule. Although small, their location is problematic because they sit precisely in the path of light traveling to the retina. They significantly affect near vision and reading ability, and cause intense glare and light sensitivity. Unlike other types, these cataracts often progress quickly, with noticeable vision changes occurring within months.
Causes and Risk Factors
While aging is the most common factor leading to the slow breakdown of lens proteins, several external and medical conditions accelerate cataract development. Prolonged exposure to ultraviolet (UV) radiation from sunlight damages the lens fibers and is a well-established environmental risk. Wearing sunglasses that block UV rays can help mitigate this cumulative damage over time.
Smoking tobacco is strongly associated with an increased likelihood of developing age-related and nuclear cataracts. Medical conditions like diabetes raise the risk significantly, as high blood sugar levels can alter the lens’s chemistry, leading to earlier cataract formation. Long-term use of certain medications, such as oral corticosteroids, is also a recognized contributor to cataract development.
Treatment Overview
Cataracts that have progressed to interfere with daily activities, such as driving or reading, necessitate treatment. Surgery is the only effective method for resolving the condition and restoring clear vision. The procedure is typically performed on an outpatient basis and is one of the most common surgeries globally.
During the surgery, the ophthalmologist makes a small incision in the cornea to access the lens. The cloudy natural lens is then broken up, often using an ultrasound probe in a technique called phacoemulsification, and the fragments are gently suctioned out. Once the cataract is removed, a clear, artificial intraocular lens (IOL) is implanted into the lens capsule to permanently replace the natural lens.