A cataract is the clouding of the eye’s natural lens, a structure normally clear and positioned just behind the iris. This clouding prevents light from focusing correctly onto the retina, leading to impaired vision. Cataracts manifest in numerous forms, classified by their location in the lens and their underlying cause. Understanding these different kinds is the first step toward recognizing the specific visual changes they cause.
Structural Classification: Location in the Lens
Cataracts are commonly categorized by the exact location of the opacity within the lens structure. The lens is composed of the central nucleus, the outer cortex, and the encompassing capsule. Clouding in each of these three regions results in a different type of cataract, each with a unique impact on vision.
Nuclear sclerotic cataracts are the most prevalent type, forming deep in the central nucleus of the lens. This type is characterized by the gradual hardening and yellowing of the lens core, a process known as nuclear sclerosis. As the nucleus becomes denser, it often causes a shift toward nearsightedness, temporarily improving close-up vision—a phenomenon sometimes called “second sight.” However, this improvement is not sustainable, and the central clouding eventually causes blurred distance vision and dulling of color perception.
Cortical cataracts begin in the lens cortex, the layer surrounding the nucleus, and are characterized by their shape. They present as white, wedge-shaped opacities that start at the outer edge and progress inward toward the center, resembling spokes on a wheel. Because these opacities scatter light, a primary symptom is glare, especially noticeable at night or in bright light. This interference affects contrast sensitivity, making it challenging to distinguish objects against similar backgrounds.
Posterior subcapsular cataracts form rapidly, appearing as a small, opaque area directly beneath the lens capsule at the back of the lens. This location is troublesome because it sits directly in the path of light focusing on the retina, even when the cataract is small. People with this type often report difficulty with reading and experience intense glare and halos around lights, severely impacting night driving. Unlike the other two types, posterior subcapsular cataracts can progress to severe visual impairment within months.
Primary Etiological Types: Aging and Congenital Factors
Beyond structural location, cataracts are also classified by their underlying cause. Age-related cataracts, also known as senile cataracts, are the most common form, developing due to the cumulative effects of time. As a person ages, the proteins within the lens denature and clump together, causing the lens to lose transparency. The changes associated with aging can result in any of the structural types—nuclear, cortical, or posterior subcapsular—making age a risk factor for all of them.
Congenital cataracts are present either at birth or develop shortly thereafter during infancy. These cases are relatively rare and can stem from genetic factors, metabolic disorders, or infections contracted by the mother during pregnancy. While some congenital opacities are small and do not affect vision, larger ones must be treated urgently to allow for normal visual system development. If a child’s brain does not receive clear visual input early in life, it can lead to a permanent condition called amblyopia, or “lazy eye.”
Secondary and Induced Cataracts
Cataracts that develop as a consequence of external factors, systemic diseases, or medical treatments are grouped as secondary or induced. Traumatic cataracts result from a blunt or penetrating injury to the eye, damaging the lens structure. The clouding may appear immediately following the accident or take several years to fully develop.
Systemic health conditions, most notably diabetes, also induce cataracts by accelerating protein changes in the lens due to fluctuating blood sugar levels. Diabetes increases the risk for both cortical and posterior subcapsular cataracts. Other eye diseases, such as chronic inflammation (uveitis) or complications from glaucoma, can also lead to opacities, often classified as complicated cataracts.
Specific medications are also known to induce cataracts, particularly corticosteroids (steroids) when used long-term or at high doses. Both systemic and inhaled corticosteroids are linked to the development of posterior subcapsular opacities. The risk is dose-dependent and increases with the duration of treatment, posing a challenge for individuals with chronic inflammatory or autoimmune conditions.