A cataract is a clouding of the eye’s natural lens, which is normally clear and works like a camera lens to focus light onto the retina. This loss of transparency causes vision to become blurry, hazy, or less colorful. Cataracts represent a group of distinct disorders classified by where the opacity forms, what caused the clouding, or when the condition first appeared. Understanding these different forms is helpful because the type often dictates the specific visual symptoms and how quickly the condition may progress.
Classifying Cataracts Based on Location
The most common way eye care professionals categorize cataracts is by their physical location inside the lens structure. The lens is composed of a central core (the nucleus), the surrounding cortex, and an outer capsule. Age-related cataracts, the most frequent type, are divided into three categories based on which of these zones is affected.
Nuclear Cataracts
Nuclear cataracts, or nuclear sclerosis, form deep in the center of the lens. This causes the nucleus to gradually harden and acquire a yellowish or brownish tint. This hardening can temporarily shift the eye’s focal point, leading to a brief improvement in near vision known as “second sight.” As the condition progresses, distance vision diminishes significantly, and patients struggle to distinguish between colors due to the lens discoloration.
Cortical Cataracts
Cortical cataracts begin in the lens cortex, the layer surrounding the central nucleus. These opacities appear as white, wedge-shaped streaks that start at the outer edge and point inward, resembling the spokes of a wheel. Because the clear areas between the spokes scatter light rays, this type causes significant glare and halos, particularly when driving at night. The scattered light interferes with the clarity of the image reaching the retina.
Posterior Subcapsular Cataracts (PSC)
Posterior subcapsular cataracts (PSC) develop on the back surface of the lens, directly beneath the capsule near the path of light. This location places the opacity precisely at the visual axis, meaning even a small PSC causes disproportionately severe symptoms. Patients often report an early loss of reading vision and heightened sensitivity to glare from bright lights. This type tends to mature and affect vision much faster than the other location-based forms.
Cataracts Caused by Specific Diseases or Trauma
Cataracts can also be acquired as a consequence of external factors, underlying systemic diseases, or injury, rather than simply aging. These acquired forms result from metabolic disorders, physical trauma, or certain medications.
Metabolic Cataracts
Metabolic cataracts are most notably linked to diabetes mellitus, developing due to the effect of high blood sugar on the lens. Excess glucose activates the polyol pathway, where the enzyme aldose reductase converts glucose into sorbitol. Since the lens cannot easily metabolize or remove sorbitol, this sugar alcohol accumulates inside the lens fibers. The trapped sorbitol creates an osmotic pressure gradient, drawing water into the lens fibers. This causes the fibers to swell, rupture, and ultimately become opaque.
Traumatic Cataract
A traumatic cataract occurs following a direct injury to the eye, such as blunt force or a penetrating wound. The trauma damages the lens capsule, allowing fluid to rush in and disrupt the alignment of the lens fibers. Blunt trauma can sometimes cause a distinctive star-shaped or rosette-like pattern of opacity to form under the capsule. These cataracts may appear immediately after the injury or develop gradually over time.
Secondary Cataracts
Secondary cataracts form as a side effect of medical treatment or pre-existing eye conditions. For example, long-term or high-dose use of corticosteroid medications significantly increases the risk of forming posterior subcapsular cataracts. Exposure to certain types of radiation, such as therapeutic radiation for tumors, can also accelerate the formation of opacities in the lens.
Inherited and Developmental Cataracts
This distinct category includes cataracts present at birth or developing shortly thereafter, often due to genetic or prenatal factors. Congenital cataracts are detected at birth, while infantile cataracts are found within the first year of life.
Genetic Factors
When a congenital cataract affects both eyes, the cause is often a genetic mutation involving genes that code for lens proteins like crystallins. These mutations disrupt the structural integrity of the lens fibers, leading to clouding. Genetic syndromes, such as Down syndrome, are also associated with these early-onset opacities.
Other Developmental Causes
Other developmental cataracts can stem from serious metabolic disorders, such as galactosemia, where the body cannot properly process the sugar galactose. This leads to toxic accumulation in the lens. Prenatal infections are another significant factor, particularly those grouped in the TORCH complex, with the rubella virus being a well-known cause of cataract development if contracted by the mother during pregnancy.