A cataract is a clouding of the lens inside the eye which leads to a decrease in vision. It is the most common cause of blindness and is conventionally treated with surgery. Visual loss occurs because opacification of the lens obstructs light from passing and being focused on to the retina at the back of the eye.

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It is most commonly due to biological aging but there are a wide variety of other causes. Over time, yellow-brown pigment is deposited within the lens and this, together with disruption of the normal architecture of the lens fibres, leads to reduced transmission of light, which in turn leads to visual problems.

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Those with cataracts commonly experience difficulty in appreciating colours and changes in contrast, driving, reading, recognizing faces, and coping with glare from bright lights. Cataracts may be partial or complete, stationary or progressive, or hard or soft. The main types of age-related cataracts are nuclear sclerosis, cortical, and posterior sub capsular.

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Nuclear sclerosis is the most common type of cataract and involves the central or ‘nuclear’ part of the lens. Over time, this becomes hard or ‘sclerotic’ due to condensation of lens nucleus and deposition of brown pigment within the lens. In advanced stages it is called brunescent cataract. This type of cataract can present with a shift to nearsightedness and causes problems with distance vision while reading is less affected.

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Cortical cataracts are due to opacification of the lens cortex (outer layer). They occur when changes in the water content of the periphery of the lens causes fissuring. When these cataracts are viewed through an ophthalmoscope or other magnification system, the appearance is similar to white spokes of a wheel pointing inwards. Symptoms often include problems with glare and light scatter at night. Posterior sub capsular cataracts are cloudy at back of the lens adjacent to the capsule (or bag) in which the lens sits. Because light becomes more focused toward the back of the lens, they can cause disproportionate symptoms for their size.

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A mature cataract is one in which all of the lens protein is opaque while the immature cataract has some transparent protein. In the hyper mature cataract, also known as Morgagnian cataract the lens proteins have become liquid. Congenital cataract, which may be detected in adults, has a different classification and includes lamellar, polar, and sutural cataract. Cataracts can be classified by using Lens Opacities Classification System III. In this system, cataracts are classified based on type as nuclear, cortical, or posterior. The cataracts are further classified based on severity on a scale from 1 to 5. Research has demonstrated that the LOCS III system is highly reproducible.