Cataract

A cataract is a cloudy area in the lens of the eye that leads to a decrease in vision of the eye. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colours, blurry or double vision, halos around light, trouble with bright lights, and difficulty seeing at night. This may result in difficulty driving, reading and recognizing faces. Poor vision caused by cataracts may also result in an increased risk of falling and depression. In 2020 Cataracts caused 39.6% of all cases of blindness and 28.3% of visual impairment worldwide. Cataracts remain the single most common cause of global blindness.
Cataracts are most commonly due to aging but may also be due to trauma or radiation exposure, be present from birth or occur following eye surgery for other problems. Risk factors include diabetes, longstanding use of corticosteroid medication, smoking tobacco, prolonged exposure to sunlight and alcohol. In addition, poor nutrition, obesity, chronic kidney disease and autoimmune diseases have been recognized in various studies as contributing to the development of cataracts. Cataract formation is primarily driven by oxidative stress, which damages lens proteins, leading to their aggregation and the accumulation of clumps of protein or yellow-brown pigment in the lens. This reduces the transmission of light to the retina at the back of the eye, impairing vision. Additionally, alterations in the lens's metabolic processes, including imbalances in calcium and other ions, contribute to cataract development. Diagnosis is typically through an eye examination, with ophthalmoscopy and slit-lamp examination being the most effective methods. During ophthalmoscopy the pupil is dilated and the red reflex is examined for any opacities in the lens. Slit-lamp examination provides further details on the characteristics, location and extent of the cataract.
Wearing sunglasses with UV protection and a wide brimmed hat, eating leafy vegetables and fruits and avoiding smoking may reduce the risk of developing cataracts or slow the process. Early on, the symptoms may be improved with glasses. If this does not help, surgery to remove the cloudy lens and replace it with an artificial lens is the only effective treatment. Cataract surgery is not readily available in many countries, and surgery is needed only if the cataracts are causing problems and generally results in an improved quality of life.
About 20 million people worldwide are blind owing to cataracts. They are the cause of approximately 5% of blindness in the United States and nearly 60% of blindness in parts of Africa and South America. Blindness from cataracts occurs in 10 to 40 per 100,000 children in the developing world and 1 to 4 per 100,000 children in the developed world. Cataracts become more common with age. In the United States, cataracts occur in 68% of those over the age of 80 years. They are more common in women and less common in Hispanic and Black people.

Signs and symptoms

Signs and symptoms vary depending on the type of cataract, though considerable overlap occurs. People with [|nuclear sclerotic] or [|brunescent] cataracts often notice a reduction of vision. Nuclear cataracts typically cause greater impairment of distance vision than of near vision. Those with posterior subcapsular cataracts usually complain of glare as their major symptom.
The severity of cataract formation, assuming no other ocular disease is present, is primarily assessed using a visual acuity test. Additional symptoms may include frequent changes in glasses prescription and the appearance of colored halos around lights, which can be attributed to the hydration of the lens.
Congenital cataracts can result in amblyopia if not treated in a timely manner.

Causes

Age

Age is the most common cause of cataracts. Lens proteins denature and degrade over time, and this process is accelerated by diseases such as diabetes mellitus and hypertension. Environmental factors, including toxins, radiation, and ultraviolet light have cumulative effects which are worsened by the loss of protective and restorative mechanisms due to alterations in gene expression and chemical processes within the eye.
Oxidative stress associated with lipid peroxidation is an important pathogenic mechanism in cataract formation. Senile cataracts are associated with a decrease in antioxidant capacity in the lens. An increase in oxidative stress in the lens or a decrease in the ability to remove reactive oxygen species can lead to the lens becoming more opaque.

Trauma

Blunt trauma causes swelling, thickening, and whitening of the lens fibers. While the swelling normally resolves with time, the white color may remain. In severe blunt trauma, or in injuries that penetrate the eye, the capsule in which the lens sits can be damaged. This damage allows fluid from other parts of the eye to rapidly enter the lens leading to swelling and then whitening, obstructing light from reaching the retina at the back of the eye. Cataracts may develop in 0.7 to 8.0% of cases following electrical injuries. Blunt trauma can also result in star- or petal-shaped cataracts.

Radiation

Cataracts can arise as an effect of exposure to various types of radiation. X-rays, one form of ionizing radiation, may damage the DNA of lens cells. Ultraviolet light, specifically UVB, has also been shown to cause cataracts, and some evidence indicates sunglasses worn at an early age can slow its development in later life. Microwaves, a type of nonionizing radiation, may cause harm by denaturing protective enzymes, by oxidizing protein thiol groups, or by damaging lens cells via thermoelastic expansion. The protein coagulation caused by electric and heat injuries whitens the lens. This same process is what makes the clear albumen of an egg become white and opaque during cooking.

Genetics

The genetic component is strong in the development of cataracts, most commonly through mechanisms that protect and maintain the lens. The presence of cataracts in childhood or early life can occasionally be due to a particular syndrome. Examples of chromosome abnormalities associated with cataracts include 1q21.1 deletion syndrome, cri-du-chat syndrome, Down syndrome, Patau's syndrome, trisomy 18, and Turner's syndrome, and in the case of neurofibromatosis type 2, juvenile cataract on one or both sides may be noted. Examples of single-gene disorder include Alport's syndrome, Conradi's syndrome, cerebrotendineous xanthomatosis, myotonic dystrophy, and oculocerebrorenal syndrome or Lowe syndrome.

Skin diseases

The skin and the lens have the same embryological origin and so can be affected by similar diseases. Those with atopic dermatitis and eczema occasionally develop shield ulcer cataracts. Ichthyosis is an autosomal recessive disorder associated with cuneiform cataracts and nuclear sclerosis. Basal-cell nevus and pemphigus have similar associations.

Smoking and alcohol

has been shown to increase the risk of age-related cataract and nuclear cataract. Evidence is conflicting over the effect of alcohol. Some surveys have shown a link, but others that followed people over longer terms have not.

Inadequate vitamin C

Low vitamin C intake and serum levels have been associated with greater cataract rates. However use of supplements of vitamin C has not demonstrated benefit.

Medications

Some medications, such as systemic, topical, or inhaled corticosteroids, may increase the risk of cataract development. Corticosteroids most commonly cause posterior subcapsular cataracts. People with schizophrenia often have risk factors for lens opacities. Second-generation antipsychotic medications are unlikely to contribute to cataract formation. Miotics and triparanol may increase the risk.

Post-operative

Nearly every person who undergoes a vitrectomy—without ever having had cataract surgery—will experience progression of nuclear sclerosis after the operation. This may be because the native vitreous humor is different from the solutions used to replace the vitreous, such as BSS Plus. This may also be because the native vitreous humour contains ascorbic acid which helps neutralize oxidative damage to the lens and because conventional vitreous substitutes do not contain ascorbic acid. Accordingly, for phakic patients requiring a vitrectomy it is becoming increasingly common for ophthalmologists to offer the vitrectomy combined with prophylactic cataract surgery to prevent cataract formation.

Hyperbaric oxygen therapy

is the administration of 100% oxygen at pressures greater than one-atmosphere absolute pressure for a therapeutic purpose. HBOT can have several side effects, including the long-term development of cataracts. This is rare and generally associated with multiple HBOT exposures over a long period. As it does not usually become symptomatic during HBOT, it may often go unrecognised and is probably under-reported. Evidence is emerging that lifetime dosage of oxygen may be a precipitating factor in the development of age-related cataracts. Nuclear cataracts have been hypothesized to be the end stage of the far better known phenomenon of hyperbaric myopic shift.