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.
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 fibers, leads to reduced transmission of light, which in turn leads to visual problems.
Those with cataract commonly experience difficulty appreciating colors and changes in contrast, driving, reading, recognizing faces, and experience problems coping with glare from bright lights.
Signs and symptoms 
Bilateral cataracts in an infant due to congenital rubella syndrome
Signs and symptoms vary depending on the type of cataract, though there is considerable overlap. People with nuclear sclerotic or brunescent cataract, often notice a reduction of vision. Those with posterior supcapsular cataract usually complain of glare as their major symptom.
The severity of cataract formation, assuming that no other eye disease is present, is judged primarily by visual acuity test. The appropriateness of surgery depends on a patient's particular functional and visual needs and other risk factors, all of which may vary widely.
Age is the most common cause. Lens proteins denature and degrade over time and this process is accelerated by diseases such as diabetes and hypertension. With the passage of time, environmental factors including toxins, radiation and UV light have an accumulative effect. These effects are worsened by the loss of protective and restorative mechanisms due to alterations in gene expression and chemical processes within the eye.
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 injuries which penetrate the eye, the capsule in which the lens sits can be damaged. This allows water 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.
Ultraviolet light, specifically UV-B, has been shown to cause cataract and there is some evidence that sunglasses worn at an early age can slow its development in later life. Most UV light from the sun is filtered out by the atmosphere but airline pilots often have high rates of cataract because of the increased levels of UV radiation in the upper atmosphere. It is hypothesised that depletion of the ozone layer and a consequent increase in levels of UV light on the ground may increase future rates of cataracts. It has also been recognized, from experimental animal studies and epidemiological studies in humans, that microwaves can cause cataract. The mechanism is unclear but may include changes in heat sensitive enzymes that normally protect cell proteins in the lens. Another mechanism that has been advanced is direct damage to the lens from pressure waves induced in the aqueous humor. Cataracts have also been associated with ionizing radiation such as X-rays. In addition to the mechanisms already mentioned, the addition of damage to the DNA of the lens cells has been considered. Finally, electric and heat injuries denature and whiten the lens itself as a result of direct protein coagulation. This is the same process through which the clear albumin of an egg becomes white and opaque after cooking. These types of cataract are often seen in glass blowers and furnace workers.
There is a strong genetic component in the development of cataract, most commonly through mechanisms that protect and maintain the lens. The presence of cataract in childhood or early life can occasionally be due to a particular syndrome.
Examples of Chromosome abnormalities associated with cataract include: 1q21.1 deletion syndrome, Cri-du-chat syndrome, Down syndrome, Patau's syndrome, Trisomy 18 (Edward's syndrome) and Turner's syndrome.
Examples of Single-gene disorder include: Alport's syndrome, Conradi's syndrome, Myotonic dystrophy, Oculocerebrorenal syndrome or Lowe syndrome
Skin diseases 
The skin and the lens have the same embryological origin and can be affected by similar diseases. Those with Atopic dermatitis and Eczema will occasionally develop shield ulcers cataract. Ichthyosis is an autosomal recessive disorder associated with cuneiform cataract and nuclear sclerosis. Basal-cell nevus and Pemphigus have similar associations.
Drug use 
Smoking has been shown to lead to a two-fold increase in the rate of nuclear sclerotic cataract and a three-fold increase in posterior subcapsular cataract. There is conflicting evidence over the effect of alcohol. Some surveys have shown a link but others that have followed patients over time have not.
Some drugs, such as corticosteroids, and the antipsychotic drug quetiapine can induce cataract development, as may haloperidol, miotics, and triparanol. Cataracts can also be caused by iodine deficiency.
Other disease 
Metabolic and nutrition diseases
Aminoaciduria or Lowe's syndrome
Galactosemia / galactosemic cataract
Cytomegalic inclusion disease
Secondary to other eye diseases:
Retinopathy of prematurity
Cross-sectional view, showing the position of the human lens
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 subcapsular.
Nuclear sclerosis is the commonest 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.
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 subcapsular 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.
A mature cataract is one in which all of the lens protein is opaque while the immature cataract has some transparent protein. In the hypermature 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.
Risk factors such as UV-B exposure and smoking can be addressed but are unlikely to make large difference to visual function. Although there has been no scientifically proven means of preventing cataracts, wearing ultraviolet-protecting sunglasses may slow the development. While it had been thought that regular intake of antioxidants (such as vitamins A, C and E) would protect against the risk of cataracts, clinical trials have shown that their use as a supplement is not. On the other hand, research is mixed, but weakly positive, for a potential protective effect of the nutrients lutein and zeaxanthin. There is some evidence that statin use is associated with a lower risk of nuclear sclerotic cataract.
Main article: Cataract surgery
Cataract surgery, using a temporal approach phacoemulsification probe (in right hand) and "chopper" (in left hand) being done under operating microscope at a Navy medical center
Slit lamp photo of posterior capsular opacification visible a few months after implantation of intraocular lens in eye, seen on retroillumination
Cataract removal can be performed at any stage and no longer requires ripening of the lens. Surgery is usually 'outpatient' and performed using local anesthesia. Approximately 90% of patients can achieve a corrected vision of 20/40 or better after surgery.
Several recent evaluations found that surgery can only meet expectations when there is significant functional impairment from poor vision prior to surgery. Visual function estimates such as VF-14 have been found to give more realistic estimates than visual acuity testing alone. In some developed countries a trend to overuse cataract surgery has been noted which may lead to disappointing results.
Phacoemulsification, typically comprises five steps, not including the anaesthetic.
Anaesthetic - The eye is numbed with either a subtenon injection around the eye or using simple eye drops.
Corneal Incision - Two cuts are made through the clear cornea to allow insertion of instruments into the eye.
Capsulorhexis - A needle or small pair of forceps is used to create a circular hole in the capsule (or bag) in which the lens sits.
Phacoemulsification - A handheld probe is used to break up and emulsify the lens into liquid using the energy of ultrasound waves. The resulting 'emulsion' is sucked away.
Irrigation and Aspiration - The cortex which is the soft outer layer of the cataract is aspirated or sucked away. Fluid removed is continually replaced with a salt solution to prevent collapse of the structure of the anterior chamber (the front part of the eye).
Lens insertion - A plastic foldable lens is inserted to the capsular bag that is used to contain the natural lens. Some surgeons will also inject an antibiotic in to the eye to reduce the risk of infection. The final step is to inject salt water in to the corneal wounds to cause the area to swell and seal the incision.
Extracapsular cataract extraction (ECCE), consists of removing the lens manually, but leaving the majority of the capsule intact. The lens is expressed through a 10–12 mm incision which is closed with sutures at the end of surgery. Extracapsular extraction is less frequently performed than phacoemulsificaction but can be useful when dealing with very hard cataracts or other situations where emulsification is problematic. Manual small incision cataract surgery (MICS) has evolved from extracapsular cataract extraction. In MSICS, the lens is removed through a self-sealing scleral tunnel wound in the sclera which, ideally, is watertight and does not require suturing. Although "small", the incision is still markedly larger than the portal in phacoemulsion. This surgery is increasingly popular in the developing world where access to phacoemulsification is still limited.
Intracapsular cataract extraction (ICCE) is rarely performed. The lens and surrounding capsule are removed in one piece through a large incision while pressure is applied to the vitreous. The surgery has a high rate of complications.
Post-operative care 
Slit lamp photo of anterior capsular opacification visible a few months after implantation of intraocular lens in eye, magnified view
The post-operative recovery period (the period after cataract extraction is done) is usually short. The patient is usually ambulatory on the day of surgery but is advised to move cautiously and avoid straining or heavy lifting for about a month. The eye is usually patched on the day of surgery and at night using an eye shield is often suggested for several days after surgery.
In all types of surgery, the cataractous lens is removed and replaced with an artificial lens, known as intraocular lens, which stays in the eye permanently. Intraocular lenses are usually monofocal, correcting for either distance or near vision, however, multifocal lenses may be implanted to improve near and distance vision simultaneously, but these lenses may increase the chance of unsatisfactory vision.
Complications of Surgery 
Serious complications of cataract surgery are retinal detachment and endophthalmitis. In both cases, patients will notice a sudden decrease in vision. In endophthalmitis, patients will often describe pain. Retinal detachment frequently presents with unilateral visual field defects, blurring of vision, flashes of light or floating spots.
The risk of retinal detachment was estimated as approximately 0.4% within 5.5 years, corresponding to a 2.3x risk increase compared to naturally expected incidence, older studies reporting a substantially higher risk. The incidence is increasing in approximately linear manner and the risk remains increased for at least 20 years after the procedure. Particular risk factors are younger age, male sex, longer axial length and complications during surgery. In highest risk group of patients the incidence of pseudophakic retinal detachment may be as high as 20%.
The risk of endophthalmitis occurring after surgery is less than 1 in 1000.
Corneal oedema and cystoid macular oedema are less serious but more common and occur because of persistent swelling at the front of the eye in corneal oedema or back of the eye in cystoid macular oedema. They are normally the result of excessive inflammation following surgery and in both cases, patients may notice blurred, foggy vision. They normally improve with time and with application of anti-inflammatory drops. The risk of either occurring is around 1 in 100.
Posterior capsular opacification, also known as after cataract, is a condition in which months or years after successful cataract surgery, vision deteriorates or problems with glare and light scattering recur. This is usually due to thickening of the back or posterior capsule surrounding the implanted lens, so-called ' posterior lens capsule opacification'. Growth of natural lens cells remaining after the natural lens was removed may be the cause, and the younger the patient, the greater the chance. Management involves cutting a small, circular area in the posterior capsule with targeted beams of energy from a laser, a procedure called YAG laser capsulotomy, after the type of laser used. The laser can be aimed very accurately and the small part of the capsule which is cut falls harmlessly to the bottom of the inside of the eye. This procedure leaves sufficient capsule to hold the lens in place but removes enough to allow light to pass directly through to the retina. Serious side effects are rare. Posterior capsular opacification is common and occurs following up to 1 in 4 operations but these rates are decreasing following the introduction of modern intraocular lenses together with a better understanding of the causes.
Disability-adjusted life year for cataracts per 100,000 inhabitants in 2004.
Age-related cataracts are responsible for 51% of world blindness, about 20 million people. Globally, cataracts cause moderate to severe disability in 53.8 million (2004), 52.2 million of whom are in low and middle income countries.
In many countries surgical services are inadequate, and cataracts remain the leading cause of blindness. Even where surgical services are available, low vision associated with cataracts may still be prevalent as a result of long waits for, and barriers to, surgery - such as cost, lack of information and transportation problems.
In the United States, age-related lens changes have been reported in 42% between the ages of 52 and 64, 60% between the ages 65 and 74, and 91% between the ages of 75 and 85. Cataracts affect 22.3 million Americans with a direct medical cost of $6.8 billion annually.
In the Eastern Mediterranean Region, Cataract is responsible for over 51% of blindness. Access to eye care in many countries on this region is limited.
The first references to cataract and its treatment in Ancient Rome are found in 29 AD in De Medicinae, the work of the Latin encyclopedist Aulus Cornelius Celsus. The Romans were pioneers in the health arena - particularly in the area of eye care.
Other early accounts are found in Sanskrit. Cataract surgery was described by the Indian physician, Suśruta (fl. ca. 200 AD). A translation from the original Sanskrit can be found at Cataract Surgery.
The Muslim ophthalmologist Ammar ibn Ali, in his Choice of Eye Diseases, written circa 1000 CE, wrote of his invention of a syringe and the technique of cataract extraction while experimenting with it on a patient.
Cataract is derived from the Latin cataracta meaning "waterfall" and from the Greek καταράκτης (kataraktēs) or καταρράκτης (katarrhaktēs), "down-rushing", from καταράσσω (katarassō) meaning "to dash down" (from kata-, "down"; arassein, "to strike, dash"). As rapidly running water turns white, so the term may have been used metaphorically to describe the similar appearance of mature ocular opacities. In Latin, cataracta had the alternate meaning "portcullis" and the name possibly passed through French to form the English meaning "eye disease" (early 15th century), on the notion of "obstruction". Early Persian physicians called the term nazul-i-ah, or "descent of the water" - vulgarised into waterfall disease or cataract - believing such blindness to be caused by an outpouring of corrupt humour into the eye.
N-acetylcarnosine have been investigated as a medical treatment for cataract. The drops are believed to work by reducing oxidation and glycation damage in the lens, particularly reducing crystallin crosslinking. Some benefit has been shown in small manufacturer sponsored randomized controlled trials but further independent corroboration is still required.
Femtosecond laser has been used during cataract surgery. This technique known as femtosecond laser technology and originally used to cut accurate and predictable flaps in LASIK surgery, has been introduced to cataract surgery. The incision at the junction of the sclera and cornea and the hole in capsule during capsulorhexis, traditionally made with a handheld blade, needle and forceps are dependent on skill and experience of the surgeon. Sophisticated 3-D image of the eyes can be used to guide lasers to make these incisions, Nd:YAG laser can also then break up the cataract as in phacoemulsification.