Opacification of the crystalline lens
Results from the normal ageing process, trauma, metabolic disorders (hereditary or acquired), medications, or congenital problems
Long-term ocular corticosteroid use
Infectious / Inflammatory cause
The WHO estimates that cataracts account for 48% of reversible blindness worldwide, which translates to about 18 million people
There are estimates that by 2020, that number could be as high as 40 million.
The socio-economic impact of the effect of cataracts is particularly important in developing countries, since 1 blind person usually takes 2 people out of the workforce.
While cataracts can be congenital or due to trauma or metabolic conditions, senile cataracts are the most common, and therefore have the greatest impact.
In the US, 20.5 million people ≥40 years old are affected by cataracts. 
The Beaver Dam Eye Study in the US found that 23.5% of women and 14.3% of men had a visually significant cataract by the age of 65 years. 
The most common cause of cataract is the normal ageing process
Other conditions that can contribute to opacification of the lens include:
metabolic disorders (hereditary or acquired)
infections (e.g., rubella)
While there is still some debate, smoking, alcohol, and exposure to UV radiation have also been indicated as factors that may cause cataract progression, especially nuclear sclerotic cataracts
presence of risk factors
subjective decrease in vision
The classic presentation is a gradual decrease in vision over many years that the patient may be very slow to recognise until there is some visual impairment.
However, under certain circumstances (e.g., diabetes mellitus), a relatively sudden reduction in vision may be reported
blurred or cloudy vision
Changes in the lens lead to a decrease in its transparency, increased light scatter, and reduced vision
A frequent complaint, especially when driving at night
washed-out colour vision
In nuclear cataracts, the change in lens proteins often causes a yellowish hue to the cataract.
Patients notice a decreased richness in colours, especially blues.
reduced visual acuity
The patient with a significant cataract exhibits a reduced best-corrected visual acuity.
Distance vision should be checked using the best-corrected spectacle prescription for distance and a formal eye chart.
defects in the red reflex
May be seen during direct ophthalmoscopy.
inadequate glasses prescription
A patient with a progressing nuclear sclerotic cataract may also complain of inadequate glasses prescription.
The thickening of the lens can cause an increase in refractive power and make the patient appear to be increasingly myopic.
Changes in the lens proteins (crystallins) affect how the lens refracts light and reduce its clarity, therefore decreasing visual acuity.
Chemical modification of these lens proteins leads to the change in lens colour.
New cortical fibres are produced concentrically and lead to thickening and hardening of the lens in nuclear sclerosis
Often appears yellow and can increase the focusing power of the natural lens.
Increasing myopia can also be evidence of a progressing nuclear sclerotic cataract.
In an experimental model, oxidative stress contributed to cataract formation, causing a decrease in the level of adenosine triphosphate and glutathione disulphide. 
Cortical cataracts are most often seen as whitish spokes peripherally in the lens, separated by fluid.
Vacuoles and water clefts can also be seen in these lenses.
Posterior sub-capsular cataracts are due to the migration and enlargement of lens epithelial cells (Wedl cells) posteriorly.
Diabetes is a major factor in the formation of this type of cataract.
Osmotic stress due to sorbitol accumulation has been linked with sudden worsening in patients with uncontrolled hyperglycaemia.
However, research has also found that when sorbitol dehydrogenase was blocked, preventing sorbitol accumulation, oxidative stress was connected with slow-developing cataracts. 
Retina and optic nerve should appear normal
Should be normal
Glare vision test
Slit lamp examination of the anterior chamber
The simple presence of a lens opacity or cataract on its own is not an indication for intervention.
Treatment is indicated if:
Cataract is causing other ocular diseases
Opacification is sufficiently dense, despite best-spectacle correction, to functionally impair the patient's vision
Opacification is sufficiently dense to preclude visualisation of the ocular fundus in the setting of active posterior segment disease or planned posterior segment surgery.
The presence of a cataract in most circumstances will not affect the health of the eye.
The only treatment option for cataract is surgical, and therefore a patient should be referred to an ophthalmologist if this is the suspected diagnosis.
The decision in favour of surgery is patient specific and cannot be generalised to the population as a whole.
There is some question as to whether cataract surgery may increase the risk of progression of underlying age-related macular degeneration (AMD) and thus have deleterious effects on vision.
However, reliable conclusions cannot be drawn from the available data to determine whether cataract surgery is beneficial or harmful in people with cataracts and AMD, and decisions must be based on best clinical judgement. 
Surgery for cataract is generally performed in an outpatient setting under local anaesthesia.
It requires about 20 to 30 minutes.
In most cases, the surgical incision is small (<3.5 mm) and the technique used is termed phacoemulsification.
This approach uses a rapidly vibrating (ultrasonic) cutting needle combined in a single hand-piece, with both irrigation inflow and aspiration outflow to maintain the pressures within the eye and to flush out the fragmented cataract.
In the vast majority of cases performed in developed countries, an artificial lens (intra-ocular lens [IOL]) is implanted within the eye to restore the optical converging power lost with the removal of the cataractous crystalline lens
The power of the IOL is calculated from preoperative measurements of the corneal curvature and the axial length of the eye.
In developing countries, the procedure may be performed using a larger incision to remove the cataract in one piece.
If no IOL is available, aphakic glasses are prescribed postoperatively.
If either an implant lens is not used or there is remaining refractive error (e.g., astigmatism uncorrected by the implant lens), the patient may need to wear either a contact lens or spectacles to achieve good postoperative vision.
In some cases, adjunctive procedures can be done at the time of surgery or after surgery to correct residual refractive error.
Most patients do well after cataract surgery provided they adhere to postoperative instructions and medication regimens.
Regular eye examination will detect any cataract development in the other eye.
Many patients need a spectacle correction to achieve their best acuity following cataract surgery, even after the implantation of an intra-ocular lens.
About 10% of patients develop a gradual opacification of the posterior capsule that can reduce the patient's vision (secondary cataract).
If the visual reduction is significant, an opening can be made in the capsule with a neodymium-doped yttrium aluminium garnet (Nd:YAG) laser.
Once a capsular opening has been made, the capsule will not regenerate and a second treatment is rarely necessary.