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 cataracts commonly experience difficulty in appreciating colors and changes in contrast,
driving, reading, recognizing faces, and coping with glare from bright lights.
Signs and symptoms Signs and symptoms vary depending on the type
of cataract, though there is considerable overlap. People with nuclear sclerotic or
brunescent cataracts often notice a reduction of vision. Those with posterior subcapsular
cataracts 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 a cumulative 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.
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 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. Following electrical injuries, cataracts may develop
in 0.7 to 8%. Radiation
Ultraviolet light, specifically UV-B, has been shown to cause cataracts and there is
some evidence that sunglasses worn at an early age can slow its development in later life.
The lens filters UV light, so once that is removed via surgery, one may be able to see
UV light. It has also been recognized, from experimental animal studies and epidemiological
studies in humans, that microwaves can cause cataracts. 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 cataracts are often seen in glass blowers and furnace workers. See Glassblower’s
cataract. Lasers of sufficient power output are known
to damage the eyes and skin. Also see Laser safety.
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 (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 cataracts.
Ichthyosis is an autosomal recessive disorder associated with cuneiform cataracts and nuclear
sclerosis. Basal-cell nevus and Pemphigus have similar associations.
Drug use Cigarette smoking has been shown to lead to
a two-fold increase in the rate of nuclear sclerotic cataracts and a three-fold increase
in posterior subcapsular cataracts. 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. Medications
Some drugs, such as corticosteroids, can induce cataract development. Patients with schizophrenia
often have risk factors for lens opacities (such as Diabetes, hypertension and poor nutrition)
but it is unlikely that antipsychotic medications contribute to cataract formation. Miotics
and triparanol may increase the risk. Other diseases
Classification 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 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. 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. Prevention
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 does 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.
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 is the most widely used
cataract surgery today. This procedure uses ultrasonic energy (U/S) to emulsify the cataract
lens. Phacoemulsification typically comprises five steps:
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 phacoemulsification but can be useful when dealing with very hard
cataracts or other situations where emulsification is problematic. Manual small incision cataract
surgery (MSICS) 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 membrane. The surgery has a high rate of complications.
Post-operative care 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 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 over time in approximately linear manner and the risk increase lasts 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 of this occurring. 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. Epidemiology
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 nearly 22 million Americans
age 40 and older. By age 80, more than half of all Americans have cataracts. Direct medical
costs for cataract treatment are estimated at $6.8 billion annually.
In the Eastern Mediterranean Region, Cataracts are responsible for over 51% of blindness.
Access to eye care in many countries on this region is limited.
History The first references to cataracts and their
treatment in Ancient Rome are found in 29 AD in De Medicinae, the work of the Latin
encyclopedist Aulus Cornelius Celsus. Archaeological evidence of eye surgery in the Roman era also
exists. 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. Etymology
Cataract is derived from the Latin cataracta meaning “waterfall” and from the Ancient Greek
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 alternative 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.
Research N-acetylcarnosine have been investigated as
a medical treatment for cataracts. 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