Cataract0

Cataract

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.

Causes

Age

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

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.

Radiation

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.

Genetics

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.

Medications

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.

 

Treatment

Surgical

Posterior_capsular_opacification_on_retroillumination 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

Cataract SurgerySlit 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.[18]

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.[18][26] In some developed countries a trend to overuse cataract surgery has been noted which may lead to disappointing results.[27]

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.

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