Understanding Cataracts

by | Sep 11, 2013 | News

Cataracts affect more than 20 million people and are the leading cause of visual impairment worldwide.  Fifty percent of Americans either have cataracts or have had cataract removed by the time they are 80 years old.  In order to understand what a cataract is, we must follow the path of light into the eye.

cataracts eye diagram copyThe cornea is the front most portion of the eyeball. It is a clear, 5 layered, curved tissue which accounts for the majority of the eye’s focusing, or refractive power.

Behind the cornea lies the iris, the colored part of the eye. The pupil is a hole in the center of the iris and appears as the black part of the eye. The pupil controls how much light enters the eye, getting larger in dim illumination and smaller when it is bright.

Directly behind the iris is the crystalline lens.  This is an oval shaped, transparent structure measuring roughly 10mm in diameter which can change the focus of the eye by changing it’s thickness. The crystalline lens provides approximately one third of the refractive power of the eye. When the eye is focused on a distant object, the lens is thin and long. The closer an object comes the shorter and thicker the lens becomes, allowing the object to be focused on the retina.

The retina is the light sensitive tissue in the back of the eye which transmits information from the visual cataracts lens focus copyworld to the brain via the optic nerve. If the image of an object is not properly focused on the retina, either falling in front of it or behind it, the image will appear blurry. The process of adjustment occurring in the lens to keep objects at different distances in focus is called accommodation. The lens is surrounded by the Zonnular fibers which keep it suspended in place behind the iris. The fibers connect the lens to the ciliary body, a muscle which allows the lens to change its thickness. The lens, along with the cornea, also functions in blocking ultraviolet light from reaching the retina as this can be harmful to the tissue.

The lens consists of three anatomical layers. The lens fibers are long, thin, firmly packed transparent protein cells and form the bulk of the lens itself. They are lined by a single layer of cells called the epithelium on the front half of the fibers. These two portions are encased in a collagen capsule. The epithelium on the front of the lens constantly produces new lens fibers, allowing the lens to grow and thicken throughout life. The new fibers are added to the outer part of the already present fibers splitting the lens into regions.

The crystalline protein fibers are packed very tightly, thus allowing for the transparency of the lens. Cataract is the opacification of a transparent lens. The opacities can be small, on the outskirts of the lens and inconsequential or they can be large, central and detrimental to a person’s vision.

TP illustratiomCataracts most commonly occur secondary to the natural aging process but can also occur at birth, due to trauma, or a side effect of certain medications or diseases. As an individual ages, the crystalline protein fibers start to degrade and condense causing an opacification of the previously transparent lens. Deposition of brown pigment within the lens makes it appear gradually more dense and brown with time under a microscope. This is worsened by the weakening of the restorative actions of the body due to age. The lens is responsible for the ability to focus objects at near and this aging of the lens decreases its plasticity and focusing ability, thus requiring people to wear reading glasses as they get older.

Cataracts occur in one of three layers of the lens. A nuclear cataract, or nuclear sclerosis, is the most common form and occurs when the central portion of the lens becomes opacified. A cortical cataract occurs when the cortex, or outer layer of the lens is opacified and a subcapsular cataract occurs when the opacification is located just underneath the outer capsule shell. No matter the cause of the cataract, the symptoms include decrease in distance and near vision even with glasses, change in your distance eyeglass prescription, glare especially at night, haloes around lights, and/or decrease in contrast and need for increased illumination. 

All cataracts form and progress differently from person to person. Even within the same individual, cataracts can be slightly different between the eyes. Although cataracts are largely considered an age related condition, the following can either cause a cataract to form earlier in life or for the cataract to progress faster: ultraviolet exposure, microwave exposure, face or eye trauma, smoking, diabetes, hypertension, Fabry’s disease, homocystinuria, hypothyroidism, congenital rubella, congenital syphilis, toxoplasmosis, varicella, hypocalcaemia, and corticosteroid or antipsychotic use. If for whatever reason the tightly packed protein fibers of the lens become damaged or displaced, the lens loses its total transparency and a cataract forms.

Extensive research has been done regarding prevention of cataracts. Although there is some conflicting evidance, protecting the eyes from ultraviolet rays by wearing a hat and ultraviolet protecting sunglasses is thought to help delay the development of a cataract, especially if the protection is consistent and started early in life. It was previously thought that regular use of antioxidants such as vitamin A, C, and E had a protective factor but research has proven that to be false. Recent research has found that the use of the nutrients lutein and zeaxanthin and the class of drugs called statins which are used for high cholesterol may have some action in lowering the risk of cataract formation.

Currently, the only treatment for cataracts is surgical removal. Once a cataract becomes opacified enough to start affecting the patient’s quality of life, where they are unable to perform their daily functions satisfactorily due to the cataract, the optometrist or ophthalmologist starts to consider extraction of the cataract. Every person responds to a cataract differently, someone may have 20/30 vision and extreme glare and light sensitivity and be unable to function while another patient may have 20/80 vision and be perfectly satisfied. Surgeons used to wait for a cataract to ‘ripen’ to perform the removal, however, due to new surgical techniques that is no longer necessary nor recommended. A surgical extraction is also considered when the health of the eye is in question, for example when the optometrist or ophthalmologist is unable to get a sharp look into the back of an eye that they need to see clearly because the cataract is clouding the view in.

Research published in September 2013 studied people 49 and over who were diagnosed with cataract-related vision impairment. The large scale study found a 40 percent lower long-term mortality risk in those who undergo cataract surgery.

Currently, cataracts are largely removed through a process called phacoemulsification. The process is performed with the patient awake and laying down. The eye is dilated with eye drops and then numbed with either drops or a local injection, no general anesthetic is required. There are two small incisions made in the cornea, the clear part of the eye, to allow instrument insertion.

A small round hole is made in the front capsule which surrounds the entire lens. A probe is then used to break up the opacified lens fibers by using ultrasound waves and the pieces are sucked out with a vacuum in the probe. When the operating ophthalmologist is certain that all pieces of the lens fibers and the epithelium are removed, a folded plastic lens is inserted into the now empty capsule, unfolded, and situated in place. The procedure usually requires less than 30 minutes and stitches are not regularly required. If you think of the lens as a grape, the procedure can be described as cutting a small hole in the skin of the grape, sucking out the fruit on the inside, and inserting a plastic ball into the leftover skin. 

cataract steps 1 to 4 TP illustration

Step 1: small hole is made in the anterior capsule

Step 2: the lens fibers and epithelium are broken apart

Step 3: the broken pieces are vacuumed out

Step 4: a new, clear, plastic lens is inserted into the capsule sac

Other, more complicated procedures may be performed if phacoemulsification is not feasible. The recovery period after surgery is relatively short. The patient is usually up and about on the same day, although caution in activities is required. The operated eye is usually patched and an eye shield is often worn at night when sleeping. Medications in the form of eyedrops may be prescribed to use for several days to weeks. Regular follow up by your optometrist or ophthalmologist is required after the procedure to insure that no infection or inflammation is present and that the operation was successful. If cataracts are present in both eyes, the surgeon will usually perform the extraction on one eye, wait for everything to heal, and then perform the second extraction.

The new plastic lens, called an intraocular lens, that is inserted into the capsule is custom made for the patient’s eye.  The lens power is manufactured to allow minimum need for distance vision glasses after the operation. This way a person would only have to wear glasses for reading. Recently, bifocal intraocular lenses have been introduced allowing for clear distance and near vision after the surgery, although the chances of 20/20 vision are less likely with these. Although many individuals no longer require glasses after cataract surgery, a patient may still need to wear glasses after the procedure. 

Cataract surgery has become one of the most commonly performed operations in the United States, however, it is still a surgical procedure and the risks and benefits should be discussed with the surgeon before a decision is made. Several months or years after a successful cataract surgery, a secondary cataract may form. This occurs when some of the cells from the natural lens are left over and start to grow over the back capsule, causing decrease in vision and glare. If this occurs, a quick and painless procedure is performed with a laser to cut out a small, round hole in the posterior capsule and improve the symptoms.