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Cataract: what is it?
The word cataract literally means ‘curtain of water’ or ‘water fall.’ It is an almost universal occurrence in the elderly, produced because of the absorption of water, which causes the fibers of the natural, transparent lens to swell and become opaque. In the early stages, the swelling of lens fibers can lead to decreased vision, glare, colored haloes around light bulbs, and diminished sense of contrast. Driving at night becomes especially difficult because of scattering of light by the swollen and partially opaque lens fibers. In some cases, unequal cataract progression in different parts of the lens results in a change in the type or power of spectacles that the patient has been using.
For instance, a person who has been using reading glasses may find that he no longer needs them and may feel that his vision is improving. This ‘second sight’ is actually the result of faster progression of cataract in the center of the lens than in the periphery and gradually, after the initial stages, will produce decreased vision at all distances and necessitate cataract surgery.
Besides the above-described ‘senile cataract,’ cataract may also occur in young people because of ocular trauma, local eye disease or systemic diseases like diabetes. The main symptom produced is generally only gradually progressive painless diminution of vision, but in some cases as in those because of trauma or when the cataract has far progressed, pain, glaucoma, inflammation, and even total irreversible loss of sight may be encountered.
Thus far there is no medicine which can cure cataract, i.e., make an opaque lens transparent. The logical solution that remains then is removal of the diseased lens and its replacement with a new, artificial, transparent one. This is precisely what is done. An incision is made in the eye, the natural lens is removed, an artificial lens is implanted, and the incision closed with fine stitches. The problem that now arises is that the incision and its closure with sutures causes a change in the original, natural curve of the eye leading to some cylindrical power becoming necessary in the patient’s glasses after the operation. Larger the size of the incision, greater the change in the curvature of the eye (astigmatism) and more the cylindrical power needed. To overcome this, smaller and smaller incisions are now being made and the size of the natural lens is reduced as much as possible to enable it to come out of the small incision. At the same time, the artificial lenses to be implanted are also made such that they can easily be inserted through small incisions.
Phacoemulsification is the technique currently in use and enables the whole procedure to be performed through an incision as small as 3 mm as compared with incision sizes of 8-12 mm essential in earlier techniques. In this procedure, the natural lens is emulsified within the eye with the help of ultrasonic energy and is then aspirated through the same probe. The incision being very small and beveled, sutures are not required thus further decreasing the astigmatism.
Ironically, the ideal material for intra ocular lens implantation was discovered with observations made during World War II. The canopy that covered the British Spitfire fighter planes was made of a plastic material known as polymethyl methacrylate (PMMA or Perspex). When shattered by enemy gunfire, fragments of this material occasionally lodged in the eyes of fighter pilots but produced no reaction. Even today PMMA remains the most common material for intra ocular lenses. However, with smaller incisions, more pliable lenses are needed which can be folded or compressed and introduced into the eye where they regain their original shape and so silicon, hydrogel, and acrylic lenses are also coming into use.
Another problem with cataract extraction and intra ocular lens implantation is that the power to accommodate, i.e., see both far and near objects is lost. The natural lens has the ability to change its shape and focus clearly objects at a distance (6 meters) as well as near ones (25 cm). The artificial lens cannot do so, and therefore, reading glasses have to be prescribed after surgery. However, with patients becoming more aware and demanding, multifocal lenses are now coming into picture. These have two or more focal points and can focus both distant and near objects. They are still experimental, and till date, there is no guarantee that a post surgical spectacle correction will not be required.
Sometimes, a few years after cataract surgery, residual lens fibers proliferate and again produce an opacity, which hinders vision. This opacity, termed ‘after cataract,’ can be easily removed with laser; alternatively, specially designed or coated intra ocular lenses, claimed to lower the incidence of after cataract, can be employed at the time of the initial surgery.
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