Also referred to as shortsighted. This describes a condition where the light which should be perfectly focused on the retina in the back of the eye, is focused “short” if the retina, hence shortsighted. People with this error cannot see far without correction (Spectacles or contact lenses).
Also referred to as long-sighted. In this instance the light is focused behind the retina in the back of the eye. This means that these people in higher grades of such an error cannot see near without the aid of spectacles or contact lenses.
This condition is most readily explained by means of the shape distortion that causes the problem. Astigmatism can be described as a refractive error which is most commonly brought about because of a shape distortion on the front surface of the eye, the cornea. For the eye to focus light perfectly, the shape of the cornea needs to be perfectly spherical. Sometimes however this is not the case and the cornea is somewhat flattened, like a soccer ball being squeezed (flattened). This ball is no longer round but flatter on one side and more rounded perpendicular to the flattened side. More like the shape of a Rugby ball. If you imagine this distorted shape of the rugby ball to be the front surface of the eye, then light focused through this surface is described as astigmatism. People with astigmatism require correction (spectacles or contact lenses) to see both far and near. Astigmatism can occur in conjunction with farsightedness of shortsightedness i.e. a person can be shortsighted with astigmatism.
Ageing eyes - (Presbyopia)
This describes a visual effect that affects most people after the age of forty-five. It is often referred to as “Long-arm syndrome” because these people have grave difficulty in maintaining their focus at near when reading or working close-up, like sewing or working on a PC. This condition is a function of age and is not directly dependent on whether the person is long- or short-sighted. People whom have had perfect vision all their lives may suddenly become aware of a near vision problem one day. They will then report to their optometrist with great concerns about their “failing” vision.
This phenomenon is caused through loss of flexibility of the focal lens within the eye. This lens loses its flexibility because of age and also because of an increase in size which takes place throughout life.
Refractive laser surgery at this stage is more successful when performed on the outside of the eye and therefore Presbyopia or “Long-arm Syndrome” is not correctable by means of this laser procedure at this time. Research specific to this condition is however constantly taking place, which in future may offer procedures and results which may be of benefit to people suffering from this condition.
How does the laser work?
The laser is used to alter the shape/curve of the front surface of the eye, by removal if very thin layers of tissue. This removal or ablation of the layers of the eye can bring about a steepening or flattening of the front surface of the eye. Or in the case of astigmatism, the shape of the cornea can be altered in different areas to allow light to pass through the cornea and be perfectly focused on the retina at the back. Different laser techniques exist to achieve this alteration in shape of the cornea, the front layer of the eye.
PRK - Photo Refractive Keratectomy
This technique utilizes a laser bean applied directly to the front surface of the eye. A thin layer of cells is removed to achieve the desired shape change. Because the laser alters the very superficial layers of the eye, the patient experiences discomfort for a few days. This technique is not that popular at this stage because of discomfort the patient experiences and also because of haze caused by the procedure. The haze does take a while to dissipate and in higher prescriptions this may take months.
LASIK - Laser in situ Keratomileusis
This procedure is by far the most poplar, reliable and less uncomfortable. The procedure has been performed for more than two decades. The fundamental difference between this procedure and PRK (described above) is that the laser beam is applied in the deeper layers of the cornea by means of entry into the cornea through a very thin flap on the surface of the cornea. Because the laser is applied to deeper layers of the cornea, the procedure is firstly less painful and secondly because the corneal surface is restored with replacement of the original flap, the visual recovery is instantaneous and there is no scarring or haze.
RK - Radial Keratotomy
This procedure is very seldom used nowadays. In the early nineties this was the only procedure to correct shortsightedness. This was done by slicing through the cornea with a diamond scalpel. The slices were in a radial shape away from the centre of the cornea like the spokes of a wheel. This procedure has proven to be very unpredictable in its outcome and has fallen in to disfavour by most refractive surgeons.
Laser Surgery vs. Contact lenses/Spectacles
We are often asked whether we recommend laser surgery for our patients wearing contact lenses or spectacles. Advice or even a decision in this regard is no simple matter since we have come to realise through experience that the patient’s lifestyle and expectation are the two most powerful forces in the perceived success of these procedures. These two factors supersede prescription limitations (see prescription limitations) where the candidate is highly motivated. This means that if a person has very high or a marginal refractive error*, he or she may feel that the procedure was a fantastic success because what was expected has been achieved. Here the success is in the eye of the beholder so to speak, in spite of the clinical results post-operatively.
Disillusionment generally is the result of expectations that were not met nor predetermined. This simply means that the patient expected more than what he got. Patients with very low or marginal prescriptions often are disappointed with the outcome of RLS (Refractive Laser Surgery) purely because the prescription requirement was very low in the first place, normally part-time spectacle wearers or “Old Eye” patients.
Who can have laser surgery?
Here we would like to rather rephrase the question to “Who cannot have laser surgery?” Anyone wearing spectacles of contact lenses can have surgery but the outcome may not be predictable or desirable.
We have through experience with different prescription types found that there certainly are prescription types that are better suited to laser surgery than others. Fundamentally it is important to realise that the laser procedure requires removing tissue form the cornea to alter the shape of the eye. It therefore stands to reason that the higher the prescription, the more tissue has to be removed. A rule-of-thumb suggests that the best prescriptions types for RLS are short-sighted people with prescriptions varying between -1.00 Dioptre Sphere and -6.00 Dioptre Sphere with Astigmatism of less than -1.50 Dioptre Cylinder.
Farsighted patients can also have the procedure done and their prescription range would be below +2.00 Dioptre Sphere and less than -1.00 Dioptre Cylinder.
High Prescriptions - Prescriptions that fall outside of the parameters mention earlier may not be corrected to the desired expectation. As mentioned earlier, to achieve the desired correction, the shape of the eye needs to be altered and this is done through the removal of tissue. Therefore the higher the prescription the more tissue has to be removed. Some corneas just are too thin to have so much tissue removed. This does not exclude them from having laser performed. For people with prescriptions higher than what we can predictably correct, laser still may be an option because of the relative improvement in lifestyle. People with very high prescriptions may become less spectacle dependent and their spectacles may be for more comfortable because the lenses are now much thinner and not as heavy. These people may even be able to play sport without their spectacles on all the time. In their frame of reference this may be a huge improvement in life style.
Other limiting factors
Disease - If a person suffers form corneal disease i.e. recurrent infections or Dystrophies such as Herpes Simplex or Keratoconus. Worth noting that some of these conditions may be present without the person actually being aware of it. Dormancy of such a condition may e recalled and the person may then present with a full blown disease which would never have posed a problem pre-laser. For this reason we require a laser pre-assessment.
Prescription Stability - A patient’s prescription needs to be stable for years in order for the laser result to have any long-term significance. Do keep in mind that the laser does not alter any normal prescriptive change. If your prescription changes on an annual basis, this change will not be altered by the laser. So should you have laser done with an unstable prescription. Your post-laser correction may change shortly after resulting in spectacle or contact lens dependence in the near future.
Laser Pre-Assessment - This pre-assessment is done to rule out any of the above-mentioned conditions, as well as making sure that there will definitely be enough healthy tissue to perform the Laser procedure accurately and predictably with desired outcome. Where we find any possible factor that could affect the success of the procedure, we will immediately disqualify the candidate and no further procedures would be required. Our approach is conservative and preclusive in this regard. If we cannot do it perfectly we will not do it all.
Age - The age of the patient really is irrelevant when it comes to the success of RLS. The ultimate goal of the patient needs to be carefully considered though. If the patient aims to be totally spectacle free, this cannot be guaranteed at all. Laser does not change the natural aging process of the human body or eyes - therefore the long-term changes, which were going to take place within the visual system, will still take place regardless of laser surgery. This is why it is imperative to have a stable prescription before laser is performed. Ageing Eyes do pose a problem where spectacle independence is the goal. People within and beyond the forties age group with “Long-arm Syndrome” will require spectacles irrespective if the success of their laser procedure, because, as explained earlier “Long-arm Syndrome” is not corrected by RLS and in some cases the symptoms of Presbyopia or “Long-arm Syndrome” may even be enhanced through RLS.
Visual acuity with spectacles or contact lenses.
It is important to note that more often than not the best measure and achieved visual acuity with spectacles and/or contact lenses are better than the best post-operative measured visual acuity. This is likely to be caused by haze and minor scarring as a result of the laser ablation process. It is however important to consider the improvement in lifestyle post-operatively. From this perspective a slight reduction in finest visual acuity is a relative small price to for spectacle independence. Again, most important, the patient expectation needs to be ascertained and met if at all possible, otherwise stay with your glasses or contact lenses. At least you know what you’ve got.
* Marginal refractive error refers to a refractive error that may not have the desired outcome, i.e. the person may still be required to wear spectacles for certain visual events such as night driving or computer work etc.