The Optical Assistant (OA) is often the first point of contact when a patient enters the practice to enquire about contact lenses, and they may have many questions. This article will explore some advantages and disadvantages of contact lenses and explain the reasons behind them.
It is important to remember that any questions regarding suitability of contact lenses for a specific patient should be left to the contact lens optician (CLO), or the optometrist, but the OA can answer questions, such as advantages and disadvantages, and give some general information.
There are many advantages of contact lenses compared to spectacles. From the patient’s perspective, these include the cosmetic benefit if the patient does not like wearing spectacles – and a wider choice of different sunglasses that can be worn over the lenses.
They can also be good for different sports as there is no frame to potentially cause a hazard, and they provide a wider field of view with no blind spots due to the restriction of the frame rim or sides. They won’t steam up or become wet and difficult to see through when out in the rain.
There are optical benefits as well, including no prismatic effects encountered as the eye looks away from the optical centre of the lens. This can be especially beneficial if the patient has a difference of at least one dioptre between the two eyes. This condition is called anisometropia, and the larger the difference in prescription between the two eyes, the greater the differential prismatic effect will be when the patient looks away from the optical centre of the spectacle lenses.
For simplicity, if we assume a patient looks down through their spectacles at a point 1cm below the distance optical centre, they will encounter one prism dioptre. If the difference in prescription between the two eyes is 3.00D, this will create a differential prismatic effect of three prism dioptres, which can potentially cause double vision or blurred vision when reading, headaches, eyestrain, or difficulty changing focus when looking up.
As contact lenses sit on the eye and move with the eye, they do not cause this problem, and so can be a good solution for anisometropic prescriptions. Another possible problem with an anisometropic prescription can be the difference in retinal image sizes (aniseikonia).
In hypermetropic prescriptions, the higher plus lens will magnify an image more than the lower plus lens, and in a myopic prescription the higher minus lens will minify the image more than the lower minus lens. This can sometimes cause the two images to not be fully fused into a single image, which can cause double or blurred vision when looking straight ahead, and possibly headaches and eyestrain.
Aniseikonia does not occur in all prescriptions with a difference in power as it depends whether the refractive error is due to the axial length of the eye or the refractive power of the eye being defective. As contact lenses sit on the eye, there will be no difference in magnification between the two lenses, and so this can be an advantage for the anisometropic patient.
For highly myopic patients, the minification of the spectacle lenses can make the smaller letters on the test chart too small for the patient to see. These patients may benefit from an increase in visual acuity with contact lenses as they do not minify the image in the way that spectacle lenses do, as they sit on the eye rather than at a distance in front of the eye.
The opposite is true for higher hypermetropic prescriptions, as the plus spectacle lens will magnify the image, whereas contact lenses will not, and so there may be a reduction in visual acuity.
It should be explained to the patient that contact lenses do not replace spectacles and there are several reasons for this. Most contact lenses should not be worn all day, every day, and some time should be spent without wearing the lenses, in order to maintain eye health.
Care is needed to clean and look after contact lenses, and if this is not done the eyes may develop an infection. Contact lenses should not be worn if there is an eye infection, and so back-up spectacles will be needed for this. Wearing time may be limited for different reasons, including working in a dry environment, and so spectacles will also be needed.
The patient needs to be competent at handling lenses and has to learn how to insert and remove them. Most patients manage this after one or two sessions, but it can take others longer, and a small minority of patients may never be able to master this technique.
These are a few examples of contact lens advantages and disadvantages. In next month’s article, we will look at the different types of contact lenses available and their suitability for different prescriptions.
Sue Deal FBDO R is a practising dispensing optician, ABDO College examiner, senior tutor and supervisor for dispensing opticians. She is also a practice visitor and external moderator for ABDO. She was recently awarded the ABDO Medal of Excellence for her outstanding services to the profession.