Subjective refraction? Objective refraction? Cycloplegic refraction?
These are terms that you may hear when you visit your ophthalmologist and initially they can seem a bit complicated. But once broken down into their basic parts, they become less intimidating and more useful. One of the important questions that your ophthalmologist will want to answer is: “how well do you see?” It is crucial that the most accurate ‘reading’ is calculated so that he or she knows whether there are any errors in your vision and if so, how severe. This will enable precise correction with either glasses or contact lenses.
The process used to determine just how well you see is called “refraction”.
There are different ways in which one can work this out and the tests may vary according to your age or that of your child.The tests can also vary according to just how much co-operation or input is needed to perform them.
Did you know that your ophthalmologist can accurately test your infant’s vision before they are able to speak?
“Subjective refraction” requires full co-operation from the patient. As an adult, you may be asked to look through the lenses in a machine called a phoropter and identify letters of varying sizes on a chart.
In this setting, you are telling the doctor what you see and your eyes are using every structure at their disposal, to help you give the correct answers.
In children, letter tests can be adapted to a more appropriate level, by substituting them with pictures. Once again, the child’s help will be required; they will be telling the doctor what they can see.
In preverbal children, this co-operation becomes a bit trickier to deliver. The ophthalmologist will need to take different tests out of his or her toolbox to get the job done.
Because the infant cannot respond to questioning, the doctor needs to observe the child carefully while performing specific, painless tests. This style of testing shifts the focus from gaining information subjectively to one of observation. One of the tests that can be performed is called retinoscopy. In this technique, the ophthalmologist uses a retinoscope to shine a beam of light into each eye. He or she then observes the reflection of light from the retina and can accurately calculate the vision error in each eye. These calculations are then used to prescribe glasses.
In order to see objects that are either near to us or at a distance from us, we need to change the shape of our lenses in each eye. This is done by altering the degree of contraction in a circular or ring muscle, called the ciliary muscle. This muscle forms a narrow ring in the eye just behind the iris. The lens is suspended from this muscle. When this muscle contracts, the lens relaxes and we are able to see objects that are near to us, like the words on a page. When the muscle relaxes, the lens becomes more elongated, enabling us to see objects in the distance, like a tree on the horizon.
In some children, the vision error can be so great that the eye needs to relax the “ring” or ciliary muscle maximally to try to see objects in the distance clearly.
This may not be enough, however. There may still be a tiny degree of tension in the ciliary muscle, changing the lens shape. This means that when the vision is tested, there may be a small “hidden” error that the doctor is not able to pick up.
Myopia is common in premature infants, but less so in full-term babies. By 6 months of age, it becomes uncommon. In adolescence however, roughly a quarter are affected, with the degree of myopia peaking at 20 years. It is the most common eye problem in this age group. After 20 years of age, most cases stabilize on their own.
Did you know that the term “cycloplegia” originates from Greek? Cyclo- means circle or ring, and plegia- means paralysis.
But there is a way of exposing this hidden error and this technique is called cycloplegic refraction. The aim in this test is to completely relax the circular muscle so that there is absolutely no change to the lens shape at all. This will enable the ophthalmologist to acquire an accurate calculation of the error and to prescribe the correct lenses to ensure good vision.
Remember that the ophthalmologist will test the vision before the drops are instilled and then again after the medication has taken effect. This means that there will be a waiting period between the two tests.
To temporarily paralyze the ciliary muscles, specialised drops are instilled into the eyes.
There are two different groups of medications that may be used. The first is cylopentolate. It takes approximately 30 mins for the muscle to relax completely. The ciliary muscle returns to normal within two to three hours. You will notice that the pupil dilates too. It will return to its normal size within 24 hours.
When you book your appointment, remember to ask if there will be a waiting period during the examination and plan accordingly.
The second drug that may be used is called atropine. It is used in special cases where the cyclopentolate may not be strong enough. It is instilled in both eyes for 3 days prior to the visit to the ophthalmologist. It is important to note that it must not be given to the child on the day of the examination.
Recovery occurs between 3 and 10 days after the last drop is instilled. If the child develops side effects such as flushing, fever or restlessness, the drug must be stopped immediately, and the doctor informed.
So, in summary, subjective refraction requires the patient’s co-operation. Objective refraction can establish an accurate calculation of the vision error without feedback from the patient and cycloplegic refraction is used to find “hidden” errors in vision.