There is a Yoruba myth that says, “If your hand is itchy, it means some good money is coming your way soon. Some have extrapolated this to mean that if you have twitching of the lids, you are not just going to be stupendously rich, you probably must be seeing things that others don’t see! If you don’t handle it probably, it may cause you some embarrassment and may land you in a psychiatry ward!
But the question is, “Is it really possible to see what others don’t see? Technically, it is possible. There are a few people with visual acuity of 6/3 or 20/10. Normal visual acuity is 6/6 or 20/20. This means standing at a distance of six metres (20feet), one is capable of seeing what a normal eye should see at six metres.
A visual acuity of 6/3 (20/10) means that standing at a distance of six metres one is capable of seeing what a normal eye can see at three metres (10feet). Someone like that would be said to be seeing what others don’t see!
A visual acuity of 6/3(20/10) would be regarded as super normal and could be an asset if you play Golf or shoot arrows or are a marksman. One of Tiger Woods’, the great golfer’s, secret is said to be a Visual Acuity of 6/3. Surprisingly a person with a normal or super normal visual acuity may not have a normal eye!
The reason is simple. Visual acuity is only one of three measures by which one can know if an eye is normal or not. So having a normal visual acuity means you have scored 33.3 per cent of the marks! And three marks out of 10 are definitely far from the pass mark! What then are the other two measures that can tell you if your eye is normal or abnormal? The second is Colour Vision – ability to perceive and appreciate colours.
Can you imagine a world without colours? You see the green grass as grey and at the traffic light you cannot differentiate between the Red and the Green. When do you stop and when do you move on? Fortunately colour blindness is rare and people with colour vision abnormality have developed coping mechanisms. But a colour blind pilot or a train or bus driver could cause serious accidents and for these occupations, it is mandatory to be certified as having normal colour vision before one is employed.
The third measure that can tell if an eye is functionally normal is the Visual Field. The Visual Field can be simply described as a measure of the area you can see ahead of you compared with the area you cannot see. This is not just in quantity but also in quality.
There are elaborate instruments used to measure visual acuity and the outcome could tell one if in the presence of a normal or super normal visual acuity one has glaucoma or a brain tumour or some other diseases.
Those who have had Visual Field measurements know that it is laborious and time-consuming. Consequently, it is not done routinely but in cases where there is suspicion of some damage to the optic nerve.
How then do I answer the question if I have a normal visual field? If we can’t easily measure the function of the optic nerve which visual field is, we can simply have a look at the structure of the nerve. If the structure is normal we can then assume the function (visual field) is normal.
Are you wondering how we can see a nerve in a human? Yes, the optic nerve is the only nerve in the body you can view directly with ease and without any surgical manipulations.
Using the ophthalmoscope and looking through the pupil, we can see the entrance of the nerve into the eye. This is called the optic nerve head or simply, the optic disc. We know what a normal optic nerve head looks like. Certain features will distinguish one disease entity from the other. These features give us the clue as to the type of visual field changes we expect. Then for these people we insist on visual field measurements and if need be other ancillary tests.
Does a normal structure always mean normal function? No! In early stages of many diseases, changes may not be detected and herein lies the folly. Also, waiting for these changes means waiting for the disease to be well established before doing something about it. We therefore often use a combination of the patient’s history (his story), our clinical findings and the outcome of various tests to make the final decision.