How is near vision tested?
A near-vision chart usually has test types based on the printer’s ‘N’ series. The smallest is numbered N5 and successive numbers indicate larger types. The usual distance for gauging near reading, till the age of 40 years, is 14 inches.
How is colour vision tested?
Colour vision is tested on special test plates termed “Ishiara test plates” in which numbers are written in coloured dots against a background of dots of a different colour. If there is total colour blindness, no numbers are visible, but if only one colour is deficient, the number read will be different from the correct one.
An alternative is to present variable colours to a patient via a coloured light projection device, devised by Edridge Green. However it is open to misrepresentations and is now rarely used.
How are spectacles tested?
Usually the preliminary information is judged and a raw number is determined using a system termed retinoscopy.
The room is darkened and a streak of light is projected into the pupil of the eye. By a skilled combination of judgement of the interplay of light and shadow in the pupil of your eye, the doctor can gauge your number.
Subsequently, a metal spectacle frame is worn, which has chambers to take the lenses being tried (hence called trial lenses). Various combinations are put in until the vision improves to its maximum. It must however be stressed that refraction, or the technique by which the spectacle power of the eye is deduced, is an exact science and at the same time an art.
Usually, the final number prescribed will depend upon the status of the eye: whether it should have a combination of lower numbers; or to which ratio the number should be varied to give better and at the same time safer, vision.
Patients are often afraid that a wrong response by them may lead to a wrong prescription. Just relax and tell as you see it. You really cannot make a mistake since all the answers are double-checked.
How is examination of the muscle balance done?
Muscle balance is present when the two eyes work together in a synchronous co-ordinated manner. The first test for muscle balance is simple: rotating the eyes in different positions to follow the light. Next, individual eyes are rapidly covered and opened while fixed on a light to evaluate if the resting posture of the eye is in excess of the normally accepted position. Subsequently the convergence ability of the eye is checked.
If these tests prove deficiencies then, using more a sophisticated instrument like the synoptophore, the exact deficiencies are plotted out.
Deficiencies in muscle balance are the most frequent cause of headaches in otherwise normal eyes, especially in school-age children and young adults.
Detection and cure by simple exercises removes this frustrating nagging problem.
How is the eye tested structurally?
The eye has first to be examined externally. This includes the lids and the lashes. Subsequently the front of the eyeball upto the back surface of the lens is examined with a very high powered microscope coupled with a source of light which can be made narrow or broad. This combination of a lamp projecting a slit of light with a microscope (for short, a slit lamp) enables the front layers of the eye to be examined section by section. It is also very useful in evaluating any dense areas or opacities in the lens of the eye.
If the corneal front surface has a fair degree of astigmatism, especially after a cataract surgery, a keratometer is used, which measures the astigmatism and the axis rapidly.
Next, usually, the posterior segment, in the back of the eye, is evaluated. The usual instrument is an ophthalmoscope, a small torch like instrument which has a large battery of lenses. An ophthalmoscope has a coaxial viewing and light system of high accuracy, such that one can easily illuminate the inside and look down on a 1/2 mm wide tube with extreme ease. The instrument lights up the inside of the eye through the pupil and enlarges the image. The optic nerve head, the retinal arteries and vessels and the macula of the eye are examined in turn.
Very often, from a judgement of the vessel walls and small changes in the eye an accurate assessment of various diseases can be made.
For certain persons, there are special tests which will be carried out over and above the usual routine examination.
What special tests are there in an eye examination and when are they carried out?
(1) Intraocular pressure (or tension) or glaucoma check.
This is a routine test which should be carried out on all patients over the age of 40 years unless for specific reasons the examining doctor feels it is not indicated.
An eyeball can be likened to a football. The football maintains its shape due to the pressure of the air pumped in. In the eyeball, the shape is maintained by the pressure of the liquid inside the eye. If the pressure increases beyond normal, the eye may be damaged and the condition is called glaucoma.
There are two basic ways of checking the pressure. Both are fairly accurate for clinical purposes. Usually the newer technique of aplanation mounted on the slit lamp is used after a drop to dull the sensation of the eye is put in. It causes no discomfort.
(2) Field of vision tests
These are important tests to check the extent of the peripheral or central field of vision. They are very useful in various retinal disorders and in glaucoma. There are 2 types of apparatus for checking the field. The larger unit uses a projected beam of light and is meant to measure peripheral field and is termed the projection perimeter of Aimark.
The second unit termed the Friedman analyser measures the central field of vision and is particularly useful for glaucoma. It electronically scans, using a controlled flash of light, different areas of the central 30° of vision and can accurately assess the function of the retina.
(3) Synoptophore test
The synotophore is an instrument both for the detection of the degree of muscular imbalance as well as for the treatment of a small degree of squint without surgery. Essentially there are 2 optical tubes which can be set at precisely graded positions relative to each other.
It is also very useful in building up the binocular capacity for vision in certain cases.
The complete examination of the eye can thus easily run into twenty or more individual readings, most of which are concerned with the way the eyes work.
The day is long past when examination of the eyes consisted of calling off a few letters from a wall chart. In our modern complex society with its tremendous emphasis on near work, the eye must be looked after with care and sophisticated precision.
