Diagnosing Disease and Advanced Instrumentation

An important part of the routine eye examination is the search for any abnormal eye conditions. This is done by ophthalmoscopy, which is when the optometrist uses his hand held torch and closely examines the back of the eye. The most common types of eye disease that optometrists encounter are glaucoma, macular degeneration, cataract, and diabetic eye disease. In many cases, it is useful to have additional, specialist instruments to further examine the eyes, as early disease can be difficult to identify. Advanced instrumentation enables the optometrist to better decide whether there is actually a problem, or if the patient has an unusual but normal appearance to their eye.

Glaucoma, Tonometry and Visual Field Screeners

Glaucoma is a disease of the eye which results in an insidious and progressive loss of vision. Those who suffer from it are not aware of it until loss of vision has become extensive. Regular eye examinations minimise the possibility that glaucoma will progress to this stage, as optometrists have several instruments which help in identifying early glaucoma. At this practice, we also have additional specialist instruments which enable us to further examine the patient and thereby more accurately determine the possibility of glaucoma.

There are typically three examination techniques which are used to identify persons suffering from glaucoma. These are: carefully assessing the back of the eye by ophthalmoscopy, and in particular examining the optic nerve head; determining intraocular pressure (tonometry), and plotting the visual field (perimetry).

More recently pachimetry, the measurement of corneal thickness, is regarded as another important measurement because a thicker cornea will show a relatively higher pressure than a thinner cornea. Corneal thickness is quite variable among the general population, and this variability is increasing with the growing popularity of refractive laser surgery, which achieves its result by thinning the cornea.

Although all of these methods are important, the measurement of the visual field is most important, because it is the loss of visual field that defines glaucoma.

Below are two photos of the optic nerve head, called the optic disc. The left photo is of a normal disc, with blood vessels which emerge centrally, and a thick ring of neural tissue surrounding the blood vessels. The photo on the right shows a glaucomatous disc, with vessels which emerge from the disc edge, due to the loss of the thick ring of neural tissue. It is the loss of the neural tissue which results in the loss of visual field and hence the diagnosis of glaucoma.

Healthy Optic Disc

Glaucomatous Optic Disc

Typically glaucoma causes loss of peripheral vision so that if the disease progresses unchecked, in the final stages of the disease, vision is reduced to a central tunnel of vision. This is where the layman's name for glaucoma, 'tunnel vision', comes from.

Below is an illustration of what it might be like to suffer from glaucoma. The scene of the previous practice front is reduced to a small central field. Vision is clear in this central area, but the peripheral field, so important in localisation within a visual scene, has been lost. Fortunately, glaucoma sufferrers these days rarely lose this much vision, as more is now known about the disease and it is usually identified within the early stages. Also, within the last ten years, ocular medication has greatly improved in effectiveness, enabling most persons with glaucoma to retain most of their sight.

The field of view of a person suffering from advanced glaucoma. Note the reduced field, or effective 'tunnel vision'.

Tonometry, or the measurement of the pressure within the eye, is important in the diagnosis of glaucoma and in monitoring the effectiveness of medication used to control it. Glaucoma medications work by lowering the Intraocular Pressure (IOP). IOP is measured by a tonometer, which will be one of two types: either a contact tonometer or a non-contact tonometer. Contact tonometry, as the name suggests, measures the IOP by directly touching the (anaesthetized) eye. It is considered to be the most accurate method of IOP measurement. (Pictured below)

Measurement of intraocular pressure using a contact tonometer.

Non-Contact tonometry measures the IOP indirectly, by measuring the characteristics of a pulse of air hitting the front of the eye. This test, "the puff of air" test as patients like to call it, is the method most often used at this practice to measure pressure. Our preference for non-contact tonometry is long-standing, as it does not disturb the corneal surface, and thereby does not interfere with visual field results.

Another important instrument in diagnosing and monitoring glaucoma is the visual field plotter. There are many types of visual field plotters, and at this practice we generally use the Humphrey Visual Field Analyser (VFA). There are two Humphrey VFAs at the practice, one of which is the latest model and uses the newer SITA test method, which checks for glaucomatous visual field loss. These Humphrey instruments are the types used by most hospitals. As a result, if a patient is referred on to hospital from here for further evaluation, the ophthalmologist will readily recognise the visual field plot made and will probably be able to directly compare the hospital visual field plot results with the plot(s) produced here. The Humphrey VFA measures the visual field of each eye in approximately 5 minutes. It is easy for patients to use and therefore produces reliable results, thus it is accurate in its representation of abnormalities of the visual field.

The Humphrey Visual Field Analyser.

For those patients who find the computerized Humphrey visual field plotters difficult, we also have two Friedmann Visual Field Analysers, which are manual versions of the computerized Humphrey instruments. With the Friedmann VFA, the operator waits for the patient’s response before proceeding, which ensures that patient confusion is minimal and that the speed of the test is appropriate for each subject. Children and occasional elderly patients can be tested in this way.

It is worth mentioning here that perimetry is not only used for the detection of glaucoma. It is also useful in detecting subtle defects within the visual field caused by a different problem, a problem either within the retina or the visual pathway within the brain. A defect in the retina is sometimes not detectable by looking directly into the eye (e.g. a shallow, peripheral retinal detachment), and a defect in the visual pathway (behind the eye) is rarely detectable by looking into the eye (e.g., damage from a stroke, or multiple sclerosis). This is why perimetry is a very important part of the routine eye examination. Thus visual field testing is performed here on almost all adult patients and often on children presenting with headaches or unusual symptoms.