Jude Furber and Andy Levy
Departments of Othalmology and Medicine, Bristol Royal Infirmary

The definition of the visual field was given by Tate and Lynn in 1977 as "All the space that one eye can see at any given instant".

Why bother to test the visual field?

Pituitary tumours lead to three principal problems:

Firstly, they can secrete excessive amounts of hormones and cause conditions such as Cushing's disease or Acromegaly.

Secondly, they can render the adjacent normal Pituitary gland underactive.

Thirdly, as the head is full of brain and as the skull itself cannot expand, there is very little room for the pituitary to expand without adversely affecting surrounding structures. This last problem is the reason that, if you have a Pituitary tumour, your doctors are so interested in your vision.

The anatomy of the visual pathway

The optic nerves carry visual signals from each eye into the head. As the optic nerves run backwards towards the visual processing part of the brain at the back, they run together, join and then separate once again to form an 'X' shape. The crossing point of the two nerves is called the optic chiasm and it allows the nerve fibres to be re-arranged so that everything you see to the right side, such as oncoming traffic, is processed by the left side of the brain and all the vision on the left, pedestrians and parked cars, for example, is processed on the right. It so happens that the optic chiasm is located immediately above the position of the normal pituitary, so that if the pituitary enlarges because of tumour growth, it is not unusual for the chiasm to be compressed from below. The pattern of visual changes produced by compression of the chiasm is very characteristic and consists of the loss of visual field. Visual field is the area of vision that you perceive "out of the corner of your eye" while you are looking at an object, rather than how well you can see something when you are looking directly at it. If you wear spectacles, for example, taking them off would affect visual acuity but would have no effect on visual fields. Wearing blinkers, however, would reduce your visual fields but would have no effect on visual acuity.

How sensitive are visual fields?

Field loss usually develops very slowly and the effects in the early stages are so subtle that they often go unnoticed. By the time a problem is identified, a large amount of visual field might have been lost and the effects can be very disabling. A change in tumour size small enough to be detected on MRI or CT scanning may nevertheless have a profound effect on visual fields. Therefore even a small change in visual fields is a very, very important sign and in many patients it is the number one criterion on which decisions about further surgical treatment are based.

Is it easy to measure visual fields?

In view of its importance, special techniques are used to determine with great accuracy the position and extent of visual loss. This sounds straightforward but in fact it can be quite difficult as the size of the visual target the patient is asked to detect, its colour, its brightness, where it is on the visual field, how long it is shown for and whether it is kept still or moved into the field of view from outside, all have marked effects on the results. When the test is repeated months or years later by a different operator using a different method, it can be very difficult to compare the two sets of results without making mistakes.

How are visual fields measured?

Various methods have been used to assess the visual fields. Doctors used to laboriously move a white spot in front of a 2m square black felt screen to find areas of poor vision. Nowadays, however, computer control systems are much more common and we use an extremely accurate system called a Humphrey Allergan Field Analyzer. The test is painless but fairly time consuming. The patient sits in front of a hemispherical, lit screen onto which a computer sequentially projects a series of dots of light in different positions. The patient is asked to focus on a fixed target in the middle of the screen and click a handheld button each time a dot is seen "out of the corner of his or her eye". Each eye is tested separately with the intensity of light varying according to the patients' responses, for the same reason that sound volume is adjusted in a hearing test. After 15 minutes or so, 300 to 400 dots of light of different intensities will have been presented to the patient in different areas of vision and a picture of areas of visual loss will have been constructed. Because the results are digital, the computer is able to identify areas of poor vision and also compare the results with normal vision, which is more sensitive to light in some areas than others. Wearing sunglasses, for example would reduce overall sensitivity, but not alter which area of vision is most sensitive. A pituitary tumour pushing on the optic chiasm may well leave some areas of vision normal, but reduce other areas - typically (but by no means invariably) the upper, outer part of vision in each eye. Because the test is extremely accurate and depends more on the computer programme used than an operator, it is possible to repeat a patient's visual field tests a few weeks or months later and determine whether there have been changes. For this reason, visual field testing is one of the most useful of all tests in pituitary disease.

Can I drive?

As well as the statutory numberplate test, the DVLA and Department of Transport recognises that an adequate field of vision is also necessary for driving. Drivers with field defects in both eyes, and thus poor peripheral vision, do have a higher incidence of accidents. The Royal College of Ophthalmologists advice to the DVLA is that peripheral visual fields with both eyes open should cover an angle of 120 from left to right and that there should be no blind spots within an angle of 20 from the centre of vision. These angles can be easily calculated by the computer.