Visual Field Testing

How the test works

You sit at a bowl-shaped instrument with one eye covered and fix your gaze on a central target. Points of light appear briefly at positions across the bowl, at varying brightness, and you press a button each time you see one. The instrument finds the dimmest light you can detect at each location, checks that you are fixating and not guessing, and produces a map of sensitivity with any areas of loss marked. It also compares the result with normal values for your age and with your previous tests.

What the results show

Glaucoma produces characteristic patterns: arcs of loss above or below the center, a step at the nasal edge, and, late, a shrinking central island. Optic nerve disease produces central or altitudinal loss. Damage to the visual pathways in the brain produces loss on the same side in both eyes. The pattern locates the problem, and its size and depth measure the damage.

Getting a reliable result

• Rest beforehand; the test is done early in the visit, before dilation
• Keep looking at the central target; do not search for the lights
• Press for any light you think you saw; do not wait to be sure
• Blink normally and tell the technician if you need a pause
• Wear the correct near correction, which the technician sets

The first test is often less reliable while you learn it, and results are confirmed on repeat before decisions are made from them.

How often

At diagnosis, two or three tests establish a baseline. Stable glaucoma is tested every six to twelve months; suspected progression more often. The instrument analyzes the series statistically to distinguish true progression from the normal variation between tests.