Visual field defects: what they are, causes, testing and driving

Your visual field is everything you can see without moving your eyes: the point you are looking at, the area around it and the edges of your vision. Each eye contributes a field and the two overlap, so a patch missing in one eye may be covered by the other. A visual field defect means part of that area is dim, patchy or missing.

Cambridge University Hospitals explains that visual field (peripheral vision) testing is used to help diagnose and monitor conditions that affect the optic nerve or the visual pathway. That phrase is a useful reminder that the problem is not always in the eye itself. The pathway runs from the retina, along the optic nerve and through the brain to the visual cortex at the back of the head, and damage at different points leaves characteristic patterns.

• Central scotoma: a blind or blurred patch at or near the point of fixation, often described as a smudge you cannot look around.

• Peripheral loss or tunnel vision: the edges close in so that you see as if through a tube.

• Hemianopia: loss of half of the field. In a homonymous hemianopia both eyes lose the same side.

• Quadrantanopia: loss of roughly a quarter of the field, often the upper or lower corner on one side.

• Altitudinal loss: the upper or lower half of the field is affected.

• Patchy or arcuate loss: scattered islands or arc-shaped gaps, as seen in some optic nerve conditions.

The pattern of loss gives clues about where the problem sits, which is why clinicians chart it rather than relying on a description alone. Causes fall broadly into three groups.

Eye conditions include glaucoma, where the NHS says the nerve connecting the eye to the brain becomes damaged, usually because pressure builds up in the eye. It most commonly affects adults aged 50 and over and can cause vision loss if it is not diagnosed and treated early. Retinal disease, such as a detached retina or diabetic eye disease, can also remove areas of the field.

Optic nerve conditions, such as optic neuritis or an interruption of the nerve's blood supply, tend to produce patchy or central loss, often in one eye. Brain conditions, including stroke, tumours and head injury, damage the pathway behind the eyes and typically affect the same side of the field in both eyes.

Stroke is an important cause to recognise. The NHS lists blurred vision or loss of sight in one or both eyes among the symptoms of stroke, alongside face weakness, arm weakness and speech problems, and advises calling 999 straight away, even if the symptoms stop after a short time.

Many people do not describe a "black area". The brain tends to fill in missing parts with the surroundings, so the first sign is often practical: walking into door frames, misjudging kerbs, missing objects on one side of a plate, or finding that you keep losing your place when reading. People around you may spot the problem before you do.

Sudden defects are different. A curtain or shadow that comes across the vision, a large new gap or loss of sight in one eye should be treated as urgent, because sight can sometimes be saved if the cause is treated quickly.

Call 999 or go to A&E if loss of vision comes on suddenly together with face drooping, arm weakness, slurred speech or confusion. The NHS says not to drive yourself to A&E when you suspect a stroke.

The NHS also says to call 999 or go to A&E if symptoms of glaucoma develop suddenly, such as intense eye pain, a red eye, rainbow-coloured circles around lights, feeling sick and a headache. For a gradual change, or any concern about your vision, see an optician or a GP. The NHS advises everyone to have an eye test at least every 2 years, and more often if they are at higher risk, for example with a close relative who has glaucoma.

An optometrist at a high-street practice may test your field as part of a sight test, and a hospital eye clinic or orthoptic department can test it in more detail. Cambridge University Hospitals describes a Humphrey visual field test, in which you sit in a dimly lit room with one eye covered and look at a fixed light while small white lights of varying brightness appear in different places. You press a button when you see one.

The same trust says the technician will ask which glasses you use, that you should keep your gaze on the fixation light and your head still, and that you should only press the button when you are sure you see a light. A standard test takes between 4 and 8 minutes. Both eyes are tested, even if only one seems affected, so that any change can be detected. Results can vary from one session to the next, which is why tests are repeated to monitor a condition over time.

Treatment aims at the cause. In glaucoma the NHS says there is currently no cure, but eye drops, laser treatment or surgery can slow damage and help stop vision getting worse, and follow-up appointments should not be missed. Detached retinas and some optic nerve problems are treated urgently, and a stroke is managed by a stroke team.

Where the loss is permanent, rehabilitation focuses on adapting. Many hospital eye and stroke services teach scanning, which means deliberately moving the head and eyes towards the missing side, and an orthoptist or occupational therapist can suggest practical changes at home, in the kitchen and when reading. A low-vision service can offer equipment and advice. The NHS also points people with sight loss towards charities such as RNIB for support and ways to adapt daily life.

The DVLA sets a minimum field of vision for all drivers: at least 120 degrees on the horizontal, with an extension of at least 50 degrees left and right, and no significant defect within 20 degrees of fixation above or below the horizontal. Its guidance says homonymous or bitemporal defects that come close to fixation, whether hemianopic or quadrantanopic, are not usually acceptable for driving.

The DVLA guidance lists conditions that produce a field defect, including bilateral glaucoma and partial or complete homonymous hemianopia, as ones that must be notified to the DVLA. If a field assessment is needed for licensing, it requires a binocular Esterman field test. Anyone who has been told they have a field defect, or who is unsure, should speak to their eye clinic and check the current DVLA guidance before driving.

On the NHS the usual route is an optometrist's sight test, which includes a field screen in many practices, followed by a referral through the GP or optometrist to a hospital eye clinic if a defect is found. Field testing, glaucoma review and orthoptic assessment are all part of NHS eye care, and urgent cases are seen through eye casualty or A&E.

Some people choose a private ophthalmologist or optometrist for quicker access or a second opinion. The tests themselves are the same machines and methods, so the main practical differences are waiting times and continuity. Whichever route you use, ask for a copy of your field plots so that changes can be compared over time.