Visual Field Test – How It Is Done and How to Read the Results

A visual field test checks what the eye takes in around the point you are looking at – not only what you see straight ahead. You can read the bottom line of the eye chart and still be missing part of your field, which is why this is a basic test in the diagnosis and follow-up of glaucoma, optic nerve disease and neurological problems. This page explains what the test measures, how to take it so that the result is reliable, and how to read the printout you receive at the end.

What the test measures

When your gaze is fixed on one point, the eye still takes in a wide picture to the sides, above and below. A visual field test (perimetry) maps that picture: at each point it measures the faintest light the eye can detect. The result is a map of light sensitivity in every area, showing whether there are areas where sensitivity has dropped or which are not seen at all.

The brain “fills in” small gaps in the picture, so a visual field defect is usually not noticed until a late stage – especially when it is in one eye and the other eye covers for it. That is why the test matters even when there are no complaints.

Who needs a visual field test

• Glaucoma – for diagnosis and follow-up. Glaucoma affects the peripheral field first, and the test shows whether the disease is stable or progressing.
• Optic nerve disease – inflammation, pressure on the nerve or damage to the blood vessels that supply it.
• Neurological problems – a stroke, a tumour or a head injury can damage the visual pathways in the brain, and the pattern of the defect helps locate the damage.
• Retinal disease – and monitoring during certain medications that can harm the retina, such as hydroxychloroquine.
• Driving licence – when you need to show an adequate visual field. The requirements are explained in driving vision requirements.

How the test is done

The most common test today is computerised. You sit in front of a bowl-shaped machine with one eye covered and your gaze fixed on a point in the centre. Small lights of different brightness appear at different points, and each time you see a light you press a button. Some of the lights are deliberately very faint, so it is normal not to see all of them.

A few things help to get a reliable result:

• Bring your glasses and an up-to-date prescription – the machine uses a lens based on your prescription, and an incorrect lens lowers sensitivity across the whole field.
• Come alert – tiredness affects the result. You can ask for a short break in the middle.
• Look only at the central point and do not “search” for the lights with your eyes.
• Press only when you really see a light – guessing distorts the result.
• Tell the technician if your upper eyelid droops or covers part of the eye.

Types of test

• 24-2 – the most common test, checking the centre and the field around it to about twenty-four degrees in each direction. Used mainly in the diagnosis and follow-up of glaucoma.
• 30-2 – similar, but reaches a little further into the periphery.
• 10-2 – checks only the central ten degrees, at high density. Suited to suspected damage close to the centre, advanced glaucoma and macular disease.
• Kinetic testing (Goldmann) – a light moves in from the periphery and the examiner plots the borders of the field. Useful in neurological problems and for people who find the computerised test difficult.
• Binocular testing – both eyes open together, to test the combined field, for example for driving.

How to read the results

The printout looks complicated, but it can be read in a fixed order.

1. Reliability. At the top of the page there are three indices: fixation losses – how often the gaze moved away from the centre; false positives – pressing when no light appeared, which makes the field look better than it really is; false negatives – failing to see a bright light at a point that was already seen earlier, usually from tiredness or loss of concentration. When these indices are high, the rest of the page cannot be relied on, and the test is repeated.

2. Global indices. MD is the average deviation from the normal values for your age – the more negative it is, the lower the overall sensitivity. PSD measures how uneven the damage is, that is, whether there is a local defect. VFI expresses the part of the field that is preserved, and is used in follow-up.

3. Deviation maps. The Total Deviation map compares each point with people of your age. The Pattern Deviation map removes a general reduction – for example from a cataract or a small pupil – and so reveals a local defect that would otherwise be hidden. The dark symbols mark abnormal points: the darker the symbol, the further the point is from normal values.

4. Glaucoma Hemifield Test (GHT). Compares the upper and lower halves of the field, and sums up in words: “within normal limits”, “borderline” or “outside normal limits”.

5. The grey-scale map. The grey-scale picture is useful for illustration, but the test should not be interpreted from it alone.

What an abnormal test means

Before drawing a conclusion, the first question is whether the finding is real. A first test often comes out abnormal only because the person is not yet familiar with the test, so an abnormal finding on a first test is confirmed with a repeat test. Factors that create a false “defect”:

• a drooping upper eyelid or a low brow – a defect in the upper part of the field;
• the rim of the lens in the machine – a defect at the edges;
• an incorrect lens, a cataract or a small pupil – a general reduction across the whole field;
• tiredness and loss of concentration – an irregular pattern, sometimes shaped like a “cloverleaf”.

When the finding repeats and is reliable, the shape of the defect points to the cause:

• Glaucoma – an arc-shaped defect around the centre, a “step” on the nasal side, or areas of loss near the centre. These defects usually stop at the horizontal line.
• Damage to the visual pathways in the brain – the defect stops at the vertical line. Loss on both outer sides points to the area where the optic nerves cross, usually because of a mass pressing on it; loss on the same side in both eyes points to damage behind the crossing, for example a stroke.
• Damage to the optic nerve or the macula – usually a defect in the centre of the field.

A visual field test on its own is not a diagnosis. It is interpreted together with the eye examination, measurement of eye pressure, examination of the optic nerve and an OCT scan, and sometimes together with imaging of the brain.

Follow-up over time

In a chronic disease such as glaucoma, the important question is not only how the test looks today but whether there is progression. That requires a series of reliable tests over time, and the comparison software in the machine marks points that have worsened compared with the baseline tests. A change on one test is confirmed with a repeat test before treatment is changed. It is worth doing follow-up with the same type of test, so that the results can be compared.

Assessing visual field loss for disability purposes is a separate subject, with its own rules; it is covered in visual field loss.

When to seek urgent care

A sudden change in the visual field does not wait for a planned test:

• A curtain or shadow spreading in from one side, especially with flashes of light and a shower of floaters – suspected retinal detachment, seek care the same day;
• A sudden loss of half of the field, or a sudden drop in vision with weakness, difficulty speaking or a severe headache – go to the emergency room immediately;
• a sudden drop in vision in one eye, with or without pain.

For the difference between situations that need the emergency room, a same-day examination or a regular appointment, see urgent eye problems.

When to be examined by Prof. Michael Mimouni

Prof. Michael Mimouni, Director of the Cornea Unit at Rambam, sees patients at his clinics in Herzliya Pituach and Haifa for a full eye examination. The examination makes it possible to interpret a visual field test in the right context: whether the finding is reliable, whether it is explained by the eye surface, the eyelid or the lens, and what the next step is – a repeat test, further tests or referral to a doctor who specialises in glaucoma or neuro-ophthalmology. It is worth booking an examination when you have received an abnormal result and are not sure what it means, when you have risk factors for glaucoma, or when you feel that part of your field is “missing”. Contact the clinic to book an examination.

The information on this page is general and is not a substitute for an examination and personal medical advice. Diagnosis and treatment decisions are made only after a full eye examination. With a sudden change in the visual field or a sudden drop in vision – seek urgent care.