Glaucoma check for a parent: what the pressure test, the optic nerve scan and the field test show
A parent has been told to get checked for glaucoma, and the appointment lists tests with unfamiliar names. Here is what each one measures, what the result means in plain words, and how often the checks repeat.
Go to an eye doctor or hospital the same day if…
I see parents and grandparents for squint, cataract and everyday eye problems, so one family can keep one eye doctor. Family eye care Where I practise
- A glaucoma check is a full eye examination, not one test: eye pressure, the drainage angle, the optic nerve, the side vision, a scan of the nerve and the thickness of the cornea (American Academy of Ophthalmology).
- One high pressure reading is not a diagnosis. The AAO says a single pressure test is not enough, and some people have glaucoma with normal pressure while others have high pressure and no damage.
- The optic nerve examination and the OCT scan look for loss of nerve fibres. A first scan is a baseline that later scans are compared with.
- The visual field test maps the side vision and is often repeated to confirm a gap, because early blind spots are not felt.
- How often the checks repeat is set by the doctor from the results. For people already on treatment the AAO says about every 3 to 6 months, though this can vary.
Someone in the family has been told to get a parent checked for glaucoma, and now there is an appointment and a list of strange-sounding tests. Nobody has explained what any of them are for. This page does that, in the order they usually happen: what is measured, what the result means in plain words, why a single number never settles the question, and how often the checks repeat.
It is written for the adult who will sit in the chair, and for the son or daughter who is booking it. Who should go, and from what age, is covered in glaucoma runs in families, so it is not repeated here.
What happens at a glaucoma check?
A glaucoma check is a full eye examination, not one test. The American Academy of Ophthalmology (AAO) says "the only sure way to diagnose glaucoma is with a complete eye exam", and lists six things the eye doctor does: measure the eye pressure, inspect the drainage angle, examine the optic nerve, test the side vision, take a picture or computer measurement of the optic nerve, and measure the thickness of the cornea.
Glaucoma is damage to the optic nerve, the cable of more than a million tiny nerve fibres that carries what the eye sees to the brain. Open-angle glaucoma, the common type, is painless and causes no change in vision at first. The AAO says that most people notice nothing until the damage is quite severe, and that half the people with glaucoma do not know they have it. That is why the check looks at the nerve itself and does not wait for symptoms.
The action: bring your glasses, a list of your eye drops and other medicines, and the name of the relative with glaucoma and roughly when it was found.
What does the eye pressure test show, and is one high reading glaucoma?
The pressure test, called tonometry, measures the pressure of the fluid inside the eye. One high reading is not a diagnosis. The AAO states plainly that "a single eye-pressure test by itself is not enough", and a screening that checks only pressure "is not enough to find glaucoma".
There are two reasons. First, one reading is a snapshot of a single moment. Second, pressure and damage do not line up neatly. Some people have higher than normal pressure and no damage at all: the AAO calls this ocular hypertension, and these people are "glaucoma suspects" who need careful monitoring rather than a label.
Others have glaucoma with pressure in the normal range, which the AAO calls normal tension glaucoma, because their optic nerve is more sensitive to pressure. The European Glaucoma Society guidelines make the same point in their foreword: a normal-looking number is only rough guidance and is often wrong for the individual.
So the pressure is read together with the nerve and the side vision, and sometimes measured again on another day. The action: if a screening booth or a clinic gave you one high number, do not panic and do not ignore it. Take it to a full examination.
What do the optic nerve examination and the OCT scan show?
They show whether the nerve has lost fibres, which is the damage that defines glaucoma. The doctor looks at the optic nerve head at the back of the eye, usually after dilating drops, and notes its shape, including how large the central dip, called the cup, is compared with the whole disc. A scan then turns that look into a measurement.
The AAO describes this as taking "a picture or computer measurement of your optic nerve". The scan most clinics use is optical coherence tomography, OCT, a painless imaging test that uses light to measure the thickness of the nerve fibre layer. Think of it as a ruler for the cable. A first scan gives a baseline that later scans can be compared with.
A scan finding is read by the doctor alongside the other tests. It is not a verdict by itself.
What does the visual field test show, and why is it done more than once?
The visual field test maps your side vision, and it shows whether nerve damage has already cost you any of it. The AAO lists it as "tests your peripheral (side) vision". You rest your chin in a bowl-shaped machine, look at a central point, and press a button each time you notice a small light flash at the edges. Each eye is done separately.
Glaucoma blind spots start at the edges, and the brain fills them in, which is why people do not feel them. The AAO says blind spots may not be noticed "until most of your optic nerve fibers have died". The field test is where an early gap can first be seen.
Doctors often repeat a field test to confirm a gap before acting on it, because the test depends on attention as well as on sight. A gap that appears again, in the place the nerve scan suggests, is far more meaningful than a one-off. The action: do not worry if the first result looks patchy, and tell the person running the test if you lose your place.
What are gonioscopy and the corneal thickness measurement for?
Gonioscopy looks at the drainage angle, the channel where the fluid leaves the eye, and corneal thickness helps interpret the pressure reading. Both are on the AAO list. Neither hurts.
For gonioscopy the doctor numbs the eye with a drop and rests a small mirrored lens on the surface so the angle can be seen from the side. It tells the doctor whether the angle is open or narrow.
This matters because narrow angles are a different type of glaucoma, and the AAO says an acute attack of angle-closure glaucoma, with severe eye pain, a red eye, blurred vision, halos around lights, headache and vomiting, "is a true eye emergency". If that happens, go to an eye doctor or hospital at once and do not wait for a booked appointment.
Corneal thickness, called pachymetry, takes a few seconds. The AAO lists a cornea that is thin in the centre as a risk factor for glaucoma, and the thickness also helps the doctor judge whether the pressure number read a little high or a little low. It is another piece of the picture, never a result on its own.
How often are the checks repeated?
There is no single schedule, because the doctor sets it from what the first set of tests shows. The AAO says "your ophthalmologist can tell you how often you should be examined", and for people already on glaucoma treatment it says to expect a visit about every 3 to 6 months, though this can vary.
In practice the follow-up falls into three groups. If every test is normal but you have a relative with glaucoma, the answer is a repeat check at an interval the doctor chooses, because a normal result today cannot promise a normal result in ten years.
If the pressure is high or the nerve looks suspicious but there is no damage yet, you are a glaucoma suspect and are checked more often so that any change is caught early. If glaucoma is confirmed, treatment is started and the visits become regular. The AAO is clear that glaucoma damage is permanent and cannot be reversed, which is the reason all of this is aimed at catching it before the sight is lost.
The Malaysian Ministry of Health has a clinical practice guideline on the management of glaucoma, now in its second edition. This page does not quote it, and the intervals above come from the AAO.
What should the rest of the family do?
Book your own check if a parent, brother or sister has glaucoma, and mention the family history at every eye visit, including your children's. The AAO lists a family history, being over 40, short-sightedness or long-sightedness, diabetes and Asian heritage among the risks. If you have diabetes in the family as well, the eye checks overlap, and diabetes in the family explains what to ask for. For a simple map of who in the household needs which check, see eye checks for the whole family.
- There is severe eye pain, especially with a red eye, headache, nausea or vomiting.
- Vision suddenly blurs, or rainbow-coloured rings appear around lights.
- A grey curtain, a shadow at the side or a sudden loss of part of the vision appears in either eye.
- You have glaucoma treatment and have run out of drops, or stopped them, and the eye becomes painful.
Common questions
Is one high eye pressure reading glaucoma?
Can I have glaucoma if my eye pressure is normal?
What does an OCT scan of the optic nerve show?
Why is the visual field test repeated?
Do I need to feel any symptoms before I get checked?
How often should the glaucoma check be repeated?
Can the damage from glaucoma be reversed?
- American Academy of Ophthalmology · Glaucoma diagnosis · www.aao.org
- American Academy of Ophthalmology · What is glaucoma? · www.aao.org
- American Academy of Ophthalmology · Glaucoma symptoms · www.aao.org
- European Glaucoma Society · Terminology and Guidelines for Glaucoma, 5th edition (Br J Ophthalmol 2021) · pubmed.ncbi.nlm.nih.gov
- Ministry of Health Malaysia · Clinical practice guidelines: management of glaucoma, 2nd edition (2017), catalogue record · localcontent.library.uitm.edu.my
- National Eye Institute · Glaucoma · www.nei.nih.gov