فحص مجال النظر: ماذا يقيس؟ ولماذا يطلبه طبيب العيون؟

English version

Visual Field Testing: What It Measures and Why an Eye Specialist Orders It

What does a visual field test measure?

Visual acuity tells us how well you see fine detail straight ahead, such as letters on a chart. A visual field test looks for areas that may be weaker or missing around a fixation point, including peripheral vision and parts of the field near the center. Because both eyes work together in daily life, the machine tests one eye at a time to make the result specific.

The most common version is automated static perimetry: faint lights of different brightness appear while the machine estimates the dimmest light noticed at each location. You should not search for lights with your eyes. Keep looking at the center and respond only when a light is seen without moving the eye.

What happens during the test?

You sit in front of a bowl-shaped machine and cover one eye while the other keeps looking at a central target. Press the button once when you see a light. Some lights are deliberately very dim, so do not guess or press when you have not seen one. If you need to blink or rest briefly, tell the technician instead of continuing while tense.

The clinic may place a trial lens in the machine to correct near focus. Test length varies by program and can feel longer the first time. Normal learning, anxiety and fatigue can affect performance, so a first test is not a final verdict on the eye.

Why might the doctor order it?

In glaucoma, visual field testing helps determine whether optic-nerve function is affected and whether it is changing over time. Eye pressure alone is not enough, because some people have pressure in the usual range with optic-nerve or field changes, while others have elevated pressure without clear damage on an initial test.

The test may also be used when an optic-nerve problem, symptoms or other findings raise concern about the visual pathways in the brain, and in selected retinal disorders. The program and repeat schedule depend on the medical question, so a result should not be compared directly with someone else's or with a different test pattern.

Does it replace OCT or optic-nerve examination?

No. OCT images the optic-nerve or retinal layers, whereas visual field testing measures visual function. Change can appear in one before the other, and either result can be influenced by image quality, concentration or refractive blur. The clinician combines symptoms, pressure, optic-nerve appearance, OCT and serial fields instead of relying on one test.

The printout contains colors, numbers and a gray map, but the important question is whether there is a repeatable pattern that matches the rest of the examination. A red area or a single missed point does not automatically mean glaucoma or vision loss, and a normal sheet alone does not exclude every early problem.

How can you prepare for a useful result?

Sleep as well as you can, bring your current glasses and tell the team if the prescription has changed or vision is blurred. Take usual medicines unless the clinician advises otherwise. If you have significant dry eye or irritation, mention it before testing because appropriate comfort measures may help you keep steady fixation.

Do not worry about missing one or two lights. The goal is an honest, calm response, not a perfect score. Do not change your button-pressing strategy or look around the bowl. If you become tired, ask for a brief pause. The clinician may repeat the test at another visit rather than make an important decision from an unreliable field.

When should you not wait for a scheduled field test?

Visual fields are valuable for follow-up, but they do not replace urgent assessment. Sudden loss or a curtain in vision, a sudden shower of flashes and floaters, severe pain or redness, or a new headache with double vision or neurological symptoms requires urgent contact with an eye specialist or emergency care as appropriate.

If you have glaucoma, do not stop drops or change the dose because of one printout. Ask the clinician to interpret the result alongside prior measurements and the long-term plan to protect the optic nerve.

Frequently asked questions

Is a visual field test painful or does it use radiation?

No. It is a light-based test that does not touch the eye and uses no radiation. It can feel tiring because it requires concentration, but a short break is possible.

Do my pupils need to be dilated before the test?

The field test itself usually does not require dilation. Drops may be used for another part of the visit, and the clinic will explain the appropriate order.

Does an abnormal result mean I have glaucoma?

Not necessarily. The clinician reviews test quality, whether the pattern repeats, and how it fits with pressure, optic-nerve appearance, OCT and the rest of the examination.

How often will I need the test?

There is no one schedule for everyone. Frequency depends on why it was ordered, risk factors, previous results and whether a possible change needs confirmation.

Medical note: This content is for education and does not replace an ophthalmic examination. Sudden vision loss, severe pain or new flashes require urgent assessment.