Interpreting visual fields

The visual field is everything the eye can see while it looks at one point. A normal monocular field extends about sixty degrees up and in, seventy down and ninety out, with the blind spot fifteen degrees temporal to fixation. Automated perimetry maps the central field, usually the middle twenty four or thirty degrees, by presenting a dim spot at each of a grid of points and finding the faintest one the patient sees. Sensitivity is recorded in decibels: higher means the patient saw a dimmer stimulus, and the field is a hill of vision, highest at fixation and falling to the edges.

The test is only as good as the patient's attention. A first field is often worse than the second because the patient is learning, and a tired, anxious or drowsy patient produces a field full of spurious loss. Read the reliability before the result.

Three indices are printed at the top. Fixation losses: the machine flashes a stimulus in the blind spot from time to time, and a patient who sees it has looked away; more than a fifth is unreliable. False positives: responses when nothing was shown, the trigger-happy patient, and a field full of white spots with sensitivities above normal is the giveaway; above about fifteen per cent, distrust the field. False negatives: a stimulus missed at a point where a much dimmer one was seen earlier, the tired or inattentive patient, though a truly damaged field also produces them. Then look at the test duration and the number of times fixation was checked, and at the patient's pupil size and refraction, because a small pupil or the wrong lens depresses the whole field.

A field with poor reliability is not wrong, it is uninterpretable, and the right response is to repeat it.

• Fixation losses above about 20 per cent: the patient was looking around.
• False positives above about 15 per cent: the patient pressed the button without seeing.
• High false negatives with a normal field: inattention; with a damaged field: possibly genuine.
• A first-ever test: expect a learning effect, and repeat before acting on it.

A standard printout shows the same data five ways. The raw sensitivities are the numbers at each point. The greyscale turns them into a picture, darker where sensitivity is lower: it is the plot for shape, and the plot that exaggerates. The total deviation plot compares each point with the age-matched normal and marks the ones that fall below it with probability symbols. The pattern deviation plot does the same after removing any general depression of the whole hill, so that a cataract, a small pupil or a wrong lens no longer darkens everything, and what remains is the localised loss that matters. Read pattern deviation for what is real.

The global indices summarise the field. Mean deviation is the average loss across the field, negative in disease. Pattern standard deviation measures how irregular the loss is: high when there is a localised scotoma, low when the field is uniformly depressed. The glaucoma hemifield test compares the upper and lower halves of the field in mirrored zones, because glaucoma respects the horizontal midline and rarely damages both halves equally at first.

Behind the chiasm every lesion produces a defect in both eyes, so read the fields side by side. A defect on the same side of the vertical midline in both eyes is homonymous: a right homonymous hemianopia means a lesion on the left of the brain, in the optic tract, the lateral geniculate body, the optic radiation or the occipital cortex. The more alike the two eyes' defects, the more congruous, and the further back the lesion: optic tract defects are incongruous, occipital ones nearly identical. An occipital lesion often spares the macula, leaving a few degrees of central field on the blind side. A quadrantanopia points to the radiation: the upper quadrant to the temporal lobe, where the lower fibres loop forward, the lower quadrant to the parietal lobe.

A defect on opposite sides in the two eyes, both temporal fields, is heteronymous: the bitemporal hemianopia of a chiasmal lesion, where the crossing nasal fibres are compressed from below by a pituitary tumour or from above by a craniopharyngioma. A junctional scotoma, a central defect in one eye with a temporal defect in the other, places a lesion where the optic nerve meets the chiasm.