Visual Field Testing
Visual field testing measures the whole area that the eye can view while it looks at a fixed point, and maps how sensitive each part of it is. It finds loss of vision in the periphery and centre that the patient may not notice, and the pattern of loss points to the site of disease in the eye, optic nerve, optic chiasm or brain.
Several methods are used. Confrontation testing is a bedside test in which the examiner compares the patient's field with their own, using finger counting, a moving finger, or a red target. Automated static perimetry is the main clinical method.
The patient looks at a central light in a bowl, and presses a button each time a faint spot of light appears in the field, and the machine varies the light intensity to find the threshold at each point. The 24-2 and 30-2 programmes test the central 24 to 30 degrees, and the 10-2 programme tests the central 10 degrees.
Kinetic perimetry moves a target from outside the field to inside until it is seen, and it is used for patients who cannot do automated tests, and for the far periphery. Frequency doubling and short wavelength tests and handheld or tablet based perimeters are other methods.
A full threshold test takes 3 to 8 minutes per eye with fast algorithms. The test is subjective, and depends on attention and cooperation. In glaucoma, visual field loss is found on perimetry only after about 25 to 35% of ganglion cells have been lost. In stroke, the test shows the typical patterns of hemianopia that locate the lesion.
Indications.
- Diagnosis and monitoring of glaucoma, and glaucoma suspects.
- Optic nerve disease: optic neuritis, ischaemic optic neuropathy, compressive lesions, papilloedema, and toxic neuropathies.
- Pituitary and parasellar tumours, with chiasmal compression.
- Stroke, head injury and brain tumours, with suspected hemianopia or quadrantanopia.
- Retinal disease: retinitis pigmentosa, retinal detachment, macular disease, and drug toxicity (hydroxychloroquine, vigabatrin, ethambutol).
- Unexplained visual loss, and suspected functional (non-organic) visual loss.
- Fitness to drive and certification of visual disability.
- Eyelid and orbital disease with field obstruction (ptosis, dermatochalasis), when surgery is considered.
Not indicated when the patient cannot cooperate, and other tests such as confrontation or kinetic perimetry may be used instead.
Contraindications and precautions.
- No contraindications.
- Patients with poor vision, fatigue, tremor, cognitive impairment, or learning difficulty cannot always complete automated tests, and the test is adapted or replaced.
- The correct near lens is used for the testing distance in patients over 40 years, since uncorrected refractive error causes false loss. Lens rims cause artefact in peripheral fields.
- The pupil is not dilated, since a small pupil under 3 mm reduces sensitivity, but dilation may change results, and a note is made.
- The test is done before dilating drops, and before applanation tonometry, when the cornea and tear film are not disturbed.
- Eyelid drooping and a lens holder can block the field. The lids are taped or lifted when needed.
Preparation and technique.
The test is explained, and the patient sits comfortably with the chin and forehead on the rest and one eye covered with a patch. The correct near lens is placed in front of the tested eye, and the patient fixates on the central target. The patient presses the button whenever a light is seen.
The machine monitors fixation by tracking the blind spot or by video, and records false positive (button presses without a stimulus), false negative (missed brighter stimuli) and fixation losses. The patient is given breaks, and the test is repeated if reliability is poor. The first test is often poor because of learning, so a result showing a new defect is confirmed by a repeat test.
