Visual Field Testing Strategies
The examination of the visual field in relation to glaucoma should be directed towards detecting visual field loss at the earliest possible stage of development, and carefully monitoring the visual field over time to help assess the efficacy of a treatment regimen. In this, the development of static automated perimeters, with their variety of testing strategies, the plethora of data they produce and statistical analysis packages, has greatly enhanced the examination process compared with the limited capabilities of the older kinetic methods. Newer technologies such as frequency doubling and short wavelength automated perimetry may provide even further impetus.
Broadly speaking, automated methods may be divided into screening and threshold strategies. Screening techniques are quick and easy to administer but they may have only a limited success in detecting very early loss in glaucoma patients. Patients should be tested at full threshold with automated static perimetry, for example, a 30 or 24-2 strategy with the Humphrey Field Analyser (www.meditec.zeiss.com), or a central fast threshold with the Medmont automated perimeter (www.medmont.com). A screening strategy could be used as practice for the patient, or the initial demonstration mode used. The comparison of the result with age-matched normative data, the production of global indices, global and probability maps, and hemifield analyses all aid the examiner in making a meaningful clinical decision. Furthermore, the use for example of custom grids, where stimulus spacings are reduced, allow specific areas of the visual field to be targeted, perhaps in response to findings with the ophthalmoscope such as the position of retinal nerve fibre layer defects. It is most important that visual fields are repeated whenever the results are equivocal.
Criteria for field losses/assessment
Visual field analysis is frequently relied upon in the clinical management of glaucoma and glaucoma-suspect patients. However there is significant variability in test results owing to it being a psychophysical technique. This can create uncertainty in differentiating normal variation from early losses, or variability from true progression. The following are suggestions to assist in improving reliability:
(a) Repeat visual fields should be performed, after a few weeks, to confirm the reliability of results. An area of the field should be shown defective at least on two occasions. To show progression, the changed result may need to be shown on two further occasions.
(b) Use a consistent criterion for abnormality/change. Assess the abnormal points: A
loss of < 10 dB in 3 contigious points may not be significant; a loss in the 3 adjacent points of > 15dB may be considered abnormal, and a loss between these values may be considered borderline and in need of reassessment. Points related to an existing scotoma that decline by > 10 dB may be considered significant.
(c) Consider the location of the suspicious field points. Their location will be more meaningful if it corresponds to an observed defect in the rim of the optic nerve head. Points closer to the fixation point may be of greater significance than points on the edge of field plot.
Dense localized scotoma just superior to fixation in a 50 year old female with POAG. Vision was 6/6.