Although long test sessions can fatigue a patient, experience

with the machine usually decreases variability over repeated testing sessions­ (learning curve).27,28 Thus the first visual field may be the

least accurate. Patients with experience on manual perimeters may have less of a learning curve effect.29 We tend to repeat the initial test if the results are abnormal in any way. Although computerized machines are automated, they are not automatic. Patient/technician interaction can have a substantial impact on the reliability of the examination and may also aid in patient satisfaction.

Ocular variables

Pupil size

A pupillary diameter of less than 3 mm can cause generalized depression of the visual field.30 It is usually best to test the field with a pupil that is at least 3 mm in diameter. If it is not possible to

dilate the pupil to 3 mm, the test should be performed with a pupil that is no smaller than that which existed during previous tests.31,32

Media clarity

Any opacity of the ocular media can cause a localized or generalized depression in the visual field.This is particularly problematic when following a glaucoma patient who is developing cataracts. As the lens opacity become denser, field defects may appear to enlarge or become denser because of the reduced amount of light reaching the retina or because of image distortion or light scattering.33,34 Patterns of localized loss tend to remain consistent before and after cataract extraction, however.35,36 Visual acuity, refraction, and the appearance of the lens can help in determining the influence of cataract on the field. If acuity has dropped by more than one line on the Snellen chart, the examiner should suspect that the cataract is accentuating the appearance of visual field defects. Some analysis programs compensate for this reduction by factoring out generalized depression from the visual field so that scotomata are exposed (Fig. 9-1).37 However, the pattern deviation is not perfect in identifying purely localized defects if the cataract is significant. The most reliable criteria for identifying glaucomatous loss appear to be a glaucoma hemifield test ‘outside normal limits’, two hemifield clusters that fall below the 5% population limit, and four abnormal points ( 5%) in a hemifield on the pattern standard deviation plot.38,39