Deviation from normal values

There are no published normal values for Goldmann perimetry. This is probably because results vary widely from technician to technician and from technique to technique. Certain guidelines, however, are helpful. Generally, the I2e isopter falls between 25° and 30°. The I4e isopter usually falls between 40° and 50° nasally. In the absence of lens opacity in patients older than 75 years of age, these isopters may contract 5° or so. Contraction may be greater in the presence of cataracts.

Graphic plot of points varying from normal

The Humphrey and Octopus perimeters have age-matched normal values stored in their computer memories for each point

tested.The Humphrey perimeter provides the deviation from normal in a graphic printout through its STATPAC program.52 For each point tested, STATPAC provides the statistical probability that the patient’s threshold would be found in the normal population (Fig. 10-20). The Octopus program provides a printout of how many decibels a point deviates from the norm for that age (see Fig. 10-17). Regardless of the technique used, it is unwise to depend on one or two deviant points within a field to indicate abnormality unless they are severely depressed or contiguous.

Global indexes

Measurements of single points are more likely to contain error than are averages of measurements of all points within the visual field. Therefore global indexes are helpful in recognizing deviation from

normal. The Octopus MD may be statistically abnormal with less deviation from normal than that required for a single point. For the Octopus, 95% of the population will fall within 2 dB of the normal mean sensitivity provided on the Octopus printout.38 By contrast, an individual point may need to be depressed two to three times this amount to be even mildly suspicious. The Humphrey printout provides a table indicating the lower 5% probability of normal for each of the global indexes. Deviations greater than these may becabnormal. The greater the deviation, the greater the likelihood that it is abnormal.

Comparison with the other eye

Published data53 indicate that the mean sensitivities of the two eyes fall within 1 dB 95% of the time and within 1.4 dB 99% of

the time. Therefore unexplained variation of mean sensitivity between the two eyes greater than 1 dB would be suspicious, and greater than 2 dB may be abnormal. If the lower sensitivity occurs in the eye with higher IOP or greater excavation of the optic nerve head, it would be highly suspicious of glaucomatous visual field loss.54

Localized variation within the visual field

Localized depression within the field may not be enough to cause a statistically significant reduction of mean sensitivity or be deep enough to be recognized by the computers as an increased MD. However, one of the most important types of early change in glaucoma is mild inconsistent depression in the paracentral area.9,55