Methods Of Testing The Visual Field; Patient Fixation And Test Reliability - Zeiss HUMPHREY 720i User Manual

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Methods of testing
the visual field
Patient fixation and
test reliability
REV. B 02/03 PN 51680-1
A defect (or scotoma) is categorized as either relative or absolute. A relative defect is an area
that has depressed vision or less than normal sensitivity; an absolute defect is an area where
the perception of light is absent. The point at which the optic nerve enters the retina is referred
to as the blind spot, and is an example of an absolute scotoma.
Some defect patterns are characteristic of certain diseases, a fact which makes visual field
testing a valuable part of the diagnostic process. Furthermore, by having patients repeat the
same tests at later dates, practitioners gain insight into the progression of the disease and the
effectiveness of treatment.
Over the years, visual field testing devices have varied in size, complexity, and testing
methodology. The fundamental premise has remained the same, however; patients must
respond when they see a stimulus.
In kinetic testing, a target of fixed stimulus characteristics is moved into the visual field from a
non-seeing area, until it is detected by the patient. Typically, the target is brought toward the
center from several directions and the operator marks the location at which the patient first
detects the target (threshold point).
Kinetic test results can only be reliably related to specific parts of the visual field if points are
joined to form an isopter, or ring of equal contrast sensitivity. Targets of varying size and
brightness are used during one kinetic test, and for each different target, a different isopter is
mapped. When reviewing several isopters, the clinician is visualizing different tiers in the hill
of vision.
A second method of evaluating retinal function is known as static threshold testing. The term
"static" refers to a stationary (rather than moving) stimulus being used.
In static testing, predefined test locations in the visual field are probed. Through a series of
stimulus presentations of varying brightness intensities, the threshold value is determined for
each test point. When evaluating static test results, clinicians are looking at the topography or
contour of the hill of vision, and whether depressions are evident.
In order for any visual field test to be clinically useful, it must yield reliable results. One
important factor affecting reliability is the steadiness of patient fixation. Unless the eye being
tested accurately fixates on the target while responding to stimuli, the results are unreliable.
Other factors adversely affecting reliability are:
• patient fatigue and anxiety
• poor test instructions
• patient discomfort
• improper near vision correction for central testing.
INTRODUCTION / INSTRUMENT SETUP
HUMPHREY FIELD ANALYZER II -
i
series