Like a painter, the practitioner of perimetry must learn his or her profession from experience. Just as a painting does not spring from the paint and brushes alone, the perimetrist does [not] get his or her diagnosis from just a printout of the field test. Rather, the perimetrist’s experience in interpreting field test results, his clinical skill in examining the validity of the patient’s performance, and his selection of the needed field technique chosen under the appropriate clinical circumstances combine to produce a suitable test and interpretation of the results.
Thomas J. Walsh, M.D., pg. 3
What Is Perimetry?
Perimetry, the measuring and documentation of the visual fields, has a long and exhaustive history (Thompson HS, Wall M. History of Perimetry. Imaging and Perimetry Society website). There are many methods of testing perimetry, which have evolved with technology and greater understanding of neuroanatomy and functional vision.
As the quote above indicates, the interpretation of visual fields requires more than simply looking at the output from the test. A key point in this chapter is that we’ve grown accustomed to looking at perimetry as an objective test, when a great deal of clinical context and understanding of the human elements of the test are necessary to properly interpret the data. We are also inclined to view automated perimetry as superior to manual techniques, forgetting that there may be a time and place for utilizing other means of measuring the visual field.
…a well-performed tangent screen is more valuable on a given occasion than a poorly performed computerized field examination.
Thomas J. Walsh, M.D., pg. 3
The Function Of Perimetry
Perimetry functions to indicate the site and extent of involvement of the visual pathway.
Another way to put this: perimetry maps out areas of existent and nonexistent vision.
Perimetry can help localize abnormalities of vision and may also be used to track/serially follow progression of various diseases that affect vision.
Why Is Learning How To Interpret Perimetry So Important?
Defects in the visual field indicate a problem with the afferent visual pathway (retina to occipital cortex).
Specific visual field patterns help localize the problem.
Visual field defects that look similar may have different causes, and some of those etiologies may include testing errors.
Example: Enlarged blind spot may be secondary to optic nerve edema, slow responder on test taking, large myopic crescent of peripapillary atrophy, etc.
Example: Cataracts can cause contraction of the visual field, as can small pupils (miosis). These can falsely lead to an interpretation of worsening glaucoma.
Knowing how to capture the information accurately is also important.
Example: Deciding which eye to test first. Testing the better-seeing eye may be helpful in some cases (learning effect), but in other cases testing the worse-seeing eye may be better (less fatigue).
Example: Selecting the most appropriate method for visual field testing for a patient (choosing to use a tangent screen vs. automated perimetry).
Terminology
Isopter: A line that indicates the outer limit of an area within which a specific target is recognized.
Best demonstrated on a Goldmann perimeter.
Represented by series of concentric lines; crossing isopters suggests either unreliable test-taking or non-organic disease.
Computerized perimetry does not use isopters to delineate extent of visual fields but instead uses decibels based on pre-specified light intensities to describe the visual field.
Threshold: The ability to see a specific test object in a specified area of the visual field.
The threshold varies depending on how far away the area of the visual field is from fixation.
Kinetic perimetry: Testing that involves moving a test object of the same size and brightness from a non-seeing to a seeing area.
Static perimetry: Testing that involves increasing the brightness of a test object until the patient can see the object in that specific area.
Suprathreshold testing: The test object used in that area of the field is above the expected threshold for that area.
Scotoma: Region of defective/depressed vision within areas of normal vision.
Reference
Walsh TJ, ed. Visual Fields: Examination and Interpretation. Third Edition. New York: Oxford University Press, 2011: xix-6.