visual fields

ANSWERS IN A Flash

A look at perimetry and guidelines for the proper evaluation of visual field results.

Chris A. Johnson, PH.D., D.SC., F.A.A.O., Iowa City, Iowa

An O.D. is to vision loss as a coroner and law enforcement is to human death. An O.D. investigates and determines vision loss cause(s) by using his education, visual acuity testing and most recently perimetry, so he/she can decide how to treat the case. A coroner investigates and determines cause of death through his education and specific tools, such as a scalpel. Law enforcement, based on the corner's findings, decides how to treat the case.

Here, I discuss perimetry, the factors that can affect perimetry result accuracy, and I talk about the diagnostic guidelines for glaucoma, retinal disease and neuro-ophthalmologic disorders, such as optic neuropathies.

Perimetry vs. visual acuity

A perimeter is a tool that permits you, the O.D., to diagnose, monitor and evaluate the efficacy of treatment for various ocular and neurologic disorders that affect visual function.

Specifically, the patient looks at the center of the perimeter, which resembles a bowl, and presses a button each time he sees lights flash. The perimeter then records the flash location and whether the patient pressed the button when the flash occurred in that location. Once the flashes cease, the device provides a printout, which reveals whether areas of the patient's vision missed any of these flashes. These areas show vision loss.

The location, pattern and shape of visual field sensitivity loss you observe for both eyes can provide you with useful information concerning the ocular, neurologic or systemic disease process influencing visual function. This contrasts with visual acuity testing, which is sensitive to central vision status, but can result in highly similar results from insults to different portions of the visual pathways.

Influencing factors

Four factors can influence perimetry result accuracy:

1. A patient's mental/physical condition(s). Learning or practice and fatigue can influence visual field test results: Patients with prior visual field testing experience are usually more consistent and prepared for this evaluation, which produces better, more reliable results. Alternatively, a longer test often produces a reduction in performance and results in less reliable test results. For these reasons, give patients clear instructions and a demonstration of the test prior to initiating formal testing. In addition, you want to prevent a “trigger-happy” patient. Such a patient can cause high false positive results, which result in fixation losses.
Some patients may have short attention spans — get rid of all possible distractions within the testing room — require multiple rest periods, need realignment of the head and eye being tested, etc. Therefore, it's imperative you, the examiner, be present throughout the perimetry test and be responsive to providing an individualized test procedure.
2. Overlooking information. All the visual field printout data are important, and it is always useful to determine whether test aspects, such as the proper test procedure and threshold estimation strategy, were correct.
3. Overlooking the factors that can affect sensitivity to light. Visual pathology is just one factor that can affect a patient's sensitivity to light, and therefore, his/her response to the perimeter. As a result, you, the practitioner, must be aware of all these factors (e.g. background lighting, ptosis, small pupil during testing [less than 3mm diameter], refractive issues, such as trial lens rim artifact [complete or partial], incorrect trial lenses used and eccentric fixation, etc.) prior to having the patient undergo testing. It is possible to compensate for these factors. If the patient has ptosis, use surgical tape for the upper lid so it does not obstruct part of their visual field. You can overcome a small pupil by dilating the patient. Finally, an optimal lens correction for the test distance (refined by a quick subjective refraction at the perimeter bowl) can minimize the refractive error effects. Also, look meticulously for early signs of pathology or changes in perimetry results at follow-up visits.^1-8
4. Incorrect date of birth. Age-related changes in the visual pathway's optics and neural structures produce reductions in light sensitivity. So, a 25 year-old patient usually has better light sensitivity than a 75 year-old patient. If the birthday for a 75 year-old patient is entered into the perimeter to reflect a 25 year-old, the visual field test procedure begins with incorrect light levels and compares the patient's results to individuals 50 years younger. This results in an incorrect assessment of test outcomes.

Proper evaluation guidelines

I have found a systematic approach to visual field interpretation permits the proper diagnosis of the vast majority of cases. Caveats: One must include all other aspects of the clinical eye exam to determine the visual field loss cause(s). Also, a few exceptions to this systematic approach exist, such as patients who have lesions of the lateral geniculate nucleus, diabetic patients with significant background retinopathy and patients who have gyrate atrophy. Although perimetry may not be as helpful in identifying damage location in the visual pathways in these patients, it is still important to document their visual field characteristics to determine whether the visual field is stable, improving or deteriorating at future vision exams.

1. For each eye, determine whether the visual field results are within normal limits. If the results are outside normal limits, move on to the next step.
2. Assess whether visual field loss exists in one or both eyes. If it exists in one eye, the loss is located before the optic chiasm, indicating a problem with the optics, retina or the eye's optic nerves. If visual field loss exists in both eyes, it is at the optic chiasm or beyond the optic chiasm. This indicates the problem is in the brain beyond the optic nerve, or it is an optical, retinal or optic nerve disease.
3. Determine the location of visual field loss (superior, inferior, nasal temporal). If the defect is extensive, focus on the most prominent area of damage on the visual field.
4. Determine the shape and pattern of the visual field loss.
► If the loss is nasal or binasal, the deficit is either retinal, in the optic nerve or possibly optical. The most common source is glaucoma (see Figure 1, page 28), but one must look at the pattern of visual loss to get a clear idea, as described below.
► If the loss is bitemporal, the patient likely has a chiasmal lesion (see Figure 2, above).
► If the loss is nasal in one eye and temporal in the other eye, the defect is homonymous, which suggests it is located after the optic chiasm. This indicates the damage is occurring to the visual pathways in the temporal, parietal and/or occipital lobe (See Figure 3, below).
► If the visual field loss is central, the defect is likely an optical, retinal or optic nerve problem.
► If the defect respects the horizontal meridian of the visual field, especially in the nasal visual field, the patient likely has glaucoma, another optic neuropathy or a retinal vascular problem.
► If the visual field loss appears as a candle flame in the central visual field, the defect is probably an optic neuropathy.
► If the visual field loss “points” to the blind spot, the defect is probably glaucoma or some other optic neuropathy, optic atrophy, compression of the optic nerve, optic neuritis or other disease that is affecting optic nerve function.
► If the visual field loss “points” to fixation, the defect is probably a deficit at the optic chiasm or after the optic chiasm. This indicates the damage is occurring beyond the optic nerve.
► If the visual field loss respects the vertical meridian of the visual field, the defect is either at the optic chiasm or beyond the optic chiasm. This indicates the damage is present beyond the optic nerve.
► If the pattern of homonymous visual field loss between the eyes is highly congruous (nearly identical), the defect is probably an occipital lobe lesion. Moderate congruity is consistent with a parietal lobe lesion, and temporal lobe lesions are less congruous.
► If a homonymous visual field loss is less than a hemifield in size and appears like a piece of removed pie from the superior visual field, the defect is a temporal lobe lesion. If a homonymous visual field loss is less than a hemifield in size and appears like a piece of removed pie from the inferior visual field, the defect is a parietal lobe lesion.
► If the region of the visual field loss appears similar to previous multiple visual fields based on the size and depth of the region of loss, the patient is stable. If it's worse, the patient has progressed. If it's better, the patient has improved.

I have personally found that these guidelines make it possible to properly evaluate most perimetry results. For more detailed information, consult the many excellent books on perimetry and visual field testing.^1-8

When determining the cause of vision loss, you, the optometrist, really do play the roles of coroner and law enforcement: First, you determine the cause of vision loss and, based on these findings, you determine how to treat the case. OM

1. Duke-Elder S, Scott GI. System of Ophthalmology. Vol VII - The Foundations of Ophthalmology, Visual Fields, London: Henry Plimpton, 1971, pp. 393-425.
2. Aulhorn E, Harms H. Visual Perimetry. In Visual Psychophysics: Handbook of Sensory Physiology, (Jameson D, Hurvich L, eds), Voll VII/4, New York: Springer-Verlag, 1972.
3. Harrington DO, Drake MV. The Visual Fields: Text and Atlas of Clinical Perimetry. St Louis: CV Mosby, 1990.
4. Anderson DR. Perimetry with and without automation. St. Louis: CV Mosby, 1987.
5. Greve EL. Single and multiple stimulus static perimetry in glaucoma: The two phases of perimetry. The Hague: Dr. W. Junk Publishers, 1973.
6. Wall M and Johnson CA. Principles and Techniques of the Examination of the Visual Sensory System, Chapter 2, Walsh and Hoyt's Textbook of Neuro-Ophthalmology (Volume 1), Philadelphia: Lippincott, Williams and Wilkens, 2005, pp 83-149.
7. Johnson CA. Evaluation of visual function (Chapter 17), Duane's Ophthalmology Solutions (Tasman and Jaeger, eds), Lippincott: Philadelphia, 2008, pp 1-51 (ebook, available at www.duanes solution.com/pt/re/duanes/home.htm;j sessionid=L3LDvpnsshpTz9HHM9T6qS2ZdndyHRwbhTgFXQdnpQy4W189qhpY! 536197444! 181195628!8091!-1)
8. Johnson CA, Wall M. The Visual Field. Chapter 35 in Adler's Physiology of the Eye, 11th ed ( Levin, Nilsson, Ver Hoeve, Wu, Kaufman and Alm, eds), 2011, pp 655-676.

Dr. Johnson is a professor and the director of the Visual Field Reading Center in the Department of Ophthalmology and Visual Sciences at the University of Iowa Hospitals and Clinics. E-mail him at chris-a-johnson@uiowa.edu, or send comments to opto metricmanagement@gmail.com.