“We will accomplish this test together.” “I’ll be here with you.” “The machine is big and cannot move when you move, so we ask you to hold still for the test.” “No one can score 100%.” “The doctor knows which dots you should be missing, so don’t be afraid when there are no dots.” “I have done this test myself. I know what you are going through.” And this: “Please do not click the clicker while your eyes are closed.”
As coach/cheerleader/clinician combined, the ophthalmic technician relays to the patient such words of encouragement to accomplish visual field testing. This test subjectively measures central and peripheral vision to detect blind spots and other visual field defects. The test can detect signs of glaucoma, retinal disease, optic nerve damage and other diseases and conditions. Although it seems simple to administer, there is a science to acquiring a reliable field test. This article will discuss the reasons for visual field testing, the types of tests, and what the tests tell the doctor.
When the test is necessary
Though scheduled on a regular basis for some patients to follow their disease progression, the patient undergoes a visual field test when there is a diagnosis, a suspected field loss, or a ruling-out being considered by the doctor.
For example, it is not unusual for a doctor to order visual field testing to be done immediately when confirmation of a differential diagnosis is needed. These diagnoses may include pre-surgical necessity for selective laser trabeculoplasty or laser peripheral iridotomy. These two pressure-lowering laser treatments may be done in the office or ambulatory surgical center for medical necessity, including poor aqueous flow through the trabecular mesh, narrow angle when the anterior segment is not deep and clear, or failure of the patient to reach the targeted IOP with glaucoma eye drop therapy.
Other glaucoma procedures include the surgical bleb-producing trabeculoplasty and microincisional glaucoma surgery; these are sterile procedures, which lower IOP, and are performed in an outpatient surgical setting. Visual field testing may confirm disease progression necessitating any of these procedures.
Blepharoplasty, an elective eyelid surgical procedure, requires documentation of visual field loss to determine the medical, and not aesthetic, necessity of this procedure. Visual field testing for patients who have ptosis (a drooping eyelid) is accomplished with eyelids in a normal-to-relaxed state and repeated with the eyelids lifted with surgical tape. This documentation of functional field loss, due to a drooping eyelid, may qualify the patient’s surgery to be covered by the patient’s insurance carrier, within the limitations of his or her policy.
Types of visual field tests
There are two classifications of visual field tests: static and kinetic. An automated perimeter is considered static perimetry when the stimuli remain in one place, gradually increasing the intensity of the light, until the patient signals having seen it. Static testing can show retinal sensitivity in a given location and is useful in picking up subtle visual field defects.
Kinetic perimetry uses a moving stimulus to map the entire field of vision. Here, the patient looks at a central fixation target signaling the technician as the stimulus enters into his or her field of vision. Types of kinetic perimetry include:
- Confrontation screening. This gross measurement of visual field defects during exam pre-testing is simple yet effective and indicates if more detailed testing is necessary. In the confrontation test, the tester sits, with arms straight out to the sides, facing the patient. As the patient looks straight ahead, the tester moves one hand or the other inward. The patient signals when the hand is seen.
- The tangent screen. This test uses a black cloth on the wall measuring the central thirty degrees of vision. The patient signals when he or she sees a white stimulus that approaches from the temporal aspect. The stimulus size can be made larger for patients with compromised vision.
- Goldmann perimetry. This test begins with filtered lights first presented in the non-seeing area until they are observed.
The visual field and testing
The normal field of vision is 90 degrees temporally, 60 degrees nasally and superiorly and 70 degrees inferiorly, from the central fixation point. The 24-2 threshhold exam encompasses the topographic arrangement of photoreceptors from 24 degrees out from fixation. The 30-2 exam encompasses from 30 degrees to fixation. Macula testing is 10 degrees.
Static and kinetic perimetry each identify the physiological blind spot, the place in the visual field that corresponds to the lack of light-detecting photoreceptors on the optic disc of the retina where the optic nerve passes through the optic disc. Reduction in vision is noted by darker spots on the printout. A scotoma, a partial loss of vision, is indicated when the intensity of the stimulus is greater than the threshold; an absolute scotoma represents an area of no vision (a blind spot) and appears black. The optic nerve is an example of an absolute scotoma. The photoreceptors of the eye will see the stimulation of light during this sensitive time if the eye moves, sometimes requiring the technician to repeat the test.
Pathology, multiple sclerosis, pituitary adenoma, brain tumor, and defect lesions of the optic chiasm are disorders that can be documented with a visual field test. The delineations are exact for hemianopsia (a visual field loss on the left or right side of the vertical midline) and quadrantanopsia (defective vision or blindness in one fourth of the visual field). There are other field losses described as punched-out (located on the occipital lobe), pie-in-the-sky (temporal lobe), and scotoma islands (islands of vision loss surrounded by normal vision).
Many patients find the visual field test to be stressful but may be put at ease by an engaged, caring technician. In addition to stress, patients may complain they are fatigued during testing, which can run as long as 20 minutes. Your direction and words of encouragement can help the patient maintain proper position and concentration during testing.
Engagement and attention
Aside from being good caretakers for individuals who undergo visual field testing is the fact that the test is not boring. The designated clinicians, with moderate engagement and attention to detail, may see the result lie before them as the validity plays out on the test screen. OP