Visual Field Testing

Visual field testing is a crucial component of the neurologic, and more specifically the ophthalmologic, examination. A lesion or disruption may occur anywhere in the pathway from the striate cortex of the occipital lobe to the retina, causing a specific visual field defect. Identification of that deficit leads to an appropriate workup and ultimate diagnosis. Visual field testing can be performed by the non-ophthalmologist proficiently and quickly as part of the bedside examination. Detailed plotting devices used by ophthalmologists also exist to more precisely locate the field deficit (see the image below).[1, 2, 3, 4, 5, 6, 7]

Indications for visual field testing include visual field deficits, vision loss, headache, and neurologic deficits.

Patients with the following medical conditions should undergo regular visual field testing by an ophthalmologist to monitor progression:

• Glaucoma
• Multiple sclerosis and neuromyelitis optica spectrum disorders (NMOSD)
• Thyroid eye disease (TED) and other orbital tissue diseases
• Pituitary gland disorders
• Central nervous system disorders (eg, tumors, Idiopathic intracranial hypertension IIH)
• Stroke
• Uveitis 

It is important that the treating physician explains to patients why perimetry is important in treatment decisions. For glaucoma patients, for instance, the doctor should explain that the visual field is the indicator of the quality of life as well as that it determines how the patient sees now and in the future.

No equipment is necessary for visual fielding. A white, green, or red disc mounted on a stick is optional, as well as a perimetry device (listed below).

Goldmann perimeter

This perimeter uses static or kinetic perimetry and tests the entire visual field. The device is a white bowl that is positioned in front of the patient. This type of device uses a light that the examiner presents from behind the bowl as the stimulus.

Tangent screen

This perimeter uses static or kinetic perimetry to test the central 30º of the visual field. The tangent screen consists of a black screen that is placed in front of the patient. The examiner presents pins as the stimuli, in front of the screen while the patient focuses on a central target.

Amsler grid

This device tests the central 20º of the visual field. The Amsler grid is a grid of transecting lines with a central target marked on the grid. This device is used more for testing macular function. The patient focuses their gaze on the central target and notes whether or not the lines on the grid appear wavy or if any spots on the grid appear to be missing.

Computerized automated perimeter

This perimeter tests the entire visual field by using only static perimetry. Computerized automated perimeters (CAPs) are improved technologies that use a bowl placed in front of the patient. Computer generated light stimuli are created throughout the visual field, and when the patient signals visualization of each stimulus, the computer generates a numerical score related to that specific stimulus and the area of the field in which it was displayed and visualized. An overall numeric score is created at the conclusion of testing that corresponds to the degree of visual field deficit. A person's individual score may be compared to their own past scores or to the scores of patients with normal visual fields. These perimeters have a limitation in testing patients with macular (central) field deficits. Patients with central field deficits typically have difficulty focusing vision on a specific spot, a requirement of this technology.

Microperimeters

Also known as fundus-driven perimetry, microperimetry is the next iteration of visual field testing. These perimeters solve a substantial limitation of CAPs. Microperimetry is performed with simultaneous fundus viewing via an infrared camera, allowing accurate quantification of visual acuity at specific points on the retina. Significant deviations of fixation of the fundus cause testing to be interrupted until fixation is re-established. They can also be used to evaluate stability and location of fixation, important in evaluation of the progression of certain diseases. Many models of microperimeters allow automated static perimetry to be performed in addition to kinetic perimetry, fixation tasks, and reading tasks. Limitations include increased examination time and susceptibility to fatigue, although newer models are resolving many of these issues.

Positioning

The patient should be seated in a chair or on the examination table in an adequately lit room. The examiner should assume a position directly across from the patient at an arm's length, so that their eyes align on the same horizontal and vertical plane.

First, the patient has to understand the purpose and the nature of the examination by the trained staff to perform the perimetry. Second, new patients must be given specific instructions how to actually take the test. It has to be explained that it is a boring exam that needs time, so the pateint has to be relaxed and not in hurry.

In some indications, such as glaucoma and some neurological diseases, the visual field tests are performed on a regular basis (usually every 6-12 months) to detect any progression. 

The visual fields of both eyes overlap; therefore each eye is tested independently. The patient should cover their right eye with their right hand (vice versa when testing the opposite eye). With the examiner seated directly across from the patient, the patient should direct their gaze to the corresponding eye of the examiner. The testing itself can be performed using stationary or moving targets (disk mounted on a stick or examiner's fingers).

The normal visual field reaches 180º in the horizontal plane (160º for monocular vision) and 135º in the vertical plane. A moving target should start outside the usual 180º visual field, then move slowly to a more central position until the patient confirms visualization of the target. To perform stationary testing, the examiner holds up a certain number of fingers peripherally, equidistant between the examiner and the patient. The patient is asked to correctly identify the number of fingers. All four quadrants (upper and lower, temporal and nasal) should be tested. Stationary targets are more precise because they present a finer stimulus to the retina and are less easily identifiable relative to a moving target. In addition, for unknown reasons, colored targets such as red or green discs are more sensitive in detecting deficits when compared to a white test object (cotton disc mounted on a stick).

Visual neglect, a defect in central processing (usually the right parietal lobe), may not be apparent in conventional confrontation testing. This type of testing is performed with both eyes open. The examiner presents stimuli in the right and left visual fields simultaneously. A patient with a right parietal lesion, who exhibits neglect, may be able to accurately identify a visual stimulus when each quadrant is tested independently; however, if the examiner presents a stimulus in the right and left temporal region simultaneously, the patient with neglect will only be able to identify the stimulus in the right temporal field. The image in the left field may appear blurry or may not be visualized at all.

Perimetry is used to measure the peripheral and central visual fields of each eye in a more detailed and sophisticated manner. With the use of perimetry, the visual field is plotted in degrees of arc. Several devices exist to aid in the plotting of the visual field. With all perimetry devices, the testing involves the patient fixing their gaze on a central target, while stimuli, usually flashes of light, are presented throughout the central and peripheral visual fields. The patient is directed to respond verbally or by raising their hand if they see the stimulus.

Two general methods of perimetry exist: static and kinetic. Static perimetry tests individual locations throughout the visual field. First, a dim light is flashed on one area of the visual field. The intensity or size of the light is increased until the patient visualizes it. This is repeated throughout all areas of the visual field until a complete visual profile can be created. Kinetic perimetry differs in that the test stimulus is constant throughout the testing (the light is a fixed size and intensity). The stimulus is presented much like in confrontation testing in the periphery first, and then moved gradually to a more central location until the patient is able to visualize the stimulus. A so-called visual field boundary is then mapped based on this testing.

Visual field testing may prove to be difficult as a simple bedside test in children and obtunded patients. However, techniques exist to help the examiner elucidate the defects. In children, the examiner should stand behind the patient and slowly bring an object, such as a toy or game, around to the front of the child, until it is noticed. In obtunded patients, the examiner should assess the patient's blink response to a visual threat, such as the examiner's finger or any small object that is quickly moved toward the patient's eye in all regions of the visual field. In addition, static perimetry testing has also been shown to be possible at age 5 years, increasing in reliability with age.