2020–2021 BCSC Basic and Clinical Science Course™

Shop

Clinical Education /

Book Excerpts /

Basic and Clinical Science Course - Excerpt

2020–2021 BCSC Basic and Clinical Science Course™

Go to Academy Store Learn more and Purchase.

5 Neuro-Ophthalmology

Chapter 3: The Patient With Decreased Vision: Evaluation

Examination

Visual Field Evaluation

Evaluation of the visual field helps localize a lesion along the afferent visual pathway, defines patterns of vision loss, and quantifies the defect, enabling measurement of change over time. The choice of technique depends on the degree of detail required and the patient’s ability to cooperate. Patterns of visual field loss are discussed in detail in Chapter 4.

Confrontation testing

Confrontation testing, which is easily performed at a patient’s bedside or in the clinic, should take place during every ophthalmic examination. It is only a screening test and does not replace formal perimetry. The accuracy of confrontation testing depends on the density of the visual field defect. Accepted methods of confrontation testing include:

Description of the examiner’s face. The examiner sits 1 m from the patient. The patient covers 1 eye and fixates on the examiner’s nose. The examiner asks the patient whether he or she is unable to see specific parts of the examiner’s face. Affirmative responses may aid in identifying central or altitudinal (superior or inferior hemifield) visual field defects.

Finger counting in the 4 quadrants. The patient is asked to count 1 or 2 static fingers as they are presented sequentially in each of the 4 quadrants. The fingers are presented approximately 20° eccentric to fixation and equidistant from the quadrant borders. This technique tests for both altitudinal defects of the anterior visual pathways and homonymous defects due to retrochiasmal lesions.

Kinetic red target test. A 5-mm, red-topped pin is moved inward from beyond the boundary of each quadrant along a line bisecting the horizontal and vertical meridians. The patient is asked to report when the pin is first perceived to be red.

Finger or red comparison test. With the finger comparison test, the examiner’s index fingers are presented simultaneously on either side of the vertical meridian, first in the superior and then in the inferior quadrants. The patient is asked to report whether the fingers are equally in focus. During finger testing, children and non-verbal adults may be asked to mimic the examiner’s finger movements. If the patient cannot identify fingers to mimic finger movements, the examiner presents progressively stronger stimuli, such as hand movement or light perception in each quadrant. In the red comparison test, 2 identical red targets are presented in the same manner as the fingers in the finger comparison test. The patient is asked whether the targets are equally red. Color may appear altered, washed out, or absent in a damaged hemifield. Slow movement of the target across the vertical meridian may identify an abnormality in color vision, which suggests damage to the chiasmal or retrochiasmal pathway.

Although these confrontation tests offer high sensitivity, their use is likely to result in many false-positives due to the low specificity of the tests. The single test with the best combination of sensitivity (74%) and specificity (93%) is kinetic testing with a red target.

Extinction refers to the inability to see a target in an affected hemifield only during simultaneous stimulation of both hemifields. A target presented only in the affected hemifield can be seen. This finding of visual neglect is characteristic of parietal lobe lesions.

Kerr NM, Chew SS, Eady EK, Gamble GD, Danesh-Meyer HV. Diagnostic accuracy of confrontation visual field tests. Neurology. 2010;74(15):1184–1190.

Amsler grid testing

Amsler grid testing screens the central 20° of the visual field (10° from fixation). The patient, with optical correction for near vision as needed, holds the Amsler grid at one-third of a meter, covers 1 eye, and looks at a fixation point in the center of the grid without scanning the grid. The patient describes any areas of distortion (ie, metamorphopsia); such areas suggest macular rather than optic nerve disease. Peripheral “bending” of the grid may represent optical aberration from spectacles and should be disregarded. The patient can also identify any scotoma. Although Amsler grid testing is rapid and simple, its sensitivity is low.

Perimetry

Perimetry provides more detailed evaluation of the visual field. In both static and kinetic techniques, the visual field is analyzed for areas of decreased sensitivity, both in location and in degree. In static testing, stimuli turn on and off at designated points within the region of the visual field being tested. In kinetic testing, a stimulus moves from a nonseeing to a seeing area of the visual field to determine the location at which it is consistently detected by the patient. All points of equal sensitivity for a specific stimulus are connected to form an isopter, which represents the outer limit of visibility for that stimulus. Analysis of several isopters (plotted with different stimuli) produces a “contour map” of the island of vision.

Kinetic perimetry

Kinetic perimetry (eg, Goldmann perimetry, the kinetic program on an Octopus perimeter [Haag-Streit, Köniz, Switzerland]) can be used to evaluate the entire visual field. Stimuli of varying sizes and intensities are moved along each radial meridian from a peripheral to central location. Typically, 2 or 3 isopters are plotted. Varying the stimulus size, intensity, and location can delineate the depths and borders of defects. Kinetic perimetry requires a skilled and knowledgeable perimetrist who can interact with patients to elicit optimal cooperation.

Automated static perimetry

Automated static perimetry is considered the gold standard for evaluating visual field defects. Although this method is particularly difficult to use with older or inattentive patients, it possesses numerous advantages over manual kinetic perimetry techniques:

standardized testing conditions, which improve serial and inter-institutional comparisons of results

less dependence on technicians

better sensitivity

numerical data amenable to statistical analysis for comparisons and clinical studies

results amenable to electronic data storage

Most automated perimeters use static stimuli that are similar in size to the kinetic perimeter size III stimulus. The perimeter randomly presents stimuli at predetermined locations within a specified region of the visual field. Because nearly 80% of the visual cortex correlates to the central visual field, testing the central 24° or 30° of the visual field is typically adequate for detecting most visual defects (Fig 3-5). It is also critical to measure the foveal threshold (normal range 32–40 dB) because it estimates central visual function. Because of the 6° test spacing of the 30-2 and 24-2 programs, 10-2 perimetry may better delineate a small central or paracentral scotoma.

Figure 3-5 Kinetic versus automated perimetry. Diagrammatic representation of the extent of the visual field evaluated by kinetic perimetry versus the 30° central program in automated static perimetry. The largest isopter in kinetic perimetry testing extends 90° temporally and 60° in other quadrants; typical automated static perimetry evaluates only the central 30°.

(Courtesy of Anthony C. Arnold, MD.)

The stimuli vary in brightness, and patient responses determine the minimum visible stimulus at each location—the sensitivity threshold. This threshold is defined as the intensity of the dimmest target identified 50% of the time at a given location. For each region tested, the report displays the threshold value in decibels (on a logarithmic scale of intensity, measuring attenuation from the maximum stimulus of the perimeter). For a given stimulus location, a higher value indicates that the patient can see a dimmer stimulus, reflecting greater visual sensitivity at that location. The measured values are not absolute numbers and are not equivalent among perimeters because the machines have different maximum intensities, backgrounds, and durations of presentation.

A symbolic representation of the threshold values, the grayscale map, depicts an overall topographic impression of the visual field data by using darker symbols for low-sensitivity points and lighter symbols for high-sensitivity points. The computer program interpolates between tested points to provide a user-friendly picture (Fig 3-6). For clinical interpretation, the perimeter calculates for each value the statistical probability that the value falls outside the normal range among age-matched control subjects; the results are placed in a total-deviation plot. Optic neuropathy may cause substantial total-deviation depression with few or no pattern-deviation abnormalities. Because ocular media abnormalities (eg, refractive error, cataract) may depress the sensitivity of the entire visual field, the program produces a pattern-deviation plot by determining the sensitivity values for all points shifted (by the seventh-highest point) and reanalyzes them based on age-expected values. This reanalysis compensates for the overall sensitivity depression, allowing recognition of abnormal patterns (eg, scotomata, arcuate defects, homonymous defects) that might have been otherwise masked. Abnormal values are depicted topographically according to statistical probability: Darker squares represent higher probability, and lighter squares represent lower probability.

The long duration and repetitiveness of the original full-threshold perimetry test can fatigue patients, reducing the accuracy of the test results. Use of the Swedish interactive threshold algorithm (SITA) shortens the time needed to complete the full-threshold test by half but maintains the accuracy necessary for validity. (See BCSC Section 10, Glaucoma.)

The reliability of perimetry test results is assessed by identifying the following patient response characteristics:

false-positive response rate: how frequently the patient signals when no target is displayed (The acceptable rate is typically <25% on threshold testing and <15% on SITA testing.)

false-negative response rate: how often the patient fails to signal when a target brighter than the previously determined threshold for that spot is displayed (The acceptable rate is typically <25%, but the rate increases in regions of true visual field loss, because the patient is unable to reproduce responses accurately.)

fixation losses: how often the patient identifies the stimulus in the previously plotted physiologic blind spot (an unexpected response), indicating that the eye is not aligned with the fixation target

Global indices are calculated to help determine changes in sensitivity over time. Such indices include a center-weighted mean of the sensitivity depressions across all points (ie, mean deviation) and different methods of addressing localized defects (eg, pattern standard deviation, corrected pattern deviation, loss variance).

Figure 3-6 Report from a Humphrey 30-2 automated static perimetry program, with explanations of statistical analysis, grayscale, and probability plots (red type).

(Courtesy of Anthony C. Arnold, MD.)

Barton JJS, Benatar M. Field of Vision: A Manual and Atlas of Perimetry. Totowa (NJ): Humana Press; 2003.

Bettis DI, Johnson CA. Updated on automated perimetry. Focal Points: Clinical Practice Perspectives. San Francisco: American Academy of Ophthalmology; 2016, module 12.

Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.

5 Neuro-Ophthalmology

Chapter 3: The Patient With Decreased Vision: Evaluation

Examination

Visual Field Evaluation

Confrontation testing

Amsler grid testing

Perimetry

Kinetic perimetry

Automated static perimetry

Figure 3-6 Report from a Humphrey 30-2 automated static perimetry program, with explanations of statistical analysis, grayscale, and probability plots (red type).

OUR SITES

FOLLOW THE ACADEMY