Visual Field Testing
Please visit https://www.youtube.com/watch?v=ByprS4vGRD0 for a full lecture on Visual Field Testing
Visual Field Testing
Diane Siegel, Malik Y. Kahook, Monica K. Ertel, Leonard K. Seibold, Cara E. Capitena Young
Glaucoma is one of the most common causes of blindness worldwide.1 While loss of visual function is irreversible, morbidity is preventable with proper diagnosis and treatment. Assessment and documentation of both the structure (through both examination and imaging) and function of the nerve (through perimetry or visual field assessment) are essential to both the diagnosis as well as the follow-up and treatment of glaucoma. This chapter focuses on functional testing and monitoring of patients with glaucoma and presents basic concepts behind the various modalities along with pearls for practice.
The field of vision is defined as the area that is perceived simultaneously by a fixating eye. In the light adapted state, the island of vision has a steep central peak that corresponds to the fovea, the area of greatest retinal sensitivity.
The point of fixation is the highest point of retinal sensitivity, which typically corresponds to the fovea. The limits of the normal visual field are 60 degrees superiorly and nasally, 70 to 75 degrees inferiorly, and 100 to 110 degrees temporally.
Types of Visual Field Testing
Kinetic vs Static
Kinetic perimetry involves a moving stimulus, where sensitivity thresholds are determined by moving stimuli of various sizes and light intensities from a non-seeing area of the visual field into a seeing area. The trajectory of the stimulus is called a vector. The location where the light is seen is then recorded. The specific location at which that response occurs has a sensitivity threshold equal to the specific light intensity used. The process is repeated so that all regions of the visual field are evaluated with the same light intensity and stimulus size. Then, stimuli of different intensities and sizes are shown, creating a map of visual field sensitivity to these various stimuli.
When enough vectors are tested throughout the same visual field with the same stimulus, the response points of each vector can be connected to create a boundary of equal sensitivity, which is known as an isopter. These boundary lines are analogous to a contour line on a topographic map. For a patient with normal vision, the area inside the boundary line is an area of “seeing” for a given stimulus and the area outside of the boundary line is an area of “non-seeing”. The luminance and the size of the target is varied to plot subsequent isopters.
In contrast, static perimetry uses non-moving stimuli and determines sensitivity thresholds at a specified number of test locations. In this type of perimetry, the size of the test target remains constant as it is set prior to the start of the test. The location of the stimulus moves throughout the test and the patient’s response to the stimulus is recorded. The retinal sensitivity threshold at a specific location is then determined by varying the brightness of the test target. These thresholds are compared to age-matched controls. Thus, small changes in sensitivity can be detected with high accuracy.
Manual vs Automated
Visual field testing is either manual or automated. Although all perimetry testing requires a technician to administer the test, manual perimetry requires more interaction between the technician and the patient to map the patient’s visual field. In contrast, automated perimetry uses a computer algorithm to determine the threshold retinal sensitivity at a specific location the visual field. Statistical analyses compare these results to age-matched controls while tracking changes over time.
Goldmann Visual Field
Goldmann perimetry is the classic manual, kinetic perimetry. The Goldmann visual field allows for different sizes of the test stimulus as well as varying luminance. Different Roman numerals, numbers, and letters indicate the various isopters and targets that are seen in each defined area. The number and letter represent the intensity of the stimulus. A change of one number represents a 5 dB (0.5 log unit) change in intensity, and each letter represents a 1 dB (0.1 log unit) change in intensity.
Goldmann visual fields are particularly useful with cognitively impaired patients or children who are unable to perform automated perimetry. In addition, Goldmann visual fields can provide higher spatial resolution, which can be valuable in certain clinical situations such as defining the size of scotoma. However, Goldmann fields required a skilled technician to perform the field accurately and can take significantly longer to perform, leading to test fatigue.
Standard Automated Perimetry
Standard automated perimetry (SAP), is the current standard method for assessing visual function in glaucoma. It employs static perimetry to measure the ability of the eye to a detect a difference in contrast between a test target and background luminance at multiple pre-defined points in the visual field. Advantages of automated perimetry over manual perimetry include: more sensitive and reproducible results, quantitative data regarding visual field loss, and typically shorter testing times.
SAP includes a variety of testing strategies. Suprathreshold tests present a stimulus brighter than what is usually seen at a particular location by a normal subject of a particular age and determine whether the subject being tested can see it or not. The actual sensitivity to light is not determined. Suprathreshold testing may be appropriate in a community screening setting or when assessing visual disability but is not recommended for glaucoma diagnosis or follow-up.
In contrast, a full threshold testing strategy defines the threshold sensitivity to light—or the dimmest stimulus that is seen 50% of the time—at all points in the testing field. It employs a bracketing strategy to estimate threshold. Most commonly, an algorithm moving in brackets of four and two decibels is employed. Testing starts with either a suprathreshold or infrathreshold stimulus. The intensity of the stimulus is for instance decreased in 4 decibel steps until the stimulus is no longer seen, which indicates that the threshold has been crossed. The stimulus then reverses becoming brighter in 2 decibel steps until the stimulus is seen again. A full thresholding program is the most accurate way of evaluating and following glaucomatous visual field defects. Unfortunately, this strategy is time-intensive. More recently, various strategies have been developed to obtain reliable and efficient estimates of the visual threshold in order to replace a full thresholding program.
For example, the Humphrey Field Analyzer (HFA) perimeter (Carl-Zeiss Meditect, Dublin, CA) uses the Swedish interactive threshold algorithm (SITA). SITA is a Bayesian test strategy that generates a probability distribution function representing the probability that the visual field sensitivity will be of a particular value at a particular visual field location based on data from healthy individuals and those with disease. Similar to the SITA test strategy for the HFA, the tendency-oriented perimeter (TOP) algorithm was developed for the Octopus perimeter (Haag-Streit, Mason, OH) as an alternative to lengthy staircase threshold procedures. It shows one stimulus at a single location of the visual field and estimates threshold sensitivity at a particular location by averaging information from nearby test points.
Selective Functional Testing
One downside of the conventional perimetry algorithms described above is that they are non-selective. They activate a variety of retinal ganglion cell processing pathways, leading to redundancy that may be responsible for the reduced sensitivity of standard perimetry. Selective perimetry is a way to target individual ganglion cell pathways in an attempt to detect disease earlier.
· Short Wavelength Automated Perimetry (SWAP): uses a blue stimulus against a bright yellow background and targets the small bistratified ganglion cells, which project axons to the koniocellular layers of the lateral geniculate nucleus. SWAP also now utilizes a SITA testing strategy to shorten testing time. In several studies, SWAP has been shown to detect glaucomatous visual field loss up to 3 to 4 years prior to standard automated perimetry, predictive of both the size and location of visual field defect.2,3 However this predictive effect has not been borne out in all studies.4 SWAP testing however is vulnerable to media opacities, such as cataract which can act as a blue light filter, and thus falsely depress the field.
· Frequency Doubling Technology (FDT): determines the contrast sensitivity for detecting a high temporal frequency counter-phase flicker stimulus, which emphasizes the responses of magnocellular retinal ganglion cells. FDT in glaucoma testing has been shown to correlate well with SAP and can detect visual field loss 3 to 5 years earlier than SAP.5,6 However, data validating its use are limited thus far. FDT advantages include low cost and portability with a short testing time. Disadvantages of FDT are that increasing sensitivity requires increasing the number of targets presented and therefore increasing testing time. Cataracts also limit sensitivity. Furthermore, the test is not well suited for following progression.
· Flicker Defined Form (FDF): believed to stimulate the magnocellular pathway and early evidence suggests that it may be useful for early glaucoma diagnosis.
SAP remains the gold standard for monitoring glaucomatous changes and is best used to follow patients over time and establish a baseline. Still, selective functional testing can be used to detect glaucomatous field defects earlier than SAP. Used properly, all of these tests can be useful and provide complementary information at various stages of glaucoma.
Humphrey Visual Field Basics
The Humphrey Visual Field is the most commonly used static perimeter for glaucoma. There are several different test patterns that are utilized, including the 24-2, 30-2, and 10-2. The most common test pattern used for glaucoma is the 24-2 protocol.
The first number of the protocol refers to the number of degrees of field tested from the point of fixation. For instance, 10-2 means that the machine tests points 10 degrees from the center of vision and a 24-2 tests points 24 degrees around fixation. The 24-2 and 30-2 programs test the central field using a 6 degree grid. They test points 3 degrees above and 3 degrees below the horizontal midline. The 24-2 eliminates the most peripheral ring of test locations compared to the 30-2, except in the nasal region. This is useful because the peripheral ring provides less reliable data and eliminating that area shortens overall testing time. The nasal 30 degrees is preserved in the 24-2 protocol to aid in the early diagnosis of glaucoma since glaucomatous defects often originate in the nasal visual field. For patients with advanced visual field loss or with primarily paracentral defects, the 10-2 protocol can also be used. These concentrate on the central 10 degrees of the visual field and test points every 1 to 2 degrees. As a result, the 10-2 protocol can improve detection of progression or better delineate a paracentral scotoma by assessing a larger amount of points within the central island.
The second number refers to the testing protocol. The Humphrey visual field historically has two testing protocols: protocols 1 and 2. Protocol 1 tests points directly on the horizontal and vertical axes. Protocol 2 eliminates these points since they can be difficult to interpret and is the more commonly used of the two.
Reading the HVF Report
On the HVF report (Figure 1), there is a wealth of information and indices that should be carefully examined when interpreting the field. Reliability data, including fixation losses, false positives and false negatives are given in the top left. Information regarding the testing algorithm used, type of stimulus, pupil size, refractive error, date of test and pertinent patient demographic information is provided across the top margin. In addition, a threshold sensitivity map with the numerical threshold sensitivities for each location and a corresponding grayscale map, a total deviation plot with a corresponding total deviation probability map, and a pattern deviation map with corresponding pattern deviation probability map are shown.
During the test, a stimulus of maximum luminance is periodically presented in the patient's blind spot, where it should not be seen. If the patient responds to the stimulus, indicating poor fixation, it is counted as a fixation loss. Fixation losses increase if the patient does not maintain fixation, the blind spot was incorrectly mapped at the start of the test, and/or the patient’s head moves. A fixation loss rate less than 33% is considered acceptable if the technician feels the patient maintained good fixation.
In automated tests, a sound cue is given before each visual stimulus. Periodically the sound cue is given but no test stimulus is presented. A false positive result occurs if the patient responds to the sound cue alone, i.e. no visual stimulus was presented. Patients with high false positive rates are often referred to as “trigger happy” and the gray scale plot may appear lighter than normal or “whited out”. The technician should inform the patient that it is normal to have periods of no stimulus and to wait patiently through them Elevated false positives artifactually improve the appearance of the field.
A false negative is recorded if a patient does not respond to a brighter stimulus presented at a location that was previously seen earlier in the test with a dimmer stimulus. A high number of false negatives may indicate inattentiveness or fatigue and an unreliable visual field. The false negative response rate is often higher in eyes with extensive visual field defects than in those with normal visual fields.
Testing time is also provided for each eye on the printout. This average time length varies based on the algorithm used. Experienced test takers should be able to perform the SITA FAST algorithm in under 5 minutes per eye. The various SITA protocols are described below. Tests that are abnormally long or short may be unreliable.
The Roman numeral represents the size of the stimulus, from Goldmann size 00 (1/16 mm2) to Goldman size V (64 mm2). Each size increment equals a two-fold increase in diameter and a four-fold increase in area. A size III (4 mm2) stimulus is most commonly used, generally in patients with visual acuity of 20/200 or better. The size V stimulus Is generally reserved for patients with visual acuity worse than 20/200.
Typically in SAP, a white stimuli is projected on a white background. However, other forms of perimetry can use different stimuli. For example, short wavelength automated perimetry (SWAP) testing projects a size V blue stimulus on a yellow background, which allows for selective activation of a select group of ganglion cells in the koniocellular pathway.
The luminance of the stimulus can be varied from 0 decibels (dB), which is the brightest stimulus, to 50 dB, the dimmest. The decibel scale is an inverted logarithmic scale created by the manufacturers of automated perimeters to measure the sensitivity of the island of vision.
The Humphrey Field Analyzer (HFA) perimeter uses the Swedish Interactive Threshold Algorithm (SITATM) automated perimetry strategy. As described above, SITA uses prior information from healthy individuals and those with disease to generate a probability distribution function (PDF) representing the probability that the visual field sensitivity will be of a particular value at a particular visual field location. As the test progresses, the distribution is then adjusted, according to whether or not the patient responds to the stimulus. This continues until the PDF is within a small range, and then the mean of the distribution is selected as the threshold sensitivity estimate. Thus, threshold values are constantly calculated throughout the test at the same points. If the results are too different, those points are tested again. Therefore, the faster SITATM protocols are generally preferred to full threshold testing to avoid patient fatigue.
The SITA strategy is available in Humphrey perimeters in a variety of different formats:
· SITA Standard: usually the strategy of choice. SITA Standard is about 50% faster than full threshold programs, usually requiring about 3 to 7 minutes per eye.7 Most glaucoma specialists rely on SITA Standard because it expedites test time without eliminating significant data. Patients with difficulties with SITA Standard should not be shifted to SITA Fast. These patients benefit from careful instructions from the technician, closer surveillance and positive feedback.
· SITA FAST: faster than the SITA Standard (about 2 to 5 minutes per eye) and typically has similar levels of accuracy and reliability, particularly in experienced test takers and in younger patients.7 However, this may be a more difficult test for the patient because the stimuli presented are closer to the patient’s threshold, thus offering less positive feedback to the patient.
· SITA FASTER: is approximately 50% faster than SITA Standard and about 30% faster than SITA Fast.8 This strategy decreases stimulus presentation delay and makes modifications to the starting stimulus intensity and staircase reversals to arrive at the sensitivity threshold more quickly. With SITA Faster, the average test time was around 2 minutes in eyes with early glaucomatous field loss and was sometimes even shorter in eyes with normal fields.8
· SITA FASTER 24-2C: includes more information in the central 10 degrees, where macular visual field defects reside. The 24-2C pattern combines all 24-2 points plus ten selected from the 10-2 pattern that cover areas known to be susceptible to glaucomatous defects both from structural and functional studies.9 The SITA Faster 24-2C test takes about 20% less time than the SITA Fast 24-2 test10. An example printout from a SITA Faster 24-2C is included below (Figure 2).
Pupil size should be consistent across tests. Pupils <2mm and >6 mm may influence the testing outcome by introducing artifacts due to light diffraction or induced aberrations.
To perform the test accurately, the refractive error of the eye must be corrected with lenses in addition to the necessary spherical power addition for near, as determined by the patient’s age and the diameter of the perimeter’s cupola. The refraction used is listed at the top of the printout. Uncorrected refractive errors cause defocusing of the test target and apparent depression of retinal sensitivity. Each diopter of uncorrected refraction causes a 1.26-db depression of retinal sensitivity. Astigmatism of more than 1.25 D should be corrected in addition to the sphere measurements to prevent depression of sensitivity as well. Further, these lenses must be positioned properly to prevent artifactual defects caused by the rim of the lens.
Age adjusted charts are available to technicians to correct for the target distance (30 cm). The machine will also calculate the appropriate lens add needed if the patient’s age and distance correction are recorded into the machine.
The correct age of the patient must be input into the machine and is essential since the patient’s responses are compared to age matched controls during test analysis. After age 20 there is a loss of differential light sensitivity of 0.6 db per decade which is more pronounced in the peripheral visual field. There can be a significant change in the probability plot as the patient crosses into a new decade. Errors in listed age can cause significant errors in analysis and reporting.
The threshold sensitivity map expresses the patient’s responses in decibels. The dBs tested by the Humphrey analyzer range between 0 and 50 db (0 is the brightest and 50 is the dimmest). Higher numbers mean the patient was able to see a more attenuated light, and thus has more sensitive vision at that location. For instance, a value of 0 means the patient could not see the brightest target and a 50 means the dimmest target was seen. The software analyzes this information and gives age adjusted significance in the total deviation and pattern deviation plots, described further below.
The grayscale in the upper right of the report is useful for patient education but should not be used to interpret the visual field. Lighter regions indicate higher sensitivity and darker regions reflect lower sensitivity. These are not compared to a normative database. The gray scale and probability plots may not be consistent. Thus, the raw threshold data should always be assessed in conjunction with the grayscale representation.
Total deviation and probability plot
Total deviation map and the corresponding probability plot compare the patient’s responses with that of known normal patients of corresponding age to highlight test locations that are outside normal limits. It is useful to compare with age-matched normal thresholds as sensitivity normally decreases gradually with age. The total deviation numerical map compares the patient’s threshold sensitivity to age-matched control patients. Positive values represent areas of the field where the patient can see dimmer stimuli than the average individual of that age. Negative values represent decreased sensitivity from normal.
The total deviation probability plot highlights deviations when they are worse than those found in the bottom 5%, 2%, 1% or 0.5% of threshold sensitivities compared to age-matched control patients. A key showing the meaning of the symbols is shown near the bottom of HVF report. The legend contains different colored boxes that correspond to the degree to which that test point differs from the age matched control. The darkest boxes indicate the worst depression, seen in less than 0.5% of the normal population.
Pattern deviation and probability plot
The pattern deviation map and probability plot is considered one of the most useful pieces of analysis on the Humphrey Visual Field report.7 Certain condition, such as cataracts, cause a generalized depression of the field of vision and could mask underlying defects such as subtle changes secondary to glaucoma. The pattern deviation factors out these generalized depressions leaving only glaucomatous defects for analysis. Again, decibel deviations from the expected values are shown in the upper numeric plot, while the statistical significance of these deviations is shown in the below probability plot. The pattern deviation probability plot uses the same symbols as the total deviation plot to identify points deviating by statistically significant amounts from age-matched control patients.
When examining these plots, it is important to carefully examine the depressed points and pay particular attention to the number of contiguous depressed points, the location, and the severity of the depressed points. Clustering of change among adjacent data points in the best way for the practitioner to evaluate areas of possible progression.
Glaucoma hemifield test (GHT)
The glaucoma hemifield test (GHT) compares similar areas in the superior and inferior visual fields to one another and reports several different outcomes: outside normal limits, borderline, general depression, abnormally high sensitivity, or within normal limits. Specifically, it compares visual field sensitivities in 5 pre-determined zones (shown in the Figure 3 below) in corresponding areas of the superior and inferior hemifields. “Outside normal limits” indicates that the upper and lower fields are significantly different, in a pattern that was found in less than 1 percent of normal controls. This suggests that glaucoma is more likely. “Borderline” indicates that the upper and lower fields were different to the extent found in less than 3% of normal controls. “General depression” or “abnormally high sensitivity” are presented when the test point locations are either so low or so high that they are at levels seen in fewer than 0.5% of normal subjects. As glaucoma frequently causes asymmetric defects between the superior and inferior hemifields, the GHT can be a valuable indicator of glaucomatous visual field defects. However, the GHT was not designed to be sensitive to neurological or retinal visual field loss.7
Visual Field Indices
· Visual Field Index (VFI): is a more recently developed index, designed to be less affected by cataract and also to provide improved correspondence to ganglion cell loss compared to MD.7 VFI is approximately 100% in normal fields and approaches 0% in fields with severe glaucomatous damage.
· Mean deviation (MD): is a weighted average of all the numbers in the total deviation plot. It indicates the overall deviation of the visual field from normal. Numbers that are positive indicate an “elevated” field, seeing more than would be expected for an age matched control. Negative numbers indicate a “depressed” field that is seeing less than an age matched control. A p-value is given for the probability that the mean deviation would occur in a normal individual. MD does not account for global depression from other sources such as cataracts or vitreous hemorrhage. Typically, an MD of -2.00 or less could indicate glaucoma. The difference in the mean sensitivity between a patient's two eyes is usually less than one dB difference 95% of the time and less than 1.4 dB 99% of the time. Inter-eye differences greater than these values are suspicious if they are unexplained by non-glaucomatous factors, such as unilateral cataract or miosis.
· Pattern standard deviation (PSD): is a measurement of the degree to which the shape of the patient’s measured field departs from the normal, age-corrected reference field. A low PSD indicates a smooth or more normal hill of vision. A high PSD indicates an irregular hill or nonuniform sensitivity loss, which indicates that the sensitivity loss is not due to diffuse depression from cataract or vitreous hemorrhage. PSD characterizes localized changes in the visual field. It indicates the degree to which the numbers differ from each other and calls attention to scotomas. In the Ocular Hypertension Treatment Study, PSD was identified as one of five important contributors to the risk of developing glaucoma.11 In advanced glaucoma, PSD can actually normalize once damage is no longer focal.
A gaze tracker is printed at the bottom of the print-out. During testing, the pupil is tracked. The gaze tracker uses the pupil to measure how often and how far from fixation the patient looked during testing. Upward spikes indicate eye movements away from the fixation target and the magnitude of the spike indicates the degree of the deviation, up to an angle of 10 degrees. Downward deviations indicate that the gaze tracking system was unable to detect the patient’s pupil. Large downward deviations are usually from patient blinking or ptosis.
Five Rules for Visual Field Interpretation: “The 5 Rs”
We propose five critical steps to interpreting visual field reports and describe them further below:
1. Right test
3. Review probability plots
4. RNFL pattern of loss
5. Re-affirm the diagnosis
To start, always take your time when analyzing visual field reports as significant information can be gleaned from them. Place the right eye report on the right side, next to the left eye report which should be on the left. Compare the current test with prior tests and educate your patient on the findings when possible.
Remember that even with improved testing strategies, these remain subjective tests. Confirmation of a new defect or worsening of an existing defect by repeating the visual field test is usually necessary to validate the clinical implication of a visual field, in conjunction with other pertinent clinical data.
Rule #1: Right Test
· Confirm patient’s name and age, ID number, and date of examination. Recall that patient age is critical because the normative database that is referenced is based on age. There can be a significant change in the probability plot as the patient crosses into a new decade.
· Identify which testing algorithm (24-2, 10-2 etc) and strategy (SITA Standard, SITA Fast etc) was used and whether this is consistent with prior tests. Be careful when comparing tests which used different algorithms or strategies to one another.
· Confirm that the appropriate stimulus size was used for the test.
· Ensure that pupil diameter is consistent when comparing across tests
· Confirm the appropriate refraction was used and is listed.
Rule #2: Reliability
· Review the reliability indices. Understand the fixation loss rate, false positive rate, false negative rate and review the gaze tracker.
· In general, fixation losses, false positives, and false negatives should be less than 20% to 30% for the test to be considered clinically valuable. Use caution when interpreting fields with fixation losses, false positive and false negatives greater than 20%. Fields with reliability indices greater than 33% should be considered unreliable.
o Note that a higher rate of false positives and negatives may occur at the edges of scotomas due to glaucoma, which must be accounted for when evaluating reliability.
· Note the test duration. Tests that are abnormally long or short for the particular testing strategy used may be unreliable.
Rule #3: Review the probability plots
· The threshold sensitivity map should be studied next. A value of 0 means the patient could not see the brightest target and a 50 means the dimmest target was seen. Most values are around 30 dB, and any numbers below this range imply a possible visual field defect. A value of 40 dB or higher on this graph indicates the patient may have been “trigger happy” or have a high false positive rate.
· Review the mean deviation (MD), pattern standard deviation (PSD), and the Glaucoma Hemifield Test (GHT) indices. One widely accepted criterion for establishing a visual field defect is the presence of a PSD with p< 5% or if the results of the GHT are outside normal limits (P<0.01). This criterion was used in the Ocular Hypertension Treatment Study (OHTS).12
· Examine the two probability plots at the bottom of the visual field report: the total deviation and the pattern deviation. The best place to look for early glaucomatous visual field defects is the pattern deviation plot rather than the grayscale or total deviation plot. The presence of a cluster of at least 3 abnormal points (p<5%) on the pattern deviation plot, with at least 1 of those points with p< 1%, has also been used as a criterion for a visual field defect.13
· Recall that the grayscale in the upper right of the report can be used to educate patients on their disease but should not otherwise be used to interpret the visual field as the gray scale and probability plots may not be consistent.
Rule #4: RNFL pattern of loss
When evaluating for glaucoma, the visual field defects should correspond to specific retinal nerve fiber layer defect patterns. In general, glaucomatous visual field defects are almost always localized initially, respect the horizontal meridian, begin nasal to the blind spot, and are almost always detectable within the central 24 to 30 degrees of vision.13 The pattern deviation plot should be consistent with glaucomatous damage of nerve fiber layer bundles.
· Nasal step (Figure 4): Focal nasal visual field loss is common in early glaucoma, which is why the 24-2 visual field preserves 30 degrees of the nasal visual field.
· Arcuate scotoma is another common pattern of visual field loss in glaucoma. An inferior arcuate scotoma is demonstrated in Figure 5.
· Paracentral scotoma: Paracentral scotomas may be small and difficult to appreciate on a 24-2 test and may be better defined on a 10-2 test due to emphasis on the central visual field. A comparison of a scotoma seen on a 24-2 test compared to a 10-2 test in the same eye is demonstrated in Figures 6a and 6B. These defects can progress to form an arcuate appearance with time.
· Altitudinal defect: Hemifield loss is common in advanced glaucoma, but can be seen in other conditions such as retinal vessel occlusions or ischemic optic neuropathies.
· Generalized depression: Can occur in advanced glaucoma. However, cataracts can create an artifactual depression of the visual field as well. in such cases, after cataract surgery, the mean deviation may decrease in magnitude, and the pattern deviation may increase as more focal glaucoma defects are revealed.
· Temporal wedge defects can be seen in advanced glaucoma however, by themselves, temporal defects are rarely due to glaucoma.
The superior and inferior poles of the optic nerve are most susceptible to glaucomatous damage. Combinations of superior and inferior visual field loss, such as double arcuate scotomas may occur, resulting in profound peripheral field loss. Typically, the central island of vision and the inferotemporal visual field are retained until late in the course of the disease.
Rule #5: Reaffirm the diagnosis
Re-assess the retina and optic nerve to seek consistency with the visual field. It is important to confirm an abnormal field even in experienced visual field test takers. In the OHTS, 85.9% of abnormal fields reverted to normal on subsequent testing.14 Re-frame the disease severity if appropriate. Remember that substantial structural damage may exist by the time visual field loss is detected by standard automated perimetry.
Common Errors and Artifacts
There are several common artifacts that can be seen on automated perimetry and should be identified to avoid pitfalls in interpreting the visual field results
· Lens rim: If the patient’s corrective lens is decentered or set too far from the eye, the lens rim may project into the central 30 degrees. Traditionally this is seen as a ring-shaped scotoma around the edge of the field all of equal depths of depression (Figure 7).
· Incorrect corrective lens: If an incorrect corrective lens is used, the resultant field will be generally depressed.
· Eyelid artifact: Partial eyelid ptosis may lead to an artifactual superior visual field defect which should improve with lid taping or manual lifting during subsequent tests.
· Cloverleaf pattern: If a patient stops paying attention and ceases to respond partway through a visual field test, a distinctive visual field pattern may develop in a cloverleaf pattern. This is the result of the testing order of the Humphrey perimeter which begins with points clustered in the central areas of each quadrant and subsequently proceeds outwards.
· High false positive rates: Recall that if a patient responds when no stimulus is presented, a false positive is recorded. False positive rates greater than 20% suggest an unreliable test that can mask or minimize an actual scotoma and can, in extreme cases, result in a visual field with impossibly high threshold values. In such cases they gray scale may look “lighter” than normal, or “whited out” (Figure 8). Careful instruction to the patient may sometimes resolve this.
Future Directions in Visual Field Testing
Recent advances in visual field testing include creating devices that enable perimetry testing with a smaller physical footprint and may even allow for at home visual field testing. Both Virtual Field and Olleyes (links below) are examples of newer devices that uses a virtual reality headset to allow for mobile perimetry in the clinic setting and potentially at home. Similarly, Micro Medical Devices has also developed a mobile perimeter in a virtual reality headset that can be used in the clinic or at home, which is currently undergoing further clinical study. M&S Technologies has launched a web-based visual field testing platform that enables visual field testing in the clinic or in patients’ homes using a computer or laptop. These newer devices are not in widespread clinical use, but may have intriguing future applications in situations such as the COVID-19 pandemic where coming to a clinic appointment can be unsafe for certain patients and in low resource settings.
Virtual Field: https://home.virtualfield.io/
Micro medical Devices: https://micromedinc.com/our-devices/palmscan-vf2000-visual-field-perimeter/
M&S Technologies: http://www.mstech-eyes.com/products/category/melbourne-rapid-fields