Basic Perimetry

of vision. Since sensitivity is maximum at the fovea, the height of “the hill island of vision”

(z) is also maximum at the fovea. The retinal sensitivity drops to sea level 15 degrees temporal to fixation (blind spot).

Types of Perimetry

Kinetic Perimetry

Perimetry aims to draw the map of the island of vision, such that it is a true representation for each eye and also aims to present it in a way which is clinically useful. Earlier methods defined the outer limits of the visual field by moving objects from the non-seeing area to the center. This technique of perimetry, called kinetic perimetry, it utilizes a moving object of a fixed size and intensity (e.g. Tangent screen or Goldmann perimeter) to define the boundary of the island of vision at a fixed height. This line representing the outer boundary for a given size of the test stimulus is called isopter. An isopter is synonymous to a horizontal slice through the hill island of vision.

Manual kinetic perimetry allows large areas to be traversed in a fairly short order. One can move quickly over areas of little interest and spend relatively more time in examining critical regions. Equipment is inexpensive and durable. Since the perimetrist is constantly communicating with the patient, the patient is more comfortable. However, reproducible and reliable examinations require technical skill and early or subtle changes are more likely to be overlooked on manual kinetic perimetry. Isopters which are stylized representations of the visual field, making quantification and statistical analysis difficult.

Static Perimetry

The outer boundary of the island of vision can also be determined by measuring the retinal

Basic Perimetry 129

sensitivity (z) at each point (x,y). This technique of perimetry is called static perimetry because the test location is fixed, while the intensity of the test object of known size is varied, e.g. Tubinger, Octopus and Humphrey perimeters. Static perimetry provides a vertical slice through the hill island of vision.

Because of the difficulty, inability and a potential for lack of reproducibility with kinetic perimetry, static perimetry is preferred for detecting and following subtle non-geographic defects in the diagnosis and follow-up of glaucoma patients. One can perform effective static perimetry with the tangent screen or the Goldmann perimeter. However, manual static perimetry is tedious, cumbersome and at times boring. Both the patient and the examiner find it difficult to concentrate for 30 to 90 minutes at a stretch. Automated /computerized perimetry presents targets at a random sequence undecipherable by the patient. It can test the same patient with the same methodology year after year and still does not get bored. Kinetic testing is difficult to computerize particularly with regard to the decisions regarding same speed and direction of presentation. A static test, on the other hand is relatively straight forward, since the target does not move, the machine has only to choose a site, target intensity and then record whether the patient responds, yes or no.

Computers have revolutionized perimetry by allowing precise repetition and meticulous attention to detail, testing the patient’s response under optimal conditions repeatedly by allowing a binary yes/no answer from the patient. All this makes perimetry tailor made for computerization.

Stimulus Presentation

During static visual field measurement the stimulus can be presented by projection or nonprojection. In the projection system a simple

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computer video monitor is used to present dark or light combinations of stimuli against a diffuse background. This system has the advantage of being more flexible and allows kinetic color perimetry. Drawbacks pertain to the mechanical aspects of presenting and moving the test target such as mechanical failures, periodic maintenance and servicing. Also, the combination of mirrors, shutters and the rotational unit produces an unsuitable clinking noise with each projection. This was used to advantage in earlier models, to assess the reliability of a given field (false positives). Newer models elicit the false positive response by omitting the light stimulus and assessing the pace of the patient to the rhythm of the testing.

In the non-projection system stimuli are generated by the turning on and off of Light Emitting diodes (LED) which are placed into the surface of the perimetric bowl. Advantages of LEDs include silent operation, no moving parts, multiple stimuli presentation and inexpensive and durable equipment. However, in the LED system stimuli are fixed in the bowl surface at the time of manufacture, inability to vary stimulus size and color, test site location or resolution pattern. Further fixed LED positions cannot be expanded to accommodate new programs. LEDs have a condensed light output. Slight variations in positioning and mounting of the LED result in different directional light intensities. All LEDs need to be calibrated individually. This needs to be done at the factory and on a routine basis.

In the non-projection system a high resolution, flat video monitor can also be used to present the stimuli. In this method, the patient fixes on a pseudo-infinite target and stimuli are presented throughout the visual field. With this method of presentation, test site location is infinitely variable, kinetic perimetry is possible, and stimulus presentation is without the audible click. Additionally the video monitor projection does away need for a perimeter bowl and the projection

device allows greater flexibility and durability. They also occupy less space. However, video monitor systems are able to assess only the central 30 degrees.

Projected stimuli are usually white and of variable size and intensity. The size of the stimuli in automated perimeters is similar to that used for Goldmann perimeter. There are five different sizes designated by Roman numerals I to V. One very often uses stimulus size III. Failure to recognize target size III necessitates testing with stimulus size V. However, tests using stimulus size V cannot be processed statistically by STATPAC 2 on the Humphrey perimeter.

In static perimetry, the patient has to respond to a stimulus of predetermined size, color and location projected for a fixed duration at a given intensity level. The patient responds with the button in two ways: stimulus seen or stimulus not seen. Any such response is only suggestive but not actual proof, that the light was seen or not seen. For a stimulus of a fixed size and location to be seen depends on its intensity. This probability of a stimulus of fixed size and location when plotted against the intensity of the stimulus is called probability of seeing curve. That is to say, the intensity of the stimulus where it is seen 50% of the time and missed 50% is called threshold. Similarly, the intensity at which the stimulus is seen 95% of the projected times, is called suprathreshold. A low intensity stimulus which is seen only 5% of the times when projected is called infrathreshold.

Bracketing

Determining the threshold for each point in the field would require thousands of stimulii of varying intensity. However, the number of stimuli for threshold determination has been conveniently reduced by a testing algorithm which is also accurate. At a given point on the visual field, the patient responds to a given stimulus

Fig. 9.2: The probability of seeing curve

Fig. 9.3: Threshold determination at the point P (Staircase technique)

Basic Perimetry 131

intensity (P1). The intensity of the stimulus is then decreased in steps of 4 dB till the stimulus is not seen (3). The threshold lies between 2 and 3. The intensity of the stimulus is then increased in steps of 2 dB till the patient is able to perceive the stimulus. Herein the threshold for the point is lying between 4 and 5, and is a more accurate assessment of the threshold value at that point. This technique of threshold determination is called 4-2 bracketing (Staircase technique). In the Octopus perimeter, the thresholding strategy continues, until a third reversal, in steps of 1 dB, called 4-2-1 algorithm (Fig. 9.3).

Normal threshold values are dependant on the location of the point on the visual field and also the age of the patient.

Fovea, the most sensitive point of the visual field corresponds to 0 degree of eccentricity. As the point moves from the fovea, the threshold value (sensitivity) decreases by 0.3 dB for every

132Diagnostic Procedures in Ophthalmology

degree of eccentricity outside the macula. Sensitivity drops to zero, 15 degrees temporal to fixation (blind spot). Sensitivity also decreases with the age; 0.6-1dB per decade of life. Since, threshold is age-related, the patients date of birth should be correctly entered as the results are compared to age-matched normals.

The intensity of light which reflects off the surface is expressed as apostillbs (unit of luminance). The sensitivity of the human visual system varies from 1 to more than 1,000,000 apostillbs(asb).Themaximumstimulusintensity of the Octopus Field Analyzer is 1000 asb and for theHumphreyFieldAnalysisis10,000asb.Hence, large numbers representing the listed sensitivity on the printout would be cumbersome. A convenientwayofexpressingthresholdvaluesis in terms of a relative logarithmic scale where the intensityofthestimulusisvariedbypowersof10

1

1dB = ————————

log unit (asb)

Increasing dB numbers on the printout imply that dimmer stimuli have been perceived. Thresholds corresponding to a dimmer stimulus mean greater retinal sensitivity. In a report of the measured thresholds, large dB values correspond to better sensitivity and small dB numbers indicate reduction in sensitivity.

Testing Strategy

With the inherent ability to vary the intensity of the light stimulus, static perimeters explore the visual field in three ways:

1.Suprathreshold screening.

2.Threshold related screening.

3.Full threshold determination.

Suprathreshold screening: Very bright stimuli (suprathreshold) intense enough to be seen easily by most normal people are presented. The patient has simply to respond (yes / no) to the presence

of the target. The role of such examinations is related to quick screening of large populations and also to define gross pathology quickly. However, such examinations can miss early changes suggestive of glaucoma.

Threshold related screening: Herein, the intensity of the light presented is 5dB brighter than the actual threshold at the test point in question. This allows the entire field to be screened quickly. Threshold related screening is at best a variant of suprathreshold tests which allow for an approximation of the true sensitivity of the visual field. It can be used as a screening test for detection and follow-up of known pathologies.

Threshold determination: A more time consuming way of determining the sensitivity of the visual field is by determining the threshold value at each point by the bracketing technique described earlier. After presenting a light stimulus the machine waits for a yes / no response. If the stimulus is not seen, the intensity of the light seen is increased in steps of 4dB till it is visible (machine records this as suprathreshold level). Subsequently, light stimuli are decreased in steps of 2dB till the stimulus is not seen (infrathreshold). The actual threshold is between the suprathreshold and infrathreshold.

Newer Strategies

Threshold determination at each point of the visual field is tedious and time consuming. Because by definition threshold is tested by the staircase algorithm, where every patient can see only half of the stimuli presented, newer techniques aim to make the procedure as short as possible, to ensure that the patient maintains concentration and thus provides better reliability.

Swedish Interactive Thresholding Algorithm (SITA) is similarly based on the fact that a response at one location has implications at the point tested

and also its neighboring points. Just as one tested point is normal, other points on the visual field are likely to be normal too.

Tendency Oriented Perimetry (TOP) is available on the Octopus perimeter and takes advantage of each response of the patient five-fold. It tests and adjusts the location where the stimulus is presented and assesses the threshold of the four neighboring locations by interpolation.

Several threshold tests are available on the two commonly available Octopus and Humphrey perimeters. In each test a certain number of points can be tested. The number of points tested in a given test is actually a compromise between the time applied and precision, which depends on the type of damage looked for as well as the diagnostic and therapeutic implications resulting therein. The response at each thresholded point is compared with a group of normal individuals. The likelihood of such a response in this population of normal patients is expressed as a probability symbol for each tested point. These probability symbols increase in significance from a set of 4 dots to a black box, p<5%, <2%, <1% and 0.5%. A black box indicates that few normal subjects will have that score; it does not necessarily correspond to an absolute defect. Many points with p<0.5% are relative defects; their actual threshold is available from the raw data.

Test Programs

The standard programs on the Humphrey are the 30-2, 24-2, 10-2 and the macular grid program. In the 30-2 the central 30 degrees of the visual field are tested. It consists of 76 points 6 degrees apart on either side of the vertical and horizontal axes, such that the innermost points are three degrees from fixation. In the 24-2 program 54 points are examined. It is near similar to the 30-2 except the two peripheral nasal points at 30 degrees on either side of the horizontal axis are included while testing the central 24 degrees.

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The 10-2 program tests 68 points 2 degrees apart in the central 10 degrees. This program helps to assess and follow-up fixation characteristics in patients with an advanced disease along with the macular test which examines 16 points in the central 5 degrees, each being 2 degrees apart. The efficiency and results of an examination are reflected by the location of the points tested.

The two commonly used programs on the Octopus are the G1X and the G2 which test 59 locations in the central 30 degrees. Here the test points are concentrated in the central field, arcuate region and nasal midperiphery to maximize detection of significant changes. Fixation characteristics are assessed with the macular program M2X which tests 45 locations in the central 4 degrees, which are 0.7 degrees apart.

Automated perimetry provides a large amount of data which is quantifiable, reproducible and amenable to statistical manipulation. However, the magnitude of the data makes interpretation complex, but a logical, consistent and sequential approach helps to make this less complex.

The earliest injury in open-angle glaucoma is localized to the nerve fiber bundle, usually in the paracentral nasal region. The initial defect may be seen as a fluctuation in a cluster of points or as a relative defect with normal surroundings. This small area of increased scatter or threshold instability is often overlooked at the initial examination, since it does not meet the criteria for a valid visual field loss. Based on the other clinical data, a subtle area of unstable sensitivity may be suspected as being glaucomatous. It becomes more manifest when progression occurs and a serial review of fields shows that the area in question has changed with time. Progression of visual field defects occur in several ways – increase in density of scotomas, expansion of areas of depression and the development of new ones. Uncontrolled glaucoma will eventually affect all areas of the field.

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The challenge in automated perimetry is to locate and document areas of subtle glaucomatous damage and carefully follow any progression. Finally a diffuse generalized depression affecting the entire visual field is rarely associated with early glaucoma, and is usually due to other conditions such as cataract or uncorrected refractive errors. Visual fields are usually analyzed by using a printout that contains different elements of data. Although several different visual field analyzers are in current use, there are sufficient similarities in the printout to permit interpretation and comparison of the results. However, difference between instruments does not permit direct correlation of their absolute scores.

Gaze monitoring is a high precision gazetracking system on the newer Humphrey models which uses real time image analysis to measure the distance between the center of the pupil and the first corneal reflex. It is unaffected by head motion. A continuous record is available on the printout.

An upward deflection is indicative of eye movement during stimulus presentation. Downward deflections imply that the gaze could not be detected. The 750 model of the Humphrey perimeter also offers head-tracking wherein the chin rest is automatically moved in increments of 0.3 mm to bring the head back to the initially gaze tracked position and the Vortex monitor wherein a beep as well as a message is produced on the screen when the patients head moves back by more than 7 mm.

Statistical Analysis

The Statpac program introduced first in 1987 and then upgraded to Statpac Plus in 1988, was derived from a group of normal patients and helped answer the question: Are the field in question normal or not? It introduced the Global Indices along with the Single Field Printout,

Change Analysis and the Overview format. In 1989 Statpac-2 was introduced. It was formed from a database of patients known to have visual field loss due to glaucoma which was otherwise stable.To detect early changes of glaucoma, groups of points in the superior and inferior hemispheres were also compared to produce the Glaucoma Hemifield Test.

An interpretation of the single visual field performed with the Humphrey visual field analyzer (Humphrey Instruments, Inc, San Leandro, C.A) and the Octopus 1 –2 – 3 visual field analyzer (Interzeag AG , Switzerland) is presented.

Components of Automated Visual Field

Humphrey Single Field Printout

There are eight parts to the single field printout (Fig. 9.5). Each has to be examined serially before drawing a conclusion.

First assess the reproducibility ( Zone-1 ) of the concerned fields (Consistency). At the onset, check the printed information at the top of the page, to ensure listing of the correct patient, the type of test done (30-2, 24-2, 10-2), eye in question and date of birth (the software package statistically compares the patients response with age corrected normal population). The recorded visual acuity, refraction and pupil size are important parameters as they all can affect the data. When pupils are miotic, or smaller than 2.5 mm, dilatation is required so as to prevent generalized depression from occurring. The decision to dilate patients with large pupils rests with the clinician, but consistency for all visual fields must be maintained.

Next scans the reliability indices (Zone-2).

Fixation losses are noted as the ratio of the number of times the patient responded when he saw a target placed in the blind spot against the total number of times fixation was tested. In automated

136Diagnostic Procedures in Ophthalmology perimetry fixation is assessed and monitored by

i.Sensors,

ii.Closed circuit TV monitors

iii.Heijl-Krakau method.

Sensors are used to detect minute shift in eye position. They are highly sensitive to slight movement in eye position but are expensive, too sensitive, such that insignificant physiological fixation shifts induced by respiration, systole and involuntary head movements get registered as fixation losses.

Closed circuit TV monitor displays the image taken by an infrared camera. This allows the examiner to view the patient’s eye and judge and assist in fixation quality. Advantages of this system are continuous monitoring of fixation throughout the test with no extra time spent in monitoring fixation per se (Blind Spot Projection Technique). However, continuous video monitoring is expensive, prone to hardware failure and there exists a potential for the machine to disregard fixation losses in patients with fairly good but not excellent fixation.

Heijl-Krakau method: In this method, the machine assumes or plots the blind spot at the beginning of the test and then retests after every eight to twelve stimuli by projecting a suprathreshold stimuli in the blind spot. A positive response indicates fixation loss. This, however, does not work well when significant field loss is adjacent to or involving the blind spot.

When fixation losses are more than 20%, it is bracketed (XX) and is indicative of questionable reliability. However, not all fixation losses are due to unsteady gaze. A “pseudo-loss” of fixation is seen when there is an improper location of the blind spot, or when the initial blind spot is present near the edge of a scotoma, so even though it is presented throughout the test, it is occasionally visible. Also, a head tilt or change in head position occurring during the test will lead to a faulty blind spot location. Finally a

patient who is continually responding even when a light is not flashed will have a number of fixation losses. For these reasons the fixation loss score is not considered in isolation, but rather compared to the other reliability scores.

False positives (FP) result when the patient responds to the audible click of the perimeter with no stimulus projected (trigger happy). It is also expressed as a ratio of the number of times the patient responds to a pause in the testing sequence without presentation of the target against the total numbers of pauses. It is the single most significant reliability indicator. Bracketing occurs when FP’s are 33% but often 15-20% rate can also destroy the credibility of a field. A high rate can also occur due to a poor understanding of the test requirements by the patient. A high FP ratio , will be accompanied by a high positive mean defect, white areas on the gray scale indication of very high threshold levels (white scotomas), a high number of fixation losses and a message of abnormally high sensitivity on GHT.

False negatives (FN) are expressed as a ratio, and occur when the patient does not respond when a point previously thresholded is retested with a brighter stimulus. High FN ratio occurs when the patient tires as in the later part of the examination, when he changes his internal criterion on whether or not he sees a point or when the edge points of a scotoma are tested. A 33% FN ratio is considered excessive and makes the test suspect. However, the presence of a scotoma and a high number of FN, with all other reliability measures being normal, is indicative of a reliable field.

Foveal threshold measures over 30 dB for a visual acuity of 6/12 or better. A normal foveal value and a poorly recorded acuity indicates need for a refraction or mild amblyopia. Likewise a good visual acuity and a depressed foveal value suggest early damage.

The Gray scale (Zone-3) is a rough indicator of the extent of field damage, but can be misleading. Each point on the gray scale is represented by a symbol of varying darkness which corresponds to the threshold level at that point. These are not indicative of disease. A normal elderly patient will have a darker gray scale than a younger patient because of reduced sensitivity in aging eyes. Additionally, there are a fewer points tested in the periphery, each of which occupies a larger space on the gray scale. For these reasons, the gray scale should not be the sole criterion for assessing the visual field.

The Total deviation plot (Zone-4) is created by subtracting the actual raw data from the expected value for age matched controls, at each point. This depending on whether the patient did better or worse than expected is expressed as a positive or negative number. The corresponding probability symbols seen below the data indicate the statistical probability of finding such a point in normal subjects. These probability symbols increase in significance from a set of 4 dots to a black box, p<5%, <2%, <1% and 0.5%. The presence of a black box indicating that a few normal subjects will have that score, it does not necessarily correspond to an absolute defect. Many points with p<0.5% are relative defects their actual threshold is available from the raw data.

The Pattern deviation plot (Zone-5) based on further calculations, is derived from the total deviation data and the overall depression of the visual field. It highlights focal changes which are concealed within diffuse changes, after making adjustment for the height of the hill of vision. Whereas the statistical significance, expressed as probability symbols, is measured for each point, the total deviation and pattern deviation probability maps are analyzed by taking the entire field into account and identifying how clusters of affected points occur, the number of points involved, their density and location.

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The Pattern and Total Deviation need to be compared and a difference if present should be explained. Corneal opacity, cataract and small pupil are the usual causes.

Raw data / numeric data (Zone-6): It is the actual threshold score for each thresholded point. Areas flagged in the Pattern and Total Deviation plot should be inspected carefully for confirmatory signs like double thresholded points of abnormal or foci of high local fluctuation. This should be followed by a geographic survey of the entire numeric data.

Global indices (Zone-7) are presented in the lower right hand corner of the printout and include:

Mean deviation (MD): It is the weighted score of all the points on the total deviation plot. It takes into account both the severity of loss and amount of field affected. A positive MD indicates that the patient scored better than expected for his age, a negative number indicates that the score was worse than expected.

Pattern standard deviation (PSD): It measures the extent to which the damaged points vary from the expected hill of vision (localized loss).

Short term fluctuation (SF): Though listed under global indices it is a good indicator of intra test reliability. It measures the variation at each point on repeated thresholding in the same test. A SF from a patient with poor reliability scores is high, further indicating a poor test taker.

Corrected pattern standard deviation (CPSD): It is calculated with the help of SF to adjust the PSD. It is a more accurate indicator of the extent of damage.

Glaucoma Hemifield test (Zone-8) is a sophisticated analysis of 5 geometric point clusters in the superior and the inferior arcuate regions whose probability maps are compared with one another. It is very sensitive and specific at detecting asymmetry between these regions as well as symmetric deviations from normal data. The GHT can be within normal limits,

138 Diagnostic Procedures in Ophthalmology

Fig. 9.6: Glaucoma hemifield test

outside normal limits, borderline sensitivity, generalized reduction or abnormally high sensitivity (Fig. 9.6).

Octopus Single Field Printout

The commonly used seven in one printout is near identical to the Humphrey single field printout (Fig. 9.7). Again, a systematic and sequential approach helps in interpretation. As before there are eight parts to the single field printout. Each has to be examined serially before drawing a conclusion.

Reproducibility (Zone-1): Before looking at other components of the printout, one needs to identify the field in question to the concerned patient. One quickly checks the name and date of birth as printed on the upper part of the printout. Test parameters such as the size of the stimulus and pupil, type of test strategy, and test program used are also looked at along with the eye and date of examination.

Reliability factors (Zone-2): To assess the reliability of the concerned examination, one assesses the catch trials just beneath zone I and

also the reliability factor listed below, with the visual indices. False positives (FP) result when the patient responds to the audible click of the perimeter with no stimulus projected. It is also expressed as a ratio of the number of times the patient responds to a pause in the testing sequence without presentation of the target against the total numbers of pauses. False negatives (FN) are expressed as a ratio, and occur when the patient does not respond when a previously thresholded point is retested with a brighter stimuli. Each of these should be less than 10%. The reliability factor (value) is determined by the outcome of the catch trials and ideally it should be less than 15%.

The Octopus 1-2-3 takes a video photograph of the pupil and stores this in its memory. If the eye deviates or the lid closes, the machine registers the loss of fixation and disregards the patients response till fixation is restored. Loss of fixation for more than two seconds halts the program. Hence the Octopus printout does not document fixation losses.

Gray scale (Zone-3): This is the most colorful part of the printout but like its counterpart in the Humphrey single field, it is the least informative since it is obtained by the interpolation of the actually tested sensitivities. Lighter colors are suggestive of higher sensitivities and darker areas suggest depression. Hence only a cursory look is required. Black depicts an absolute loss of sensitivity.

Comparison (Zone-4): It is synonymous with the total deviation plot on the Humphrey single field printout. The lower left part of the printout, one on top of the other, is the comparison display, with a numeric display above a probability map. The comparison values are the difference between the patients test results and age-matched normals. The ‘+’ symbol indicates a normal sensitivity. The probability map is displayed graphically below this. Defects are marked as symbols of different shades. Darker the marking,

140Diagnostic Procedures in Ophthalmology

less likely it is to being normal. Values tagged as ‘p’<0.5% mean are less than 0.5% of the normal population may show such a defect without it being significant. Comparison values represent the total depression of the visual field.

Corrected comparison (Zone-5): It is synonymous with the pattern deviation plot on the Humphrey single field printout. The corrected comparison represents the localized defect after removing the generalized depression of the visual field from the total depression. Similar to the comparison they are represented by values, ‘+’ for normal sensitivity and the probability map is displayed graphically below this.

Numeric data/raw data (Zone-6): They represent the actual thresholded points from which the entire statistical calculation is done. Close to fixation, values are in their late twenties or early thirties. In the mid periphery, threshold values are in their mid twenties and in the late teens in the periphery.

Visual field indices (Zone-7): They were first introduced on the Octopus perimeters in 1985 and include:

Mean sensitivity is the average of retinal sensitivities that are measured at all points.

Mean defect (called mean deviation on the Humphrey printout) is the average defect of all thresholded points from the age-matched normals, as shown in the comparison chart. It is indicative of the height of the hill island of vision.

Loss variance (called pattern standard deviation on the Humphrey printout) is obtained from individual deviations of all measured locations with the mean defect value. These are indicators of localized damage.

Short term fluctuation is a reliability factor suggestive of an intra test variation. A value of more than 2.5 is significant. The difference between individual deviations on the numeric

display is best assessed by removing the normal shorttermfluctuationfromthelossvariance.This gives us the corrected loss variance (called loss variance on the Humphrey single field printout) which is more sensitive to localized defects.

Bebie’s curve (Zone-8): In the presence of a local defect, it is often difficult to quantify an additional diffuse defect in a particular visual field. The cumulative curve was introduced by Dr. H. Bebie in the late 1980’s to help assess the overall condition of the visual field at a glance. In the Bebie’s curve test locations are arranged according to the extent of their difference from the normal values. The individual test locations are arranged on the x-axis, and the defects in decibels on the y-axis. The test locations with the least difference are found on the left side of the figure, while those with the greatest are on the right side. With this graphical representation, it is simple to differentiate localized from diffuse damage.

Analysis of Single Field Printout

After ensuring good reproducibility the visual field is analyzed using each of the eight areas alone or in combination. The reliability indices give an indication of the credibility and accuracy of the fields. The gray scale gives a rough over view of the field, but is not used in the actual field interpretation. Any suspected change must be confirmed by inspecting other parts of the printout. The total deviation and pattern deviation (Comparison and Corrected Comparison on the Octopus printout) should be compared in tandem. A difference between them if seen must be explained. The pattern deviation symbols are used in the interpretation of the field with the arrangement and severity of the points or clusters analyzed. The greater the number of points involved and greater the depression the more severe the defect is. After a quick look at the

numeric data, the global indices (visual field indices on the Octopus printout) are analyzed next, with the mean deviation (mean defect on the Octopus printout) being an indicator of the overall depression of the field. The pattern standard deviation (loss variance) or corrected pattern standard deviation (corrected loss variance) is considered significant when a score of p < 5% is noted. The short term fluctuation is analyzed as a part of the reliability indices and with the total and pattern deviation symbols. The glaucoma hemifield test is analyzed at the end, a reading outside normal limits is significant. The interpretation should also include allowance for artifacts such as drooping lid, prominent brow, or improper positioning of the patient/trial lens. Other mimics of glaucomatous field loss include retinal and neurological disorders along with disorders affecting the clarity of the ocular media. These need to be ruled out by a detailed ocular examination.

The minimum criteria for the diagnosis of glaucoma are listed in Table 9.1.

Criteria should be fulfilled on at least two occasions

Non-characteristic visual field defects (Figs 9.8 to 9.11) must be substantiated by clinical examination of the retina and optic nerve head. The first visual field test in an inexperienced patient should be taken with caution. After first test the patient becomes more proficient; the resulting improvement in the visual field is known as learning curve. It is, therefore, desirable to test two or more visual fields before proper interpretation. To be clinically significant, the visual field should be reproducible.

Basic Perimetry 141

While assessing single field printout, the presence of miotic pupil and media opacities should be taken into consideration because they can cause generalized depression of visual field. The interpretation should also include allowance for artifacts such as position of the patient, correcting lens (Fig. 9.12), drooping of the lid and prominent brow. It is not rare to find that visual field changes in neurological disorders (Fig. 9.13) may mimic the glaucomatous field defects.

The visual field examination is a useful tool to study the course of an eye disease as well as to monitor the therapy. Periodic visual field testing is usually recommended for all glaucoma patients especially with a view to evaluate the desired target intraocular pressure. In spite of good control of the pressure, the patient’s visual fields may show deterioration on follow-up (Fig. 9.14) while in some patients the fields remain stationary (Fig. 9.15). Assessment of progression is difficult because of the long-term fluctuations. One needs to repeat the field test when in doubt. In clinical practice the recent fields are compared with the earlier baseline fields to judge the progression.

Conclusion

In conclusion automated perimetry is an extremely useful tool which has also become the standard technique for evaluating the visual field in patients with glaucoma or glaucoma suspects. Interpretation of the results is difficult and requires experience in addition to a detailed understanding of the underlying principles of automatic static perimetry and the applied statistical analysis.

A word of caution is necessary. Automated perimetry should never be used in isolation. Treatment of patients requires combining the results of automated perimetry with an

150Diagnostic Procedures in Ophthalmology

examination of the complete eye especially the retina, optic nerve and visual pathway.

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3.Flammer J. The concept of visual field indices.

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4.Heijl A, Lindgren G, Olsson J. A package for the statistical analysis of visual fields. Doc Ophthalmol Proc Ser 1987;49:153.

5.Humphrey Field Analyzer User’s guide. Humphrey Instruments, Inc. San Leandro, 1994.

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7.Johnson CA, Keltner J. Automated suprathreshold static threshold perimetry. Am J Ophthalmol 1980;89:731.

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