Module 15 

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Module 15:

The Threshold Strategy

Screening Strategies

SITA

SWAP

Functional Field Loss

Binocular Field Testing

Esterman Functional Tests

The SSA Kinetic Test

Fluctuation

Steep and Sloping Margins

The Threshold Strategy

Automated perimeters employ predetermined strategies to test the visual field.  Since it is impractical to test every point in the visual field, each strategy uses a grid of test points that adequately covers the area of the field most likely to show defects in the particular condition being tested for.  The most common example is the Humphrey Visual Analyzer 24-2 threshold strategy for glaucoma.  In this strategy, a grid of 54 points is tested in the central 24 degrees.

Each point in the grid is tested to determine the sensitivity of the retina to light stimulus.  Just as it is not practical to test each point in the visual field, it is not practical to test each point in the grid with a wide range of stimulus intensities.  A threshold strategy was developed to define a standard level of sensitivity and to find that level with the fewest number of trials.

The threshold strategy defines the threshold as being that level of light intensity that the patient responds to 50% of the time.  A stimulus that is bright enough to be easily seen should be seen and responded to100% of time by the patient.  A stimulus that is too dim to be seen by the patient should never elicit a response from the patient.  Somewhere in-between there is a stimulus intensity that will elicit a response from the patient in half of the presentations.  In other words,  if the particular stimulus intensity is presented 10 times, the patient will think she saw the light and she will respond in 5 of the presentations.  In the other 5 presentations the patient will think that she really did not see a light and she will not respond.

If a stimulus is above (brighter) the threshold level,  it is said to be suprathreshold.  If a stimulus is below (dimmer) the threshold level it is said to be infrathreshold. The number of presentation needed to determine the threshold are reduced by moving the stimulus intensities above and below the threshold.  This is called bracketing the presentations.  

• If the first stimulus presentation is suprathreshold, then the computer presents the next stimulus at a level 4 dB lower.

• If the second stimulus is responded to, then the computer lowers the stimulus level once again by 4 dB.

• This lowering of the stimulus level continues until the stimulus is not responded to, indicating an infrathreshold stimulus has been found. This is called crossing the threshold.

• The next stimulus is raised by 2 dB.  If the stimulus is not responded to, then the next stimulus is raised again by 2 dB.  This continues until the stimulus is once again responded to (suprathreshold).

• Thus, the threshold is crossed twice.  After the second crossing has occurred, the trials end.  The threshold value lies between the lowest value of the suprathreshold (seen) stimuli presented and the highest value of the infrathreshold (not seen) stimuli presented.

• For an attentive patient, the average number of presentations needed to determine the threshold is five.

As is evident from the bracketing procedure, the computerized perimeter uses the patient's responses to speed up the testing procedure.  The computer further speeds up the testing process by using normal population curves.  This means that many normal people in each age group are tested with the machine.  This testing produces a "normal" threshold value for each age group at each point in the grid.  The computer uses this information to decide what the initial testing value will be at each point according to the patient's age.  Thus, inputing the wrong age for the patient would slow down the testing procedure.

The Suprathreshold or Screening Strategy

Automated perimeter screening strategies are designed to quickly determine if there is a significant defect present.  If a defect is detected, then a more comprehensive threshold strategy is used to characterize the defect.

Screening strategies use knowledge of the normal threshold values to present only suprathreshold stimuli that are just above the normal threshold values.  If the patient misses a significant number of these stimuli, then the program is considered to have detected a defect that warrants further testing.

The Humphrey Field Analyzers have three different "zone" strategies for each screening grid:

• Two Zone:  The stimulus presented at each point is 6dB higher than the expected hill of vision.  The printout uses a "0" for seen points and a black square for missed points.

• Three Zone:  Same as two zone except missed points are also tested at the maximum stimulus intensity level.  The strategy thus characterizes defects as relative or absolute.  The printout uses a "0" for seen points, an X for no response to 6dB (relative defect), and a black square for no response to the brightest stimulus (absolute defect).

• Quantify Defects:  Same as two zone except missed points are also tested for threshold value.

SITA

SITA (seat-ah) stands for Swedish Interactive Thresholding Algorithm. 

The Humphrey Visual Field Analyzer has become a standard instrument for perimetric testing of the visual fields, commonly performed to detect field losses due to damage to the visual pathways.  Anyone who has monitored patients being tested with the common Full Threshold 30-2 algorithm knows how much patients dislike the test.  They dislike it because it is long and tedious. This leads to fatigue and inattentiveness, which results in an unreliable test.

The SITA algorithm was designed to reduce testing time while still providing an adequate test of visual sensitivity.  Reduced test time should increase attentiveness and result in a more reliable test.

There are two different SITA programs, SITA Standard, and SITA Fast:

• SITA Standard was designed to replace the Full Threshold program (e.g. Full Threshold 30-2).

• SITA Fast was designed to replace Fastpac, which is a simplified Threshold program.

Consider that a Full Threshold 30-2 visual field test on an eye with significant pathology might take 16 minutes to complete.  The same test with SITA Standard would take about 8 minutes, and the same test with SITA Fast would take about 4.5 minutes.  

Examination time can be further reduced by running SITA with the 24-2 pattern instead of the 30-2 pattern.  The 24-2 pattern gives adequate coverage for detecting and following glaucomatous field defects.

Some doctors are not comfortable with using SITA Fast as a standard field test for glaucoma.  They prefer to use SITA Standard as the usual test and use SITA Fast in special situations.  The SITA Fast test can be used for patients to "learn" on.  Once the patient is comfortable with the testing procedure, she can be switched to the SITA Standard test.  The SITA Fast test can also be reserved for the patient who cannot even tolerate the speed of the SITA Standard test.

The SITA algorithm works by using patient responses and the timing of the responses to customize the test for each patient during the testing procedure.  It uses normal visual field models, probability functions, and mathematical analysis to estimate the threshold values.  It can also determine false-positive errors during the course of the test without additional trials.  At the end of test the algorithm corrects threshold values according to false-positive, false-negative, and reaction time results.  All of this processing gives a reasonably accurate estimate of the threshold values while cutting the testing time significantly.

As a technician monitoring the SITA program for the first time, be aware that the average threshold sensitivity for SITA is different than it is for the Full Threshold program.  This means that the dB numbers may generally be higher for the SITA test as compared to a Full Threshold test that the patient may have taken in the previous field exam.  Abnormal defects on the grayscale printout may not appear as deep or black as they appeared on the Full Threshold test.  You would not want to communicate to this patient that he appeared to be getting better or that the results are better, as this may not be the case.  Although this may seem to be a weakness of the SITA algorithm, the "shallower" defects are actually more statistically significant.  Upon the switch to SITA, your doctor will be comparing the current SITA results to future SITA results and not to results from previous Full Threshold tests.

SWAP

SWAP stands for Short Wavelength Automated Perimetry

The SWAP algorithm is a threshold strategy developed to detect visual field defects at an earlier stage than other currently available testing algorithms.  Instead of using a white target on a white background, SWAP uses a blue target on a yellow background. 

Cones are the light sensitive cells that are concentrated in the macular area.  Cones are sensitive to color.  There is a specialized cone for each of the blue, red, and green wavelengths.  The yellow background color minimizes the response of the red cones, the green cones, and the rods.  The blue stimulus thus isolates the response to the blue cones.  

It is thought that the ganglion cells associated with the blue cones are relatively few in number as compared to the ganglion cells associated with the red or green cones.  Therefore, damage to blue ganglion cells should produce a detectable field defect at an earlier stage than would be produced if testing the other cones or all the cones together with the rods.

Another theory suggests the the blue cone ganglion cells are damaged earlier than the ganglion cells of the other cones in response to eye pressure changes.

Studies have shown that SWAP does detect field loss sooner than other methods, but there are some problems with the test:

• the variability of the threshold estimate is higher with SWAP than with other threshold tests

• the variability from test to test is higher with SWAP as compared to other threshold tests

• the test takes longer than standard perimetry (SWAP testing takes about 14 minutes for a normal eye)

• cataracts and nuclear sclerosis significantly reduce the visibility of the blue stimulus

The current SWAP algorithm is proving most useful for young patients who are glaucoma suspects.  Older patients, especially those with significant field loss, do not tolerate the length and tedious nature of the procedure well.  

SWAP testing requires the use of a visor that extends out above the patient's forehead.  Be sure to retract the visor when SWAP testing is complete.  SWAP testing also requires careful explanation to the patient as to what will be seen.  The demonstration mode is very useful.  The blue stimulus will be large and relatively dim against the yellow background.

If you perform SWAP testing, be aware that the patient will likely have a higher number of false positive and false negative errors than would be expected with full threshold or SITA testing.

SWAP testing is not available with the SITA strategy.  SITA and SWAP testing are only available with Humphrey Field Analyzers.

Functional Field Loss

Sometimes a patient will complain of visual field loss when there is no organic (physical) evidence to explain the loss.  This is also called non-physiologic or non-organic vision loss. This may occur in the following situations:

• Hysteria:  The patient suffers from a psychological illness (perhaps from stress or trauma) that has visual manifestations.  

• Malingering:  A patient may fake illness in order to receive some benefit.  The motive could be sympathy, revenge, time off from work, or monetary gain from an insurance company settlement of an injury claim.

A manifestation of functional loss can be the loss of peripheral vision, with or without a complaint of decreased visual acuity.  The hallmark of functional field loss is that it does not follow the "rules" of the physiology of the visual pathways.  For example:

• If the visual field loss is a one eyed complaint, the patient can be tested with the fellow eye un-patched.  Ordinarily the visual field defect will disappear because the stimuli can be seen with the fellow eye.  The patient with a functional loss may maintain the defect.

• When tested repeatedly, functional field defects may not maintain consistent boundaries, as you would expect with an organic defect.  An isopter boundary in kinetic testing may have a spiral appearance as the boundary moves closer in with repeated testing.

• When tested at a different distance, the size of a defect should change proportionally. For example, a scotoma or area of seeing should double in size if tested again at a distance twice as far away with a stimulus twice as large.  The functional defect may maintain the same boundary size at different test distances.  

The tangent screen is particularly useful when testing for a functional defect.  See the discussion of the tangent screen in Module 12 Section 3.

Binocular Field Testing

Some patients with double vision do not have double vision in every position of gaze.  The areas of gaze with single vision and the areas with double vision can be mapped using a Goldmann perimeter. 

• The patient is tested with a large stimulus size and with both eyes open, thus the name "binocular" field testing. 

• A correction is used if necessary for the patient to see the stimulus well, but it is best to test without a lens in place to avoid the obstruction of the lens rim.  Contact lenses are a good means of correction.

• It is very important that the patient be instructed not to move his head during testing.  Some patients are able to compensate for double vision by the use of a head tilt.  We are trying to map areas of double vision that occur without a head tilt.

• The patient follows the stimulus as it is presented in the cardinal positions of gaze.  The patient indicates if the stimulus appears double or single. 

• Areas of single vision are explored in a fashion similar to mapping a scotoma.  The stimulus is presented in an area of single vision and it is moved outward until double vision is reported.  At this point a mark is made on the graph and the process is repeated until the boundaries of the areas of single vision are adequately mapped. 

• The areas of single vision and double vision are clearly marked on the Goldmann graph paper and it is labeled as being a binocular field test.

Esterman Functional Tests

Some government agencies require information regarding the functional ability of a person's vision.  Examples would be visual acuity and visual field limits for disability determination or a pilot's license.   Typically, the requesting agency will have a form to be completed with specific guidelines for what information is needed and how it is to be obtained.  

A common guideline has been to test the visual field limits with a Goldmann perimeter using the III-4-e stimulus.  The testing can be performed quickly and efficiently with the Goldmann perimeter.  However, with the advancement of automated perimeters, the Goldmann perimeter is becoming a rare bird.

An international standard was developed for evaluating visual capability with an automated perimeter; The Esterman Functional Tests.  The Humphey Visual Analyzer II has this test as an option, and it can be used if the particular agency accepts the score from the Esterman Test.

The monocular Esterman tests a 100 point grid with a Goldmann equivalent III-4-e (10 dB) stimulus from 75 degrees temporally to 60 degrees nasally.  The binocular Esterman tests a 120 point grid to 150 degrees bi-temporally.

When the test is called up on the HFA, instructions are given on the screen.  No trial lenses are used.  If the patient can function without glasses, then no glasses are used for the test.  Glasses are used if necessary.  A patch is used for the monocular test.  No patch is used for the binocular test and the patient is centered in the bowl.

The SSA Kinetic Test

The SSA Kinetic test is a relatively new testing procedure available on the 750 series HFAs.  It is designed for testing the outer limits of the visual field for government disability exams.  This test replaces the Esterman test for many agencies.  

The Esterman procedure uses a grid of static testing points.  The SSA test is a kinetic procedure that is very close to the old Goldmann procedure for testing field limits.  A Goldmann equivalent III 4 E stimulus is kinetically moved from the non-seeing periphery toward the fixation point along the following degree lines: 0, 45, 90, 135, 180, 225, 270, and 315.  The procedure can be performed automatically or an operator controlled step-by-step procedure can be used.

A numeric printout is an option, or the standard field map can be printed.  The isopter connecting lines can be drawn-in as an option, as shown in the printout below.  

An "SSA Efficiency Score" is computed and it is included in the printout.  This score is essentially a summary of the visual field performance compared to a normal field.  A score of 100% would represent a normal visual field in terms of the outer limits.  A score of 0% would represent no response to any of the stimuli.  An agency can use this score as part of the disability determination process.

To find out if your 750 series HFA has this procedure installed, click on "show test library" from the main screen. Click on "kinetic" and look for "SSA".  If it is not installed, you can download the software free of charge from  humphrey.com.

Fluctuation

Fluctuation is a measure used by the automated perimeter to gauge how consistent the patient's responses are.  The Humphrey Field Analyzer measures the threshold at 10 different points and compares the values.  

A low fluctuation value means the patient had very consistent responses.  Consistent responses mean a more reliable field test.  A high fluctuation value may mean that the patient is inattentive, but a reliable patient may have high fluctuation values if there is impending visual field loss.  The ophthalmologist uses fluctuation values with other reliability indicators to sort out the significance of the fluctuation values.

Fluctuation measurements add about 10% to the testing time and it is not available with the SITA strategies.

Steep and Sloping Margins

A scotoma can be characterized as having steep or sloping margins.  

The blind spot caused by the normal optic nerve head is a good example of a scotoma that has steep margins.  Since there are no photoreceptors overlying the optic nerve head, this area has zero sensitivity to light.  The surrounding area of sensitivity ends abruptly at the margin of the nerve head.  Look at the map of retinal sensitivity below.  The area of the blind spot (B) forms a deep well, abruptly going to zero.

The grayscale image printed by the Humphrey Field analyzer depicts the scale of sensitivity with shades of gray.  Areas of very poor or no vision are shaded black, and areas of good retinal sensitivity are white or have very little shading.  There are various shades of gray in-between depending upon where you are on the scale.

In the printout below is a threshold field of the right eye of a glaucoma patient.  We notice that there is an extension of the blind spot as the area of black extends above what would be the normal blind spot.  This black area is surrounded by a dense gray area.  The dense gray area is surrounded by an area of less dense gray shading.  This variation in shading tells us that this area of defect has sloping margins.  There is an area of dense scotoma (black) surrounded by a relative scotoma (shades of gray).