Module 14 

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Post-test

Key Words:  These key words and phrases are used in this module.  Use these links to skip to a particular subject of interest.

age adjusted data

background

central 24-2 theshold test

dB graph

false negative

false positive

fixation losses

fixation target

foveal threshold testing

gaze tracking

greyscale graph

Heijl-Krakau method

pupil diameter

Rx calculation

single field analysis

stimulus

stragegy

What are "false pos errors" and "false neg errors"? Why are "fixation losses" and "questions asked" important? Do I need to pay attention to these?

The reliability (accuracy) of any given visual field exam is dependent upon the manor in which the patient responds to the test. If the patient is alert, understands what is expected of him, and follows directions, the chances of an accurate measure of his visual field are good. If the patient falls asleep during the exam, the chances are not so good.

Visual field testing accuracy is important, especially when following glaucoma patients.  The ophthalmologist has three primary sources of information that aid her in the diagnosis and treatment of glaucoma: the intraocular pressure reading, the appearance of the optic nerve head, and the results of visual field testing.  The intraocular pressure is useful in gauging the effectiveness of therapy, but it does not tell much about the progress of the disease.  Thus, the ophthalmologist relies heavily on the appearance of the optic nerve head and the results of visual field testing to tell him if the patient's vision is getting worse or not.  An accurate visual field test is very important tool in glaucoma assessment.

Automated perimeters have several methods of keeping track of the accuracy of the examination. These indicators can tell the operator if the test is going smoothly, or if adjustments need to be made.   The indicators on the printout  help the ophthalmologist decide if the test was a valid measurement.

By far, the most widely used automated perimeters are the Humphrey Field Analyzers, and this is the machine that we will use for our discussion and examples.

First, let's look at the information that is available to us on the HVA printout.  Some of this information is available to the operator while the test is in progress.  There are four columns of information at the top of the page.  The two columns on the left appear as printed below.  This representation will be repeated several times in the module for easy reference.

NAME:  JOHN DOE                                                      ID: 1234                           

CENTRAL 24-2 THRESHOLD TEST                                  

FIXATION MONITOR: GAZE/BLINDSPOT                 STIMULUS: III, WHITE

FIXATION TARGET: CENTRAL                                      BACKGROUND: 31.5 ASB

FIXATION LOSSES: 0/15                                                 STRATEGY:  SITA-STANDARD

FALSE POS ERRORS: 0%

FALSE NEG ERRORS: 38%

TEST DURATION: 05:34

FOVEA: OFF

The patient's name, ID number, and date of birth (not pictured above) are given on the next line.  When printing out the results on your patient, you will want to confirm that this information is correct.  It is particularly important that the date of birth is correct.  The Humphrey Field Analyzer performs calculations and analysis based upon age adjusted data.

"Central 24-2 Threshold Test"  is the type of test that was chosen at startup for this patient.  This particular test is a threshold test of the central 24 degrees of the visual field.  This is a common test for glaucoma patients.  Almost all of the field defects caused by glaucoma occur within the central 30 degrees, with the majority falling within the central 24 degrees.  The 24 degree test is significantly faster than the 30 degree test, thus decreasing patient fatigue and increasing reliability.  The type of test used is something that the ophthalmologist decides.

NAME:  JOHN DOE                                                      ID: 1234                           

CENTRAL 24-2 THRESHOLD TEST                                  

FIXATION MONITOR: GAZE/BLINDSPOT                 STIMULUS: III, WHITE

FIXATION TARGET: CENTRAL                                      BACKGROUND: 31.5 ASB

FIXATION LOSSES: 0/15                                                 STRATEGY:  SITA-STANDARD

FALSE POS ERRORS: 0%

FALSE NEG ERRORS: 38%

TEST DURATION: 05:34

FOVEA: OFF

"Fixation Target: Central" indicates that the patient was directed to fix on the yellow light at the center of the bowl.  This is the most often used target.  If the patient does not have central fixation, perhaps because she has a central scotoma from macular degeneration, then the small diamond or the large diamond can be used.  

NAME:  JOHN DOE                                                      ID: 1234                           

CENTRAL 24-2 THRESHOLD TEST                                  

FIXATION MONITOR: GAZE/BLINDSPOT                 STIMULUS: III, WHITE

FIXATION TARGET: CENTRAL                                      BACKGROUND: 31.5 ASB

FIXATION LOSSES: 0/15                                                 STRATEGY:  SITA-STANDARD

FALSE POS ERRORS: 0%

FALSE NEG ERRORS: 38%

TEST DURATION: 05:34

FOVEA: OFF

In order to get an accurate measurement of the sensitivity of any given portion of the peripheral retina, using an automated perimeter, the retina must remain stationary while it is repeatedly simulated. This is accomplished by providing the patient with a fixation target to stare at. If the patient maintains fixation (looks at the target all the time), then the eye does not move.

Try staring at a spot for 10 to 15 minutes. It is not easy to do. The patient wants to move her eye. The eye has a natural reflex that tells it to track something that it picks up in the peripheral vision. If you see a fly in the corner of your eye, do you wait until it crosses your central vision to look at it? Have mercy on those patients who have difficulty with fixation and give them gentle encouragement.

The ophthalmologist interpreting the results of the field test will want to know how often that poor fellow lost concentration and looked away from the fixation target, possibly producing an inaccurate measurement at those particular moments.

The Humphrey Field Analyzer II uses an optical tracking device called "Gaze Tracking" and the Heijl- Krakau method for monitoring patient fixation.  The machine will alert the operator and the patient (usually with a beep) that a fixation deviation has taken place. The machine also counts the occurrences and reports them as "fixation losses".

Gaze Tracking can only be initiated at the beginning of the procedure but may be turned off at any time.  Gaze tracking does not work with small pupils, strong prescriptions in the lens holder, droopy eyelids, and eyelashes that get in the optical path.  The tracker prints out a real-time graph on the screen and a final graph on the printout.  The graph gives an accurate indication of the quality of the patients fixation.

The Heijl- Krakau method simply flashes a bright stimulus in the area of the physiological blind spot. If the patient is fixing, she will not see the light. If she is not fixing, she probably will see the light, because she has moved her blind spot by looking away from the fixation target.

The blind spot is "located" in an automatic calibration sequence at the beginning of the test. The screen may say "Locating blind spot". This is a very important part of the test. Make sure the patient is centered correctly and is fixing on the target during this phase of the test.

If your machine is using this "blind spot check" method, the results will be expressed as a fraction that is given on the screen during the test and on the printout. The numerator represents the number of fixation losses, and the denominator represents the number of times fixation was checked by flashing the light in the location of the blind spot. Fixation losses exceding 20% will have an "XX" printed after the fraction.

High fixation losses may indicate poor fixation, but a high number can also indicate that the blind spot was not properly located during the initialization procedure.  Suspect the latter if the patient appears to be fixing well in the monitor and on the Gaze Tracker.  The test can be paused to re-plot the blind spot.

Gaze Tracking and fixation monitoring can be used together, or Gaze Tracking can be used alone.  If the patient has demonstrated good fixation, the test taking time can be shortened by not using the Heijl-Krakau fixation monitoring.

As the operator, don’t forget to monitor fixation and centering yourself by looking at the monitor or by looking through the peep-hole (600 Series HVA).

NAME:  JOHN DOE                                                      ID: 1234                           

CENTRAL 24-2 THRESHOLD TEST                                  

FIXATION MONITOR: GAZE/BLINDSPOT                 STIMULUS: III, WHITE

FIXATION TARGET: CENTRAL                                      BACKGROUND: 31.5 ASB

FIXATION LOSSES: 0/15                                                 STRATEGY:  SITA-STANDARD

FALSE POS ERRORS: 0%

FALSE NEG ERRORS: 38%

TEST DURATION: 05:34

FOVEA: OFF

False positives

False positives occur when the patient responds at a time when no stimulus is being presented. Some automated perimeters make a noise due to movement of the projector when the stimulus is presented. Patients can learn to push the button when they hear the noise, rather than when they see the light. Automated perimeters are tricky in that they sometimes move the projector, but they don’t present the light. When the patient responds during one of these "tricks", the response is counted as a false positive. 

The machine won’t pick up all of the false positives. Sometimes a patient will get disgusted with the test and simply fire off responses at random, or in a rhythm. It is up to the operator to be alert to this problem and remind the patient of the importance of the test.

For standard threshold testing, the printout gives the result as a fraction with total number of false positive responses followed by the total number of trials. If the number of responses exceeds 33%, an "XX" follows the fraction.  Some glaucoma specialists consider false positive responses over 15% to be indicative of an unreliable test.  Ask your ophthalmologist to give you guidelines for each of the indicators.

The SITA printouts give the false positive responses as a percentage, making it easier to determine unreliability. 

A false negative occurs when a patient fails to respond to a stimulus of greater brightness in a particular location that was previously responded to. The patient saw the test spot before; he should see it again. It is an indication that the patient is not paying attention. A patient with high false negatives may also be snoring. You have to be careful though, an alert person with significant field loss can also have high false negatives.

For standard threshold testing, the printout gives the result as a fraction with total number of false negative responses followed by the total number of trials. If the number of responses exceeds 33%, an "XX" follows the fraction.  Some glaucoma specialists consider false negative responses over 15% to be indicative of an unreliable test.  Ask your ophthalmologist to give you guidelines for each of the indicators.

The SITA printouts give the false negative responses as a percentage, making it easier to determine unreliability.  See Module 15 for information on SITA testing.

Here is a memory trick to help you remember which is which. For false negatives picture someone snoring. For false positives picture a gunfighter who is trigger happy.

NAME:  JOHN DOE                                                      ID: 1234                           

CENTRAL 24-2 THRESHOLD TEST                                  

FIXATION MONITOR: GAZE/BLINDSPOT                 STIMULUS: III, WHITE

FIXATION TARGET: CENTRAL                                      BACKGROUND: 31.5 ASB

FIXATION LOSSES: 0/15                                                 STRATEGY:  SITA-STANDARD

FALSE POS ERRORS: 0%

FALSE NEG ERRORS: 38%

TEST DURATION: 05:34

FOVEA: OFF

Test Duration

Test duration refers to the time it takes the patient to complete the test from start to finish.  The Humphrey Field Analyzer can speed up or slow down the testing depending on the patient's response time.  While performing visual field testing, make a mental note of what an "average" test time should be.  Exceptionally long testing times may lead to patient fatigue and unreliable results.  Field tests on an eye with significant field loss will take longer than a test done on a normal eye.  For more on test duration, see the SITA strategy in Module 15.

Fovea

This line indicates whether or not Foveal Threshold Testing was used, or was off.  Your ophthalmologist will tell you if this procedure should be used or not. Check this indicator to make sure it was in the proper mode.  

This feature increases the time that it takes to complete the test.  This testing is performed before the actual field test.  The patient looks at the center of the diamond fixation target and responds to stimuli within the diamond.

Stimulus and Background

These are usually standard settings that do not change unless a special test is used or if the patient cannot see the default stimulus.  The stimulus size and color can be changed (see Module 13).  The background refers to the intensity of the background illumination of the bowl. Make sure they are on the usual settings unless they were changed intentionally.

If the SWAP strategy is used, the stimulus will be listed as "V. Blue" and the background will be listed as "Yellow".  For more information on SWAP, see Module 15.

Strategy

The test strategy is chosen by your ophthalmologist.  Make sure that the testing was carried out with the expected strategy.  There are various test patterns designed for specific problems, such as glaucoma and neurological defects.  For more information on strategies, see Module 15.

The right side of the visual field printout:

                                                                                                  EYE: LEFT

                                                 ID: 1234                                     DOB: 08-29-32

                                         PUPIL DIAMETER:  2.4mm             DATE: 09-22-01

                                         VISUAL ACUITY: 20/30                    TIME: 2:35 PM

                                         RX: +2.00 OS                                      AGE: 69

ID:  Make sure the patient ID number is correct

DOB (Date of Birth):  Be sure this is the correct birth date for the patient.  This is particularly important because the Analyzer does glasses correction calculations and VF adjustments and calculations based upon this date.

Pupil Diameter:  Small pupil size has an adverse affect on the overall sensitivity of the visual field test.  A pupil size of 2mm or less will cause a "diffuse depression" of the overall field.  This means that the dB levels will be generally lower at each point.  Ask your ophthalmologist if these patients should be dilated before having the visual field test.  If you do dilate these patients,  make sure that you note it on the printout.

Also be sure to give the patient the full +3.00 add in the lens power calculation because the patient's accommodation will be paralyzed from the dilating drops.  You will need to calculate this manually, as the automatic calculation that the machine does is age adjusted.  If the patient is over 55 years old, you will not need to modify the automatic calculation.

Visual Acuity:  This reading comes from the visual acuity measured during the patient workup.  Ideally, this would be the patient's best corrected distance vision.

RX:  This is the lens power that was either calculated by the Analyzer or entered by the operator during the setup procedure.  Presumably,  it is the lens power that was placed in the lens holder for the patient to view through.

This Rx ideally should have been determined in the following manner:

1. The patient is refracted on the exam day to determine the Rx that gives the best distance vision.

2. There is an addition to the sphere power of the Rx that is determined by the patient's age and distance from the eye to the bowl.  The Analyzer will automatically calculate the final Rx according to the patient's age if the correct distance Rx is entered.  

When looking at the printout,  you can do a quick calculation to determine if this Rx number is what it should be, or reasonably close.

The Analyzer does the Rx calculation according to the following rules:

1. Drop .25 D cylinder powers.  For example,  +4.00+0.25X90 becomes a +4.00 sphere.

2. For cylinder powers from .50 to 1.25, use the spherical equivalent.  This would yield:

   • For +.50, add .25  to the sphere power

   • For +.75, add .37 to the sphere power

   • For +1.00, add .50 to the sphere power

   • For +1.25, add .62 to the sphere power

   • If minus cylinders are used, the number would be subtracted from the sphere power.  For example,  +1.00-1.00X90 becomes a +0.50 sphere.

3. The full cylinder correction is used for cylinder powers 1.50 D and above.

4. A correction for the distance from the eye to the bowl, calculated according to the patient's age, is then added to the sphere power of the distance correction, as follows:

   • Under age 30, no add

   • Age 30 to 39, add +1.00

   • Age 40 to 44, add +1.50

   • Age 45 to 49, add +2.00

   • Age 50 to 54, add +2.50

   • Age 55 to 59, add +3.00

   • Age 60 and up, add +3.25

   Example:  For a 40 year old,  +1.00+1.50X90 becomes +2.50+1.50X90.

An example from start to finish would be as follows:  The patient is 50 years old and has a distance correction of -2.00+0.75X90.  Since the cylinder power is between .50 and 1.25, we add the spherical equivalent of the cylinder power (+0.37) to the sphere power (-2.00) and we get -1.63.  You can round off to -1.50 if you like.  Our patient is 50 years old, so +2.50 is added to -1.50 to get +1.00.  A +1.00 D lens is placed in the lens holder.

Now that you now how the calculation is made,  you can quickly mentally check that the correct lens was used when you check the information on printout.  Sure, you say.  How I am supposed to memorize all that information?  Well, steps 1 through 3 aren't all that difficult.  It is the age table that is a bit much.  The table is not so hard to figure out if you remember just 3 ages.  Remember that age 30 is a +1.00 add,  age 45 is a +2.00 add, and age 55 is a +3.00 add.  Now remember that the range for each group is 5 years.  By remembering the above you can quickly figure out the add for the in-between groups.  

So, you still don't want to tax your brain that much.  Why not copy the table information and tape it to your VF machine?  Here is a link to a text file that you can print out and tape up:  Rxcalc.txt

What if you are double checking this information on the printout and you figure out that the patient was supposed to have a +1.00 lens and actually was looking through a +2.00 lens?  Would it make that big a difference in the results of the field?  A one diopter error may make a difference depending upon other factors such as the patient's visual acuity and the strength of the correction.  You would want to discuss the error with the physician and let her decide if the field test should be repeated.

Should a  visual field test be done before or after preliminary exam procedures such as a refraction?  Obviously, it would be best to have the proper correction before starting the visual field test.  But patient flow issues sometimes make it more practical to perform the visual field test first.  If this is the case,  ask the patient if he has had any changes in vision since the last visit.  A "yes" answer may necessitate performing a refraction on this patient before the visual field test is done.  If a significant change in refractive error is discovered after the field test is done,  then the field test may have to be repeated depending upon the opinion of the physician.

For more on Rx correction, see Module 13.

What the graphic printouts tells us about reliability

The dB graph

The top left graph on the Single Field Analysis printout is the dB graph.  The "dB" stands for decibel.  The intensity of the light projected onto and reflected off of the inner dome of the perimeter is expressed in units called apostilbs (or asb).  The maximum intensity of the stimulus on the Humphrey Field Analyzer is 10,000 asb.  The human eye can detect light intensity values from 1 asb up through and above 10,000 asb.  In order to better understand changes in this broad range of values,  the apostilbs units are expressed in powers of 10.  These new units are called decibels.   The decibels tested by the Humphrey unit range from 0 to 50 decibels.  It is important to remember that 0 represents the brightest stimulus and 50 represents the dimmest stimulus.

Threshold testing means that a point on the retina is systematically tested with varying intensities of light until the machine determines the dimmest stimulus that the patient responds to 50% of the time.  This "dimmest stimulus seen" value is expressed as a decibel, which is printed on the dB graph.  

So a value of "0" would mean that our patient saw little or nothing when tested at that particular point on the retina.  For example, the area of the optic nerve head should have a "0" or "<0" dB value,  because the optic nerve head has no photoreceptors.  The "<0" value means that the patient did not respond to any of the stimuli, not even the brightest one.

A value of "50" would mean that our patient consistently saw the dimmest stimulus that the Humphrey Field Analyzer can project.  Accurate values at or above 40 dB are very unusual.

When evaluating test reliability, be sure to look at the numbers printed on the Numeric Results (dB) graph.  A value of 40 dB or higher on this graph indicates that the patient is "trigger happy".  That is, the patient is anticipating the presentation of the stimulus and is responding before the stimulus is seen.  Readings of 40 dB or higher indicate an unreliable field that will need to be repeated.  It is best to catch these reading while the test is in progress and re-start the test.  A typical "normal" dB reading is around 30.

The dB readings on this graph indicate significant field loss.  The highest numbers are below 26, indicating a generalized decrease in field sensitivity.

The Grayscale graph

The graph of the top right of the Single Field Analysis printout is the "grayscale" graph.  The grayscale graph converts the dB values to a "gray" scale, making it easier to visualize the data.  For example,  values less than or equal to "0" are represented by solid black, meaning the patient did not respond to the brightest stimulus.  Values above 40 are represented by total white,  meaning the patient could see the dimmest stimuli.  The values in-between are represented by varying shades of gray, made up of increasing densities of dots.

Do you remember the discussion about false negatives?  A false negative occurs when a patient fails to respond to a stimulus of greater brightness in a particular location that was previously responded to.  This means that our patient could be falling asleep.  However, you have to be careful when evaluating false negatives because an alert person with significant field loss can also have high false negatives.

If all of our measures of reliability look good except false negatives, look at the grayscale printout.  Significant areas of dark gray or black on the grayscale printout indicate significant field loss, which could account for isolated high false negative readings.  In this case, the field test would not need to be repeated.

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