Introduction

This module uses the Humphrey Field Analyzer Series 600 and Series 700 as the models for discussing automated perimetry.  Although the HFA can be used for many types of visual field tests, it is by far used most often for visual field testing of glaucoma patients.

The ophthalmologist has three major sources of information that are used to monitor patients with chronic open angle glaucoma.  They are: the intraocular pressure readings, the appearance of the optic nerve head, and the results of visual field testing.  Thus, the reliability of the visual field test becomes very important.

It is the responsibility of the ophthalmologist to interpret the results of the visual field test.  It is our responsibility as the technicians to make sure that the test is a reasonably reliable measure of the patient’s visual field; to make sure that the test is valid. There are four general areas that affect test reliability.

• Machine set-up

• Patient compliance

• Patient positioning

• Patient fixation and response

These areas can be subdivided as follows:

• Select the proper testing mode (stategy) and enter patient data

• Determine what (if any) trial lens is needed

• Properly position the trial lens in the holder

• Change parameters if necessary

Instruct the patient regarding what to expect, how to respond, maintaining fixation, and proper forehead and chin position.

• Occlude the eye not being tested

• Position the patient in the chin rest such that the eye being tested is properly aligned with the fixation target

• Position the patient’s forehead against the forehead rest

• Align the trial lens holder with the patient’s eye

• Be sure the patient is holding the response button

• Adjust the patient’s positioning for comfort

• Monitor and maintain the correct eye position

• Re-instruct the patient as necessary

• Keep the patient motivated

• Use the tools and information available to you

Each of the above procedures will be discussed in detail on the following pages. The procedures will be reviewed for the HFA 600 series machines and the 700 series machines.

Setting up the instrument is primarily a set of software functions.  You are first asked to choose which test will be performed.  This is decided by the ophthalmologist.  

After selecting the testing mode, you will then be asked to enter patient data, including name, ID number, and birth date.  It is, of course, important that this information be entered accurately.  Pay close attention to the accuracy of the patient's birth date.  The birth date is used by the HVA to calculate the trial lens power and to do other calculations.

If you use the data recall feature for a patient who has previously had a visual field test, be sure to confirm the accuracy of the information and edit as necessary.  It is not unusual to have to change the trial lens information.

On the 700 series machines, from the main "Patient Data 1" screen you are asked to either "Proceed" or go to "More Patient Data".  Enter data on the "Patient Data 2" screen as guided by your ophthalmologist.  It is particularly useful in terms of reliability to enter the visual acuity and the pupil diameter. As you might suspect, decreased central vision may have an effect on the results of the field test.  It has been shown that a pupil diameter less than 2mm will adversely affect the visual field results.  Measure the pupil in normal room lighting conditions and enter the data here.  This information will also be on the printout, giving the ophthalmologist another indicator of reliability.  If the patient's pupil measures 2mm or less, ask the ophthalmologist if the patient should be dilated.

The Model 750 HVA has an autopupil feature that automatically measures the pupil diameter when the Gaze Tracking is activated.

The patient’s vision must be corrected for the distance between the eye and the bowl. The patient’s glasses are not used, although the patient may wear contact lenses. The correction is accomplished by using trial lenses that fit into a trial lens holder in front of the patient’s eye.  The trial lens power must also be adjusted for the patient's age.

Although you are given the option of manually inputting the trial lens information,  the newer Humphrey machines give you the option of having the HVA calculate the trial lens power for you.  To improve accuracy, always choose to have the machine do the calculation unless there is a specific reason not to.  If you choose to do it manually,  here are the guidelines that the HVA uses to do the calculation: rxcalc.txt

Here is a specific reason why you would want to enter the trial lens correction manually.  Remember the situation in which the patient has a pupil diameter of 2mm or less?  Your ophthalmologist may want you to dilate this patient.  The dilating drops will also paralyze the patient's accommodation.  If you look at the guidelines on the rxcalc.txt link, you will notice that patients below the age of 55 receive less than the full dioptric correction of +3.00 D (or +3.25) for the eye-to-bowl distance.  This is because they are assumed to have some accommodative ability.  If you take away that ability by using dilating drops, you will have to give the dilated patient who is under 55 years old the full correction of +3.25 D added to the distance correction.

If the machine does the calculation for you, you will be asked to enter the patient's distance glasses correction.  Ideally, this should be the patient's best correction on the day of the test.  It is best to perform some preliminary exam procedures, such as visual acuity and refraction, before the patient does visual field testing.  Because of patient flow considerations, this is not always the case.  If visual field testing is performed first, make it a habit to ask the patient if her vision has changed since the last visit.  If so, it may be best to perform a refraction before the patient has the visual field test.

If the patient will be wearing contact lenses for the test, the distance correction is entered as "0" or plano if the contact lenses are a correction for best distance vision.  Contact lenses should not be used if the patient does not have optimal distance vision with the contact lenses.  For example, a contact lens that does not correct for significant astigmatism would not be used if the patient has better visual acuity with the glasses correction.  In this case the contact lens would be removed and the patient's glasses distance correction information would be used in the trial lens calculation.

Be aware that some patients wear one contact lens for near correction (monovision).  This patient should not wear the near contact lens for automated perimetry unless you are confident that you can calculate the trial lens correction needed for this eye to focus at the eye-to-bowl distance.

Patients with very high corrections, such as an aphakic patient with a glasses correction above +8.00, probably test best with a contact lens correction, if this is an option.

1 Move the trial lens holder from the storage position into the ready position in the bowl. Remember that trial lenses are only used for central field tests (within 30 degrees).  If a  peripheral screening test is being performed, the HVA will prompt you to remove the lens for testing the field outside of the central 30 degrees.

2 Place the cylinder lens (if any) into the slot farthest away from the patient. Align the axis mark of the lens with the axis of the prescription.

3 Place the sphere lens into the slot closest to the patient.

For each strategy (e.g., Full Theshold 30-2) there is a set of standard parameters.  These are determined by the ophthalmologist.  For the 700 Series HVAs, the parameters are as follows:

• test speed

• fixation target

• fixation monitoring

• blue yellow

• foveal threshold

• stimulus size

• stimulus color

• fluctuation

Each of the parameters has several options.  For example, the fixation target can be "central", "small diamond", "large diamond", or "bottom LED". The "central" target is used most of the time, so it is set as the standard or default option for the fixation target parameter. The parameters that are set as standard should be the ones that the ophthalmologist wants to use in most testing situations for that particular strategy.  In most testing situations you will not change these parameters.

You, as the technician performing the test, should be aware that it is best, in terms of reliability, to keep the parameters consistent from one test to the next.  This means that you will need to review the previous field test for any changes in parameters from the standard.  The parameters most likely to be changed are the test speed and the fixation target.  The test speed can be changed from "normal" to "slow" for the slow responding patient.  However, in most situations the 700 Series HVAs automatically adjust the test speed.

The fixation target can be changed from central to "large diamond" or "small diamond" to try to improve the fixation of the patient with a central scotoma (most often caused by macular degeneration).  If a diamond was used in the previous field test, you can change this parameter from the "Start test" screen or from the "Pause" screen.  For more on the diamond fixation, see Module 14.

Your instruction should include the following:

The following instructions are quoted from the Humphrey Field Analyzer II User’s Guide. Use the Humphrey instructions (which you can read from the screen), or use your own, but be sure to include each of the four subjects of instruction as outlined above.

"This test will measure your central and side vision. It is important that you always look straight ahead at the steady light (Point to the fixation light). Other lights will flash one at a time off to the side. Some will be bright, some dim. Press the button whenever you see one of these lights (Give patient the response button). You are not expected to see all of them. The test is designed so that you may see fewer than half of them."

" If you want to rest, hold down on the button (demonstrate to patient). The test will resume when you release the button. We test one eye at a time. Blink normally so your eye does not get dry. A good time to blink is whenever you push the response button."

The part in the monologue about not seeing all of the lights is particularly important.  With threshold testing, by definition, the patient will not be able to see all of the stimuli.  It is important that the patient understands this.  Some patients become concerned and anxious when they think that a stimulus is being presented and they cannot see it.

You may have notice that the patient is not instructed on proper forehead and chin position in the above monologue. Be sure to instruct the patient to keep her forehead against the band and her chin in the chinrest while in the testing mode.

Factors affecting test reliability

Machine set-up

Patient compliance

Patient positioning

Patient fixation and response

1 Occlude the eye not being tested

Be sure the patch is positioned such that the patient cannot peak around the edge with the occluded eye. The band should not come down over the eyelid of the tested eye.

3 Position the patient’s forehead against the forehead rest

Adjust the trial lens holder so that the lens is as close to the patient’s eye as possible without the patient’s eyelashes touching the lens.

The trial lens holder on the 700 series machine is simply adjusted forward or backward manually.

Once the patient is in position in the chin rest, with forehead against the band, you will want to adjust the patient’s position for improved posture. Recheck the patient’s eye alignment.

On the 600 series this is accomplished by adjusting the chair position forward or backward, and by adjusting the table height by using the power switch on the table. Pictured below.

Although both the 600 and 700 series HVAs have devices for monitoring patient fixation and response, it is the operator’s responsibility to make sure the patient is maintaining fixation and is responding appropriately. If a problem is detected during testing, you the operator must re-instruct the patient and/or make adjustments to the HVA to insure the reliability of the test.

Demo — Once the patient has been positioned and you are ready to start the test, you will have the menu option of choosing to show the patient a demonstration of the test. This is a good idea if this is the patient’s first experience on the HVF. The demo lasts for one minute and can be canceled at any time.

Ask the patient if she can see the fixation target.  If it is not seen well or not seen at all, proceed as follows:

1. Reposition the patient if necessary and make sure that the trial lens is close to the eye.  If it is still not seen well:

2. Re-calculate the trial lens power and make sure it is the optimum correction for the eye-to-bowl distance.  If it is still not seen well:

3. Re-evaluate the patient's visual acuity.  If the patient has a known central scotoma (blind spot), change the fixation parameter to the small or large diamond.

This is also the time to change the stimulus size if necessary.  The default stimulus size is size III white.  On most HVA models there are two larger stimulus sizes that can be used to low vision patients if necessary.  The parameters can be changed from a button link on the main screen.

Encourage the patient to blink normally to avoid drying out the cornea.

The Eye Monitor — The eye monitor is not only used for the initial set up of the patient, but should also be used by the operator during testing to visually confirm that the patient is maintaining the proper position and is maintaining fixation. You do not have to wait for the machine to sound a fixation alarm to re-instruct the patient regarding proper fixation.

Re-positioning and Pause Control — During the course of the test periodically re-check the patient’s chin and forehead position. Minor adjustments can be made using the headrest controls while the test is in progress. If major adjustments and/or re-instruction are necessary, the pause control (menu choice, or the patient can hold down the response button) can be used.

Blind Spot Monitor — Both the 600 and 700 series HVAs monitor fixation by presenting about 10% of all stimulus presentations in the blind spot. When the test begins, the machine "locates" the blind spot by an initialization process. During testing, if the patient responds when the stimulus is presented in the blind spot, then it is an indication that the patient has lost fixation. The event is recorded on the screen as a "fixation loss". Fixation losses are presented as a fraction with the numerator representing the number of losses and the denominator representing the number of presentations. For example: 2/7 indicates that fixation has been tested 7 times and there were 2 fixation losses.

Fixation Alarm — If the patient has 2 fixation losses in the past 5 presentations, the HVF will sound a fixation alarm. At this time is is necessary for the operator to re-check the position of the patient’s eye alignment in the eye monitor and to re-instruct and encourage the patient regarding maintaining proper fixation.  Make sure the patient can see the fixation target well.

Know what your doctor considers to be acceptable fixation losses.  If your doctor considers five fixation losses to be an unreliable test, then you will need to restart the test if your patient reaches 5.

Before restarting the test, make sure that:

• the correct trial lens is being used

• the patient can see the fixation target (change it if necessary)

• the opposite eye is well occluded

• the eye is properly aligned close to the trial lens

• the forehead is against the bar and the chin is in the chinrest

• the pupil is centered in the eye monitor

• the patient understands what is expected

False positive errors — This is another reliability indicator. This indicates that the patient is responding when no stimulus has been presented. The patient is "trigger happy". Results are presented as a fraction. A patient with high false positives should be instructed to respond only when a stimulus is seen and that she will not be able to see all the stimuli that are presented.

False positives are not shown on the SITA screen because the SITA program does not use trials to determine false positives.  With the SITA protocol, you can monitor the decibel graph to catch high false positives.  See the discussion of decibels below.  See Module 15 for more information on SITA.

False negative errors — This is a reliability catch trial that indicates the patient is not responding to stimuli that were previously seen. This likely indicates that the patient is becoming fatigued or is inattentive and should be re-instructed and encouraged. The pause control can be used to give the patient a rest. 

This indicator can also be high in reliable patients with significant field loss.  If fixation losses and false positives are low, but false negatives are adding up, look at a previous field test or check the dB readings on the screen for significant field loss.  If there is significant field loss, then the high false negatives don't necessarily mean inattentiveness, and the field test can be continued.  

Decibels --  The dB (decibel) numbers on the screen graph are a measure of the patient's ability to see dim stimuli.  The dBs range from 50 down to zero.  A reading around 30 would be a fairly normal reading.  Readings from zero up to 20 indicate low sensitivity and significant field loss.  Numbers above 40 are "hypersensitive" and usually indicate that the patient is "trigger happy" and is pressing the button before, or just as the stimulus is being presented.  Readings above 40 are indicative of an unreliable test. 

On Screen Information — Both the 600 and 700 series HVFs present fixation losses and errors on the screen in the bottom right corner.

The 700 Series HVFs have additional features for monitoring and maintaining the patient’s eye position. 

Gaze Tracking (models 740-750) — Gaze tracking is an electronic system that graphs the movement of the patient’s eye during the test. The operator can tell at a glance on the screen how well the patient has been fixating during the course of the test. The graph is also on the printout.

If you do not want gaze tracking: At the "Start of Test" menu screen, press "Change Parameters". Turn gaze tracking off.

If you do use gaze tracking: Follow the initialization procedure that

pops up on the screen after you press the "Start" button.

Gaze Tracking can be used without the Blind Spot Monitor.  This setup saves testing time but should only be used if the patient has a history of good fixation.  This setup can be specified from the Parameter Setup window during initial setup.

Head Tracking (model 750) — Using the head tracking feature, the model 750 can automatically make small adjustments in the chin rest to keep the patient properly aligned with the fixation point. Head tracking only works when the trial lens holder is being used and when gaze tracking is turned on. If you turned gaze tracking off at the beginning of the test, then you do not need to be concerned about head tracking.

If head tracking is being used, a pop-up window may appear on the screen and a beep will sound if head tracking loses its position. At this point you should re-position and re-instruct the patient. The pop-up window will give you the option of turning head tracking off.

Vertex Monitor (model 750) — The model 750 will display a pop-up screen and sound an alarm if the patient backs away more than 7mm from his original position. This feature only works when the trial lens holder is being used and when the gaze monitor is being used. If the pop-up screen appears, re-position and re-instruct the patient. If the vertex alarm continues to sound, press "re-initialize vertex" on the pop-up menu. Make sure the patient is properly positioned. The vertex monitor may be turned off from the pop-up menu.

To be (with the patient),

or not to be (with the patient) --  Whether or not you stay with the patient the entire testing time is primarily up to your doctor.  If the doctor gives you the option, then you must use your judgment as to the reliability of the patient.  If the patient is headed toward numerous fixation losses and/or false negatives, then you will need to stay and encourage the patient to produce a reliable test result.