Fundamentals of hfa perimetry

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Humphrey Field Analyzer II/IIiFundamentals of Perimetry

Measurement

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Measurement

There are three International Standards of measurement for Perimetry:

ASBASB

dBdBFT/LFT/L

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Measurement

ASBASB

AApopoSStiltilBB

0 (Dimmest) – 10,000 (Brightest)

A measurement of light, dealing with the brightness of the

surface of the bowl

International StandardInternational Standard

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FT/LFT/L

Foot LambertFoot Lambert

HFA Range

0 (dimmest) – 929 (Brightest)

FT/L

Higher number = Brighter light


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dBdB

DecibelDecibel

HFA Range

0.1 dB (Brightest) – 51dB (Dimmest)

Higher number = Dimmer Spot


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International Standards Comparison

Dimmest Brightest

0 ASB 10,000 ASB

0 FT/L 929 FT/L

51 dB 0.1 dB

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All Perimeters use the same standards

These standards are based on the

Goldmann Perimeter

ASBASB

ASB

ASB ASBASB

AS

BA

SB

ASBASB

FT

/LF

T/L

FT/LFT/L

FT/LFT/L

FT/LFT/L

FT/LFT/L

FT/L

FT/L

dBdB

dBdB

dBdB

dBdB

dBdB

dBdB

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The Goldmann Perimeter, which defined the Standards of Perimetry. These standards are still in use today.

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The standards of Perimetry are:

Spot IntensitySpot Intensity

Background IlluminationBackground Illumination

Spot DurationSpot Duration

Spot SizeSpot Size

Spot SpeedSpot Speed

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Background Illumination

The standard value is 31.5 ASB.The standard value is 31.5 ASB.

The illumination in the bowl must remain The illumination in the bowl must remain constant throughout the test.constant throughout the test.

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Why 31.5 ASB?

Originally used by Goldmann Perimeter

Adopted as standard by International Peremetric Society (International Council of Ophthalmology 1979)

Approximates minimum level for photopic or daylight vision

Photopic vision relies on retinal cone function instead of rods

Cones – Object Contrast Rods – Absolute Brightness

Small changes in pupil size or clarity of media do not have an effect on Contrast, so have little effect on test results.

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Measurement

Spot IntensitySpot Intensity

Glass Wedge/Film WedgeGlass Wedge/Film Wedge

The Spot intensity is controlled by filter wheelsThe Spot intensity is controlled by filter wheels

The spot intensity is directly related to the The spot intensity is directly related to the bowl intensity.bowl intensity.

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Measurement

Spot Size

Size V spot is the largestSize V spot is the largest

The size of the spot can be size I,II,III,IV,or VThe size of the spot can be size I,II,III,IV,or V

The default spot size is size IIIThe default spot size is size III

Mr. Default Mr. Default to you!to you!

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Measurement

Spot size 3 is .43 degrees

Spot Size is smaller in the HFA II bowl than in the HFA I, because the bowl

radius is smaller.

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Measurement

Spot Duration

The default time duration is 200mS The default time duration is 200mS (milliseconds) + or – 10mS(milliseconds) + or – 10mS

How long the spot is displayedHow long the spot is displayed

Duration can be changed to 500 mS for older Duration can be changed to 500 mS for older patients.patients.

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The principle of temporal summation holds that for very short durations, the visibility of a stimulus increases with duration; when a stimulus lasts more than about 0.5 seconds, on the other hand, its visibility is basically independent of duration.

200 ms – long enough for visibility to not be affected by small variations in duration. But less than …

250 ms – latency for voluntary eye movement

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Measurement

Spot Speed

This only applies to This only applies to KineticKinetic testing. testing.

How fast the spot movesHow fast the spot moves

Default speed for the HFA is 4 degrees per second.Default speed for the HFA is 4 degrees per second.

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Goldmann vs. Humphrey Parameters

Goldmann HumphreyGoldmann Humphrey

Spot Size I,II,III,IV, V I,II,III,IV,VSpot Size I,II,III,IV, V I,II,III,IV,V

(V is the largest) (V is the largest)(V is the largest) (V is the largest)

Filters 1,2,3,4 Glass WedgeFilters 1,2,3,4 Glass Wedge

(4 is the brightest) Film Wedge(4 is the brightest) Film Wedge

a,b,c,d,ea,b,c,d,e

(e is the brightest)(e is the brightest)

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Kinetic TestingSingle intensity; Moving Target

A light spot (Stimulus) is introduced along a particular meridian, following a straight line until a patient response (sees the light spot) is indicated.

MeridianMeridian

Patient ResponsePatient Response

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Static Testing

Varies the intensity of the spot over the entire Hill of Vision. Patient does not see the spot move. The spots are projected at different positions on the bowl, but the instrument will return to the spots at different intensities.

Variable Intensity; Stationary Target

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Measurement

The “Island” of Vision

A A StaticStatic test on a test on a normal eye will normal eye will produce a pattern produce a pattern similar to the one at similar to the one at right.right.

DimDim

BrightBright

Blind SpotBlind Spot

FoveaFovea

NasalNasal TemporalTemporal

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Scotomas

There are two types ofThere are two types of ScotomasScotomas::

Definition: A defect on the Retina. An area or spot on Definition: A defect on the Retina. An area or spot on the Retina that is not as sensitive to light as it should the Retina that is not as sensitive to light as it should be.be.

1.1. Relative Relative – An area or spot on the Retina that – An area or spot on the Retina that can detect light, detect light, but not as good as when compared to a normal eye at that same but not as good as when compared to a normal eye at that same spot.spot.

2. Absolute – An area or spot on the Retina that cannot detect light, no matter how bright it is.

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Measurement

Scotoma

Is this Scotoma Relative Is this Scotoma Relative or Absolute?or Absolute?

Relative!!Relative!!

BlindspotBlindspot

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Hey man, like, ALL my Hey man, like, ALL my relatives are Scotomas!relatives are Scotomas!

Absolutely!!!Absolutely!!!

Can you dig it??!!Can you dig it??!!

Cool!!Cool!!

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The Blind spot exhibits the same characteristics as an ________ ________?

Absolute Absolute Scotoma!Scotoma!

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Thresholding and Bracketing

What is thresholding?

Threshold (or Threshold Level) is the minimum amount of light that the eye can detect at a particular point on

the retina.

Bracketing is the process of determining the Threshold value.

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Measurement

Thresholding by Bracketing

For our purposes, we will assume that the instrument begins the threshold test with a spot of 36db. The spot or stimulus is presented at a particular spot on the bowl to see if there is a patient response.

0 dB36 dB55 dB

Indicates a negative (did not see) responseIndicates a negative (did not see) response

Indicates a positive (did see) responseIndicates a positive (did see) response

DimmerDimmer BrighterBrighter

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The patient did not respond, indicating that he/she did NOT see the spot. The instrument will now introduce a spot of 32dB (a 4dB change) at the exact same location.

0 dB0 dB36 dB36 dB55 dB55 dB

Indicates a negative (did not see) responseIndicates a negative (did not see) response

Indicates a positive (did see) responseIndicates a positive (did see) response

32 dB32 dB


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The patient responded positively, indicating that he/she DID see the spot. The instrument will now introduce a spot at the same exact position, at a brightness of 34dB. (A 2dB change)

00 dB dB36 dB

55 dB

Indicates a negative (did not see) response

Indicates a positive (did see) response

32 dB


34 dB

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Once again, the patient did not see the spot. The instrument has determined by BRACKETING that the threshold for this spot is 32dB. (NOTE: Some tests may closer define the threshold by testing at 1 dB steps.)

0 dB36 dB

55 dB

Indicates a negative (did not see) response

Indicates a positive (did see) response

32 dB


34 dB34 dB


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What happens when you start a test?

1. One spot in each 1. One spot in each quadrant will be quadrant will be bracketed to determine bracketed to determine the Threshold level.the Threshold level.

At the same time, the At the same time, the location of the Blind location of the Blind Spot will be determined.Spot will be determined.

Bracketing begins at 24dBBracketing begins at 24dB

Blindspot

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2. Once the threshold value for a spot in each quadrant has been found, an Expected Hill of Vision is determined.

DimmerDimmer

Expected “Hill of Vision”

36dB (CEN)36dB (CEN)

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The “CEN” is defined asThe “CEN” is defined as the Central ReferenceCentral Reference LevelLevel, or the, or the Expected Expected Foveal SensitivityFoveal Sensitivity, or the, or the Expected Foveal Expected Foveal Threshold.Threshold.

Expected “Hill of Vision”

DimmerDimmer

36dB (CEN))


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3. As the test is run, the patients “Actual” readings are plotted.

Expected “Hill of Vision”Expected “Hill of Vision”

DimmerDimmer

36dB36dB

Actual Patient plotsActual Patient plots

Blind Spot

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Types of Tests

1. Screening Tests – A QUICK overview of the patient’s field of vision. Most screening tests tell only if the patient did or did not see the spot. In general, screening tests do not quantify. That is, they do not determine how bad a scotoma is.

Now you see it……..Now you see it…….. Now you don’t!!!!!!!!!Now you don’t!!!!!!!!!

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Types of Tests

2. 2. Threshold TestsThreshold Tests – – QuantifiesQuantifies each spot. That is, it finds the threshold each spot. That is, it finds the threshold level. Determines how bad the scotoma is by calculating the level. Determines how bad the scotoma is by calculating the exactexact light light level the patient can see at a particular spot on the Retina.level the patient can see at a particular spot on the Retina.

I don’t see it…

I don’t see it…

Now I see it!!!!!

Now I see it!!!!!

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Measurement

Short Wavelength Automated Perimetry (SWAP)

Blue Goldmann Size 5 Stimulus on bright Yellow background

Yellow Background reduces responsiveness of the red and green cone system

Blue Stimuli are seen primarily by the Blue Cone System

SWAP can detect progression of field loss earlier in patients than standard White on White perimetry

SWAP eliminates the redundancy in the visual system by reducing responsiveness from non blue cones and the rod system

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Reliability Factors

After a Field Test has been completed, a After a Field Test has been completed, a determination must be made if the test is accurate determination must be made if the test is accurate

and therefore reliable. Several Reliability and therefore reliable. Several Reliability Indicators appear on the test printout:Indicators appear on the test printout:

Fixation LossesFixation Losses False Negative ErrorsFalse Negative Errors

False Positive ErrorsFalse Positive Errors

Gaze TrackingGaze Tracking

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Measurement

Fixation Losses

During the test, the patient is told to Fixate (Stare) at a central LED. The instrument records the number of times that the patient lost this fixation and reports it on the printout.

Excessive fixation losses can render a test invalid.

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Fixation Monitoring

The HFA uses two systems for measuring patient fixation: the standard Heijl-Krakau blind-spot monitoring and the IR Gaze Tracking System. Both methods can be used, either together or alone, or they can both be turned off, as required.

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Heijl-Krakau Blind Spot Monitoring

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Gaze TrackingLocation of Corneal Reflex Marker

Corneal Reflex marker Location Digitized and Stored in Memory

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The direction of a patient's gaze is determined in two steps: 1. A reflex marker is establishedon the corneal surface.

2. The location of the pupil center is determined.

Gaze Tracking

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Gaze Tracking

Gaze tracking is initialized in the following manner when a selected test is first started:

The patient is asked to fixate on the central illumination LED. Gaze tracking turns on the reflex gaze IR LED located just under the diamond fixation pattern and turns off eye illumination briefly . Light from the LED is reflected off the cornea, and back to the IR sensitive camera.

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Gaze Tracking

The majority of the cornea appears black except for the reflected spot. This image is digitized and stored in memory. The reflected spot is referred to as the reflex marker. Because the corneal surface is rounded, the reflex marker will move very little even if the patient's eye rotates, and thus the marker becomes a (relatively) stationary reference point.

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Gaze Tracking

Next, the system locates the pupil center by illuminating the entire eye with the two IR LEDs located either in the bottom of the bowl, or in the trial lens holder (when in the raised position). The iris appears bright with a dark pupil. This image is also digitized and stored in memory. It is the relationship between the location of the reflex marker on the cornea and the location of the pupil center that determines fixation.

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Gaze Tracking

During a test, each time a spot is projected into the bowl, the locations of the reflex marker and the center of the pupil are compared to the initial images stored in memory. If the patient is fixating correctly, the positional relationship between the reflex marker and the pupil center will be the same as that of the stored images. If the patient is off fixation, the positional relationship between the reflex marker and the pupil center will be different. The greater the misalignment, the higher the mark on the Gaze Graph.

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Measurement

Gaze Tracking

Determining the Pupil Center

Patient Fixating – CornealReflex Marker and Pupil in

ProperRelationship

Patient Not Fixating –Corneal Reflex Marker and Pupil

Not in Proper Relationship

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Gaze Tracking

Gaze Graph

Upward spikes indicate that the patient has lost fixation;• a spike that reaches the top horizontal line (or higher) indicates 10 degrees (ormore) off fixation;• a spike that extends halfway to the top line indicates 5 degrees off fixation.P Downward spikes indicate as follows:• a short spike downward indicates that the gaze at that time cannot bedetermined by the software.• a long spike downward indicates that the patient blinked at the time fixationwas checked.The absence of marks on the graph indicates proper fixation.

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Gaze Tracking

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Gaze Tracking

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Gaze Tracking

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False Negative Errors

Let us say that a Let us say that a StimulusStimulus (Spot) is presented at (Spot) is presented at 26dB26dB, , and the patient responds that the spot was seen. and the patient responds that the spot was seen. Later in the same test, at the same location, a brighter Later in the same test, at the same location, a brighter spot, say spot, say 22dB22dB, is presented, and the patient does not , is presented, and the patient does not respond, that is, the patient does not see the spot.respond, that is, the patient does not see the spot.

““Yessir, I see it!”Yessir, I see it!”

““Nosir, I didn’t see that one!”Nosir, I didn’t see that one!”

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False Positive Errors

The patient responds that a spot of light was seen The patient responds that a spot of light was seen when none was presented. This sometimes can when none was presented. This sometimes can occur when the motors move on the Humphrey occur when the motors move on the Humphrey Field Analyzer, or a patient gets into a rhythm and Field Analyzer, or a patient gets into a rhythm and anticipates spots.anticipates spots.

SpotsSpots

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Fundamentals of hfa perimetry