READING A SINGLE FIELD OF AUTOMATED PERIMETRYSiddarth sain

What is the visual field?A three dimensional area seen around an object of fixationPerimetry is the procedure for estimating the extent of the visual field

Classic central full threshold testOriginal program of HFA-I introduced in 198430-2 strategy determines sensitivity in central 30 degrees at locations 6 degrees apartPatient gazes at center of a diamond projected at 10 degrees below fixation point at center of the bowlSize III stimulus for 0.2 secWait for a response for about 1.8 secIf no response, a stimulus 4 dB more intense is presentedIf there is a response, a stimulus 2 dB less intense is presented until patient fails to respond

Threshold testing

Classic central full threshold testThreshold is first determined at 4 primary locations one in each quadrant symmetrically placed 9 degrees from both horizontal and vertical meridian starting at 25 dBThese 4 points are tested twice

Classic central full threshold testIn addition to the 4 mentioned, 6 more random points are tested twice, a total of 10 points, which are used to calculate the short term fluctuationTo save time the initial stimulus intensity at each point is extrapolated from the sensitivity of the nearby pointAny deviation from the expected threshold more than 5dB leads to retesting of the point

The Fastpac strategyIntroduced in 1991Changes stimulus intensity in 3 dB stepsMore vulnerable to response errorsHigher intratest variability.Requires 30% less time than full threshold

The SITA strategyAvilable as SITA std. and SITA fastUses pt. age ,normal and abnormal database and pt. response to calculate the expected results for each pointShorter testing timeReduced normal variability

the supra threshold testScreening modeFaster then full thresholdLess informativeDo not provide quantitative dataLess often use in glaucoma

The single field print outTest informationReliability indicesProcedure and patient dataNumeric dataGray scaleTotal deviation plotPattern deviation plotGlaucoma hemi field testGlobal indicesGaze tracker

Test information

Procedure and patient data

Reliability indices

Fixation monitor and fixation targetBlind spot indicates that the blind spot was mapped first using the Heijl-Krakau technique.

The blind spot is mapped early and then test targets are intermittently placed within the mapped area.

Any response indicates loss of fixation

Fixation lossesIf a patient is having fixation losses he will see a stimulus in the originally mapped blind spot areaAn acceptable upper limit is 20% lossesA field is reasonably reliable if in presence of a high FL rate, the FP,FN,SF rates are low

False positive responsesRepresents the tendency of a patient to respond not in response to seeing a stimulus but in expectation of a stimulus or in response to a non visual clueUpper limit of acceptiblity is 33%There will be some points of unusually high sensitivitiesPresence of white scotomas with points having sensitivity thresholds of 40dB or more on the numeric scaleThe GHT will show abnormally high sensitivityMD will be strikingly high

False positive responses

False negative responsesFailure of a patient to respond to a presented visual stimulusDue to loss of concentration or fatigueThe measured threshold values come out to be very lowNegative MDSome points are randomly more affected than others so PSD and SF are abnormalPatches of depressed sensitivity occur at the edge of the field which is tested lastClassical clover leaf pattern in grey scale

False negative responses

Raw Numeric dataRepresents the actual threshold values for the various points. At least 10 points are tested twice. Any point that is 5 dB above or below the expected value is also retested.This data provides the basis for the entire visual field.

Numeric data

Gray scalePoints on the gray scale are calculated from the points on the numeric data.

They give an immediate and easily comprehensible picture.

Useful in highlighting the common artifactual field loss.

The change is gray scale may be falsely dramatic as it is a smoothened out picture based on extrapolation of just 76 points.Shallow defects can be missed ..

Gray scale

Total deviationCompares the patients visual field with a bank of normal data for a patient of same age. 2 parts : The decibel plot represents the deviation rounded to the nearest integer from the mean normal value for that persons age.

A deviation of 5dB is taken as abnormal.

The normal range is larger in the periphery and also larger superiorly.

The probability plot :Symbols depict the frequency of the value within the normal population of that age.The TD is good at identifying the overall visual field loss but of limited value in determining focal defects if a generalized depression is present.

Total deviation

Pattern deviationDetermined by correcting the field for overall depression. The seventh most sensitive non edge point is used to adjust the hill of vision. Helps in identifying a focal defect from glaucoma in a patient with a field depressed by other causes e.g. a cataract..

Pattern deviation

A catch in the PDThe PD plots are based upon the sensitivities of the best points in the TD plot. Therefore if the best points are almost blind then PD wont help. (look for the gray scale). During early stages the subtle defects will be picked up, but as the glaucoma advances the generalized depression sets in. So the PD can be misleading.In end stage glaucoma the plot might actually reverse. Have a look at the fundus.

Glaucoma hemifield testThis looks at the cluster of points above and below the horizontal to see if there is any significant difference between the mirrored points. This difference is the hallmark of glaucomatous damage. The GHT analyzes the difference in terms of deviation from controls and translates them into the probability domain, for the whole central field. Five zones are compared like this. Score is assigned to each zone depending upon the percentile deviation in the PD plot of the group of points in the zone.

Glaucoma hemifield test

Readings in the GHTGHT describes the field asWithin normal limitsGeneralized reductionAbnormally high sensitivityOutside normal limitsborderline

Global indicesMEAN DEVIATIONPATTERN STANDARD DEVIATIONSHORT TERM FLUCTUATIONCORRECTED STANDARD PATTERN DEVIATION

Any global index of p value less than 5% has a high probability of being abnormal.

MEAN DEVIATION : 1)calculates the mean of deviation in the patients results from the age corrected normal database.

2) The MD is mainly an index of the size of the visual field defect, more sensitive to generalized field defect

PATTERN STANDARD DEVIATION : It is the difference between a given point and its adjacent points.

SHORT TERM FLUCTUATION: Expression of variability between two different evaluations of the same patient. A high number indicates low patient reliability. It is estimated from the test retest differences at 10 standard locations. Usually ranges from 2 to 3 dB normally. Important to note that it may be the first sign of glaucomatous damage.( edge of scotoma)

CORRECTED PATTERN STANDARD DEVIATION: is the PSD corrected for SF. A high SF will give rise to a CPSD lower than the PSD. It is an index of localized non uniformity of the hill of vision. In general: 1) normal MD and CPSD = probably normal field 2) abnormal MD and normal CPSD= generalized loss 3) normal MD and abnormal CPSD= localized defect 4) abnormal MD and CPSD= large defect with a localized component

Gaze trackerPresent in the newer machines.Follows the patients cornea and records the movements.More spikes and taller spikes indicate greater deviation. Downward spikes represent the situation when fixation was unrecordable.

Criteria for minimum abnormality3 or more contiguous, non edge points in an expected location of the field that have p

Progression of a fieldNEW DEFECT:A new cluster of at least 3 non edge abnormal points arises in a typical location, each with threshold sensitivities occurring in fewer than 5% of the normal population(p

Progression of a fieldDEEPENING OF A PREEXISTING DEFECT: a defect has deepened or enlarged if 2 or more points within or adjacent to an existing scotoma have worsened by at least 10dB or 3 times the average of SF, whichever is larger.GENERALIZED DEPRESSION: Decline in MD that is significant at p3dB at all points on two consecutive fields.

How to follow up?Establish a baseline field/fieldsTwo or three successive fields 4 weeks apart that are reproducible are taken as representative baselineIn case of severely contracted fields concentrate on the central 20 or 10 deg with the 10-2 or the macular testsUsage of a larger pattern of points. Eg. If the diagnosis was made with a 24-2 pattern with most points abnormal,converting to a 30-2 may be helpfulUsage of a larger size V stimulus in cases with high visual loss

OVERVIEW ANALYSISDisplays aal visual fields of eye in CHRONOLOGICAL order including gray scale,numeric data,probability plots ,visual acuity,pupil sizeEasier to scan a series of examinations

OCTOPUSFankhauser(1975) was the mastermind behind the first OCTOPUS perimeter.OCTOPUS 101 and OCTOPUS 300/1-2-3 are the current available models

Test conditionsOriginally the OCTOPUS perimeters operated at 4 apostilbs.OCTOPUS 300/1-2-3 can be operated under normal environmental light conditions, hence background illumination was increased to 31.4 apostilbsGoldmann size III stimulus is usedIn the low vision program size V stimulus is used

Octopus 32 measure retinal sensitivity at 76 points in central 30Octopus G1 measures retinal sensitivity at 73 points 59 points in central 26 at threshold level and 14 points b/w 30 -60 at suprathreshold level

More concentrated test locations in center to find paracentral scotomasNo test location on horizontal and vertical axis except central pointTo avoid pseudo scotomas caused by correction lens edges 59 central test points are within 26 and not tradition 30 14 peripheral test locations with a concentration on the nasal side (to better detect nasal steps) are added if information outside the central 30 are requested without prolonging the test duration too much. In this case the complete G1 has 73 test locations. If only the center 30 are tested this program is also called G2X as in the OCTOPUS 300series

Normal test strategyBased on threshold of differential light sensitivityTesting begins at 4 anchor pointsBegins at age corrected normal values minus 4 decibels followed by 6dB increase in stimulus luminance when there is no responseThereafter process continues with brighter spots in steps of 8 dB After the first crossing of threshold the bracketing process is reversed making the stimulus luminance dimmer by 4dBAfter a NO crossing again there is an increment by 2dBFinally a 1dB adjustment is applied in opposite direction to obtain d.l. sensitivity

Tendency oriented perimetry (TOP)Reduces examination time by 80%TOP can be applied to flicker as well as blue on yellow perimetryThe anatomic and topographic interdependence of visual field establishes a tendency between the thresholds of neighbouring zonesInstead of questioning each individual point 4-6 times, the threshold at every location is adjusted 5 times with only one question per locationThis is done by one direct question and four by results from questions in neighboring locations

The examination starts at half the normal valueThen testing proceeds with bracketing applying steps in relation to patients age corrected normal value. Finally a step in each direction to determine the actual threshold of d.l. sensitivity

BEBIE /CUMULATIVE DEFECT CURVEHelp to assess the overall condition of visual field at glance59 points tested at full threshold (in G1) are ranked from highest to lowest sensitivity after age correctionA curve is obtained Points on left represent better pointsThose on the right the worse points

Octopus perimetersMEAN SENSITIVITY- average of retinal sensitivity measured at all pointsMEAN DEFECT average defect of all threshold points from age matched normals,as shown in comparison chartLOSS VARIANCE- is obtained from individual deviations of all measured locations with mean defect valueSHORT TERM FLUCTUATIONCORRECTED LOSS VARIANCE- taking into account STF

Criteria to detect abnormality staticallyOne nasal step difference of more than 10dBTwo neighboring defects of more than 10dBCluster of three non edge defects of more than 5dB not connected to blind spotThree locations with less than 5% probabilityOne location of less than 1% probability

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