Visual field is the part of environment that is visible to the steadily fixing eye. The limits of visual field for each fixing eye is 60 degrees superiorly and nasally, 75 degrees inferiorly and 110 degrees temporally.
Fig 1 shows the normal island of vision as described by TRAQUAIR. The hill is highest at fixation, where the visual sensitivity is highest and as we go peripherally the height of hill decreases due to decreasing visual sensitivity in the periphery.
Based on this principle the modern visual field testings are operated. Perimetry is the measurement of “Hill of Vision” in terms of establishing the patient’s differential light sensitivity.
There are mainly two types of perimetry test:
Kinetic (ie. Bjerrum’s screen, Goldmann, Lister perimetry)
Static /automated (ie. Humphrey, octopus)
We will be discussing about how to evaluate the Humphrey visual field (HVF).
Before delving deep into the Humphrey field testing, we should know few basic points which will keep on revolving during the whole discussion. Background Luminance- the background luminance is constant for each machine. In case of Humphrey visual field it is 31.5 apostilbs (asb)
Stimulus Intensity- this variable and for HVF it is from maximum 10000 asb to minimum 1asb. The DECIBEL (db) unit is used to denote retinal sensitivity. 0 dB corresponds to the maximum brightness of the stimulus that can be produced by the perimeter (10,000 asb) and 51dB to the weakest intensity possible (0.08 asb).
Fig 2 shows the association between light intensity and retinal sensitivity.
THRESHOLD- it is the dimmest target perceived with 50 % probability for a fixed retinal point. The Threshold point for each retinal point can be determined by bracketing technique.
SIZE OF STIMULUS - In HVF machine it is from size I to size V.
SIZE I – 1/4mm2
SIZE II – 1 mm2
SIZE III – 4mm2
SIZE IV – 16 mm2
SIZE V – 64 mm2
The most common stimulus used is size III. Whereas in case of severely depressed visual acuity patient’s size V stimuli are used most frequently.
Humphrey Visual Field Printout
For easier understanding of the HVF printouts, we would like to divide the printout into 8 segments as shown in (FIG 3)
This area includes the patient’s data. It gives us the idea about the test performed (24-2 or 30-2), the name of the patient, visual acuity, size of the stimulus. Patient’s date of birth is also entered in this zone, which is pretty important as the machine compares the data with the age-matched normative database. We should also note the pupil diameter in this column, as optimum pupil diameter required to perform a good HVF test is 2.5mm to 3 mm.
30-2 test measures 30 degrees temporally and nasally. The program tests total 76 points 6 degrees apart from each other. The points are not arranged over the horizontal and vertical meridian. These are used for general screening and neurological conditions.
24-2 test excludes the peripheral points of the 30-2 program except for the nasal one. The 24-2 program tests total 54 points. This HVF testing program is more commonly used for early detection of glaucoma.
10-2 program measures 10 degree temporally and nasally and tests total 68 points. The points are all aligned two degrees apart and not arranged over the horizontal and vertical meridian. This program is used to look for advanced glaucomatous damages.
This area which gives us an idea about the reliability criteria. The machine gives intermittent stimulus to the previously determined BLIND SPOT, where the patient is not supposed to be responding. If the patient responds to such stimuli, the machine flags the same as fixation loss. If patients respond without even seeing the visual stimuli the machine flags the same in the false-positive column. If patient is not responding to even at the previously determined threshold stimuli, it is known as false negative. A printout with high false-negative values shows a typical “CLOVERLEAF PATTERN”. Recording foveal threshold is optional. Foveal threshold gives agood idea about visual acuity and two should be always compared. The machine flags with “XX” sign if the reliability criteria are outside normal limit, but it don’t mean that particular HVF is of no use.
This zone is noted as greyscale. The machine according gives a colour coding to each point according to the actual perimetry results. This particular zone is more useful from neuro-ophthalmology point of view with their typical patterns. In glaucoma the diagnosis is never made depending on greyscale. The greyscale gives a good tool for patient explanation about the disease per se.
point by point difference from expected value for age-related normal individuals. It gives an idea about generalized depression.
In this segment, machine identifies the 7th best point from the total deviation numerical plot (the outer points of 30-2 printout are excluded except the two nasal ones) are adjusted for overall depression. By this mean the machine is able to find out the focal depressed spots. (Fig 8)
this area of visual field shows a unique indicator of HVF printout, the VISUAL FIELD INDEX (VFI). The index expresses the visual field as a percentage of normal age adjusted visual field. During the calculation the central points, near the fixation are given more importance. Along with VFI this segment gives the Mean deviation (MD) and Pattern Standard Deviation (PSD) values also. The newer machines with SITA standard program don’t give the Short term fluctuation (SF) and Corrected Pattern Standard Deviation (CPSD) values.
this segment tells us about Glaucoma Hemifield Test. We all know glaucoma is a disease respecting the horizontal midline. The machine compares five sets of points above meridian to their mirror image below the midline.
If a single cluster pair differs P value <0.5% or all cluster pair differs P value <1%, machine flags the same as OUTSIDE NORMAL LIMIT. If the P value of hemifield differs by <3%, the machine flags it as BORDERLINE.
this area is comprised of raw data of retinal sensitivities in Db ranging from 0-40 db.
CHARACTERISTICS OF GLAUCOMATOUS FIELD DEFECT-
Asymmetrical across horizontal midline
Located in mid-periphery (5-25 degrees from fixation)
Not attributable to any other pathology
Correlates with optic disc changes
Anderson and Patella Criteria for glaucomatous field defect-
IN PATTERN DEVIATION PLOT 3 or more congruous ‘non-edge points’ in typical arcuate area on 30-2 program depressed with p< 5 % with at least one point with p<1 %
PSD / CPSD @ p< 5%
GHT – outside normal limits
All this must be demonstrated on 2 consecutive field defects.
In some instances where the glaucoma is pretty advanced we may elect to choose MACULAR program to locate macular split. The Macular program is done in central 5 degrees of visual field with size III stimulus. If the visual acuity is severely depressed we may use the size V stimulus too. In conclusion it should be highlighted that with immense advancement of technology visual field has become an everyday routine diagnostic test for a glaucoma surgeon. However, the findings in the HVF printout shouldn’t be taken as last verdict and should always be compared with clinical disc evaluation as many retinal pathologies may also lead to defects in the visual fields.