A visual field encompasses the area of space or area of the external environment that is seen by a steadily fixating eye. Advances in the technology of visual field testing have changed our clinical perception of normal and abnormal fields of vision. It is of great importance to diagnose and monitor optic nerve diseases, notably glaucoma. Automated perimetry is now the current gold standard for both diagnosis and follow-up of glaucoma. The Humphreys visual field analyzer by Zeiss technology is the most widely used machine for visual field assessment. In this write-up, we will illustrate ten pearls to interpret a Humphrey’s Automated visual field printout.
For a reliable visual field test, a good understanding between the operator (usually an ophthalmic technician) and the patient is important. It is a good practice for the technicians to undergo self-testing in order to get an idea about the procedure and would help in explaining the procedure better. The patient needs to understand the task at hand, the nature of the light stimuli, the target to focus on, and that he may pause the test and take a break in case fatigued. If the patient is poorly attentive the technician could tailer the Visual Field to be done using a faster strategy like 24-2 SITA fast. Any abnormal test or unexplained progression has to be repeatable. Visual field done on two machines of different manufactures cannot be compared. For better comparison always do visual field testing by using the same program.
2. Patient information and examination details
Start at the top. Interpretation should start at the top. The top of the visual field printout contains key information about the patient including his/her name, age, ID, refraction, and pupil size. For a follow-up printout, the name has to be identical to that entered at baseline. As the test results are compared with a normative database of the same age group, one should enter the correct birthdate. One should make sure that the refractive correction and visual acuity entered is appropriate and correlating with foveal sensitivity. In order to prevent artifacts, the lens must be properly placed in the trial frame and the correcting lens should be close to the testing eye. Very high refractive errors may be corrected by contact lenses to avoid rim artifacts. A patient with central scotoma may not able to fixate with the central target. The standard size of the stimulus is size III for all routine tests, but in those with poor vision, the test may be conducted with size V.
3. Reliability indices
Examination of the reliability of indices is the next key step in the visual field interpretation. These parameters say, can you trust what you see? It is very important to see the foveal threshold at the beginning of the interpretation. If the patient has good visual acuity and a significant reduction in foveal sensitivity, one should become alert and check for uncorrected refractive error. The other Reliability Indices are as follows:
- Fixation loss. This index indicates how steady the patient gazes at the fixation stimulus. During the test, 5% of the stimuli are presented to the presumed location of the patient’s blind spot (Heijl–Krakau method), and positive responses to these stimuli indicate a fixation loss. High fixation loss (>20%) is a strong indication that the test results are unlikely reliable.
- False-positive errors This helps to identify “trigger happy patients” who click the response button to a nonprojected stimulus. The field often looks normal. FP rate exceeding >15% is indicative of unreliability.
- False Negative (FN) error. FN error is detected when the patient fails to respond to the light stimuli in an area previously detected to have sensitivity based upon an earlier response. High FN errors may indicate a patient who is not paying much attention or showing fatigability during the course of the test. In general > 20% FN is considered as abnormal.
4. Raw data and Grey Scale
This is the actual value of the retinal sensitivity measured in decibel (dB) units of the selected points calculated by the perimeter. The values are indicated between 0 to 50. The value zero indicates absolute scotoma (no response even for maximum intensity of light- 10,000 asb), and the number 50 is the highest retinal sensitivity that can be recorded by HAF (0.1 asb is the dimmest light that can be projected by HAF analyzer. Sensitivity is highest at the fovea and as we go towards the periphery the retinal sensitivity decreases. The numerical value of the raw data in dB units are represented graphically according to varying shades of grey, in which areas of lowest sensitivity are denoted by darker shades and areas of higher sensitivity are denoted by lighter shades. This part of VF printout is an easy plot to use for easy identification of scotomas as well as getting a general assessment about the locations and size of the defects. This is useful for explaining to the patient and the clinician should not use it alone for interpretation.
5. Total deviation
The total deviation is the difference between the patient’s measured retinal sensitivity and the age-matched normal value at each tested location. Total deviation numerical plot (TDNP) represents the difference between the actual retinal sensitivity of patients and age-matched normative data, expressed in terms of deviation values from normal. The value zero in TDNP at a particular point means the measured retinal sensitivity and the normative data are the same. The value with (-) sign indicates loss of retinal sensitivity at that point. For example at a particular point on raw data shows a sensitivity of 21 dB and the corresponding figure in TDNP is -6 dB, which means the normal sensitivity at that point is 27 dB. The higher deviation with (-) indicate that the defect at that point is deeper.
Total deviation probability plots (TDPP). TDPP gives the probability of each point being normal or abnormal (TDPP is the graphical representation of TDNP). It gives the extent and pattern of the field defects not the depth of the field defect. The dot sign in the TDPP is normal and all other symbols denote different p-value (<5% <2% <1% <0.5%), in general, the darker the symbol, the more the chance of being abnormal.
6. Pattern deviation plot.
In a single field analysis print out, pattern deviation plot most important and difficult to understand. The pattern deviation plot is made to identify deep scotomas (localized defects) in the generalized field defects. Pattern deviation numerical plot (PDNP): PDNP is derived from TDNP. To convert TDNP to PNDP the machine identifies the 7th best point in the TDNP. For example, after the chronological ordering of all deviation values of TDNP, if the 7th best value is -5, then add 5 to all deviation values on TDNP. So the 7th best point the TNDP becomes zero on the PNDP. Since -5 dB is the 7th best deviation point on TDNP, +5 is added to all the points of the total deviation numerical plot to convert to PNDP. Pattern deviation probability plot (PDPP) is the graphical representation of PDNP and it depicts the probability of pattern deviation being abnormal. Dot symbol in the TDPP is normal and all others symbols denotes different p-value (<5% <2% <1% <0.5%).
7. Global indices
The main global indices are 1) mean deviation (MD); 2) pattern standard deviation (PSD); 3) short term fluctuation (SF); 4) corrected patterns standard deviation (CPSD); 5 visual field index (VFI). The indices available on SITA strategies are MD, PSD, and VFI.
- Mean deviation: MD is the average of all deviation points on the total deviation numerical plot and it signifies the overall depth or severity of the field defects. It has a (-) sign and the higher the numerical value deeper the defect. The generalized defects or in advanced disease, MD will be high, whereas in a small localized field defect the MD value will be low.
- Pattern standard deviation (PSD). It is the standard deviation of all deviation in total deviation numerical plot or it is an index of the degree to which the numbers in the total deviation plot differ from each other. It gives an idea of how much the patient’s field varies across the values. A lower value of PSD indicates that the patient field is more likely to resemble the shape of the hill of vision. A higher value shows more disturbances in the hill of the vision. If the value is being given with a p-value then it may be abnormal, and in general lower the p-value more the chance of being abnormal.
- Visual field index (VFI): VFI is a relatively new index that expresses patient visual field status in the percentage of normal age-adjusted visual field. This index is derived from MD and PSD, and more weight is given to central points. The VFI ranges from 0 to 100 %.
8. Glaucoma hemifield test (GHT)
Asymmetry in defects between the upper and lower half of the visual field is the hallmark of glaucoma. GHT is helpful to pick up these dissimilarities among the deviation values of corresponding points on either side of the horizontal axis. GHT evaluates five-zone in the one half of the field and compares their mirror image zone to the other half of the field. The difference is compared with the limits taken from a normative database. Five results of GHT are as follows. 1). Borderline; 2) Outside normal limit; 3) Abnormally low sensitivity; 4) Abnormally high sensitivity; 5) Within normal limits.
9. Gaze tracker eye-tracking system
This system monitors the gaze on the SITA strategy and tells deviation from a horizontal line. Upward deviation indicates the patient’s gaze was not on the fixation target. The higher (greater the magnitude) the deviation more will be the fixation loss. A deep downward deviation indicates a blink.
10. Is the field report is abnormal or is it glaucomatous
The final tip is the ability to answer the question of whether a disturbed visual field test is glaucomatous or not. Anderson’s criteria are helpful in diagnosing early glaucomatous field defects. These are 1) Abnormal GHT; 2) Three or more non-edge points of the 30-2 printout, contiguous and with a P < 5%, out of which at least 1 has a P < 1%; 3) CPSD or PSD should be abnormal and should have a P < 5%. For diagnosing glaucomatous field report must be supported by clinical findings, optic disc changes/ retinal nerve fibre defects observed. There is a learning curve in doing HAF, and the patient might require several tests to get reliable results. The field defects should be reproducible, statistically significant, and correlate with clinical findings of the optic disc and RNFL evaluation.
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