|Year : 2021 | Volume
| Issue : 1 | Page : 38-54
Visual fields in glaucoma - An overview
Dr. G. R. Reddy Eye Care Center, Tadepalligudem, West Godavari District, Andhra Pradesh, India
|Date of Submission||12-Aug-2021|
|Date of Decision||13-Aug-2021|
|Date of Acceptance||13-Aug-2021|
|Date of Web Publication||01-Nov-2021|
Dr. G R Reddy
Dr. G. R. Reddy Eye Care Center, Tadepalligudem, West Godavari District, Andhra Pradesh
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Reddy G R. Visual fields in glaucoma - An overview. J Ophthalmol Clin Res 2021;1:38-54
Visual fields are the gold standard in the field of glaucoma which aids in the diagnosis and in setting the target intraocular pressure (IOP) to arrest progression and onset of new field defects. Understanding of single-field analysis printout is mandatory for a systematic approach to interpreting visual field printout. It is divided into 11 zones. Broadly, these 11 zones can be classified into two groups:
Group 1 consists of zones independent of normative data and STATPAC analysis.
Zone 1 Patient data/test data
Zone 2 Reliability indices
Zone 3 Raw data
Zone 4 Gray scale
Zone 11 Gaze tracking.
Group 2 consists of zones dependent on normative data and STATPAC analysis.
Zone 5 Total deviation numerical plot (TDNP)
Zone 6 Total deviation probability plot (TDPP)
Zone 7 Pattern deviation numerical plot (PDNP)
Zone 8 Pattern deviation probability plot (PDPP)
Zone 9 Global indices
Zone 10 Glaucoma hemi-field test.
| Zone 1|| |
Patient data and test data should be exactly similar to that given in the order form for visual field testing, which is mandatory to obtain an overview printout. The age of the patient should be entered accurately otherwise age-matched data will be inaccurate.
| Zone 2 and Zone 11: Reliability Indices and Gaze Tracking|| |
Performance by the patient is gauged by the reliability indices and gaze tracking. The least false positives (FPs), false negatives (FN), and fixation losses (FLs) indicate good reliability. The FP index is the most important and useful of the three available reliability indices. FP exceeding 15% is strongly associated with compromised test results and the test should be repeated. Try achieving reliability indices near to 100% perfection, especially in cases of glaucoma suspect. In gaze tracking, upward deflections indicate gaze error. Downward deflections indicate absent pupil images or corneal reflexes (usually from blinks).
| Zone 3: Raw Data|| |
It is the measured retinal sensitivity in dB units at that particular point. The numerical value is directly proportional to the retinal sensitivity. A < sign in front of 0 indicates an absolute scotoma. A < sign in front of a numerical value indicates the time to change the bulb or need to calibrate the machine. Low retinal sensitivity in the central 16 points of 24-2 (where higher sensitivity is anticipated) is an indication to repeat the test with 10-2 and assess if glaucoma is originating in the central 10° circle area.
| Zone 4: Gray Scale|| |
Gray scale is the pictorial form of the raw data where different shades of gray are given for a different range of sensitivity. Gray scale gives valuable information regarding the pattern of the field defect, the multicenter origin of glaucoma, depth of the scotoma (increase in the dark shade), and horizontal and vertical progression on follow-up tests. The main disadvantage of the gray scale is that a mild-to-moderate loss of sensitivity in the central 10° area will not be appreciated at the earliest, owing to its brighter shade.
| Zone 5: Total Deviation Numerical Plot|| |
TDNP is nothing but raw data expressed as deviation values from the normative data (deviation from the normal slope of the hill of vision). The exact depth of the scotoma is known from TDNP.
Raw data are expressed as deviation values from the normative data (the deviations from the normal slope of the hill of vision) in total deviation numerical value. The normative data at 1, 2, and 3 are 33, 30, and 30 dB, respectively (normative data − raw data = TDNP).
Points above the normal slope of vision are better sensitivity points.
Points on the normal slope of vision are no loss of sensitivity points.
Points below the normal slope of vision are loss of sensitivity points.
The numerical values of TDNP can be divided into three groups:
Group 1 - The deviation values without any sign (yellow-colored dots). These are points with sensitivity better than normal. These points will be above the normal slope of the hill of vision.
Group 2-0 deviation (green-colored dots). These are the points with no loss of sensitivity. These points are positioned on the normal slope of the hill of vision.
Group 3 - The deviation values with −ve sign (red-colored dots). These are the points with a loss of sensitivity. Higher the deviation value deeper is the scotoma and vice versa. Superficial scotomas with lesser deviation will be close to the normal slope of the hill of vision. Note that TDNP most of the time may not have Group 1 point. In an early localized scotoma, most of the points will be either on or close to the normal slope of the hill.
| Zone 6: Total Deviation Probability Plot|| |
TDPP gives the extent and pattern of the field defect but not its depth. STATPAC calculates the P value of the points with a loss of sensitivity. Each deviation value of the TDNP is given a symbol according to its P value and is plotted as a TDPP. The superficial scotomas will be represented by P values ranging from <5% to 1%. The deep scotomas are represented by P < 0.5% (black square). A black square represents a wide range of a loss of sensitivity usually from 6 dB onward. Cataract patients will have a loss of sensitivity >6 dB at most of the points. Probability plots in case of cataracts show uniform generalized depression with black squares at most of the points. If this cataract patient develops a localized field defect either due to glaucoma or any other cause, he/she will not be appreciated in TDPP as the majority of points were already black squares. Hence, pattern deviation plots are created to identify these localized field defects in generalized depression. This localized field defect can be appreciated in the raw data, gray scale, total deviation numerical plot though masked in the generalized depression of TDPP.
P < 5% indicates that this degree of loss of sensitivity at that point is seen in <5% of normal population. The P < 5% is represented by:
P < 5%
P < 2% indicates that this degree of loss of sensitivity at that point is seen in <2% of normal population. The P < 2% is represented by:
P < 2%
P < 1% indicates that this degree of loss of sensitivity at that point is seen in <1% of normal population. The P < 1% is represented by:
P < 1%
P < 0.5% indicates that this degree of loss of sensitivity at that point is seen in <5% of normal population. The P < 0.5% is represented by:
P < 0.5%
| Zone 7 and Zone 8: Pattern Deviation Plots|| |
The basic concept behind the creation of pattern plots is to remove generalized depression from total deviation plots till a certain percentage of points are not represented by any P value in PDPP. The dB value that converts the 7th best deviation point to normal sensitivity point or 0 deviation point is added to all points in TDNP to convert TDNP to PDNP. The 7th best deviation point was preferred to any other point as about 15% of the points of TDPP will not be represented by any P value symbol in PDPP. If a higher number is selected, the recent-onset scotomas will not be represented by any P value symbol and the direction of progression is likely to be missed.
Conversion of TDNP to PDNP can be explained in three steps.
Step 1: Identification of 7th best deviation point of TDNP
The most important key point for the conversion of the TDNP to PDNP is the identification of the 7th best deviation point of TDNP. Before identifying the 7th best deviation point of TDNP, the following points are to be noted.
- In 30-2 point pattern, only points of 24-2 point pattern are considered
- In 30-2 and 24-2 point patterns, the three points in the area of the blind spot are ignored
- In a 10-2 point pattern, all 68 points are considered (blind spot is present outside central 10° field).
All the points in this TDNP are with −ve sign in front of the deviation values. These points represent a loss of sensitivity. −4 dB deviation is the best deviation point, and −23 dB deviation is the worst deviation point of this TDNP. There are no normal sensitivity points or points whose sensitivity is better than normal in this TDNP. Now, the computer arranges the deviation points of the TDNP in chronological order based on deviation values from the normative data. The first point is the best deviation point and the last point being the worst deviation point. The computer selects the 7th best deviation point after ignoring the above-mentioned points, while arranging the deviation values in chronological order. All these calculations will be done by the software in the field analyzer.
The 7th best deviation value points (−5 dB) are represented by green dots. Better sensitivity points than the 7th best deviation value (<−5 dB) are represented by yellow dots, and loss of sensitivity points more than the 7th best deviation value (>−5 dB) are represented red dots. 7th best deviation point is −5 dB.
Step 2: Converting the 7th best deviation point to zero (0) deviation point or in other words bringing the 7th best deviation point to the normal contour of the hill of vision. Since the 7th best deviation point is −5 dB, we have to add +5 dB to make the 7th best deviation point to zero (0) deviation point or in other words to bring the 7th best deviation point to the normal contour of the hill of vision.
Step 3: The dB value (+5 dB) that makes the 7th best deviation point to zero (0) deviation is added to all points of TDNP to convert it to PDNP.
The yellow points (deviation values better than 7th best deviation value) of TDNP will become points without sign in PDNP. The green points (7th best deviation value points) of TDNP will become 0 deviation points in PDNP, and the red dots (deviation values worse than 7th best deviation value) in TDNP are shown as violet dots (superficial scotomas) and red dots (deep scotomas) in PDNP.
By elevating the sensitivity of each point by 5 dB value, the 7th best deviation point becomes normal (0 deviation point) and the first six best points of TDNP become 1, 1, 1, 0, 0, 0 deviations, respectively, in PDNP in this printout. From this, it is very clear that the PDPP will never show generalized depression and always will have at least seven points, without any significant P value symbol in the PDPP. The pattern and extent of the field defect will be appreciated in any situation in PDPP. By identifying the superficial scotomas (points with P value symbols except for P < 0.5%), in PDPP, the direction of progression of the field defect will be known.
Conversion of Total deviation plot to Pattern deviation plot
If the 7th best deviation point is taken into consideration to convert TDNP to PDNP, a minimum of 15% of the points of TDNP will not be represented by any significant P value symbol in PDPP. Hence, PDPP is always a localized scotoma plot and a minimum of 15% points of PDPP are without a significant P value symbol. These 15% points without a significant P value symbol in PDPP are enough to highlight the pattern, and the direction of progression of the field defect presents as a generalized depression of TDPP.
If the 10th best deviation point is taken into consideration to convert TDNP to PDNP instead of the 7th best deviation point for conversion of TDNP to PDNP, the 10th best deviation point becomes 0 deviation point in PDNP and it will be on the normal slope of the hill of vision. There will be a minimum of 10 points (around 20% of the points) in the PDPP of 24-2 point pattern will not be represented by any significant P value symbol.
If the 25th best deviation point is into consideration to convert TDNP to PDNP, 25th best deviation becomes the 0 deviation point in PDNP and it will be on the normal slope of the hill of vision. There will be a minimum of 25 points (around 50% of the points) in the PDPP of 24-2 point pattern will not be represented by any significant P value symbols. If we select the deviation points more toward the right in the chronologically arranged deviation values of TDNP to convert TDNP to PDNP, the superficial scotomas of TDPP (recent-onset scotomas) may become nonsignificant in PDPP, and hence, the direction of progression may not be appreciated. That is the reason why the 7th best deviation point was selected to convert TDNP to PDNP.
The TDPP with generalized depression and a normal PDPP indicates that it is a case of uniform generalized field defect. In such a situation, always think of the conditions such as cataracts and media opacities, and check if the fields were done with proper refractive error correction or not. Apart from glaucoma, some neurological conditions such as AION and occipital lobe infarcts can be diagnosed based on the pattern of the field defect. When these conditions are suspected in the presence of cataracts, the PDPP plays a major role in picking up these diseases. If the PDPP does not show any field defect (uniform generalized field defect) as in this case, we can eliminate the above-said conditions. If the PDPP shows a localized field defect (irregular generalized field defect), the diagnosis can be made depending on the pattern of the field defect.
In this case, the deviation values of TDNP vary from 3 dB to −33 dB. The 7th best deviation value in TDNP is (0), the 15th best deviation value is −1 dB, and the 25th best deviation value is −2 dB. In localized scotoma, the deviation values of most of the points will be minimal, and hence, there are close to the normal contour of the hill of vision. In this case, the 7th best deviation point is already (0) dB, and is on the normal contour of the hill of vision. No dB value is added to TDNP during the conversion of TDNP to PDNP, Hence, both numerical plots look similar and hence both the probability plots look similar.
In localized field defects, both probability plots (TDPP and PDPP) look similar as the 7th best deviation point of TDNP is either (0) or minimal deviation value. A lesser dB value is added to convert TDNP to PDNP. Hence, TDNP and PDNP are identical and so are the TDPP and PDPP.
Uniform generalized field defects show generalized depression in TDPP with a normal PDPP. In uniform generalized depression, the loss of retinal sensitivity is almost similar at all points, the dB value that brings the 7th best deviation point to the normal slope of the hill of vision will also bring all the remaining points either to the normal slope of the hill of vision or close to it and hence are not represented any significant P value symbol in the PDPP.
Irregular generalized field defect shows generalized depression in TDPP with a localized field defect in PDPP. Here, the dB value that brings the 7th best deviation point to the normal slope of the hill of vision, can only bring the recent onset. Scotomas nearer to the normal slope of the hill of vision and cannot change the P values of deeper scotomas and will be highlighted in PDPP.
If the sensitivity of the 7th best deviation point of TDNP is better than normative data, it will be above the normal contour of the hill of vision. To bring the point to the normal contour of the hill of vision, we have to decrease the sensitivity of the points of TDNP. Normally, during the conversion of TDNP to PDNP, the sensitivity of the points of TDNP will be elevated. However, in this example, since the 7th best deviation point of TDNP is +18 dB (better than normative data), the sensitivity of each point in TDNP is decreased by 18 dB, or in other words, a generalized depression worth of −18 dB is added to all the points of TDNP during its conversion to PDNP. As there is a decrease in sensitivity at all the points in PDNP, we see more black squares in the PDPP than the TDPP. If the 7th best deviation point of TDNP is better than normative data, we see more black squares in the PDPP than the TDPP.
| Zone 9: Global Indices|| |
The MD index is the average of all the deviation values of the TDNP except the deviation values of the two points in the area of the blind spot. It is an index developed to express the depth of the field defect, and its value is directly proportional to the depth of the field defect. Note that that the value of the MD index is always lesser than the exact depth of localized field defects. Even a small increase in MD on follow-up tests should arise suspicion regarding the progression of the localized field defect. In uniform generalized field defect, the MD index is the true index to express the depth of the field defect, and to some extent, it gives true value in irregular generalized field defect.
PSD is an index developed from TDNP to express the contour of the hill of vision whether it is smooth or irregular. In uniform generalized depression (e.g., cataract), there is uniform loss of sensitivity affecting the height of the hill of vision but not the contour of the hill of vision (smooth contour is maintained). PSD will be nonsignificant. The contour of the hill of vision will be affected when there is a localized or irregular generalized field defect and PSD will be significant. PSD is not related to the depth of field defect but only signifies the contour of the hill of vision whether it is smooth or irregular.
In early and established cases of glaucoma, PSD will be high or significant and is represented by P value. As the disease progresses, the sensitivity at all points will be nearing 0 dB and the PSD will be low and with a nonsignificant P value. The important points to note regarding PSD are: (1) it does not carry any sign in front of it; (2) it is not the index to tell the depth or severity of glaucoma; (3) it is an index developed to pick up early localized field defects.
VFI is an index developed from PDNP and hence is not affected by cataracts. It transposes deviations from the normative data into a percentage scale. 100% means the quality of life is not affected. VFI reflects the quality of life. A certain level of defect has to be reached to deviate the VFI from 100%. VFI is defined as a deviation below the 5% probability level on the pattern deviation plot. The center of the visual field has more weight than the periphery while calculating the VFI and the index switches to the use of a total deviation plot if the MD shows severe global visual field deviations. VFI is absent in 10-2 printouts. The VFI (glaucoma progression index) is a new perimetric index designed for two purposes: (1) for calculating the rate of glaucomatous progression (glaucoma progression analysis), this is the reason why the index is also named as glaucoma progression index; (2) to reflect the quality of life, hence, doctors may use this for educational purposes since patients can quickly perceive it with minimal explanation.
VFI is expressed in percentage where 100% represents a normal visual field and 0% represents a perimetrically blind field. VFI-100% means that the quality of life is not affected. The VFI cannot go beyond 100%.
| Zone 10: Glaucoma Hemifield Test|| |
This is an index to pick up early field defects due to glaucoma. Five groups of points on either side of the horizontal raphe where the glaucoma defects usually arise are designed. A score is assigned to each zone based on the location of the zones and their deviation values in the PDNP. A comparison of each upper zone is made with the corresponding lower zone, and the difference in scores between the upper and lower zones is calculated. The difference is compared with significant limits taken from a database of normal subjects, and the results are given as borderline, outside normal limits, low sensitivity, abnormally high sensitivity, and within normal limits. Glaucoma hemifield test (GHT) is absent in 10-2 printout. With the above information, one can diagnose glaucoma suspect at an early stage using Anderson's criteria.
Any localized scotoma should fulfill Anderson's criteria to be labeled as a glaucomatous field defect. In localized field defect, concentrate TDPP, and in irregular generalized field defect, concentrate PDPP.
The localized field defect should have a minimum of three nonedge cluster points of 30-2 point pattern either in TDPP (localized field defect) or in PDPP (in an irregular generalized field defect) with 2 points have P < 5% and (1) P < 1%. (2) PSD: P < 5%. (3) GHT - Outside normal limits.
30-2 point pattern: 76 points is distributed in a 30° circle area with fovea as the center. There are no points either on the horizontal or vertical axis. The distance between point to point is 6° and point to the axis is 3°. There are no points at the center 3° circle which is called a bare area.
24-2 point pattern is nothing but a subset of 30-2 point pattern. The outer set of points except for the two nasal points on either side of the horizontal axis is eliminated from the 30-2 point pattern to form a 24-2 point pattern. Hence, the total points of the 24-2 point pattern (76 – 22 = 54) will be 54 points.
24-2C new point pattern is introduced to reduce the extent of the bare area present between the 3° circle points and 9° circle points. The bare area present between 3° circle points and 9° circle points in 24-2 point pattern is filled with 10 additional points in 24-2C new point pattern.
10-2 point pattern: 68 points is distributed in a 10° circle area with fovea as the center. There are no points either on the horizontal or vertical axis. The distance from point to point is 2° and there are no points at the center 1° circle. Hence, we are testing almost up to fixation in the 10-2 point pattern program. We should not comment on foveal status till we do the test with a 10-2 point pattern.
Macular program is nothing but a subset of 10-2 point pattern. The outer 3 sets of points of the 10-2 point pattern are eliminated to form the macular program. So the macular program has 16 central points of 10-2 which are present within the 3° radius circle from the fixation point. In advance cases of glaucoma to assess the foveal status and to comment on the macular split, a macular program with stimulus size III or V can be asked.
From Flowchart 1, it is very clear the visual field testing starts with a 30-2 or 24 -2 point pattern in the case of glaucoma. If there is an indication for a 10-2 printout, repeat the test with a 10-2 point pattern.
| Interpretation of Visual Field Defects|| |
Make sure that the test is done with the correct point pattern. In a case of glaucoma either suspect or established, initially, the test should be done with a 30-2, 24-2, or 24-2C point pattern. The indications to repeat the test with a 10-2 point pattern are (1) probability plots - a black square in the 10° circle area; (2) gray scale - dark shade in any quadrant of 10° circle area; (3) raw data and TDNP: significant loss of sensitivity at any of the 16 points in 10° circle area (+); (4) fundus - direction of RNFL defect toward macula; (5) OCT- thinning of RNFL between 6 and 8 clock hours (right eye) or 4 and 6 clock hours (left eye).
Make sure the patient data and the test data are properly fed to the field analyzer by the technician as per the order form. Always put foveal threshold “ON” which correlates with visual acuity. Good foveal sensitivity should have good visual acuity and vice versa. If not correlating check if the refraction for near is accurate or if the patient has understood the method to perform the test.
Never interpret visual fields in isolation. It should always be correlated with fundus. Meticulous fundus examination is a must to identify nonglaucomatous field defects.
Pick-up findings such as disc pallor exceeding the cup, optic disc pit, tilted disc, and proper evaluation of myopic disc help identify nonglaucomatous field defects.
Identification of the artifacts due to small pupil (<3 mm), improper refractive error correction, FLs, FP errors, FN errors, dim bulb, rim artifacts is the most important step in the interpretation of SFA print out, especially in a suspected case of glaucoma.
Always look for indications to repeat the test with a 10-2 point pattern.
| Visual Field-Dependent Factors to Set Lower Target Intraocular Pressure|| |
- Location and extent of the field defect: If the glaucomatous field defect is either originating outside 24° and extended into 10° circle area or starting within 10° circle
- Direction of the field defect: Direction of progression toward fixation
- Depth of the field defect: The presence of absolute scotoma (nonedge point).
MD index represents the depth of the field defect. Do not grade glaucoma on the MD index. The presence of a field defect within a 10° circle area and direction of progression toward fixation even with low MD index needs low target IOP. One need not aim at lower target IOP even with higher MD in the absence of field defect in 10° circle or progressing away from fixation.
The novel concept in the classification of open-angle glaucomas, based on the location of the field defect will help us in locating the site of origin of glaucoma. Perimetry helps classify open-angle glaucomas into three types.
This classification helps in a rational approach not only in the interpretation of visual fields but also in the management of glaucoma. During interpretation of probability plots, always see both the probability plots as a single unit. This is one of the most important concepts in the interpretation of visual fields.
Location, extent, and direction of progression of the field defect
Glaucoma originating as a localized field defect in the upper nasal quadrant with most of the points represented by P < 0.5% (black square). When all points in the field defect in both the probability plots are represented by black squares, it is difficult to tell the direction of progression from the probability plots. TDNP gives a clue to assess the direction of progression. From the TDNP, it is very clear that the field defect started outside the 10° circle and is progressing toward fixation because the loss of sensitivity at the points outside the 10° circle is around 30 dB and the loss of sensitivity at 3° upper nasal point is 12 dB. Hence, we have to correlate both the probability plots and TDNP to know the direction of progression of the field defect. The direction of progression of the field defect toward fixation is not a good sign. We have to aim for low target IOP.
Depth of the scotoma
Loss of sensitivity at most of the points of the scotoma is around 30 dB. Here, MD index is −4.97 dB, P < 0.5%. In localized scotomas, MD index is not a true index to represent the depth of the scotoma.
Foveal sensitivity and raw data
Foveal sensitivity is 35 dB and the field defect is showing few absolute scotomas. There is one absolute scotoma among the 16 points in the 10° circle. Repeat the test with a 10-2 point pattern to know the sensitivity of the points in the macular area. In this case, though the 24-2 MD index is −4.97 dB, P < 0.5%, the current IOP is causing absolute scotomas and the field defect is progressing toward fixation. With the above features of the field defect and keeping the age of the patient (52 years) in mind, we should aim for a low target IOP.
Note that field defect in 10-2 – the minimal loss of sensitivity within the 10° circle area will be better appreciated in probability plots than gray scale.
The most important point, in this case, is to identify the location of origin of glaucoma. Identification of absolute scotomas within the 100 circles and one among the three is on the 30 circle point. It indicates glaucoma started close to fixation and progressing outward. To know the exact extent and depth of the field defect, repeat the test with a 10-2 point pattern. The 24-2 VFI misguides the treatment approach in Type 2 glaucoma.
In Type 2 glaucoma, 10-2 MD index (−9.85 dB) will be always more than 24-2 MD index (−8.18 dB) The 24-2 VFI 79% misguides the treatment approach in Type 2 glaucoma as VFI is not calculated in 10-2 SFA printouts. Low-target IOP must be aimed, and if needed, early surgical approach may be indicated in Type 2 and Type 3 glaucoma.
| The Raw Data|| |
The black square in the upper nasal quadrant in 100 circle area is an absolute scotoma and is an indication to repeat the test with a 10-2 point pattern. The lower nasal quadrant outside the 100 circle area is almost an absolute field defect. 24-2 MD index is − 13.37 dB and 24-2 VFI is 69% VFI is not calculated in 10-2 program. Remember that the progression of glaucoma starting in 100 circles cannot be analyzed based on the change in VFI. Progression analysis for glaucoma starting in 100 circles will be calculated based on the change in the MD index. In this case, glaucoma started in and outside the central 100 circles (Type 3 POAG-Primary open Angle Glaucoma).
Note that the extent of the 3 black squares in the 10° circle area in the 24-2 program will be better appreciated in the 10-2 program. The progression of the field defect into the lower nasal quadrant in the central 10° area is better appreciated in the probability plots than in the gray scale. Concentrate on the 16 points in the 10° circle area in a 24-2 point pattern to know whether the test should be repeated with a 10-2 point pattern or not. Concentrate on the 16 points in the 3° circle area in a 10-2 point pattern to assess foveal status. In this case, the field defect extended into a 3° circle. Maintain very low-target IOP till surgery is planned.
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