A GUIDE TO VISUAL FIELD PRINTOUT INTERPRETATION
Perimetry is a standard method used to assess a patient’s visual field. It provides a measure of the patient’s visual function throughout their field of vision.
Uses
-Detection of pathologies
-Evaluation of a disease status
-Follow up on pathologies overtime to determine progression or disease stability
-Determination of efficacy of treatment
-Visual ability testing
Points to note before interpretation are:
Patient Name: Check that the result you are interpreting truly belongs to the patient. The names must be spelt correctly for the purpose of future comparison.
Gender: Check that the gender is correct because certain names can go for either gender.
Age: Ensure an accurate date of Birth because the computer will compare the patient’s result with its already stored age-matched normal. The wrong date of birth will give a wrong result by either overestimating or underestimating the result.
Pupil Size: Pupil size should not be less than 2.5mm. This is to avoid ringed scotoma
Patient RX: Patient prescription should be in place to enhance VA and contrast sensitivity
To appropriately interpret the visual field result, we will employ an acronym called “WANDER”. This acronym will effectively help clinicians to interpret visual field results without missing any step.
W= What was done. A =Accuracy. N= Normal or abnormal. D=Defect. E= Evaluation. R=Reflect
1. What was done.
This answers 5 questions.
a) what was the STRATEGY used?
b) What was the AREA tested?
c) What type of STIMULUS used?
d) What type of PERIMETRY done?
e) What type of TEST PATTERN used?
a) STRATEGY
The different strategies we have in perimetry testing are-
-Full threshold Strategy
-fast threshold Strategy
-Screening Strategy
-Short Wavelength Automated Perimetry (SWAP)
-Frequency Doubling Technique (FDT)
-Pulsar
SWAP, FDT and PULSAIR are function specific tests and therefore uncover glaucoma early. For a comprehensive explanation, contact any certified FOM mentor.
b) AREA
The different areas available for testing on perimeter include: 10°, 24°, 30°, 5º etc.
The area tested is dependent on what the clinician is probing for example. area 24° and 30° are glaucoma specific due to the distribution of the Arcuate bundle on that region. Area 10° is good for glaucoma follow up while area 5º is useful for testing maculopathy.
c) What TEST PATTERN was used?
Test patterns are well standardized in order for test results to be comparable between different sessions, between different patients and even between different eye care providers.
The commonly used test patterns include:
a) The G, 32°, 30-2, 24-2 patterns for glaucoma
b) The M, 10-2 pattern for Macula
c) The 07 pattern for full field test (e.g neuro)
d) The F pattern for testing fovea
e) The N pattern for neurological disease.
f) The ET (Esterman) pattern for Driving
g) The BT pattern for Blepharoptosis
h)The BG pattern for testing for Blindness
d) What type of stimulus was used?
Standard Automated Perimetry (SAP) uses a white stimulus on a white background of the cupola is used. SWAP uses Blue on Yellow stimulus. FDT uses flickering stimulus. Pulsar uses pulsating stimulus.
2. ACCURACY
This check whether the test is reliable or not. In the printout check the reliability indices which include: -Fixation loss (FL), False negative (FN), False positive (FP), Short term fluctuation (SF)
-Fixation loss (FL): FL is the number of times a patient responds to a target placed in the blind spot. If a patient responds to the stimulus, a fixation loss is recorded. FL should be < 33% depending on the instrument.
-False Negative (FP): No response at a location that had a measurable threshold earlier in the examination. In Humphrey, FN <1/3 (33%)
-False Positive (FP) – This is when the patient responds to the sound cue alone. It is seen in trigger happy patients. (In Humphrey, FP <1/3 (33%).
Short Term Fluctuation (SF): The variability in the sensitivity which occurs when a threshold is estimated repeatedly during a single visual field examination. It is a fluctuation of measured thresholds within a visual field examination. SF < 1/4 (<25%) in Humphrey and Octopus
3. NORMAL OR ABNORMAL
Check if the field is normal or abnormal? To find out, check the printout for the following:.
a) Numerical value b) Gray scale c) Total
deviation d) Pattern deviation e) Probability graphs. f) Bebie curve g) Glaucoma hemifield. h) Visual field index i) Global indices.
The Numerical Value – These are numbers that represent the sensitivity of the retina in decibels.
Grey Scale- This is a map that represents the sensitivity across the patient’s visual field. It is not dependable but may be used in patient’s education.
Total Deviation- The map compares the patient’s visual sensitivity to a group of average normal individuals of the same age. E.g. if the sensitivity of the patient’s retina point is 20db and the normal for this age at this same point is supposed to be 25db then the difference is -5db. This is total deviation.
+ve values = more sensitivity than the average individual of that age
-ve values = represents decreased sensitivity from normal.
Pattern Deviation: PD is a corrected version of Total deviation. It highlights localized defect by correcting for diffuse changes in the hill of vision. It allows for the assessment of localized visual field loss without the influence of diffuse defects.
Probability plots- The probability plots for both the total and pattern deviation map indicate the statistical significance of the deviation for each point tested. It is represented by shades and symbols. The darker the symbol, the less likely it is that a point depressed by that amount would occur in a normal population
Cumulative Frequency Curve (Bebie Curve)- Is the graphical ranking for the defect for each point in the visual field where X axis represents the rank of the defect from smallest to largest and Y axis is the magnitude of the defect. Contraction towards the X axis shows a diffuse defect while towards Y is signifies a localised defect. Contractions of the field towards both X and Y axis is mixed field defect.
Visual Field index (VFI)- Is a new index which expresses the visual field status as a percentage of an adjusted visual field of a normal eye. It is derived from PSD and MD.
VFI ranges from 0-100% where 100% is a perfect field and 0% is a poorest field.
Glaucoma Hemifield Test (GHT)– Is the Sensitivity difference between the upper and lower hemifield. It is a hallmark of glaucomatous field
GHT Classification-
Outside normal Limit – Obvious differences between the upper and lower halves of the field.
Borderline-Early differences
Within Normal limit – No differences
Abnormal high sensitivity
General depression of sensitivity
GLOBAL INDICES
Give a summary of the visual field status and also useful for the assessment of the severity of the visual field loss. Global indices available on Humphrey/Octopus are:
–Mean deviation / Mean defect (MD)
-Pattern standard deviation (PSD) or Square root of loss variance (sLV)
-Corrected pattern standard deviation (CPSD) or Corrected Square root of loss Variance (CsLV)
-Short term Fluctuation (SF)
Mean Defect (MD)– This is the mean average of loss of sensitivity shown on the total deviation or comparisons map. It gives a sense of the overall height of the hill of vision.
Pattern standard deviation (PSD)- This is the mean Value of PD. It measures the degree to which the shape of a measured visual field or hill of vision departs from the normal age-corrected reference field model. It is much more reflective of localized loss. It gives us the impression of the SHAPE of the field and has a +VE sign.
A small PSD = Smooth uniform hill of vision
A large PSD = Irregular hill of vision.
Corrected pattern Standard Deviation (CPSD)-Just like PSD, it is the mean of PD but it does not take short term fluctuations into account.
4. DEFECT
Here we try to find out if the pattern of defect goes with any particular disease. Is it neurological defect or retinal defect?
5. EVALUATE
In evaluation, you check if the field is normal or abnormal or if there is any progression. Check if the field defect is neurological or due to glaucoma, cataract and other artifact. Also check whether the result can be accepted
6. REFLECT
Decide on what next to do. Do you need to repeat the test or interpret it? Important to note.
CONCLUSION
The VF test is a diagnostic tool that should be used with other clinical data to reach a diagnosis. Correlate VF with disc cupping, NFL loss, IOP and complete fundus Examination.