Visual Fields in Glaucoma The field analyzer primer, Essential Perimetry, fourth edition. Andreas Heijl Vincent Michael Patella and Boel Bengtsson, 2012 Questions • What is the ‘2’ in 24-2? • How is the GHT calculated? • How is VFI different from MD? • Does the Humphrey use static or kinetic perimetry? • Which reliability index is more important when evaluating glaucomatous fields, FN or FP? • When would you suggest using SITA fast, and not SITA standard? Perimetry Basic concepts Hill of vision • Height determined by • Age • Ambient light • Stimulus size • Stimulus duration • Field defect – any statistically and clinically significant departure from normal hill of vision Automated Static Vs Dynamic Perimetry • Static Vs Dynamic (kinetic) • Dynamic - Stimulus is moved from non-seeing to seeing area • Static - Stationary stimuli are presented at defined points in the visual field Automated Static Perimetry • The most important and prevalent tool for assessing visual fields • Standard of care since the mid 80s Perimetry Technical aspects Stimuli in the Humphrey Perimeter Intensity • Threshold sensitivity is acquired by varying stimulus intensity, not size • Intensity/Brightness • Varies between 0.08-10,000 apostilbs (asb) • dB refers to stimulus intensity • 0 dB – maximal intensity the perimeter can produce (10,000 asb) • 51 dB – 0.08 asb Stimuli in the Humphrey Perimeter - Size • 5 sizes available • III used almost exclusively • Normative data based on size III • V used for advanced loss Stimuli in the Humphrey Perimeter Duration • 200 micro-seconds • Long enough to pick up the signal • Shorter than the latency for voluntary movement Humphrey perimeter – Background illumination • Standard background illumination of 31.5 asb • Simulates minimum brightness for daylight vision (photopic) • Why is photopic better than scotopic? • Less effected by lens opacity and small pupils • Visibility depends more on contrast (rather that absolute brightness) Tested area 24-2 Vs 30-2 • In glaucoma the central visual field is the most diagnostically important • Why not test outside the central 30? • Infrequent to have damage only outside the central 30 • The range of normal in the periphery is very wide 24-2 30-2 54 locations 76 locations Faster Earlier detection? Fewer trial lens artifacts o ?6 24-2 Vs 30-2 Threshold • Suprathreshold • Normal Vs Abnormal • Threshold • Quantification of visual sensitivity • Full threshold Threshold • Values from one location are used as starting points for adjacent locations • Pacing is determined by patient response time • Suprathreshold stimuli at each test location (First – 1 point in each quad) • Intensity decreased (fixed increments) until not seen • Intensity increased (fixed increments) until seen • Value at that point – last seen intensity • Swedish Interactive Threshold Algorithm (SITA) • Complex mathematical model used to estimate threshold values • Starts with full threshold at 1 point in each quad • At least one reversal from descending to ascending Swedish Interactive Thresholding Algorithm SITA • Earlier strategies ended testing on a specific point after a fixed predetermined endpoint was reached • Crossing of the threshold ‘X’ times • SITA – ends testing when statistical certainty is reached at a certain point • Allows test to be significantly shorter SITA • The difference between SITA Std and SITA Fast • The amount of statistical certainty needed • Though SITA FAST is faster than SITA std, it is not easier, and shouldn’t be used in patients struggling with perimetry SITA Standard Fast More precise (but not by much..) Faster More tolerant of patient mistakes May start with dimmer stimuli Recommended standard Good for young/experienced Strategy & Time o 2 24-2 Vs 10-2 Late stage disease • When only a central island remains • 10-2 (has a separate normative database) • Stim V • Extended sensitivity range • ability to follow up advanced disease • No normative data or GPA Single field Analysis of the visual field A series of fields Demographics and testing conditions • Name, ID #, DOB, Age • Test Date • Tested eye • VA, RX • Testing strategy Avner Belkin 2018 FN, FP, FL Reliability indices Avner Belkin 2018 False positive (FP) • Measures tendency to press response button when no stimulus has been seen • Before/during stimulus • Too soon after stimulus • considering patient response time in the same test • Calculated at the end of the exam • Considered the most important reliability index • >15% excluded from GPA Avner Belkin 2018 • Designed to measure inattention • Very bright stimuli presented at location previously found normal False negative (FN) • FN errors are a known feature in glaucomatous eyes, even in attentive patients • Limited utility in glaucoma Avner Belkin 2018 Fixation Loss • Measures gaze stability • Blind spot method • Stimuli shown in area of blind spot • Many pitfalls • Blind spot not properly located • Trigger happy patients • Patients tilting their head • Gaze tracking • Upward lines - amount of gaze error during each stimulus presentation • Downward lines – Gaze direction not successfully measured (ie blink) Avner Belkin 2018 Avner Belkin 2018 Data interpretation Visual field Avner Belkin 2018 RAW DATA Numerical Value Data compared to age-matched controls Total Deviation Pattern Deviation Probability Plots Raw Data • Threshold sensitivities measured at each location • Numerical data • Grayscale map Avner Belkin 2018 Gray Scale Map Strengths Weaknesses Ruling out artifacts Not compared to normative data (so significant changes can be missed) Demonstrating profound visual loss Can miss early loss Common midperipheral loss can be over emphasized Avner Belkin 2018 Total Deviation • Identifies locations with abnormal sensitivity as compared with age-matched controls • Sensitivity range greater in the periphery • A 5dB deviation from age-normal could register as normal in the periphery, but not in the center Pattern Deviation • Sensitivity loss after adjustment for generalized depression/elevation • PD probability plots largely Ignores normal variability and thus highlights subtle, local changes • PD deemphasizes some artifactual findings • Uses 7th highest number on TD plot to account for diffuse depression • 0,0,-1,-1,-1,-2,-2,-2,-2,-3,-4,-7,-8,-11 Avner Belkin 2018 Glaucoma Hemifield Test (GHT) • Plain language classification of 30-2 and 24-2 fields based on glaucomatous field loss patterns • 5 zones in the upper hemifield are scored and compared to mirror images on the lower hemifield • Differences between hemifields are compared to normative data • Relatively high specificity & sensitivity • Depends on patient population Avner Belkin 2018 Avner Belkin 2018 • Outside normal limits • At least 1 zone pair differs like < 1% of the population • Borderline GHT • At least 1 zone pair differs 1% < 3% of the population • General depression/Abnormally high sensitivity • If best test location is so low/high as to be seen by <0.5% of normal subjects • Within normal limits • None of the above Avner Belkin 2018 MD, VFI, PSD Global Indices Avner Belkin 2018 Mean Deviation (MD) • A center-weighted average (points near fixation count for more) of the Total deviation map • Main uses • Stage VF loss • Assess rate of change • Appx ‘0’ in normal, -30 to -35 in blind eyes Avner Belkin 2018 MD Avner Belkin 2018 Visual field index (VFI) • Introduced in 2008 • Expressed in % of normal age-corrected visual function • 100% - 0% • Driving factors to develop VFI • MD too affected by cataract • MD weakly center weighted, and thus not correlated enough with visual function Am J Ophthalmol 2007 Avner Belkin 2018 Visual field index (VFI) • Based on the Pattern deviation plots • But if MD < -20, is based on the Total deviation plots • As compared to MD • Less affected by cataract • Better correlated with ganglion cell loss – much more center weighted than MD Am J Ophthalmol 2007 Avner Belkin 2018 Avner Belkin 2018 Avner Belkin 2018 Pattern Standard Deviation (PSD) • The standard deviation of all points in the total deviation map, around the MD Avner Belkin 2018 PSD Pattern Standard Deviation (PSD) Measures the irregularity, or ‘departure from normal hill of vision’ Avner Belkin 2018 MD/PSD • If we have a small local scotoma in the upper hemifield • The MD would be High Low • The PSD would be High Low • If the entire hemifield is depressed because of cataract • The MD would be High Low • The PSD would be High Low Avner Belkin 2018 Analysis of the visual field Avner Belkin 2018 Single field A series of fields Important concepts in progression 1 2 Event Analysis Trend Analysis • Has there been any significant worsening in the visual field? • Quantify rate of change • Tool for predicting future disability Avner Belkin 2018 Avner Belkin 2018 Event Analysis Glaucoma Change Probability Maps (GCPM) • Based on the PD plots • A complex mathematical model • EMGT criteria for progression • Highly sensitive & specific • Require 5 fields (2 base line, 3 follow up) • Not done for MD < -20 • Do not provide information regarding rate of change Avner Belkin 2018 • Possible progression GCPM • 3 or more test points show progression on 2 fields compared to baseline (2 tests) • Likely progression • 3 or more test points show progression on 3 fields compared to baseline (2 tests) Avner Belkin 2018 Avner Belkin 2018 Avner Belkin 2018 Trend Analysis • Goal – to identify those at risk of significant visual disability during their lifetime • Regression analysis using VFI • Requirements • 5 tests • 2 years • Acceptable 95% CI • Projections • Never > 5yrs • Never > follow up One of the most important predictors for future rate of progression, is past rate of progression Avner Belkin 2018 Avner Belkin 2018 Avner Belkin 2018 Visual field loss in glaucoma Avner Belkin 2018 Functional anatomy & clinical correlations Avner Belkin 2018 Nerve fiber layer & Optic nerve head anatomy • RGCs follow an arcuate path to the ON • Nerve fibers from the temporal retina respect the temporal raphe • Axons maintain a retinotopic layout Avner Belkin 2018 Common glaucomatous VF loss patterns • Arcuate defects (Bjerrum) • Paracentral Scotomas • Nasal Steps • It is common to see more than one pattern on one field Avner Belkin 2018 Arcuate Scotoma • VF loss Courses around fixation, ending at the midline • Connects to the blind spot • A complete arcuate defect is correlated with a full notch on the optic disc Avner Belkin 2018 Paracentral Scotoma Occurs in cases of a partial notch; “partial thickness” axonal damage Axons likely to be of similar length Will include part of the arcuate defect Nasal paracentral scotomas are most common Avner Belkin 2018 Nasal Step • Occurs due to relatively wide spread loss across the optic nerve • Asymmetrical between superior and inferior hemifields • Will manifest as a difference in sensitivity across the horizontal midline in the nasal area – NASAL STEP Avner Belkin 2018 General reduction in sensitivity • Uncorrected refractive errors • Media opacities • Miosis (but not mydriasis) Avner Belkin 2018 Early loss • Very gradual, usually develops over several years • Localized defects can come and go before settling into clear repeatable defects Avner Belkin 2018 Artifactual test results Avner Belkin 2018 Avner Belkin 2018 Frequency of testing • Common recommendation is 3 tests/year in the first 2 years • High sensitivity for catching rapid progressors • Frequent adjustments are needed due to practical/logistic reasons • Glaucoma suspects – according to risk stratification • After many tests with stable ROP – may consider lowering frequency Avner Belkin 2018 Thanks!
