An Introduction to Visual Fields Professor David Henson University of Manchester

What are they used for? Diagnosis / Management Glaucoma Space-occupying lesions of visual pathway other optic nerve disorders Functional Assessment of Vision Occupational (e.g. Driving, Flying, Police) Blindness / Low Vision registers Litigation

Extent of Visual Field 100 60 Line of sight Line of sight 60 75 Definition: All the space that any one eye can see at any given instant.

Visual field chart Limit of the visual field

Extent of binocular field 90 60 Binocular Field Monocular temporal crescent

Island of Vision Isopters, lines joining points of equal sensitivity

Sensitivity Differential threshold + Stimulus intensity (I+ΔI) Intensity Background Intensity (I) Differential threshold How much brighter than the background does the stimulus need to be to be seen +

Measuring the visual field Stimulus Angle subtended Fixation point Patient Cupular

Current test strategies Conventional Examination Strategies Unconventional Examination Strategies Kinetic Static Ring perimetry, Frequency doubling, Displacement perimetry, Noise perimetry, Resolution, Blue on yellow Threshold SITA ZATA Full Threshold Suprathreshold Single Multiple

Examination Strategies Kinetic perimetry Conventional Examination Strategies Kinetic Stimulus size and intensity kept constant for each isopter

Kinetic perimetry 3 different stimuli to plot 3 isopters

Advantages of kinetic perimetry Perimetrist has more control area examined direction and speed re-instruct patient repeat measures Severe field loss Children Elderly

Disadvantages of kinetic perimetry Time consuming Less sensitive not fail safe Too much flexibility operator dependent You can also do static testing on a Goldmann perimeter although this is rarely done

Decibel scales Widely used in perimetry High values mean dimmer test intensity High values mean the patient is more sensitive Decibel number can be thought of as the density of a filter placed in front of a fixed intensity lamp

Decibel Scales Not a measure of intensity but attenuation 3 dB down is half as bright 10 dB down 1/10 as bright Log scale Each instrument has a specified 0dB test intensity dB scales refer to this It is often difficult to transfer the findings from one instrument to another

Static perimetry Static Threshold Suprathreshold Monitoring Detection/ screening

Suprathreshold strategy Stimuli presented at intensity calculated to be above threshold If seen no other testing of that location It is very fast Developed for screening/driving test

Suprathreshold strategy Missed stimuli are normally retested and only recorded as a miss if missed on both occasions Missed stimuli may be tested at a series of higher intensities or thresholded Some suprathreshold test present more than one stimulus at a time and the patient verbally reports the number they see

How does the patient respond? Verbally Kinetic perimetry Multiple stimulus suprathreshold perimetry faster (screening) helps maintain vigilance Button press single stimulus (threshold and suprathreshold)

Suprathreshold perimetry Advantages Disadvantages Faster per test location Less than 5 mins for both eyes Ideal for detecting defects Ideal for screening Can miss shallow defects Does not give precise measure of defect depth (unless test establishes the thresholds of missed locations)

Threshold perimetry Used to monitor defects Routine strategy in eye departments Much more time consuming You can also undertake kinetic testing on some static perimeters

Threshold perimetry Measures threshold at series of predetermined locations Uses repetitive bracketing On average ~4 presentations per test location

Print out Thresholds Grey scale Defects Probability values age corrected height corrected Probability values

Visual field advantages Direct measure of visual function What you are trying to preserve Non invasive Relatively cheap, high cost of service due to time commitment High sensitivity and specificity

The End Ref: D B Henson. Visual Fields 2nd Edition, Butterworth Heinmann