Visual Fields I

Define the visual field

• extent of space in which objects are visible to an eye in a given position

Approximate monocular visual field size

• 100° temporal

• 60° nasal → nose in the way so less

• 60° superior → brows in the way

• 75° inferiorly

What is the size of the binocular field approximately

• 120° horizontally

Why do we measure the visual field?

• visual field determined by our retina + connectivity between retina and brain

• VF can tell us about abnormalities or damage in the visual system along this pathway

• can help pinpoint exactly where along pathway problem lies

What is the purpose of measuring the visual field?

• detection of abnormalities in visual function

• locating site of abnormality

• monitoring disease progression

What is discrimination threshold?

• tells us when patient just able to detect that stimulus luminance is different from background luminance → contrast perception task

• our ability to discriminate between different light intensities can get worse with disease

e.g if background luminance L → 400 units , smallest change we can detect is 50 units

50/400 =0.125

What does a smaller threshold value indicate?

• more sensitive patient is to small changes in luminance

What is sensitivity?

• The reciprocal of threshold value

1/Threshold

How is stimulus intensity expressed?

• In decibels (dB) on a logarithmic scale

What do higher dB values represent?

• Dimmer (less intense) lights and higher sensitivity

• 33dB average in young patient → good

Can dB values be compared across machines?

• No, because background luminance and max intensity of bulbs in machine differ (varies between manufacturers)

What is the Method of limits - determine threshold?

• in ascending method of limits luminance of stimulus gradually increased → patient reports when they first detect the stimulus (against the background)

• difference between luminance level of stimulus& background luminance gives threshold

What is the Descending method of limits?

• Stimulus brightness decreases until no longer seen

• threshold can also be calculated as avg of both ascending + descending values

Staircase method for determining threshold

• luminance increased in steps until patient reports that they can see it

• then luminance decreased again until they report that stimulus can no longer be seen

• it is then increased/decreased in increasingly smaller increments until each change in luminance gives reversal (bracketing technique)

• threshold avg between last few reversals

Psychometric function to determine threshold?

• stimuli presented at dif intensities + patient reports when they can see stimulus

• curve can be plotted showing dif stimulus intensities in dB (x axis) against % of times patient reports that they saw stimulus (y axis)

• when they could see stimulus 50% of time , corresponding value taken as threshold

• very time consuming

What is the Hill (island) of vision?

• higher the hill, more sensitive that part of retina is

• fovea = most sensitive

What does width of hill represent and the height?

width → Extent of the visual field

height → sensitivity of vision at various location within the field

Retinitis pigmentosa can constrict VF (tunnel vision) we might want to measure extent of px’s visual field

Define kinetic perimetry?

• Uses moving stimuli to plot the full extent of the visual field

• not good at sensitivity so not good for glaucoma etc

• measured on Octopus 900 machine or Goldmann perimeter

What is the Goldmann perimeter?

• Maps entire visual field with dif targets → lights varying in size + brightness

• time consuming

What is the Goldmann Field plot?

• different targets plotted in different colours

• larger brighter targets more easily detected than small dim ones

• blind spot drawn in black

Goldmann target size and intensity range

• target size → 0 (0.0625mm2 to V 64mm2)

• target intensity 1a (19dB (dimmest) to 4e 0dB (brightest)

• quite time consuming

What is automated static perimetry?

• Uses stationary stimuli flashed at a different number of locations within patient VF

• done on automated machine unlike Goldmann

• tests categorised as threshold or suprathreshold

Main use of automated static perimetry

• useful for measuring patients threshold or detecting localised scotoma

• tests can be categorised as threshold or supra threshold

what are the common machines of automated static perimetry?

• Humphrey Visual field analyser (Zeiss)

• Henson (Elektron Eye Technology)

• Octopus (Haag Streit)

Advantages of automated perimetry

Standardised, sensitive, good for monitoring disease.

What is the 30-2 test pattern?

• stimuli presented to 76 different locations (76 points)

• extends out to 30° from fixation

• stimuli 6° apart

• 2 in the name refers to fact that stimulus offset from vertical + horizontal midline by 3°

What is the 24-2 test pattern?

• 54 locations

• extends out to 21° in all directions except NASALLY where it goes out to 27°

→ early glaucomatous field loss can occur in nasal region

Why are points offset from midlines?

• helps interpreting & diagnosing field loss

What is a Threshold test?

• plot px exact threshold at various locations

• useful for detecting or monitoring subtle changes such as in early disease

• typically takes 3-5 mins per eye to do full threshold test

What is the Disadvantage of threshold tests?

• time consuming

• on Humphrey VFA the full threshold method employs a staircase at each location

What is the Humphrey threshold algorithm ?

• predicts px threshold based on knowledge of a normal VF

• SITA

Octopus threshold algorithm

• TOP

What is the Henson threshold algorithm?

• ZATA

What is the Suprathreshold test (single stimulus)?

• use age-matched normal values as starting point + then present stimuli which are several decibels brighter

• assumption is that all stimulus will be above the px threshold (easily seen) and someone with a normal field will get a full score

• quicker and not as difficult for px as most lights can be easily detected

Main use of suprathreshold tests

• Screening tool

• if field abnormal can be very time consuming as machine goes on to plot exact threshold of each missed point → not using speedy algorithm

Limitation of suprathreshold tests

• May miss subtle defects

What is the Multiple stimulus suprathreshold test?

• initially determines rough threshold for px, then presents subsequent stimuli several dB above threshold

• presents between 1-4 stimuli simultaneously at different locations

• px must state how many lights seen on each presentation

• any miss points →determined using clock face analogy , examiner then indicates missed stimuli on printout

• quick screening tool

What is the 10-2 test?

• concentrates on MACULAR REGION

• Testing out to 10°

• stimulus spacing of 2°

• tests 68 locations in total → can see what part of macula affected

why is the 10-2 specialist test helpful?

• mapping out CENTRAL defects

• end stage disease where only a small island of vision is remaining → 24-2 would take very long time to complete

What is the Estermann test (specialist test)

• Binocular suprathreshold test often used to determine px legality to drive → DVLA may request

• tests 120 points at 10dB (fairly bright stimuli)

• spans ~150°horizontally and 90° vertically