7: Visual Fields

what is the visual field

all the space that one eye can see at any gven instant

what does assessment of VF/perimetry enable

allows the detection and analysis of damage along the visual pathway and the monitoring of disease progression

what is perimetry

the study/ investigation of the visual field

done using a machine called the perimeter

extent of the visual field

inferior VF i bigger than the superior VF

temporal VF larger than nasal VF

with both eyes open, binocular VF is when RE and LE overlap: 120 degrees

LE/RE 40 degrees each

photopic

light conditions

most sensitive at the fovea - when looking directly at something

scotopic

low light conditions

centre of vision/ fovea lacks usefullness and sensitivity

peripheral works better , much more sensitive

sensitivity profiles

VF done with room lights off

not testing VF in scotopic environment as machine inside is lit

so expect the centre of the field to be very sensitive and pick up more stimulus than the periphery

why do we do perimetry

to assess health of visual pathway

picks up lots of dieases like tumours, glaucoma, diabetic retinopathy, cataract

mainly used for testing for glaucoma

types of perimetry

• kinetic perimetry ( moving)

1. gross (arc) perimetry

2. goldmann manual

3. octopus semi automated

• static perimetry ( non moving)

1. amsler grid

2. supathreshold

3. threshold

kinetic perimetry

approaches centre of vision laterally, moving target

constant luminance, variable position

static perimetry

constant postition

variable luminance

what is done for each stimulus on kinetic perimetry

vary eccentricty from fixation

px reports when a stimulus fisrt becomes visible and if it dissapears again

test meridians every 10 or 20 degrees

change stimulus intensity usually by changing stimulus size

who should central VF screening not be performed on

px with minimal risk factors eg those over 40 without other risk factors of POAG

this is due to porblems of false positives results when testing healthy px

who should central VF screening be performed on

if a px has a known defect, risk factor for a VF defect, or when following protocols for the management of glaucoma

IOP greater than 24mmHg, 60+ age

symptoms consistent with a neurological disease eg headahces, dizzyness

gross perimetry

type of kinetic

a target is moved from an area of non seen to seen

px reports when the target first becomes visible, and if the taregt dissapears again

bead on a stick

arc perimetry./ confrontation perimetry

arc ( gross perimetry)

practitione holds bead on a stick

px is covering eye with hand

practitioner asks px to fixate on their eyes/nose

mpving the bead form behind the px head, bringing it around into an arc to a point in which they first see the bead

keep moving until it dissapears

done at around 33cm away

done in 8 meridians

kinetic perimetry- goldmann manual

VF test that maps the px peripheral vision using a moving light stimulus

small light of specific size and brightness is moved inward from non seeing to seeing areas

the point where the px detects it is marked

connecting these points forms an isopter

kinetic semi- automated perimetry

CF test where a moving stimulus os presented bu a computor controlled perimeter, but practitioner can stilla adjust settings

px fixates centrally inside the bowl

light target moves inwards from peirphery and px presses button when first seen

computor records detection point and points are connected to form isopters

goldmann stimuli

static usually involves at III - 4mm² target / 0.15²

what is isopters

kinetic plots results in isopters

this is a line joining points of equal retinal sensitivity to a given test stimulus

blue ring is how far out px can see the light

can tell the right hand side is temporal as extends further out

colours correspond to brightness- red is the dimmest blue is the brightest

constricted visual field

dotted line is where we expect he px to see up to

advantages of kinetic perimetry

more px friendly

goes further into the periphery

often used for neurological conditions

disadvantages of kinetic perimetry

often requires an experineced examiner- need t make sure target is in stright line at constat speed

can be more time consuming

types of static perimetry

amsler grid

supratheshold

threshold

amsler grid

only looks within central 10 degrees - fovea

a grid on a piece of paper is shown to px and ask them to stare at the centre dot. then asked if all grid lines are visible/ if any are missing/ do lines look straght or wavy

types of amsler charts

chart 1: standard. 5mm square, white gris with each square subtending approx degree from 30cm on black bckground with central white fixation target

2: same as 1, but with 2 diagonal white lines to assist steady fixation

3: similar to 1, but with a red grid. useful for toxic amblyopias and optic neuritis

4: scattered white dots with central white fixation target- dtection of scotomas

5: white parallel lines only and a central white fixation target. chart can be oreinted by rotation. used to investigate metamorphospia

6: similar to 5, but black lines on white card with additional lines above and below fixation

7: similar to 2, but with additonal squares in central 8 degrees. detect subtle macular disease

what does asking the px if lines appear wavy do for practitioner

helps them detect any metamorphospia which is usually caused by macula oedema

recording of amsler

record defects or distubrnaces

note down eye tested, date of examination, and px name

if no defects found then recorded as Amser charts: central fields full R and L

static perimeters

what happens for a given stimulus area of static perimetry

stimulus doesnt move- just flashes on and off

test fixed locations, typically 6 degrees apart

px reports when they see a stimulus

2 main types of programme- suprathreshold and threshold

establishing a threshold

intensity of a stimulus that is just visible

stimuli brighter are seen and timuli dimmer than threshold not seen

centre of field most sensitive

sensitivity is the reciprocal of threshold: a high sensitivity means px can see very dim light , so threshold is low

what scale do perimeters use for recording

use a logarithmic scale

higher numbers = increasng sensitivity and dimmer light seen

lower end of scale is exaggerated

matches response of the visual system

1dB step change percieved the same along the whole range

what is the dB scale based on

the dB scale is a relative scale- it is based on the maximum luminance the instrument can generate

sensitivity of 0/ log scale/dB of 0 is max luminance the machine can produce

what is the goldmann size for static perimetry

goldman size 3

4mm²

0.15 deg²

what does static perimetry include:

usually named : standard automated perimetry (SAP)

is a white GIII stimulus

white background

stimulus displayed for 200ms

test location typically 6 degrees apart

px indicates hey have seen a stimulus by pressing a button

the central visual field

few defects with no central component

retinal detachment

retinitis pigmentosa

tumours

mainly central 30 degrees tested

test locations- grid pattern

6 degrees apart

24-2 test pattern : shape is slightly larger around temporal side. nasal goes to 30 degrees peripherally

30-2: more symmetrical

test location grid patterns: SITA Faster 24-2C

extra spots for location within the centre

full threshold

a stim is presented. px does not press the button as they havent seen it

next stim presented is brighter . the px presses the button to indicate that they have seen it

the next stim presented is less bright than the last stimulus but is brighter than the first one

or px presses button as sees light, so go dimmer, px cant see so go slightly brighter than last sitm to work out threshold

visual field plot

advantages of threshold tests

individual threshold for each location is tested

identifies small and early defects

effective for tracking changes over time

disadvantages of threshold tests

time consuming

more difficult for px as can get false positives

max 30 degrees from fixation

suprathreshold screening

stimuli is slightly brighter than expected threshold

so px is expected to see all the stimuli presented

if a px misses a stimulus it may be opresented again in case px blinked or wasnt paying attention

if px still fails to respond to the stimulus, a brighter stimulus may be presented at the same location

suprathreshold screening: how test stimulus is chosen

how bright the test stimulus is, depends mainly on age

elderly have more variable thresholds

more spots presented on left eye

eg grey spot shows 5dB greater than stimulus

black shows 12dB brighter.

advantages of suprathreshold tests

quicker if there is no defect present

easier for px

can go further into the periphery

disadvantages of suprathreshold

may miss small defects due to locations chosen my manufacturers

not good for monitoring change

stimulus luminance and/or test pattern may be sub optimal

what is the multiple stimulus test

type of suprathreshold

multiple stimuli presented at the same time

px verbally reports how many stimuli they saw and where using a clockface

operator manually records if stimuli are missed

advantages of multiple stimulus test

more px friendly

can check px understandng through questions

quicker if VF are normal

disadvantages of multiple stimulus

requires manual operation and input

can be more time consuming if defects are present

max 30 dgrees from fixation

testing closer to fixation eg 10-2

red spots in centre, 2 degrees apart in many different centre, and tests 10 degrees from fixation in all directions

done if suspect something is wrong with the macula

mainly on a nhumphry analyser

whats the 24-2 test

tests 24 degrees from fixation in most directions

test points are arranged 6 degrees apart, but offset so none lie directly on the horizontal or vertical meridians - main use is standard glaucoma test

advanced glacomatous defect

can use 10-2 to concentrate on the vision px still has left so any small changes can be picked up quicker

testing further into the periphery

eg 60 degrees from fixation

FF: full field

begins with suprathreshold then moves out as can be time consuming

esterman

binocular test

suprathreshold

120 locations/ 75 degrees either side of fixation

35 degrees superiorly

55 degrees inferiorly

used for driving standards

what is the minimum field of view for group 1 driving for esterman

a field of view of at least 120 on the horizontal measured using a targett equivalent to the white goldmann settings

the extension should be at least 50 degrees left and right. In addition there should be no significan defect in the binocular field that encroaches within 20 degrees of fixation above or below Hx meridian

SWAP: short wavelength automated perimetry

uses a blue, goldmann size V stimulus displayed on a yellow background

preferentially stimulates the short wavelength ( koniocellular) pathway

evaluation of SWAP

higher test- retest variability with swap compared with SAP

longer test time

short wavelength light absorbed by macular pigment and ocular media

no longer recommended in glaucoma management

freuency doubling technology FDT

black and white flickering stimuli

when it flickers at a specific speed the stimuli looks like it doubles

thought to stimulate the magnocellular pathway

evaluation of FDT

quick test

robust to optical blur

less robut than GIII to reduced retinal illuminance - if they have cataract or small pupils

px dont always understand it

could lead to false positives

what are you testing for when doing a visual field test

glaucoma: age and family history

disc appearance, IOP, anterior chamber angle

if already diagnosed, monitoring for chages, and if the changes have advanced

also testing if had a stroke, or concerning history or symptoms

what VF test is used for glaucoma

if suspicions of glaucoma- threshold, central 30 degrees

if no suspicions- suprathreshold, central 30 degrees

already diagnosed- advanced threshold central 10 degrees

what VF test is used for no suspicions

suprathreshold as a backup

what VF test is done for a px with suspicions- case history. other symptoms that you cant idenfity for

how far into the periphery should you test would determine what test to choose

insturment set up

some VF have a lens holder- put the trial lens in to correct the px refractive error

what is the px set up for a VF test

trial lens positoned close to the eye

px places their chin on rest, forehead against bar. would be on the left side to test the right eye

chin rest adjusted

monitor px thoughout

quiet environment

short break before left eye VF done

what is recorded on a VF test

what test was done

and if the fields are normal : WNL is wihtin normal limits

why is the size of stimulus mainly in degrees, and not mm

in mm, spot size stays the same, but the size of the image it makes on the retina is different to how far away it is

in degrees, when the image is further away, image on the retina is made bigger to maintain the same visual angle on the retina

visual field plot of a humphry field analyser

VF plot (hfa): px information

name

ID

which eye tested

DoB

also has test information: eg what test is done,like central 24-2 threshold tets

what are the reliability indices on a visual field plot of HFA

fixation monitor: gaze/ blind spot - would be able to track where px moves eyes

fixation losses: eg if recorded as 10/16 x x then would think the px moved their eyes

false POS errors: how many x the px clicked button when wasnt light

false NEG; how many lights were missed

test duration: longer test less likely px is paying full attention. bgger defects = lonnger duration time

reliability indices: fixation losses

bright stimulus periodically presented at the blind spot throughout the test

a positive px response suggests that the pc may not be fixating consistently

if exceeds 20% may suggest compromised test results

why would fixation losses not always be relliable/ work

can be artificially hgih

blind spoty could be incorrectly plotted

monitoring the px fixation using internal camera can help to distinguish between artefacts and poor fixation

reliability indices: false positives

the px presses the repsonse button in the absence of a stimulus

>15% strongly associated with compromised test results

a high fixation loss rate may actaully be indicative of excessive false positives

considered most important and useful reliability index

reliability indices: false negatives

the px fails to respond to a stimulus they shouldve seen

>15% indicative of inattention

a high false negative rate may indicate that the px is not paying attention to the test

reliability indices: gaze tracking

used with fixation loss measure to monitor fixation stability

sensitvity values : interpreting VF plot

deicebels values higher in centre

triangle is blind spot so expected to be lower

higher value: high sensitivty: dim lights

interpreting VF plot: grayscale

visual representation of the data

more condensed the spots, the brighter the spots had to be

limited use when analysing results in detail

interpreting VF plot: global indices

mathematical summaries of all sensitivity values

VFI: visual field index- approx 100% in normal fields, and approaches 0% in perimetrically blind fields

less affected by cataracts and to provide imporved correspondance to ganglion cell loss compared with MD

global indices: mean deviation MD

departure from age normal

comparing px to the normative database for specific age group

approc 0dB in normal fields and ←30dB in extreme visual field loss

global indices: pattern standard deviation PSD

ireegularities in the field

small, close to 0, both in normality and blindness

peaks at moderate levels of localised field loss

looks at what parts are less sensitive than others

evaluation of global indices

useful tools to gain a quick idea about the field

summarising enitre field plot into 1 number gives limited information

interpreting VF plot: numerical total deviation map

this is looking at each individual number and comparing it to the database

age matched comparison of sensitivity values

-ve = sensitivity below expected values

+ve= higher than expected sensitivity

interpreting VF plof: total deviation probability map

representation of TD values, based on probability of normality

eg prob of that spot being normal is less than 0.5%

evaluation of total deviation

normative databse only includes osbervers who meet certain criteria, only on the database if:

-5.00 to +5.00 mean sphere

<2.50 cyl

6/9 or better

largely caucasion/ white

means that those falling outside criteria doesnt mean their values arent normal

interpretation of VF plot: numerical pattern deviation map

compares the px to themselves

removes any generalised depression in sensitivity - eg ruling out what isnt serious but reasons why values differ

looks only for localised differences and loss

interp VF plot: pattern deviation probability map

if TD and PD maps similar, little/no generalised depression

shows that the prob of values being nomral is eg less than 1%

interp VF plot: glaucoma hemifield test GHT

machine scores : whether it is outside normal limits, within etc

specific to HFA

compares different zones of the visual field between superior and inferior hemifields

idea is that the zones comparing should be symmetrical

GHT categories: outside normal limits

at least 1 zone pair differes by an amouont fooound in fewer than 1% of normal observers

CHT catergory: borderline

at least 1 zone pair differs by an amount found in fewer than 3% but more than 1% of normal observers

GHT category: general depression or abnormally high sensitivity

whenever even the best test point locations are either so low or so high they are seen in fewer than 0.5% of normal observers

GHT category: within normal limits

whenever none of the other conditions form categories apply

interpretation of results: what is WANDER

W: what was done

A: how accurate are the results

N: are the fields normal

D: if abnormal, what defects are present

E: evaluate the field

R: is the field repeatable

what is the anatomy of the field plot

pattern of the nerve fibres of retina do not pass the horizontal line

if a defect is seen superior/inferior, it is at the level of the retina

pre chiasmal

anatomy of the field plot in visual pathway

once the information crosses the optic chaism and crossing of nerve fibres, a vertical demarcation is seen here , instead of superior / inferior

would see a pattern of vertical lines on the field plot - which shows post chiasmal

glaucoma field defects: patterns of loss

paracentral : pointed section is the 24-2 , defect at the centre near macula

temporal wedge: defect at temporal side

arcuate goes from disc sup or inf to nasal side

altitudinal: most of one half of VF defected, either superior or inferior, mainly pre chiasmal

advanced: affected majority of visual field

arcuate defect

focal notch at the optic disc, loss of rnf in the corresponding area

physiological blind spot is in the temporal VF, but the optic disc is inthe nasal retina: inversion

paracentral defect

within 10 degrees of fixation

nasal step

asymmetrical involvement of superior and inferior field