Revision lecture

Question 1

how are electroretinograms recorded

Answer 1

jet corneal electrodes are used for eegs under ga in theatre

would need topical anesthetic otherwise

disposable

9 pound each

blephrostats used to hold eyelid open

Question 2

when patients are awake what electrodes will be fitted

Answer 2

DTL fibre electrode

dawson , trick and litzkoq

used in clinic

no anaesthetic required

can be worn all day

no effect on visual acuity

suitable for all diffuse and structured stimuli

disposable

3.50 each

often easier to get in than drops

patients don’t remember them

Question 3

what type of diffuse stimuli are used for erg’s

Answer 3

light stimuli = diffuse stimuli

stimuli Is delivered via the ganzfeld stimulator

gurantees that the light goes over then entire retina evenly

standard flash luminance defined as 3cd.s.m-2 - (3 candlea flash)

which is quite bright especially if you have dilated pupils

deliver this in the dark adpated state and the light adapted state

Question 4

what are the two responses from the erg

Answer 4

rods = low light (scotopic conditon)
cones

cones = photopic - colour - light

light adapted (photooptic response)

rods suppressed by 30cd/m2 for 10 mins

then standard flash used to produce ‘‘cone response’’

dark adapted (scotopic response)-

eyes are adapted for approx 20mins

dim flash (0.01 cod’s-2) used to produce rod response (-2.5 log units below or 1/316 of standard flash luminance)

then standard flash used to produce a mixed response from both rods and cones

so 3 conditions are looked at - the rods , the cones , and then the rods and cones

Question 5

what would you expect in a normal dark adapted response 0.01

Answer 5

dim flash

dark adapted

Lower amplitude and the long b wave implicit times , small a wave , just rods

Question 6

what would you expect in a dark adapted state 3.0

Answer 6

standard flash
dark adapted
larger amplitude
larger a wave
medium b wave
implicit time
rods and cones

Question 7

what would you expect from a light adapted response 3.0

Answer 7

standard flash
light adapted
lower amplitude
short b wave implicit time
just cones- suppress the rods

Question 8

how are measurements made from the erg

Answer 8

most clinical imformation comes from the amplitudes of responses

a wave amplitude normally measured from baseline to first rough

b wave amplitude normally measured from a wave trough to next +ve peak

however timing is an important factor

period from light stimulus being applied to response peak occurring gives information about the response time and is known as the implicit time

Question 9

what happens if you vary the flash intensity/luminance in the dark adapted state

Answer 9

the waveform will evolve and it will change shape

erg amplitude increases with increasing flash luminance

waveform morphology (i.e. shape) changes due to the successive emergence of non linear saturating responses from the bipolar cells, muller cells and photoreceptors

the first response originates In the rod system and as the flashes get brighter the cone system contributes more

Question 10

in what conditions would you expect a abnormal erg

Answer 10

in a normal erg you would see a normal a wave and a normal b wave

there Is a condition called congenital stationary night blindness - same as melanoma associated retinopathy

in both conditions the on bipolar cells are defective / not there so you don’t get a b wave to a dim flash - that is because it is largely from the on bipolar cells

could be the photoreceptors that are faulty

Question 11

what would a normal a wave indicate

Answer 11

a normal a wave would indicate the rods are okay and the cones

Question 12

if the b wave is abnormal what does that tell you

Answer 12

that something is wrong after phototransduction

Question 13

what does no a wave indicate

Answer 13

cone dystrophy
no cones present

Question 14

if there is a flat line what does that indicate

Answer 14

no rod response and no cone response

Question 15

what would you expect to see on a erg with a patient with retinoschisis

Answer 15

• post phototransduction dysfunction

photoreceptors are okay but signals are not getting through the bipolar cells because the retinal layers have split

there is an a wave but not much of an b wave

Question 16

what is the source of the electrooculargram

Answer 16

dependent on the rest of the retina working - photoreceptors e.g.

i.e. if the photoreceptors are not working than the eog will be flat

Question 17

what are the recording methods for the eog

Answer 17

difficulty recording standing potentials because of the uncertainty concerning baseline position - e.g. electrode offset potentials - caused by the patient sweating

therefore signal of interest made to vary with time by voluntary eye movements

two lids in ganzifeld , subtending an arc of 30 degrees, illuminated alternately for 1 second and subject asked to track them

Question 18

where are the electrodes placed when conducting an eog

Answer 18

recording electrodes are placed on the naison and lateral canthus with a reference electrode on the ear lobe

get patient to look left and right

as the eye looks left and right it rotates between the two electrodes

signal size approx 1mv (about 30uv)

bandwidth of signal approx 0.01hz- 30hz

Question 19

how is an eog recorded

Answer 19

you record in the dark

record response for 10 seconds every minute

to avoid fatigue for 15mins during dark adaptation

amplitude ‘‘dark tough’’ occurs after typically 12 mins

then a bright light is put on 500cd/m2 steady illumination switched on

recording continued until light peak amplitude occurs typically after 10 mins

Question 20

what is the Arden ratio

Answer 20

arden ratio= ‘‘light peak/ dark trough’’- a value of greater than 1.85 is regarded as normal

Question 21

what are the clinical uses of the eog

Answer 21

best vitelliform macular dystrophy- v subnormal

pretty much essential for the diagnosis

acute quinine toxicity

retinitis pigments

(rod/ cone dystrophy)

but results parallel the erg

azoor - can have subtle changes in the electroretinogram - - eoog can therefore help to confirm the diagnosis - supernormal

adult vitelliform macular dystrophy

can be normal, but tends to be low-normal to slightly subnormal

central retinal artery occlusion - flat but erg much more informative

Question 22

how is the electrooculargram important for measuring position of gaze

Answer 22

electronystamography (eng)

saccadic velocity

horizontal angle of gaze - with electrodes either Side of the eye

vertical angle of gaze

with electrodes above and below the eye

position of gaze
can derive a vector from vertical and horizontal angles if testing both eyes

can put electrodes either Side of the eye and above and below

you can determine the position of the eye from the recordings

however there can be potential changes due to light - for it to be reliable you need the patient to be exposed to the same level of light for some time so that you can have stable conditions - however it varies so there is a need to frequently recalibrate the equipment because of that - once that patient looks up and down between a known angle the equipment is calibrated - you can determine where they are looking

Question 23

in what type of condition would you see a completely flat eog

Answer 23

in a retinitis pigmentosa you would expect to see a completely flat eog

Question 24

what do nerves conduct

Answer 24

nerves conduct digital electrical impulses around the body which can be electrically stimulated and the responses can be recorded for diagnostic purposes

Question 25

what is conduction velocity increasedbby

Answer 25

conduction velocity is increased by the presence of myelin information is coded by pulse frequency modulation

Question 26

what does the erg record

Answer 26

the erg records responses to brief visual stimuli

Question 27

how can rod and cone responses be selected

Answer 27

adaptation and stimulus luminance can be used to select rod and cone reponses

Question 28

what does the eog record

Answer 28

the eog records slow changes in the retinal trans epithelial potential in responses to general illumination levels

Question 29

what is the multifocal erg (mferg) used for

Answer 29

responses from multiple discrete areas of retina primarily used to assess spatial variations in cone function discrete retinal lesions - involving too small an area to affect the erg enlarged blind spot syndrome abs Maculopathy starboard and azzor acute zonal occult outer retinopathy

Question 30

what is the mferg useful for

Answer 30

the mferg is useful for identifying zonal dysfunction in the retina

Question 31

what stimus is used for the mferg

Answer 31

black and white circle - the spacing o fit elements get. bigger as you move away from the centre structured stimulus - multiple elements may stimulate many areas of the retina simultaneously each element flashes following a pattern of ons and offs determined by a maximum length or m sequence e.g. 00010010001011 individual responses deconvolve from mass responses to give miniature erg's for each area the concentration of cones in the retina varies with eccentricity - you have the greatest density of cones in the fovea

Question 32

what is mferg stimulus scaling

Answer 32

scaling (spatial distortion) of the stimulus patten is needed too account for the spatial variation in cone density throughout the retina elements increase in size increasing in eccentricity to give approximately equal sized responses stimulating the central 50 degrees produces a tracer ray

Question 33

what type of electrodes are used for mferg recordings

Answer 33

recorded using dtl thread electrodes to avoid interfering with vision and dilated pupils for consistent and repeatable retinal illmuninance focus/contrast is less important luminance related response not co toast related

Question 34

what would you expect to see on a normal mfERG

Answer 34

you would see traces of approximately equal amplitude you would see a smaller response which would correspond to the blind spot

Question 35

what would you expect to see in a mfERG in a patient with Maculopathy

Answer 35

you would expect to see a loss of responses from the macular region

Question 36

what is the pattern erg

Answer 36

the pattern eg is recording using a counterpoising (reversing) chequerboard stimulus don't dilate for this- undilated pupils are required because contrast is the most important parameter - highly dependent on focus mean luminance remains constant at all times typically 50cd/m-2 white squares at 100cd/m2 and black squares at 0cd/m2 so the mean luminance is 50

Question 37

how are erg recordings interpreted

Answer 37

gives information about the macula and the optic nerve p50 is measured from n35 n35 means negative trough from 35 milliseconds p50 represents macular function n95 represents retinal ganglion cell function the test is unsuitable for patients with nystagmus and patients under 6 years old (generally) 5uv= a tiny retinal response

Question 38

how would you differentiate between Maculopathy and optic neuropathy with perg

Answer 38

in a normal response you would expect n95 to be larger than p50 in Maculopathy you would expect n95 to be concomitantly reduced with p50 , p50 may also be delayed in optic neuropathy you would expect n95 to be smaller than p50

Question 39

to look at optic nerve function further what would you do

Answer 39

you would look at visual evoked potential (vep) the vep is a recording of the electrical activity that occurs in the brain in response to visual stimulation by time variant diffuse or structured stimuli

Question 40

when is the vep flash stimulus indicated for use

Answer 40

testing infants and people with poor vision / cooperation cannot estimate visual acuity to better than rudimentary though - if a response is given doesn't indicate good vision just indicates that the visual pathway is intact good for detecting misrouting - e.g. in ocular albinism

Question 41

what is a reversing chequerboard

Answer 41

similar to perg confounded by nystagmus pattern is smeared by the movement - is liar effect to reduced contrast

Question 42

what chequer boards are usually used

Answer 42

typically 1 degree chequers for macular stimulation and 15 degree chequers for foveal stimulation differentiate between foveal dysfunction and macular dysfunction by using two different chequer sizes typically 2 reversals per second stimulus field > 15 degrees steady fixation is necessary requires cooperation and focus patient must be refracted

Question 43

what is the typical arrangement for recording vep's

Answer 43

patient - electrodes - amplifier - filter - analog to digital converter - computer - stroboscope or pattern stimulator

Question 44

where are vep electrodes placed

Answer 44

one on the midline and one on either side of the hemisphere - allows us to tell which side of the brain the response is coming from

Question 45

what is the typical pattern reversal vep

Answer 45

electrodes mocc- mf n70= foveal component - if someone has good va they usually have good visual acuity p100 is macular component = measured from the n70 n135 is a particular component

Question 46

what are reps typically used for

Answer 46

demyelination large majority of patients with ms show increased peak time even in the absence of symptoms powerful at detecting sub clinical optic neuritis compression of the optic nerve from space occupying lesions function v structure advantage optic neuropathy - amplitude is attenuated functional integrity of visual pathway objective visual cortical acuity measurements

Question 47

what is the geniculostriate pathway

Answer 47

you have 3 electrodes and all 3 are referenced towards the forehead looking at activity occurring from the back of the head to the forehead you have. further channel that you connect across the lateral electrodes i.e. a and c if both half of the brains are doing the same thing you are ether going to see the same thing in the two lateral electrodes or they will cancel out if there is a difference between what the two hemispheres are doing that is either because they are stimulating 1 hemisphere selectively or because there is a visual field defect

Question 47

what is the geniculostriate pathway

Answer 47

you have 3 electrodes and all 3 are referenced towards the forehead looking at activity occurring from the back of the head to the forehead you have. further channel that you connect across the lateral electrodes i.e. a and c if both half of the brains are doing the same thing you are ether going to see the same thing in the two lateral electrodes or they will cancel out if there is a difference between what the two hemispheres are doing that is either because they are stimulating 1 hemisphere selectively or because there is a visual field defect

Question 48

how would you deliberately stimulate one half of the hemipshere

Answer 48

you would use half field stimulation in normal subject stimulus od or os will activate the left hemisphere the checkerboards are on the right side - this is right half field stimulation and this will activate the left hemisphere

Question 49

how do the responses vary under full field stimulation and half filed stimulation conditions

Answer 49

with full field stimulation you get a big response from the mid line electrode - which is between both hemispheres the lateral electrodes show a smaller signal and if you deliberately stimulate one hemisphere with a half field stimulus then you would get a response from just 1 hemisphere so the electrode placed over the left hemisphere records a positive waveform - electrode over the left records a negative waveform half field stimulation activates 1 hemisphere only p100 paradoxically recorded from side of scalp ipislateral to stimulated half field

Question 50

explain paradox lateralisation

Answer 50

the electrode placed open the left side of the brain picks up signals from the right side better so you would see positivity on the opposite side to the electrodes placed- because it is picking up positivity from the opposite hemisphere and negativity on the aide that the electrodes are placed p100 produced by dipole generators in the calcimine sulcus electrode on scalp ipsilateral to stimulated heal field better placed to detect p100 full field stimulation causes cancellation in lateral electrodes but not midline

Question 51

what is misrouting and what conditions is it associated with

Answer 51

crossed and uncrossed retinal ganglion cells in normal patients you would see decussation you would see macular fibres projecting to the other hemisphere in patients with albinism you have excessive decussation - i.e. more fibres cross than they should associated with ocular albinism - all abinos display misrouting oca oa cordial - hibachi syndrome warden burg syndrome albinoidism normal routing in carriers of x linked oca though i.e. In normal eye you have a 50- 45% of fibres crossing and In a albino you have 80- 20% of fibres crossing- so only 1 half of the hemisphere is activated

Question 52

what does vep asymmetry cause

Answer 52

mistouring in albnism results in occipital lateralisation of the vep- this is seen in all modalities but the degree to which each displays this best varies according to the age of the patient in excessive decussation you have marked asymmetry assymetrey of an opposite sense is seen in achiasmia and in compression of the crossing fingers e.g. pituitary adenoma ) not so with cranipharyngignioma though) you can see via the pattern of crossing weather it is achismia or albinism