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Electro > Revision lecture > Flashcards

Revision lecture Flashcards

1
Q

how are electroretinograms recorded

A

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

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2
Q

when patients are awake what electrodes will be fitted

A

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

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3
Q

what type of diffuse stimuli are used for erg’s

A

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

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4
Q

what are the two responses from the erg

A

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

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5
Q

what would you expect in a normal dark adapted response 0.01

A

dim flash

dark adapted

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

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6
Q

what would you expect in a dark adapted state 3.0

A

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

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7
Q

what would you expect from a light adapted response 3.0

A

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

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8
Q

how are measurements made from the erg

A

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

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9
Q

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

A

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

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10
Q

in what conditions would you expect a abnormal erg

A

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

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11
Q

what would a normal a wave indicate

A

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

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12
Q

if the b wave is abnormal what does that tell you

A

that something is wrong after phototransduction

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13
Q

what does no a wave indicate

A

cone dystrophy
no cones present

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14
Q

if there is a flat line what does that indicate

A

no rod response and no cone response

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15
Q

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

A
  • 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

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16
Q

what is the source of the electrooculargram

A

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

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17
Q

what are the recording methods for the eog

A

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

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18
Q

where are the electrodes placed when conducting an eog

A

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

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19
Q

how is an eog recorded

A

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

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20
Q

what is the Arden ratio

A

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

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21
Q

what are the clinical uses of the eog

A

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

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22
Q

how is the electrooculargram important for measuring position of gaze

A

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

23
Q

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

A

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

24
Q

what do nerves conduct

A

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

25
Q

what is conduction velocity increasedbby

A

conduction velocity is increased by the presence of myelin

information is coded by pulse frequency modulation

26
Q

what does the erg record

A

the erg records responses to brief visual stimuli

27
Q

how can rod and cone responses be selected

A

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

28
Q

what does the eog record

A

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

29
Q

what is the multifocal erg (mferg) used for

A

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

30
Q

what is the mferg useful for

A

the mferg is useful for identifying zonal dysfunction in the retina

31
Q

what stimus is used for the mferg

A

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

32
Q

what is mferg stimulus scaling

A

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

33
Q

what type of electrodes are used for mferg recordings

A

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

34
Q

what would you expect to see on a normal mfERG

A

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

35
Q

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

A

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

36
Q

what is the pattern erg

A

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

37
Q

how are erg recordings interpreted

A

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

38
Q

how would you differentiate between Maculopathy and optic neuropathy with perg

A

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

39
Q

to look at optic nerve function further what would you do

A

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

40
Q

when is the vep flash stimulus indicated for use

A

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

41
Q

what is a reversing chequerboard

A

similar to perg

confounded by nystagmus

pattern is smeared by the movement - is liar effect to reduced contrast

42
Q

what chequer boards are usually used

A

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

43
Q

what is the typical arrangement for recording vep’s

A

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

44
Q

where are vep electrodes placed

A

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

45
Q

what is the typical pattern reversal vep

A

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

46
Q

what are reps typically used for

A

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

47
Q

what is the geniculostriate pathway

A

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

47
Q

what is the geniculostriate pathway

A

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

48
Q

how would you deliberately stimulate one half of the hemipshere

A

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

49
Q

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

A

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

50
Q

explain paradox lateralisation

A

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

51
Q

what is misrouting and what conditions is it associated with

A

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

52
Q

what does vep asymmetry cause

A

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

Electro (5 decks)

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