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Electro > Revision deck > Flashcards

Revision deck Flashcards

1
Q

what is tonometry

A

the measurement of iop

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2
Q

what are the units of pressure used for iop

A

pascals (pa) or n/m2
iop is usually given in mmhg
hectopascal = hecto =100

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3
Q

what is mmhg in hpa

A

mmhg= 1.33hpa

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4
Q

what can raised intrauouclar pressure be due to

A

impaired drainage of aqueous fluid from the anterior chamber

permanent damage to optic nerve leading to loss of ganglion cells

can be considered form of optic neuropathy

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5
Q

what is the incidence of glaucoma

A

affects 1/200 aged over 50

1/10 aged over 80

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6
Q

what can be tonometry be used for in relation to glaucoma

A

tonometry can detect and monitor iop

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7
Q

how does glaucoma affect the optic nerve

A

forces optic nerve out and creates cup in optic nerve (forces it into a cup shape)

stretches glands and nerve fibres

eventually they fail which will lead to severe visual loss

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8
Q

what are the different subtypes of glaucoma

A

acute (closed angle) - sudden onset and very painful

chronic (open angle) gradual loss often of peripheral visual field - often not noticed - as gradual loss of visual field

cupping of optic disc occurs over time

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9
Q

what is the normal range of iop

A

the normal range of iop= 10-20 mmhg

mean = 15mmhg

not necessarily glaucoma - if iop is higher than 20mmhg

you can have glaucoma when iop is a normal range

don’t always need to treat if it isn’t in a normal range

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10
Q

what is atmospheric pressure equivalent to in spa and mmhg

A

1 bar = 100hpa

100hpa = 750mmhg

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11
Q

what is applantion tonometry

A

infers iop from the force required to flatten a constant area of the cornea

by flattening an area of 3.06mm so that the meniscal forces of the tear film become equivalent to that of cornel rigidity , the iop can be estimated from the force applied - relies of relationship between wall tension and pressure in elastic sphere

surface tension (y) is related to pressure difference (∆P) across a curved wall by ∆P=2y/r

if the wall of the sphere is flattened so r=∞ , then the pressure difference will be 0 and the pressure within the sphere can be found by pressure = force/ are (usually Goldman tonometer)

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12
Q

what is sodium fluorescein

A

sodium fluorescing is excited by blue light maximally at (494nm)

fluoresce green at approximately 521nm

depends on ph = 7.5 - 8.5

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13
Q

how is the Goldman tonometer used

A

Goldman tonometer = special disinfected biprism which is mounted on a tonomter head and placed against the cornea

topical anaesthetic is used

examiner uses cobalt blue light to view the meniscus formed by the fluroscein tear filmed around the probe contact area

split into two green semi circles (mires)

by a bi prism with a tonometer head

the force applied to the tonometer head is the adjusted using the green dial until the inner edges of the semi circles meet

care is needed to avoid Injury - planar movement and excessive movement can abrade the cornea

if too much pressure is added the diameter of the circle will increase

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14
Q

what are confounding factors for the use of the goldan tonometer

A

corneal thickness - (corneal hetrogentiy)- e.g. scar tissue can vary a lot - also will change after getting laser eye surgery - getting history of patient is important

corneal curvature - (keratoconus)

vibrations in tear film (runny, gooey, dry)

time of day

age

epithelial oedema

poor cooperation (blinking and movement)

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15
Q

what are problems with the Goldman tonometer

A

assumptions of sphericity , elasticity , homogeneity can differ and become untenable after surgery

therefore iop measurement can become unreliable

probe can damage cornea

flurosecein can damage tissues

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16
Q

describe principles of general electrophysiology

A

most parts of body produce electrical potentials (neural tissue , muscles , organs , skin.

they are very small signals with amplitudes of up to a few nv

far smaller than interfering signals from outside/ inside body

most potentials can be recorded to provide information about physiological function

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17
Q

what are electrodes and recording systems used for

A

to record electrical signals an electrode amplifier, filter display and recording device are needed - may also need a stimulator

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18
Q

what are electrodes used for and what are the different types

A

used to convert ionic form of current to electric flow along wire

usually metal

in many forms : skin surface, needle can be very specalised

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19
Q

how can noise be disruptive in electrodiafnosis

A

unwanted signals

caused by random motion of electrons in recording signals

magnetic fields from electrical machinery

radio signals

measurement eror

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20
Q

what are amplifiers used for

A

traditionally electrophysiology signals are amplifies prior to recording because they are very small

sensitive low noise differential amplifiers are almost always required

electrodes are connected to amplifiers by leads

1.15m long - which make good antennas for picking up interference

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21
Q

what are the two different inputs used differential amplifiers have

A

inverting and non inverting

inverting = - and non inverting +

output = the different betweenn inputs (differential signal) x gain of amplifier

any signal common to both inputs will be rejected

differential amplifiers have high differential voltage gain, AMD

very low common Mode voltage gain ACM

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22
Q

what is the common mode rejection ratio

A

differential amplifiers have

high differential voltage gain - AMD

very low common mode voltage gain - ACM

common mode rejection ratio- 20log (adm/acm)

need at least 100db cmrr

db = decibels

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23
Q

what is the frequency content of signals

A

any repetitive waveform can be synthesised by adding sine waves

the ecg is a periodic signal whose lowest frequency component is the heart rate (if hr = 1hz then lowest frequency component is 1hz)

Fourier analysis shows that the complete ecg waveform can be produced by adding sine waves of 1hz 2hz, 3hz etc

the amplitude of the components will determine the shape of the ecg

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24
Q

what are filters used for

A

even with differential amplifiers there is still acitivity being picked up by the electrodes from other parts of the body

sometimes they are of a different frequency and we can use this to get rid of them

combining a high pass filter with a low pass filter creates a band pass filter

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25
Q

what is the bandwidth of a filter

A

the bandwidth of a filter is the frequency range between -3db points e.g. 1hz to 100hzfl to fh

this bandwidth must encompass the wanted signals

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26
Q

what are the different frequency ranges for different recording systems

A

ecg - 0.5hz - 100hz

eeg - 0.5hz - 75hz

emg - 10hz - 5khz

nap- 10hz- 10khz

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27
Q

what are the effects of filtering

A

it is easier to identify and measure waveforms

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28
Q

how are recordings digitised

A

analogue to digital converters

  • which is a chip that samples several thousands times a second - feeding each measurement into a computer for further processing analysis , storage and publishing

a 32- bit a to d c an represents a signal using 2,147,483,647 values per sample

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29
Q

describe the basic arrangement of an evoked potential system

A

patient- electrodes- amplifier - filter - analogue to digital converter - computer - visual stimulator

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30
Q

what is signal averaging

A

averaging reduces the noise in a signal by a factor of square root n (high number of averages (n) means a lower % noise level

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31
Q

describe sensory and motor fibres

A

sensory and motor fibres

are bundled into nerve turns which can have up to 20,000 fibres in structure - approx 3mm in diameter

32
Q

describe the difference between myelinated and unmyelinated nerve fibres

A

long and thin approx 10nm and diameter long

myelin on some fibres insulates them except on small gaps - decreasing the area of membrane that needs to be depolarised - forcing the current to jump the gaps

33
Q

what is membrane capacitance

A

membrane capacitance is proportional to exposed area

time taken to depolarise the next section of the nerve is proportional to rc

therefore decreasing c and or / r increases conduction velocity by approx 10x up to approx 70m/s

34
Q

what does digital transmission of nerve signals avoid

A

avoids cross talk and external interference

35
Q

what is the all or nothing principle

A

impulses either occur or don’t (all or nothing)

impulses last approx 1ms

body uses up to 100ips - 100 impulses over second

more intense sensation or greater force required result in more impluses per second

36
Q

how are nerves stimulated electrically

A

current is applied via surface or needle electrodes

pulses of approx 100micro seconds used

20ma needed to stimulate through the skin and up to 250v

37
Q

what is non invasive conduction velocity measurement

A

surface electrodes are used to stimulate and record superficial nerves (needles required for deeper nerves)

measurement = total time from stimulation to muscle twitch - twitch is termed latency - which includes transit time across the neuromuscular junction
hence it is necessary to stimulate at two positions and measure at one to get a true motor measurement (approx 200microv with fibre)

myelinated fibres = approx 50m/s

38
Q

what is a erg

A

electroretinogram

electrical response of the retina to flashes of light or patterns

39
Q

what is a eog

A

an electrooculogram

= an electrical response of the retina to changes In steady state illumination

40
Q

what is the source of the erg

A

light dependant decrease in rod and cone dark current gives ‘a’ wave plus release of k+

muller cells absorb extracellular k+ resulting in the b wave rest of the b wave comes from bipolar cells

ideally there is a A and B wave from the oscillatory potentials from amacrine potentials

41
Q

what are the different erg recording methods

A

electrodes used (contact lenses or fibre electrodes with ag/agcl ground electrode on forehead and reference electrode on ipsilateral temple

local anaesthetic necessary for contact lense but not fibre

dilation of pupils so they are the same diameter for all stimuli and to let in more light

signal size approx 300microv with fibre

averaging is typically 10 responses

bandwidtth= 0.3hz - 300hz for full response

42
Q

what is the jet corneal electrode used for

A

used for ergs under general anaesthetic in theatre

would need topical anaesthetic otherwise

disposable

43
Q

what are DTL fibre electrodes used for

A

no anesthetic required

can be worn all day

no effect ion v a(suitable for all diffuse and structured stimuli

disposable

often easier to get in than drops but patients might not remember them

thread of silver across eye - patient can’t feel it

44
Q

what type of light stimuli is used for erg

A

standard flash luminance

3cdsm (quite bright , especially Wirth dilated pupils)

light adapted (photopic response)

rods suppressed by 30cd/m2 for 10mins then standard flash used to produce cone response

dark adapted (scotopic) response

eyes are adapted for approx 20mins

dim flash (0.01dsm) used to produce rod response

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

45
Q

what are mircoelectrodes used for

A

not clinical
used to record from within or close up to a cell

traditional glass tube, open at top containing saline

new tungsten-in glass electrode

46
Q

what are skin electrodes used for

A

metal - not allowed to make contact with skin

au and ag/agcl have low electrode potentials

gel interface reduces both electrode offset potential and movement artefact

47
Q

how are measurements taken from erg’s

A

most clinical information comes from the amplitudes of the responses a wave measured from baseline to trough

b wave measured from a wave trough to next positive peak

timing is also an important factor

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

48
Q

describe the luminance response you would see on a erg

A

erg amplitude increases with increasing flash luminance

wave from morphology (shape) changes due to su festive emergence of non -linear saturating responses from bipolar cells , muller cells and photoreceptors

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

49
Q

what are some of the reasons you may not have an erg response

A

retinitis pigments (severe retinal degeneration)

ophthalmic artery occlusion

to confirm total retinal detachment when imaging is possible

50
Q

what are some of the reasons you may have reduced a/b wave amplitude erg indications

A

rod/cone dystrophy

drug toxicity

chorideremia

51
Q

what are some of the reasons why you may normal a wave and a reduced b wave

A

congenital stationary night blindness

juvenile retinoschisis (splitting of retinal layers)

central retinal artery occlusion

melanoma associated retinopathy

batten disease

52
Q

what other responses might you see from an erg and what do they indicated

A

normal dark adapted response

abnormal light adapted response

cone dystrophy

normal light adapted response
abnormal dark adapted response

rod dystrophy

diminished oscilatory potentials

early retinal detachment in diabetes

ischameia

53
Q

what is the source of an eog

A

standing trans epithelial potential of approx 10mv (quite a large potential)

varies slowly with illumination

54
Q

what are the different eog recording methods

A

standing potentials difficult to measure because of uncertainty concerning baseline position

(electrode offset potentials)

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

two lids in ganzfield subtending an arc of 30 degrees, illuminated

alternately for 1 second and subject asked two track them

recording electrodes are placed on the nation and lateral cantos with a reference electrode on ear lobe - signal size approx 1mv

bandwidth of signal approx 0.01hz - 30hz

55
Q

what stimuli is used for eog’s and what responses are expected

A

record réponse for 10 seconds every mi nuitée to avoid fatigue for 15mins during dark adaptation , amplitude ‘‘dark trough’’ occurs after around 12 minutes

500cd/m2 steady illumination turned on

recording continued until ‘‘ligh peak’; amplitude occurs typically after approx 10mins

arden rattio= light peak/dark trough

value of less than 1.85 is considered normal

56
Q

what are some of the clinical uses of the eog

A

subnormal results - best vitelliform macular dystrophy (essential for diagnosis)

retinitis pigments (rod/cone dystrophy) results parallel erg

adult vittleform macular dystrophy - can be normal but tends too be slightly subnormal

central retinal artery occlusion = flat- erg = more informative

57
Q

what is an eng used for

A

eng= electronystagmography

saddaric velocity

horizontal gaze (with electrodes either side of eye)

vertical angle of gaze (electrodes above and below the eye)

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

58
Q

what is an merge

A

mferg= multifocal electro retinogram

responses from multiple discrete areas of retina

primarily used to measure spatial variations in cone function

discrete retinal lesions (involving too small of an area to affect the erg)

enlarged blind spot syndrome (EBS)

Maculppathy

acute zonal occult outer retinopathy

59
Q

what stimulus is used for mFERG

A

multiple elements stimulate many area of the retina simultaneously

each element flashes following a pattern of ons and offs determined by a maximum length (m sequence) e.g. 010201101001

individual responses deconvoloved (simplification of complex signal) for mass response to give miniature ergs’s for each area

60
Q

what scaling is used for mferg

A

mferg stimulus scaling is used

scaling (spatial distortion) of the stimulus pattern is needed to account for spatial variation in cone density throughout the retina

elements increase in size with increasing eccentricity to give approximately equal sized responses

61
Q

how are mferg’s recorded

A

recorded using DTL thread electrodes to avoid interfering with vision

dilated pupils for consistent and repeatable retinal illuminance (focus/contrast less important)

62
Q

in Maculopathy what abnormalities would be picked up by mjferg

A

loss of response from macular region

63
Q

what is enlarged blind spot syndrome

A

area of dysfunction evident in eye extending temporally from the optic disc along the vascular arcades sparing the macular

funds/oct normal

64
Q

what is a PERG

A

Pattern erg

recorded using a counterphasing

(Reversing) chequerboard stimulus

mean luminance remains constant (usually 50cd/m2)

udilated pupils are required as contrast is most important factors (highly dependent on focus)

65
Q

what is a PERG recording

A

has a tiny retinal response (differentiates macular ON disease

p50= macular function

n95 retinal ganglion cell function unsuitable for patients with nystagmus and generally under 6 years old

66
Q

what are the normal ranges for perg results

A

normal = n95 larger than p50

Maculopathy = n95 concomitantly

reduced with p50, p50 may be delayed

optic neuropathy = n95 smaller than p50

67
Q

what is a VEP

A

visual evoked system

recording of the electrical activity that occurs in the brain in response to visual stimulation by time variant diffuse or structured stimuli

68
Q

what is the vet good for

A

good for testing children/adults with poor vision/ cooperation

can’t estimate visual acuity

good for detecting misrouting

reverses chequerboard (similar to PERG)

confounded by nystagmus (pattern is ‘‘smeared’’ by movementt

chequerboard is usually 1 degree chequers (macular stimulation) and 15 chequers (foveal stimulation)

usually 2 reversals per second stimulus field .15 degrees

steady fixation is necessary (requires cooperation and focus, patient must be refracted

69
Q

deesribe the arrangement for verps

A

patient - electrodes - amplifier- filter 0 analogue to digital converter - computer - stroboscope or pattern stimulator

70
Q

what are some of the uses of veps

A

demyelination - large majority of patients with ms showed increased peak time even with absence of symptoms

compression of optic nerve from space occupying lesions

optic neuropathy - functional integrity f visual pathway

objective cortical va mesurent

71
Q

how is va measured using veps’s

A

vet’s are recorded using pattern stimuli with different element sizes to limit of visual acuity

infants found to reach adult levels of vet acuity by 6 months

72
Q

what is minimum vep acuitty

A

6/ x spatial element size in minutes of arc0

likely to underestimate actual va - if responses only recordable to flash, va is likely to be rudimentary only

patient may not be blind if no VEPS are recordable

73
Q

what is a sweep vep

A

rapid presentation of different chequer sizes - good padagraims ensure robust and objective measurement in as little as 10 seconds

74
Q

what is right hand field stimulation

A

in normal subjects, stimulus od or os will activate the left hemisphere

temporal projection os left

nasal projection od right

opposite for left - half field stimulation

75
Q

what is paradxoial lateralisation of the p100 to half field stimulation

A

half field stimulation activates 1 hemisphere only

p100 paradoxically recored from side of scalp ipsilateral to stimulated half filed

p100 produced by dipole generators in calcimine salcus

electrode on scalp ipsilateral to stimulated hard oiled better placed to detect p100

full filed stimulation causes cancellation in lateral electrodes but not midline

76
Q

what are the causes of crossed and uncrossed roc’s

A

conditions displaying misrouting oct (normal shown In carriers of x linked oca

oa

chediak hibachi syndrome

hermansky pudlak syndrome

warden bury syndrome

albinoidisim

77
Q

vep asyymetry

A

misrouting in albi ism results in occipital lateralisation of the vep

this is seen in all modalities but the degree to which each displays this best, varies on the age of the patient

asymmetry of opposite sense is seen in aschiasmia and in compression of the crossing fibres e.g. pituitary adenoma

Electro (5 decks)

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