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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
what is the bandwidth of a filter
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
26
what are the different frequency ranges for different recording systems
ecg - 0.5hz - 100hz eeg - 0.5hz - 75hz emg - 10hz - 5khz nap- 10hz- 10khz
27
what are the effects of filtering
it is easier to identify and measure waveforms
28
how are recordings digitised
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
29
describe the basic arrangement of an evoked potential system
patient- electrodes- amplifier - filter - analogue to digital converter - computer - visual stimulator
30
what is signal averaging
averaging reduces the noise in a signal by a factor of square root n (high number of averages (n) means a lower % noise level
31
describe sensory and motor fibres
sensory and motor fibres are bundled into nerve turns which can have up to 20,000 fibres in structure - approx 3mm in diameter
32
describe the difference between myelinated and unmyelinated nerve fibres
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
what is membrane capacitance
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
what does digital transmission of nerve signals avoid
avoids cross talk and external interference
35
what is the all or nothing principle
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
how are nerves stimulated electrically
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
what is non invasive conduction velocity measurement
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
what is a erg
electroretinogram electrical response of the retina to flashes of light or patterns
39
what is a eog
an electrooculogram = an electrical response of the retina to changes In steady state illumination
40
what is the source of the erg
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
what are the different erg recording methods
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
what is the jet corneal electrode used for
used for ergs under general anaesthetic in theatre would need topical anaesthetic otherwise disposable
43
what are DTL fibre electrodes used for
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
what type of light stimuli is used for erg
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
what are mircoelectrodes used for
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
what are skin electrodes used for
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
how are measurements taken from erg's
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
describe the luminance response you would see on a erg
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
what are some of the reasons you may not have an erg response
retinitis pigments (severe retinal degeneration) ophthalmic artery occlusion to confirm total retinal detachment when imaging is possible
50
what are some of the reasons you may have reduced a/b wave amplitude erg indications
rod/cone dystrophy drug toxicity chorideremia
51
what are some of the reasons why you may normal a wave and a reduced b wave
congenital stationary night blindness juvenile retinoschisis (splitting of retinal layers) central retinal artery occlusion melanoma associated retinopathy batten disease
52
what other responses might you see from an erg and what do they indicated
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
what is the source of an eog
standing trans epithelial potential of approx 10mv (quite a large potential) varies slowly with illumination
54
what are the different eog recording methods
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
what stimuli is used for eog's and what responses are expected
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
what are some of the clinical uses of the eog
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
what is an eng used for
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
what is an merge
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
what stimulus is used for mFERG
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
what scaling is used for mferg
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
how are mferg's recorded
recorded using DTL thread electrodes to avoid interfering with vision dilated pupils for consistent and repeatable retinal illuminance (focus/contrast less important)
62
in Maculopathy what abnormalities would be picked up by mjferg
loss of response from macular region
63
what is enlarged blind spot syndrome
area of dysfunction evident in eye extending temporally from the optic disc along the vascular arcades sparing the macular funds/oct normal
64
what is a PERG
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
what is a PERG recording
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
what are the normal ranges for perg results
normal = n95 larger than p50 Maculopathy = n95 concomitantly reduced with p50, p50 may be delayed optic neuropathy = n95 smaller than p50
67
what is a VEP
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
what is the vet good for
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
deesribe the arrangement for verps
patient - electrodes - amplifier- filter 0 analogue to digital converter - computer - stroboscope or pattern stimulator
70
what are some of the uses of veps
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
how is va measured using veps's
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
what is minimum vep acuitty
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
what is a sweep vep
rapid presentation of different chequer sizes - good padagraims ensure robust and objective measurement in as little as 10 seconds
74
what is right hand field stimulation
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
what is paradxoial lateralisation of the p100 to half field stimulation
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
what are the causes of crossed and uncrossed roc's
conditions displaying misrouting oct (normal shown In carriers of x linked oca oa chediak hibachi syndrome hermansky pudlak syndrome warden bury syndrome albinoidisim
77
vep asyymetry
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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