Revision deck Flashcards
what is tonometry
the measurement of iop
what are the units of pressure used for iop
pascals (pa) or n/m2
iop is usually given in mmhg
hectopascal = hecto =100
what is mmhg in hpa
mmhg= 1.33hpa
what can raised intrauouclar pressure be due to
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
what is the incidence of glaucoma
affects 1/200 aged over 50
1/10 aged over 80
what can be tonometry be used for in relation to glaucoma
tonometry can detect and monitor iop
how does glaucoma affect the optic nerve
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
what are the different subtypes of glaucoma
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
what is the normal range of iop
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
what is atmospheric pressure equivalent to in spa and mmhg
1 bar = 100hpa
100hpa = 750mmhg
what is applantion tonometry
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)
what is sodium fluorescein
sodium fluorescing is excited by blue light maximally at (494nm)
fluoresce green at approximately 521nm
depends on ph = 7.5 - 8.5
how is the Goldman tonometer used
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
what are confounding factors for the use of the goldan tonometer
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)
what are problems with the Goldman tonometer
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
describe principles of general electrophysiology
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
what are electrodes and recording systems used for
to record electrical signals an electrode amplifier, filter display and recording device are needed - may also need a stimulator
what are electrodes used for and what are the different types
used to convert ionic form of current to electric flow along wire
usually metal
in many forms : skin surface, needle can be very specalised
how can noise be disruptive in electrodiafnosis
unwanted signals
caused by random motion of electrons in recording signals
magnetic fields from electrical machinery
radio signals
measurement eror
what are amplifiers used for
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
what are the two different inputs used differential amplifiers have
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
what is the common mode rejection ratio
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
what is the frequency content of signals
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
what are filters used for
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
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
what are the different frequency ranges for different recording systems
ecg - 0.5hz - 100hz
eeg - 0.5hz - 75hz
emg - 10hz - 5khz
nap- 10hz- 10khz
what are the effects of filtering
it is easier to identify and measure waveforms
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
describe the basic arrangement of an evoked potential system
patient- electrodes- amplifier - filter - analogue to digital converter - computer - visual stimulator
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
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
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
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
what does digital transmission of nerve signals avoid
avoids cross talk and external interference
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
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
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
what is a erg
electroretinogram
electrical response of the retina to flashes of light or patterns
what is a eog
an electrooculogram
= an electrical response of the retina to changes In steady state illumination
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
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
what is the jet corneal electrode used for
used for ergs under general anaesthetic in theatre
would need topical anaesthetic otherwise
disposable
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
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
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
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
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
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
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
what are some of the reasons you may have reduced a/b wave amplitude erg indications
rod/cone dystrophy
drug toxicity
chorideremia
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
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
what is the source of an eog
standing trans epithelial potential of approx 10mv (quite a large potential)
varies slowly with illumination
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
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
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
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)
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
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
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
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)
in Maculopathy what abnormalities would be picked up by mjferg
loss of response from macular region
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
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)
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
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
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
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
deesribe the arrangement for verps
patient - electrodes - amplifier- filter 0 analogue to digital converter - computer - stroboscope or pattern stimulator
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
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
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
what is a sweep vep
rapid presentation of different chequer sizes - good padagraims ensure robust and objective measurement in as little as 10 seconds
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
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
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
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
