electrophysiology Flashcards
what is electrophysiology
- study of electrical properties of cells and tissues
- based on the prinicple of resting membrane potentials and action potentials
requires the use of electrodes to detect action potentials and a monitor to display changes in electrical current
- name a example of a electrophysiological test
- ecg - allows cliniian to asess conduction of current in the heart and is widely used
can show different cardiac pathology
myocardinal infarcatrion
arthymias- atrial fibrilation , ventricular fibrilation
conduction abnormalities -
what is a eeg
useful in assessing a variety of neurological conditions e.g. epilepsy
emg - elctromyoography
used to asess both muscular activity as well as nerve supply
malfunction of skeletal muscle by either muscle or nerve will result in differences in graphs
electrophysiology of the eye
the eye has a different electrical charge at the front
compared to the back
the cornea has a relative postive charge compared to the retina which is relativley negativley charged
positive charge anterioly and negative charge posterioly
the eye is a dipole
why is the eye described as a dipole
because the conrea has a relative postive charg and the retina has a relative negative charge
what happens when a dipole moves
when dipoles move - (something with positive charge at one end and
negative charge at the other end -
moving the eye left to right or up and down creates a electrical deflection that we can measure
what does a eog do
a electrooculogram
records the electrical diopole between the front and the back of the eye
- you place electrodes in the medial and lateral canthus and then you ask the patient if they can look from left to right and you try to measure the changes in the electrical activity
reflects rpe activity
can help distinguish localised from diffuse retinal disease e..g rod dystrophyies or cone dystrophyes which effect the entire retina
e.g. in best viteliform macular dystrophy - erg is normal but eog light peak is markedly reduced
what does the eog do
the eog measure the conreoretial standing potential - the eye is a diopole postive charge at the cornea and negative at retina - by using lateral eye movements in conditions of varying luminance
measurements between pairs of electrodes at medial and lateral canthi are taken in scotopic conditions followed by mesopic conditions
signal amplitude is minimum in the dark and maximum in light
the light peak to dark trough is calculated = arden ratio
a normal ratio is more than 1.80
a ratio less than 1.65 is significantly subnormal
what is the arden ratio
- the amplitude of the change from light to dark conditions is called the arden ratio
you calculate the light peak by the dark trough
- in a normal eye you would expect it to be over 1.8
if its below 1.6 it is abnormal
what does a electroretinogram do
erg isolates the retinal function
electrical mass response of the retina to a light stimulus
affected by
intensity of stimulus
duration of stimulus
stimulus wavelength
stimulus pattern
corneal electrodes place via a contact lens
eog electrodes also used on medial and lateral canthi
what are the graphical components of a erg
a wave - corneal -ve deflection - signifies hyperpolarisation of rods and cones (mainly outer retina)
b wave - corneal +ve deflection - signifies depolarisation of on centre bipolar cells (mainly inner retina)
what do the a wave and b waves of an erg represent
a wave represents outer retinal function and the b wave represents inner retinal function
if you have a crao what would you see on the erg
you would see the b wave affected by the cra supplies the inner retina- a wave would not be affected because the outer retina recieves its blood supply from the choroidal circulation
what is a negative erg
a negative erg is where you lose the b wave - you have a flat b wave- and you still have the a wave
what is the c wave
refelctive of the retinal pigmented epithelium activity
what is the d wave reflective off
caused by depolarisation of off centre bipolar cells
what are the different erg settings
flash erg = mixed rod- cone response - you are engaging both the rods and the cones
dim white/ blue flash in scotopic conditions = isolated rod response
bright flash with 30 hz flicker = isolated cone response (rods have poor temporal resolution ( temporal resolution refers to how fast something is able to change its light intensity) and unable to respond to a 30hz stimulus)
what is retinitis pigmentosa
- it is a rod dystrophy - the rods are responsible for dark vision and the way you isolate them is by using dim light in a scoptopic condition - i.e. a dark adapted eye
what would you exepct a erg to look like if you have retina pigmentosa
- you would expect a flat a wave
what are the uses of erg’s
can help distinguish retinal disease from optic nerve disease
cannot distinguish macular disease from optic nerve disease
can help distinguish conditions affecting choroidal circulation from conditions affecting central retinal artery occlusion
why do you use different erg settings
different erg settings can be used to isolate rods and cones
what do rod dystrohgies produce on erg scans
rod dystrophies become apparent with a dim blue/ white light in scotopic conditions (isolated rod response)
what do cone dystrophies produce in erg’s
cone dystrophies produce an abnormal flicker erg
what do conditions affecting the inner retina produce
conditions affecting the inner retina can only produce a typical ‘negative erg’’ where the wave is intact ( outer retina) but the b wave is abolished
what can be used for localiszed retinal disease/macular disease
for localised retinal disease/ macular disease , multifocal erg can be used
what are visual evoked potentials
measures visual cortex response to a visual stimulus
big use of veps= visual functional loss
based on principles of eeg
patients must have their refractive errors corrected for the test
reverse checkboard stimulus usually used
flash vep can be used in uncooperative patients e.g,. infants and coma patients
when would you use vep
flash vep is useful in asessing vision in unccoperative/preverbal/non verbal patients
can distinguish organic from functional visual loss
occasionally used in demyleinating disorders - increased p100 latency
