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An electric potential (voltage) that is measured between points in living cells, tissues, and organisms, and which accompanies all biochemical processes
Units?
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At rest, Na/K pumps in a neuron's membrane keep a higher concentration of ___ outside the cell and a ___ inside the cell, creating a voltage difference called ____.
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When stimulated, Na flows down its concentration gradient and flows ___ the cell, while K flows ___ the cell.
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The voltage required to exactly oppose the flow of any given ion is the equilibrium potential for that ion
Equilibrium Potential
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At rest, the membrane is slightly polarized: ____ inside and _____ outside.
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Measures the resting potential of the retina
Electrooculogram, EOG
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Eye movements are used as an indirect way of measuring the resting potential
EOG
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Eye movements of constant amplitude can be used to detect changes in the:
Standing potential
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Retinal illumination (can/cannot) change the standing potential
can
scotopic and photopic conditions will give different results!
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Since the eye is polarized, changes in the eye's polarization when the eye changes horizontal orientation in the socket can be detected by skin electrodes placed where?
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The resting potential of the retina makes the eye act like a polarized battery! It is more positive towards the ____ and more negative towards the _____.
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Eye movements of constant amplitude can be used to detect changes in the _______ _____.
standing potential
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____ ______ can change the standing potential
Retinal Illumination
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Can you have different results of standing potential be different if you measure it under scotopic and photopic conditions.
YES
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Resting potential is more + towards the _____ and more (-) towards the ____.
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The normal standing potential of the eye requires a healthy...
RPE
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What is a direct way to measure the health of the RPE?
measure the standing potential
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What layer of the retina has the greatest electrical potential?
RPE
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The cornea is + or - ?
The retina is + or - ?
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When light hits the photoreceptors there is a increase of decrease in extracellular K+ concentration around the photoreceptors?
decrease
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When light hits the photo receptors A change in the concentration of an unknown substance called?
"Light-Peak Substance"
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After light hits photoreceptors and causes a change in concentration, there is an increase in the flow of ___ out of the basal membr of RPE, which causes _____________.
- Cl-
- light peak polarization
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Does EOG measure the response to a stimulus?
NO (stimuli is used so the pt has something to fixate on)
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EOG tests...
eye movements
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Describe what happens during an EOG procedure
There are 2 stimuli that the pt is told to move their eyes back and forth to look at.
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A pt must have equal ______ for the measurement to be most accurate?
saccades
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Where do you attach the electrodes to for an EOG?
medial and lateral canthi and ground electrode on forehead or earlobe
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Name of bowl used in EOG and how far are fixation lights set apart?
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Fixation lights in an EOG are used to ensure the saccades have...
constant amplitude
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Movement of eyes during EOG produces a voltage swing of ~ __ microvolts that is recorded
5
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What is the lighting during an EOG?
- lights are turned off after 5 min training period; after 15 minutes lights are turned on
- (15 minutes of dark adaptatin and 15 minutes of bright light recorded)
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Explain the dar trough
voltage becomes smaller in the dark, reaching its lowest value after 8-12 minutes
less illumination of retina = decrease in resting potential of eye
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When lights are turned on, the potential rises and peaks at about 10 minutes; this is called the ?
Light rise
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IN an EOG amplitude is recorded in
microvolts
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Smallest voltage change in an EOG occurs...
during dark (dark trough)
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EOG result graph: black data points represent...
the magnitude of the voltage swing with the eye mvnt
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EOG x axis and yaxis represent?
- x axis: time in minutes
- y axis: amplitude of saccades in milivolts
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EOG signals are very small so they have to be _____
Amplified (DC or AC)
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This amplifier will more faithfully reproduce the square wave voltage changes that take place when the pt looks back and forth and gives a better record of actual eye position
DC amplifier
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Which amplication is used more frequently?
AC
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Which amplification works better when you are studying eye movements? which one when you are studying RPE health?
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What is the arden ratio?
(light peak)/(dark trough)
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What are the normal ranges for an arden ratio? abnormal?
- Normal: >1.65 -1.80
- Abnormal: <1.65-1.80
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Abnormal EOG result indicates..
unhealthy RPE
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EOG is helpful in diagnosis what disesase?
Vitelliform macular degeneration (BEst's disease)
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Best's macular dystrophy is..
affects males or females more?
]those with mild formes of the disease are called?
- autosomal dominance
- male = female
- carriers
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Best's macular dystrophy presents when with what?
childhood 1-2nd decade with a yellow macular yolk-like lesion
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Stages of Bests
Stage 0: normal fundus, abnormal EOG
Stage 1: macular RPE disturbances-foveal pigment mottling and hypopigmentation, (-)FLR
- Stage 2- "egg yolk" lesion,
- Stage 2a: scrambled or fried egg appearance (vitelliruptive stage)
Stage 3-pseudohypopion stage
- Stage 4a- atrophy of RPE produces orange red lesion in macula; resorption of vitelliform lesion
- Stage 4b: white hypertropic scar of fibrous tissue in macula; -fibrotic, gliotic-appearing scar-atrophic changes in RPE & choriocapillaris
Stage 4c: neovasc of fibrous scar
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What is the hallmark of Best's macular dystrophy?
markedly abnormal EOG in all stages of progression and in phenotypically normal carriers
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What is difference on EOG with Best's pt vs normal?
light rise will not be present
(line at bottom of graph shows when light is turned on and off)
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When does Stage 2 (egg yolk) occur?
what happens in RPE
visual acuity is around?
- first few years of life (3-15years)
- lipofuscin accumulates beneath or w/in RPE
- 20/20-20/40
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When does stage 3 occur (pseudohypopion)?
What happens to visual accuity?
What happens during an FA?
- second decade
- decreases
- early hyperfluorescence, late staining of superior lesion
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If you saw a first time elderly pt, how could you tell if they had Best's or macular degeneration??
EOG!
armd will have normal EOG, OCT could look the same
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What does and ERG look like in Best's
full field ERG is normal
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What is charactistically abnormal on EOG for Best's?
severe loss of light response
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What is a typical Arden ratio in Bests?
<1.5
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Why is dark trough in a Best's pt normal? why is a VF normal?
- Dark adaptation is normal since rod's are not affected.
- Damage is in fovea not in periphery
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What are products of oxidation of lipids and lipoprotiens that are brown pigments similar to melanin that accumulate in animal cell with age?
lipofuscin
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Best's is a mutation of what gene?
VMD2, long arm chromosome 11 (Ch11q13), encodes bestrophin-1: RPE specific protien
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Visual prognosis for Best's? What is vision threatening event that can occur?
- Usually have decent vision for reading or driving.
- CNV (choroidal neov)
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What should you do if you have a pt with Best's/carrier?
- screen family, DFE, EOG and VMD2 testing
- **Genetic counceling and monitoring
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Apporx how many carriers have normal fundus?
~5%
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What is the treatment for Best's?
- No effective treatment to stop progression of vitelliform lessions.
- photodynamic therapy if they develop a CNVM
- or
- Intravitreal bevacizumab (Avastin) to dry up leakage of neov/vessels
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