Introduction to Electrophysiology

Question 1

What is electrophysiology?

Answer 1

Study of the electrical properties of cells and tissues

Based on the principle 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

Question 2

What is Electromyography (EMG)?

Answer 2

Used to assess both muscular activity as well as nerve supply.

Malfunctioning of skeletal muscle by either muscle or nerve will result in differences in graphs.
e.g. myasthenia gravis

Question 3

How is the eye charged?

Answer 3

It’s a dipole. The cornea has a relative positive charge and the retina a relative negative charge

Question 4

What happens when dipoles move?

Answer 4

They cause electric field changes which creates an electrical deflection that we can measure

Question 5

What does an electro-oculogram (EOG) measure?

Answer 5

Records the electric dipole between the front and back of the eye (The EOG measures the corneoretinal standing potential by using lateral eye movements in conditions of varying luminance)

Reflects RPE activity (therefore tests RPE activity). Recording difference between front and back of the eye.

Can help distinguish localized from diffuse retinal disease
E.g. in Best vitelliform macular dystrophy – ERG is normal but EOG light peak is markedly reduced

Question 6

How are measurements in EOG set up?

Answer 6

Measurements between pairs of electrodes at medial and lateral canthi are taken in scotopic conditions (rod only) followed by mesopic conditions (cone and rod)

Signal amplitude is minimum in the dark and maximum in light

The light peak to dark trough is calculated = Arden ratio

Question 7

What are the normal and abnormal ratios for an EOG?

Answer 7

A normal ratio > 1.80

Ratio < 1.65 is significantly subnormal

Question 8

What is an electroretinogram (ERG)?

Answer 8

Electrical mass response of the retina to a light stimulus

See response of retina itself to a light stimulus (is the retina responding to the light)
ERG – different refractive errors respond to different lights differently even in normal retinas so must look at what it is affected by during the ERG

Question 9

What is an electroretinogram (ERG) affected by?

Answer 9

Affected by:
- Intensity of stimulus
- Duration of stimulus
- Stimulus wavelength
- Stimulus pattern

Question 10

How are ERG electrodes placed?

Answer 10

Corneal electrodes place via a contact lens

Question 11

What is an A-wave on an ERG?

Answer 11

Corneal -ve deflection. Signifies hyperpolarisation of rods and cones (mainly outer retina) – more positive

Question 12

What is a B-wave on an ERG?

Answer 12

Corneal +ve deflection. Signifies depolarisation of ON-centre bipolar cells (mainly inner retina)

Question 13

What is a C-wave on an ERG?

Answer 13

Transepithelial potential due to hyperpolarisation of inner most RPE

Question 14

What is a D-wave on an ERG?

Answer 14

Corneal +ve deflection. Caused by depolarisation of OFF-centre bipolar cells

Question 15

What does a slow-rising C wave depend on?

Answer 15

Slow-rising ‘c’ wave depends on intact pigment epithelium (RPE)

Question 16

What are the 3 ERG settings and what are they used for?

Answer 16

Flash ERG =
Mixed rod-cone response (meso)

Dim white/blue flash in scotopic conditions = Isolated rod response

Bright flash with 30 Hz flicker =
Isolated cone response (rods have poor temporal resolution and unable to respond to a 30Hz stimulus)

Question 17

What is an ERG used for?

Answer 17

Can help distinguish retinal disease from optic nerve disease; it CANNOT distinguish macular disease from optic nerve disease – Because it looks at the retina as a whole, not specifically at the macular

Can help distinguish conditions affecting choroidal circulation from conditions affecting central retinal artery circulation – How? Because the c-wave relies on intact RPE (as this is supplied by the choroid)

Question 18

What settings of an ERG isolate rods and cones?

Answer 18

Different ERG settings can be used to isolate rods and cones

Rod dystrophies become apparent with a dim blue/white light in scotopic conditions (isolated rod response)

Cone dystrophies produce an abnormal flicker ERG

Question 19

How do we see conditions affecting the inner retina on an ERG?

Answer 19

Conditions affecting the inner retina only can produce a typical ‘negative ERG” where the a wave is intact (outer retina) but the b wave is abolished

Question 20

How do we see conditions like retinal disease/macular disease on an ERG?

Answer 20

For localized retinal disease/macular disease, multifocal ERG can be used

Question 21

What are VEPs?

Answer 21

Visual Evoked Potentials

Question 22

What do VEPs measure?

Answer 22

Measures visual cortex response to a visual stimulus

Based on principle of EEG

Patients must have their refractive errors corrected for the test

Reverse checkboard stimulus used

Flash VEP can be used in uncooperative patients – e.g infants and coma patients

Question 23

What test tells us if the signal is reaching the occipital lobe?

Answer 23

VEP (visual evoked potentials)
Is the signal reaching the Occipital Lobe? This will give us a physical response from the lobe to tell us that they are viewing the image
Not as often used in clinic compared to MRI or OCT as need to ensure first point of investigation is to check it’s not organic

Question 24

What are VEP’s useful in?

Answer 24

• Flash VEP useful in assessing vision in uncooperative/preverbal/nonverbal patients
• Can distinguish organic from functional visual loss
• Occasionally used in demyelinating disorders – increased P100 latency