All You Ever Wanted to Know About ERG but Were Afraid to Ask

American Academy of Ophthalmology, Orlando, 2002 28 April 2020 1

Testing Levels of the Visual System  clinical neurophysiology allows assessment of retina, optic nerve, chiasm, and retrochiasmal visual pathways  the ERG can functionally dissect the neural retina 28 April 2020 2

Electroretinogram (ERG)  ERGs are mass electrical activity from the retina in response to visual stimulation  the ERG is objective, non-invasive measure of electroretinal function

OD OS a b

25 ms 100 Uv 28 April 2020 3

Plotter

Recording the ERG Photic stimulator

electrode

ERG Electrodes Contact Lens Electrode 28 April 2020 Microconductive Thread Electrode 5

Modern ERG Recording Electrodes DTL-Plus™ Microconductive Thread Electrode 28 April 2020 6

ERG Electrodes  neglible mass  convenient to use, disposable  no interference with pupil or optics  reliable ERG recordings  well tolerated by children Microconductive thread electrode 28 April 2020 7

Pre-Millenium ERG Technology  Ganzfeld Stimulation (full-field)  provided by xenon flash photostimulator  full visual field stimulation  stimulus intensity  stimulus wavelength  variable stimulus rate  variable background intensity 28 April 2020 8

28 April 2020 Modern ERG Stimulator 9

Mini-Ganzfeld LED Stimulator Espion Colorburst™  4.5 inch diameter  176 LEDs  “blue” (470 nm)  “green” (525 nm)  “red” (640 nm)  7 log unit luminance range  optoelectronic stimulation can be more precisely controlled and calibrated than xenon discharge tubes 28 April 2020 10

a amp

Components of the Flash ERG

b i.t.

b-wave Implicit time (i.t.)= time from stimulus until peak of activity Amplitude (amp)= voltage magnitude at peak of activity b amp a i.t.

a-wave

Generator Sites of the Flash ERG

 a-wave originates from the photoreceptor layer • • b-wave is a glial potential originating from the Müller cell and the bipolar cells OPs originate from amacrine cells • flash ERG reflects electroretinal activity distal to the ganglion cell layer 12 28 April 2020

Oscillatory Potentials (OP)

 a subcomponent of the ERG seen on ascending limb of the b-wave  Wachtmeister (1977) demonstrated OPs originate from amacrine cells in the inner plexiform layer

OD OS OP2 OP1 OP3 OP4

15 ms 5 Uv 28 April 2020 13

28 April 2020 Rod vs Cone ERGs Rod ERG OPs OP1 OP2 OP3 Cone ERG 30 Hz Flicker 14

28 April 2020 Pattern ERG (PERG)  Elicited to pattern stimulation (usually checkerboard or bar gratings)  much lower amplitude than flash ERGs  fixation is

critical

not flash ERG for PERG,  PERG reflects activity in the ganglion cell layer 15

Generators of the Pattern ERG  PERGs are generated in the ganglion cell layer  PERGs may reflect both activation of cell body and axonal activity 28 April 2020 16

ISCEV ERG Standards  International Society for Clinical Electrophysiology of Vision (ISCEV)  ERG standard established in 1989  standards for 5 commonly obtained responses  ERG developed by the rods (dark-adapted)  maximal ERG response (dark-adapted)  Oscillatory Potentials  ERG developed by the cones (light-adapted)  ERG to flicker 28 April 2020 17

Who are the Clients of an Electrodiagnostic Service?

 Patients whose eyes have:  symptoms suggestive of known neurological or ophthalmological disease  unexplained visual loss   medico-legal problems workman’s compensation  defects associated with psychiatric disturbance, mental or physical handicap  pediatric neuro-ophthalmic practice  metabolic, hereditary or neurological disease with visual impairment  non-seeing child 28 April 2020 18

Who are the Clients of an Electrodiagnostic Service?

 Patients with media opacities  prior to corneal grafting or cataract surgery  vitreous hemorrhage (in DR or following trauma, RD)  Patients with uveitis or inflammatory eye disease  Patients with suspected disease or carrier status of inherited visual disorders  Patients requiring quantitative assessment of disease progression  Assessment of retinal and optic nerve function following trauma 28 April 2020 19

Indications for ERG  aid to diagnosis in patients suspected of having stationary or progressive inherited retinal degenerations 

to rule out clinically unaffected family members



to evaluate possible female carriers of x-linked disease

  to assess patients with retinal toxicity due to metallosis (siderosis) retinotoxic or neurotoxic medications   chloroquine, Hydroxychloroquine, ethambutol, phenothiazine Desferrioxamine, Tamoxifen  Vigabatrin toxicity?

  patients with OHT/glaucoma to identify eyes that are likely to develop neovascularization in diseases such as CRVO and DR  documenting therapeutic effects of surgery or medication 28 April 2020 20

Disorder RP

RP (dom) RP (rec)

CSNB Best’s Stargardt’s

Scotopic (Rod) ERG a-Wave Ext. or  Normal b-Wave Ext. or  Delayed Delayed Ext. or  Photopic a-Wave Ext. or Normal  (Cone) ERG b-Wave Ext. or  Normal Normal or  Normal Normal Normal or  Normal Normal Normal or  Ext. or   (neg-ERG) Normal Normal or  Ext. or  Flicker ERG Ext. or Delayed Delayed  Normal Normal or  Ext. or  

Cone Dystrophy Choroideremia

Ext. or  Ext. or  Ext. or  Ext. or  Ext. or 

Fundus Albipunctatus Glaucoma

 (N) Normal  (N) Normal Normal or  Normal Normal or  Normal Normal P-ERG 

Disorder Leber’s amaurosis

Scotopic (Rod) ERG a-Wave Ext. or  b-Wave Ext. or  Photopic (Cone) ERG a-Wave Ext. or  b-Wave Ext. or  Flicker ERG Ext. or 

DR Albinism

Normal Normal Normal  Normal Normal Normal  Normal OPs  or Ext Normal

Vit-A deficit

Ext or   Ext or   

Hurler’s, Sanfilippo’s, Scheie’s MPS Batten’s, Jansky-Biel, Spiel.-Vogt CRVO

Ext Ext. or   Ext Ext. or   delayed Ext.

Ext. or  Normal or  Ext.

Ext. or   delayed Ext.

Ext. or  delayed

Electrodiagnostics of the Future  Standardized instrumentation  Standardized testing protocols  Multicenter normative data collection  Web-based telemedicine application  University of Ottawa Eye Institute announces ERG Reading Centre 28 April 2020 23