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

All You Ever Wanted to Know About ERG but Were Afraid to Ask. American Academy of Ophthalmology, Orlando, 2002. Testing Levels of the Visual System. clinical neurophysiology allows assessment of retina, optic nerve, chiasm, and retrochiasmal visual pathways

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

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  1. All You Ever Wanted to Know About ERG but Were Afraid to Ask American Academy of Ophthalmology, Orlando, 2002

  2. 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

  3. b OD OS a 100 Uv 25 ms 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

  4. Plotter Recording the ERG Photic stimulator electrode

  5. ERG Electrodes Contact Lens Electrode Microconductive Thread Electrode

  6. Modern ERG Recording Electrodes DTL-Plus™ Microconductive Thread Electrode

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

  8. 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

  9. Modern ERG Stimulator

  10. Mini-Ganzfeld LED Stimulator • 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 Espion Colorburst™

  11. Components of the Flash ERG b-wave • Implicit time (i.t.)= time from stimulus • until peak of activity b i.t. • Amplitude (amp)= voltage magnitude • at peak of activity b amp a amp a i.t. a-wave

  12. 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

  13. OP2 OP1 OP3 OD OP4 5 Uv OS 15 ms 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

  14. Rod ERG OPs OP2 OP1 OP3 Cone ERG 30 Hz Flicker Rod vs Cone ERGs

  15. Pattern ERG (PERG) • Elicited to pattern stimulation (usually checkerboard or bar gratings) • much lower amplitude than flash ERGs • fixation is critical for PERG, not flash ERG • PERG reflects activity in the ganglion cell layer

  16. Generators of the Pattern ERG • PERGs are generated in the ganglion cell layer • PERGs may reflect both activation of cell body and axonal activity

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

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