Electroretinogram (erg)

electroretinogram

1. ELECTRORETINOGRAM (ERG ) By, G.Shirisha , 13IMMO20

2. ERG ERG is the corneal measure of an action potential produced by the retina when it is stimulated by light of adequate intensity. It is the composite of electrical activity from the photoreceptors, Muller cells & RPE.

3. HISTORY 1865 : First known recording of an ERG (amphibian retina) Swedish physiologist Alarik Frithiof 1877 : Holmgren , James Dewar of Scotland (humans) 1908 : Einthoven and Jolly separated the ERG response into three components : a-wave, b-wave and c-wave 1941 : American psychologist Lorin Riggs introduced the contact-lens electrode (clinical use) 1967 : Ragnar Granit Nobel Prize for Physiology and Medicine (demonstrated the physiology of the receptor potential of each component of the ERG) 1989 : ISCEV standards( International Society for Clinical Electrophysiology of Vision) 1992 : Erich Sutter mfERG

4. PROCEDURE According to ISCEV 2015 guidelines: Maximally dilate the pupils Before Dark adapted protocols- 20-30 min of dark adaptation Before light adapted protocols- 10 min of light adaptation Present low strength flashes before stronger flashes- so that the partial light adaptation due to bright light does not occur

5. Insert corneal contact electrodes (when these are used) under dim red light after dark adaptation period. Avoid strong red light. Allow 5 min of extra dark adaptation after insertion of contact lens electrode. Allow at least 30 min recovery time in ordinary room illumination after use of strong light for retinal imaging ( fundus photography, fluorescein angiography and others). Request the patient to fix and not move eyes. Ocular movements can change the positions of electrodes, can cause blockage of light by eyelids or electrode and may induce electrical artifacts.

6. ELECTRODES GROUND ELECTRODE – FOREHEAD REFERENCE ELECTRODE – OUTER CANTHUS ACTIVE ELECTRODE -Cornea (contact lens electrode) in flash ERG Conjunctival sac – used in pattern ERG

7. TYPES OF ERG FULL FIELD ERG FOCAL ERG MULTIFOCAL ERG PATTERN ERG

8. Full-Field ERG The full-field ERG, also referred to as the standard or flash ERG Retinal potential elicited by a brief flash of light, recommended to be about 5 ms in duration, that evenly illuminates the entire retina ( Marmor and Zrenner , 1998).

9. FOCAL ERG Used for detecting small focal lesions or pathologies which are missed by standard full field ERG. A small stimulus of 4⁰ size is projected on area of retina to be tested. Due to light scattering & poor signal to noise ratio, this technique is mostly used in research setting than in clinical setting.

10. MULTIFOCAL ERG The stimuli consists densely arranged black or white hexagonal elements displayed on CRT monitor. These hexagonal elements change from light to dark independently & this change results into recording of mfERG .

11. PATTERN ERG It mainly represents inner retinal activity (especially ganglion cell activity) Useful in differentiating optic nerve disorders from macular disorders. Unlike flash ERG, pattern ERG is a very small response. Recorded with full correction of refractive errors as visualization of stimulus for extended time is essential for recording.

12. PERG WAVEFORMS P1 or P50 : Initial corneal positive response. N1 or N95 : Immediate cornea negative response. This 50 & 95 represents the time in milliseconds from the onset of stimulus to peak of positive or negative response.

13. Indications & Clinical Uses of ERG Evaluation of visual function in infants & children. To determine presence or absence of retinal function. To evaluate progression of retinal degeneration. To confirm diagnosis of a particular disease (dystrophies). For early detection of toxic retinopathies. Assisting in diagnosing the retinal conditions in which clinical findings don't match with visual complaints (unexplained visual loss).

14. Components a-wave : initial corneal-negative deflection, derived from the cones and rods of the outer photoreceptor layers b-wave:  corneal-positive deflection; derived from the inner retina, predominantly Muller and ON-bipolar cells c-wave:  derived from the retinal pigment epithelium and photoreceptors

15. d-wave : off bipolar cells. Dark adapted  Oscillatory potentials : Responses primarily from the amacrine cells/inner retina. Latency of response  refers to the onset of the stimulus to the beginning of the a-wave. Implicit time or peak time  is a measure of the time interval from onset of the stimulus to the peak of the b-wave

16. Factors affecting the ERG Physiological : Pupil, Age, Sex, Ref. Error , Diurnal Variation, Dark adaptation , anesthesia Instrumental : amplification, gain, stimulus , electrodes Artifacts : Blinking, tearing, eye movements , air bubbles under electrode .

17. ISOLATED ROD RESPONSE Produced by dark adapting patient for 20-30 min. & then stimulating retina with dim light flash(2.5 log units/24db) which is below cone threshold and the time interval is 2seconds. The resultant waveform has ‘prominent b (positive) wave ‘& no detectable ‘a (negative) wave’.

18. MAXIMAL COMBINED RESPONSE It is a larger waveform generated by using bright flash (10 seconds of interval)in dark adapted state(30 mins) which maximally stimulates both rods & cones. It results in prominent ‘a ( negative) wave & ‘b (positive) wave’ with ‘oscillatory potentials’ which are superimposed on ‘b wave’.

19. SINGLE FLASH CONE RESPONSES It is obtained by maintaining the patient in light adapted state(10 mins) & stimulating the retina with bright white flash(10 sec interval). The rods are suppressed by light adaptation & donot contribute to the waveform .

20. 30 Hz F licker Response With patient in light adapted state(10 mins), a flickering stimulus at 30 Hz(30 stimuli/sec) can also be used to filter rod response & measure cone response

21. Interpretation of ERG ERG is abnormal only if more than 30% to 40% of retina is affected A clinical correlation is necessary Media opacities, non-dilating pupils & nystagmus can cause an abnormal ERG ERG reaches its adult value after the age of 2yrs ERG size is slightly larger in women than men

22. Negative ERG Characterized by large a-wave . Causes: X-linked retinoschisis Congenital stationary night blindness Quinine, vigabatrine toxicity, siderosis Melanoma associated retinopathy

23. INTERPRETATION Non recording ERG Leber congenital Amaurosis , Retinitis pigmentosa , Total RD Retinal Aplasia Abnormal or non recordable photopic ERG (often mild rod ERG abnormalities) Cone degenerations Achromatopsia X-linked blue cone monochromatism X-linked cone dystrophy Non recordable Rod ERG (abnormal dark adapted bright flash ERG) Normal to near normal Congenital stationary blindness Early RP Barely or Non recordable Scotopic ERG (abnormal photopic B-Wave ERG) Rod Cone Degenerations Night blindness

24. Normal MFERG 2D Map 3D Map

25. RETINITIS PIGMENTOSA

26. CONE DYSTROPHY

27. Chloroquine retinopathy

28. Macular Degeneration Stargardt’s Disease

29. Limitations of ERG Since the ERG measures only the mass response of the retina , isolated lesions like a hole hemorrhage, a small patch of chorioretinitis or localized area of retinal detachment can not be detected by amplitude changes. Disorders involving ganglion cells (e.g. Tay sachs ’ disease ), optic nerve or striate cortex do not produce any ERG abnormality

30. REFERENCES eyewiki.aao.org/ Electroretinogram webvision.med.utah.edu/book/electrophysiology/the- electroretinogram -clinical-applications electroretinographic -modifications-induced-by- agomelatine -a-novel- aven -peer-reviewed- fulltext -article-NDT wonderbaby.org/articles/ whats -erg slideshare.net/ shenoyvignesh /electrophysiology-in-ophthalmology slideshare.net/ susumarashdeh /erg-21541872 intechopen.com/books/ electroretinograms / electroretinogram -in-hereditary-retinal-disorders