Electroretinogram (ERG) PDF

ELECTRORETINOGRAM (ERG) Punay pratap sah M.Optom HEI ERG Electroretinography (ERG) is an eye test that detects function of the retina (the light‐detecting portion of the eye). The retina is comprised of layers of specialized cells, includ...

ELECTRORETINOGRAM (ERG) Punay pratap sah M.Optom HEI ERG Electroretinography (ERG) is an eye test that detects function of the retina (the light‐detecting portion of the eye). The retina is comprised of layers of specialized cells, including photoreceptors (rods and cones), that detect light and ganglion cells that transmit images to the brain. Specifically, the ERG picks up electrical signals from the photoreceptors, as well as other cells (Muller cells and bipolar cells) that act as intermediaries between the photoreceptors and the ganglion cells.  Abnormal ERG readings can detect certain abnormalities of these cell layers. During the test, a medical professional places an electrode on the cornea (at the front of the eye) to measure the electrical responses to light of these cells. BASIC PRINCIPLE OF ERG Sudden illumination of retina. Simultaneous activation of all the retinal cells to generate the current. Currents generated by all the retinal cells mix, then pass through vitreous & extra cellular spaces. High RPE resistance prevents summated current from passing posteriorly. The small portion of the summated current which escapes through the cornea is recorded as ERG. COMPONENTS (ERG WAVE FORMS) 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 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 ERG RESPONSES A normal ERG has 5 distinct responses : Rod response Maximal combined response Dark adapted Oscillatory potentials Single flash cone response Light adapted 30 Hz flicker response 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’. MAXIMAL COMBINED RESPONSE It is a larger waveform generated by using bright flash (10 seconds of interval) in dark adapted state (30 min) 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’. CONE RESPONSES ‘Single flash response’ is obtained by maintaining the patient in light adapted state & stimulating the retina with bright white flash. The rods are suppressed by light adaptation & do not contribute to the waveform. 30 Hz flicker response With patient in light adapted state, a flickering stimulus at 30 Hz can also be used to filter rod response & measure cone response TYPES OF ERG (SPECIALISED FORMS) FULL FIELD ERG FOCAL ERG MULTIFOCAL ERG PATTERN ERG Full‐Field ERG (BRIGHT FLASH ERG) The full‐field ERG, also referred to as the standard or flash ERG Used for assessment of retinal function in ‘severely traumatized eye‘ or ‘eye with dense media opacity’ like dense VH, corneal opacity or advanced cataract.  In this procedure successive responses are obtained with flashes of increasing intensity, allowing the time for re‐ adaptation in between flashes. 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). A non recordable flash ERG is an ominous sign for visual prognosis. 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. Clinical uses of FERG : Early detection of cone dystrophy or macular disease before the fundus changes are evident. Can differentiate between early macular & optic nerve pathology. Can be used for evaluation of any type focal macular pathology. MULTIFOCAL ERG The stimuli consists densely arranged black or white hexagonal elements displayed on cathode‐ray tube (CRT) monitor. These hexagonal elements change from light to dark independently & this change results into recording of mfERG.  Based on retinal activity, the recorded mfERG appears in ‘topographic map form’ & also in ‘small ERG waveforms’ from various parts of retina. Both cone & rod mfERG forms can be recorded. mfERG findings also correlate with visual field defects but very small scotomas can be missed by this method. Normal MFERG 2D Map 3D Map 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. 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. ELECTRODES GROUND ELECTRODE – FOREHEAD ( neutral electrode) REFERENCE ELECTRODE – OUTER CANTHUS (negative electrode) ACTIVE ELECTRODE ‐ Cornea (contact lens electrode) in flash ERG ‐ Conjunctival sac – used in pattern ERG ( positive electrode) ELECTRODES USED IN ERG Jet Electrode Gold Plated Electrode  Skin Electrode DTL Electrode  HK Loops Burian Allen Electrode Electrode paste skin prep gel Factors affecting the ERG Physiological : Pupil, Age, Sex, Refractive Error, Diurnal Variation, Dark adaptation, anesthesia Instrumental : amplification, gain, stimulus, electrodes Artifacts : Blinking, tearing, eye movements, air bubbles under electrode. Clinical protocol (PROCEDURE) According to International Society for Clinical Electrophysiology of Vision (ISCEV) 2015 guidelines: Preparation of the patient Pupillary dilatation ‐ The pupils should be maximally dilated, and the pupil size noted before and at the end of recording the ERGs in this ISCEV Standard Pre‐adaptation to light or dark ‐ The recording conditions outlined below specify 20 min of dark adaptation before recording darkadapted ERGs, And 10 min of light adaptation before recording light‐adapted ERGs.  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. Pre‐exposure to light ‐ Fluorescein angiography, fundus photography and other imaging techniques using strong illumination systems should be avoided directly before ERG testing. If these examinations have been performed, we recommend least 30‐ min recovery time in ordinary room illumination before beginning ERG testing.  Fixation ‐ Patients should be instructed to look at a fixation point incorporated into the stimulus dome. A stable gaze is important so that eye movements do not alter the position of the electrode on the eye, produce electrical artifacts, or allow blockage of light by the electrode or eyelid. Patients who cannot see the fixation target may be instructed to look straight ahead and keep their eyes steady. Patients should be monitored to assess compliance and any difficulties in eye opening or fixation should be noted.  Dark‐adapted 0.01 ERG (rod‐system response) Dark‐adapted 3 ERG (combined rod and cone system responses) Dark‐adapted 10 ERG (combined responses to stronger flash) Light‐adapted 3.0 ERG (single‐flash cone response) Light‐adapted 30 Hz flicker ERG Single‐flash ERGs Oscillatory potentials Flicker ERGs 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). 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 INTERPRETATION Each lab should have its own normal values Adjust for age