Multifocal ERG is useful to assess the function of:
Ans. b. Macular cones (Ref: Yanoff and Duker 4/e p460)Multifocal ERG is useful to assess the function of macular cones.Multifocal ERG (mfERG)The multifocal ERG (mfERG) has supplanted focal ERG testing and records multiple local ERG responses elicited from the central 40--50 degrees of the retina under light- adapted conditions.These responses are then displayed individually so that abnormal spatial variations can be localized to their corresponding areas in the macula, perimacula, or remaining posterior pole.Clinically, mfERG testing is most useful to assess macuiar function in patients with unexplained or central loss of vision who may have a normal full-field ERG.The mfERG can aid in the diagnosis of macular diseases including Stargardt's dystrophy, cone dystrophy, occult macular dystrophy, and hydroxychloroquine toxicity.Ring ratio analysis of mfERGs provides a quantitative measure that can be helpful in early detection of diseases that cause dysfunction in the pericentral macula, such as in hydroxychloroquine toxicity.Fig. 23: Typical electroretinogram showing a, b and c waves. The dip in the lower line indicates the point of stimulationElectroretinogram (ERG)In ERG, changes in the resting potential of the eye, induced by the stimulation of the eye with a light stimulus, are measured.In the normal dark-adapted eye, after a fleeting early receptor potential, three components are seen:Negative a-wave* Possibly representing the activity of the rods and conesQPositive (composite) b-wave* Arising in the inner retinal layers: and, with strong stimuli, a secondary rise in potentialQc-wave* Related not to visual processes but to retinal metabolism, associated particularly with the pigmentary epitheliumQ.The ERG measures a global response and essentially gives an indication of the activity of the entire retina, that is, of the rods and cones and their immediate connections.The rod response is selectively tested in the dark-adapted state with a blue light stimulus (scotopic ERG).The cones are tested in bright light (photopic ERG) or with a flickering light stimulus of >20 Hz, which is higher than the critical fusion frequency of rods.The pattern ERG indicates the activity of the central macular region and its corresponding nerve connections: it is used to diagnose early glaucomatous damage.ERG (Normal ERG is biphasic)Extinguished ERG* Advanced retinitis pigmentosa. CRAO. complete old retinal detachment, advanced siderosisSubnormal ERG* Early retinitis pigmentosa, large area of retina is not functioningNegative ERG* Gross disturbances of retinal circulationPattern ERG* Indicates activity of central macular regionMultifocal ERG* For disorders of central retina/fovea
Rods
Macular cones
Ganglion ceils
Retinal pigment epithelium
Techniques of Ocular Examination and Diagnostic Tests