Introduction to the Electroretinogram

The electroretinogram (ERG) is a technique used to measure the electrical activity generated by the retina in response to a light stimulus. An ERG provides an objective measure of overall retinal function and can help detect various pathological conditions that affect the retina such as retinitis pigmentosa, macular degeneration, and diabetic retinopathy. During an ERG test, electrodes are placed on the surface of the eye and forehead/ temples to record the retinal response. Light flashes of increasing intensity are presented while the electrodes measure the summed electrical activity of different retinal cell layers.

Components of the ERG waveform

The main components of the Global Electroretinogram waveform provide insight into the functionality of different retinal cell types. The a-wave represents the function of photoreceptor cells and reflects their response to light. As photoreceptors hyperpolarize in response to a flash, a negative voltage deflection is seen as the a-wave. The b-wave arises from depolarizing bipolar cells which transmit signals from photoreceptors to the inner retinal layers and ganglion cells. A robust b-wave indicates healthy inner retinal function. In some cases such as in patients with retinal dystrophies, the a-wave may be reduced or absent while the b-wave is preserved, suggesting photoreceptor dysfunction.

Clinical applications of ERG testing

ERGs are useful for diagnosing various inherited and acquired retinal diseases. In retinitis pigmentosa, ERG typically shows a reduced a-wave arising from early photoreceptor loss followed by eventual attenuation of the b-wave as well. Diabetic retinopathy may manifest as abnormalities in the a-wave, b-wave, or both depending on the severity and location of retinal ischemia. ERGs also help detect toxoplasmosis, retinal detachments, and optic neuropathies where retinal ganglion cell function is compromised. Serial ERG monitoring allows evaluation of the progression of retinal degeneration and the impact of new treatments. It is a valuable supplementary tool to retinal imaging and aids management decisions.

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