Role of Electroretinogram in Diagnosing Retinal Diseases

Dr. Purushottam Naidu Menda, M. Optom, PhD

Principal and Professor, ITM Skills University, Navi Mumbai, India

 

Electrodiagnostic examination of the eye involves non-invasive examinations that measure electrical activity in the visual system to objectively assess eye function, using electrodes placed on the eye (Example: Burien Allen) or skin (Dawson Trick Litzkow) to detect responses to light stimuli. The main tests include the Electroretinogram (ERG) for retinal cell (Neural and Non-neural cells) function, the Visual Evoked Potential (VEP) for visual cortex activity, and the Electrooculogram (EOG) for retinal pigment epithelium function. These tests help diagnose and monitor various inherited and acquired eye conditions, providing valuable functional data that complements structural imaging. ⁽¹⁾

Electrodiagnostic examinations are essential to diagnose a few retinal diseases along with visual pathway defects. In electrodiagnostic examinations ERG is the most important examination tool to diagnose retinal diseases. Full field ERG (ffERG) is a widely used electrodiagnostic test. In 1989, a basic protocol was standardised so that certain responses could be recorded uniformly. In 1992, the International Society for Clinical Electrophysiology of Vision (ISCEV) was mentioned with standardised testing procedures and normative data.

In ERG, there are 5 commonly obtained responses that are present, as mentioned below: ⁽¹⁾

• Rod response
• Maximal response
• Oscillatory response
• Cone response
• 30Hz flicker response ⁽²⁾

Figure 1: The Electroretinogram (ERG) Response in Health and Disease. ⁽³⁾

The first 3 responses are measured under scotopic condition and the rest can be measured in photopic condition.

To measure the flash ERG (fERG) Ganzfeld simulation should be used with the help of electrodes, and the responses can be measured.

The most common electrodes used to record retinal response in ERG are the contact lens electrode, which is also called the Burian-Allen Electrode, and is widely used because it can cover the entire cornea of the eye. The referral electrode can be an ear clip, or the forehead can be used. ⁽⁴⁾

In scotopic response, we will measure rod response, and in photopic response, we measure cone response. ⁽⁵⁾

While preparing the patient, we should first dilate the pupil for a wide field of response from the retina. Dark adaptation for at least 30 minutes is required to achieve a stable physiologic condition and relatively maximal scotopic responses. If Burien-Allen electrodes are used, they can be inserted under dim red light after anaesthesia of the eye. ⁽⁶⁾

After adaptation, we can measure the waveform response from segments like rod response and oscillatory potential. Once these segment measurements are done, we can give light adaptation for 10 minutes for the photopic response. After light adaptation, we should measure cone response and 30Hz flicker response to evaluate cone function. It is advised to avoid Fundus Fluorescence Angiography (FFA) and Optical Coherence Tomography (OCT) before ERG use, but if these examinations have been performed, a period of dark adaptation of one hour is needed. A fixation point is useful but not essential; some patients will not see it, as well as the Ganzfeld drum minimises the need for accurate fixation. ⁽⁷⁾

Recording Infant ERGs

Infants up to about 2 years of age can usually be tested without sedation, with the parent holding them bundled in a blanket. It is difficult to convince a child less than 5 years of age to allow a device in their eye. Alternatively, the child is sedated or anesthetised. Oral glucose or sucrose can be used for analgesia for procedures in infants up to 18 months of age, including recording ERGs.

ERG testing is also sometimes performed as part of an exam under anaesthesia.

Factors Affecting the ERG

• Duration of stimulus
• Size of retinal area illuminated (amplitude can be reduced if the stimulus is not full field, because the patient would then be positioned too far from the stimulus source)
• Interval between stimuli
• Size of pupil
• Systemic circulation and drugs
• Development of the retina
• Clarity of ocular media (note that a mild cataract has minimal effects on the ffERG)
• Age
• Reduced ERG amplitude due to high myopia
• Anaesthesia

Diagnosis

ERG measures the electrical activity of the retina in response to light stimulation, providing objective information about the health and function of retinal cells. ⁽⁸⁾

In the absence of partial or full response of rods and cones, there is a flat response in the rod segment or a subnormal response in the rod segment. In interpretation, we will always take into consideration both amplitude and implicit time. For the practical purpose, a-wave and b-wave are measured to know each response of the segment.

The a-wave amplitude is measured from baseline to a-wave trough, the b-wave amplitude is measured from a-wave trough to b-wave peak, and the b-wave time to peak is measured from flash onset to the peak of the wave.

Figure 2: Amplitude and Latency response

In the Oscillatory potential (Ops) response, consider that 3 major peaks are followed by a smaller peak to interpret further. ⁽⁹⁾

Figure 3: Oscillatory Potential Response

SCOTOPIC RESPONSES
Component	Amplitude (µV)	Implicit time (ms)
a-wave	190–300	20–26
b-wave	400–700	40–56
PHOTOPIC RESPONSES
Component	Amplitude (µV)	Implicit time (ms)
a-wave	20–50	14–20
b-wave	90–180	26–34

Table 1: Normative data

Figure 4: Comparison of normal and abnormal ERG response in CSNB

Figure 5: Comparison of normal and abnormal ERG response in cone dystrophy

The list of retinal diseases below, where ERG is more useful for diagnosis.

• Birdshot Retin choroidopathy
• Acute Zonal Occult Ocuter Retinopathy (AZOOR)
• Retinal Artery/ Vein Occlusion
• Carotid/Oph Artery Occlusion
• Toxic Retinopathies
• Achromatopsia
• IFB
• Cancer-Associated Retinopathy
• CSNB
• Oguchi Disease
• Rod Cone Dystrophy
• RP
• RP with Syndromes
• LCA
• Choroidal Atrophy
• Vit A Deficiency
• Stargardt Macular Dystrophy
• Cone Rod Dystrophy
• Best Dystrophy

Abnormal ERG conclusion

When an ERG is abnormal, the conclusion provides specific details on which waveforms are affected to help identify the type and location of the retinal dysfunction. Abnormalities typically involve reduced amplitude, delayed timing, or a specific pattern of wave loss.

Conclusion

The ERG is an indispensable tool in ophthalmology, providing an objective and detailed assessment of retinal function that is not possible with other examination methods. The technique measures the electrical responses of various retinal cell types to light stimuli, revealing the location and extent of functional defects before structural damage may be visible. In conclusion, the ERG is crucial for the early detection, accurate diagnosis, differential diagnosis, and monitoring of a wide range of inherited and acquired retinal diseases

References

1. Calcagni, A., Neveu, M. M., Jurkute, N., & Robson, A. G. (2024). Electrodiagnostic tests of the visual pathway and applications in neuro-ophthalmology. Eye (London, England), 38(12), 2392–2405. https://doi.org/10.1038/s41433-024-03154-6
2. Odom JV, Bach M, Brigell M, Holder GE, McCulloch DL, Mizota A, et al. ISCEV standard for clinical visual evoked potentials: (2016 update). Doc Ophthalmol. 2016;133:1–9.
3. Rashwan, R., Hunt, D. M., & Carvalho, L. S. (2021). The role of voltage-gated ion channels in visual function and disease in mammalian photoreceptors. Pflügers Archiv-European Journal of Physiology, 473(9), 1455-1468.
4. Odom JV, Bach M, Brigell M, Holder GE, McCulloch DL, Mizota A, et al. ISCEV standard for clinical visual evoked potentials: (2016 update). Doc Ophthalmol. 2016;133:1–9.
5. Lankford, C. K., Umino, Y., Poria, D., Kefalov, V., Solessio, E., & Baker, S. A. (2022). Cone-Driven Retinal Responses Are Shaped by Rod But Not Cone HCN1. The Journal of Neuroscience, 42(21), 4231–4249. https://doi.org/10.1523/JNEUROSCI.2271-21.2022.
6. Jiang, X., & Mahroo, O. A. (2022). Human retinal dark adaptation tracked in vivo with the electroretinogram: insights into processes underlying recovery of cone- and rod-mediated vision. The Journal of Physiology, 600(21), 4603–4621. https://doi.org/10.1113/JP283105.
7. Nguyen, C. T., Tsai, T. I., He, Z., Vingrys, A. J., Lee, P. Y., & Bui, B. V. (2016). Simultaneous Recording of Electroretinography and Visual Evoked Potentials in Anesthetized Rats. Journal of Visualized Experiments, (113), 54158. https://doi.org/10.3791/54158
8. Ala-Laurila P, Cornwall MC, Crouch RK, Kono M. The action of 11-cis-retinol on cone opsins and intact cone photoreceptors. J Biol Chem. 2009;284:16492-16500.
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About the Author

 

Dr. Purushottam Naidu is working as a Principal and Professor at ITM Institute of Health Sciences College of Optometry for the past 14 years and specialises in Electro diagnostics, Contact Lenses, Neuro Optometry. He completed Diploma in Optometry from Sarojini Devi Eye Hospital, Hyderabad followed by Fellowship. He completed his Bachelors in Optometry from Aditya Jyot Institute of Optometry, a unit of Aditya Jyot Eye Hospital, Mumbai and completed M. Optometry from Lotus College of Optometry, Mumbai. He later went on to complete his PhD in Optometry. He was a gold medalist in Diploma and Bachelors in Optometry and was also awarded the Best Faculty from Aditya Jyot Institute of Optometry. He has completed Scleral Contact Lens course from L V Prasad Eye Institute, and various certificate courses in Research Methodology
He had successfully completed the IACLE Student Trial Exam from Australia and was awarded FIACLE. He is a member of IOA, OCI and MOA and is appointed as a Joint treasurer in Maharashtra Optometry Association (MOA). He is a visiting faculty at various Optometry colleges in India.

He has published extensively in national and international journals, presented various topics as a speaker at major conferences and conducted workshops in India and overseas. He is guiding under-graduate, post-graduate and PhD scholars for their research work, actively involved in various research projects for Optometry students and examiner of various optometry colleges in India.