Continuing Education Activity
The multifocal electroretinogram (mfERG) is a more recent advancement in electroretinographic testing, which enables a rapid assessment of retinal function from many areas at the same time. Using a contrast-reversing stimulus. There are standard protocols for eliciting the retinal electrical response. The mfERG is a valuable diagnostic tool that can aid clinicians in determining a correct diagnosis in patients with retinal disease beyond standard clinical examination capabilities. This activity reviews the mfERG procedure and technique and highlights the role of the interprofessional team in evaluating and improving care for patients using this procedure.
- Identify the indications for mfERG.
- Describe the typical mfERG findings associated with macular dysfunction.
- Review the interfering factors for the mfERG.
The multifocal electroretinogram (mfERG) is a more recent advancement in electroretinographic testing, which enables a rapid assessment of retinal function from many areas at the same time. The mfERG records multiple retinal responses simultaneously using a contrast-reversing stimulus comprising an array of 64 or 103 black-and-white hexagons over 30 to 40 degrees of the central visual field. In turn, the mfERG produces a topographic representation of the central retinal function, which provides valuable spatial information for mapping focal deficits in the retina by layer and by region.
Analysis of the mfERG waveform components can provide useful diagnostic information for distinguishing various anterior visual pathway diseases, especially when the etiology of vision dysfunction remains uncertain following standard clinical examination.
Anatomy and Physiology
The retinal structural architecture includes 10 layers constituting various cell-types along with their synaptic connections for visual processing. The inner retina comprises nerve fiber layer axons, ganglion cells and their dendritic synaptic connections, and amacrine cells. The outer retina comprises the rod and cone photoreceptors, which transfer visual information to second-order bipolar cells found in the middle retina. Cones are found in the highest concentrations in the central macula or fovea, the area of the retina responsible for visual acuity. Following phototransduction from the outer retina, the inner retinal ganglion cells then transmit this electrical information to the brain via the optic nerve for visual information processing.
The mfERG is traditionally recorded in photopic conditions. This excludes rod contributions to the signal and ensures a cone-driven response primarily. The mfERG waveform includes an initial negative deflection (N1), followed by a positive deflection (P1), and a second negative deflection (N2).
The N1-wave is the initial negative deflection corresponding to cone photoreceptor cell activity. This wave-component largely measure outer retinal function.
The P1-wave is the positive deflection following the N1-wave representing the depolarization of inner retinal Muller and bipolar cells and amacrine cells, providing a measure of phototransduction activity.
The mfERG can be analyzed according to the amplitudes and implicit times of the wave components. Generally, the amplitude and implicit time of P1 are the standard measurements. The amplitude and timing of N1 may also be measured. However, these measurements are not part of the current standard.
The amplitude is the maximal light-induced electrical response (voltage) generated by the various retinal cells. The mfERG amplitude is the trough-to-peak amplitude, measured from the trough of N1 to the peak of P1.
Implicit time (time-to-peak) refers to the time needed for the electrical response to reach maximum amplitude. Implicit time is measured from stimulus onset to the corresponding wave-component peak and reflects the rate of signal conduction.
The ISCEV standards for generating the mfERG response are designed to minimize variability between procedures, thus enabling data to be compared among laboratories. ISCEV defines the following clinical protocols for mfERG stimulus parameters and recording:
Stimulus and Recording Parameters
Stimulus Size and Number of Elements
The stimulus display should comprise an array of 61 or 103 black-and-white hexagons over 40-50 degrees (20-25-degree radius to display edge) of the central visual field, including a central fixation target.
Duration of Recording
It is recommended to record the mfERG for a minimum duration of 4 minutes for 61 element arrays and 8 min for 103 element arrays. To permit patient rest between runs, the total recording time is typically divided into shorter segments of approximately 15–30 seconds. This helps to minimize loss of data secondary to movement, noise, or other potential artifacts.
Cathode ray tube (CRT) monitors have traditionally been used for displaying mfERG stimuli. However, these are being more recently replaced with liquid crystal displays (LCDs). Since alternative sources of stimulation can impact the mfERG waveform, it is important to specify the details of the model used.
CRT frame frequencies of 75 Hz (most common) and 60 Hz may be used. Since variation in frequency may alter the mfERG response, normative values for healthy individuals must be separately determined for a given frequency. Therefore, it is important to note the frame frequency during data interpretation.
Luminance and Contrast
- When using CRTs, stimulus luminance should be at least 100 cd/m2 in the light state. In the dark state, luminance should be low enough to achieve a contrast (Michelson) of at least 90%.
- When using LCDs, a higher luminance setting may be required to achieve clear waveforms with reasonable mfERG amplitudes in the light state.
- For all recordings, the mean stimulus luminance should match the background luminance.
Since luminance and contrast affect signal recordings, it is important to calibrate the stimulus according to ISCEV guidelines. In particular, the light and dark elements are not always uniform for many monitors, and variations greater than 15% are not acceptable.
The Standard display is a hexagonal stimulus with larger peripheral hexagons and smaller central hexagons. This pattern is scaled in size, such that mfERG responses of approximately equal amplitudes are produced over the healthy retina.
The m-sequence is the standard for routine testing in mfERG. This algorithm determines the rate at which hexagonal elements change between dark and light stages with every frame.
Stimulus Size and Number of Elements
The size of the contrast-reversing stimulus may comprise an array of either 64 or 103 black-and-white hexagons over 30-40 degrees of the central visual field. The width of the stimulus field must include the blind spot. Selection of 64 versus 103 elements will depend on balancing between good spatial resolution and a high signal-to-noise ratio while also minimizing the recording time.
Stable fixation is critical for acquiring reliable mfERG recordings. To avoid a diminished response, fixation targets should minimally cover the central stimulus element. The examiner should also confirm the proper visualization of the fixation target by the patient.
Amplifiers and Filters
Amplifiers and filters are used to produce recognizable signals and removing confounding electrical noise. A filter bandpass range of 5 to 200 Hz is acceptable for basic mfERG with 3 to 10 Hz, and 100 to 300 Hz ranges for the high and low pass cutoffs, respectively. Since filter settings within these ranges can variably alter the waveform, the same settings should be uniformly applied for all patients tested in a given study.
Artifact rejection algorithms are used for removing sources of signal distortion, as may be artificially induced blinks or movement.
Spatial averaging may be used to eliminate noise and smoothen waveforms. The contribution from the averaged neighboring elements should not exceed 17% to ensure equal influence by each of the 6 neighbors for a given hexagon.
The first-order kernel is the standard response. The second-order kernel and other higher-order kernels are used in special circumstances and are occasionally reported as well.
Interpreting and Reporting the mfERG
- Carefully examine the mfERG trace array for areas of diminished or delayed signals.
- Assess mfERG normality based on the overall appearance of the waveform and comparison with locally available normative data.
- Evaluate 3-D representations and ring response plots to help further identify potentially damaged areas.
- Displays an array of the mfERG traces for visually inspecting topographic variability as well as the quality of recordings.
- Trace lengths of 100 ms or more should be used.
- It can be spatially compared to visual field tests to help evaluate potential relationships between retinal dysfunction and visual field defects.
Topographic (3-D) response density plots
- Depicts the overall signal strength per unit area of the retina.
- It can be useful for assessing the quality of fixation by observing the location and depth of the blind spot. The presence of a blind spot assures good fixation. The absence of a blind spot can be due to poor fixation or a generalized loss of signal due to disease.
- Limitations of the 3-D plot include:
- Loss of waveform information. Thus, large but abnormal or delayed responses can produce normal 3-D plots.
- A central peak in the 3-D plot can be seen in some records without any retinal signal. The appearance of the 3-D plot from a given recording is dependent on how the local amplitude is measured. For these reasons, 3-D plots should not be used without a simultaneous display of the trace array.
Ring and other regional averages
Regional responses can be averaged per hexagon and compared between affected and unaffected or control eyes. Averaged responses within successive rings may also be used to evaluate visual dysfunction in patients with radial asymmetry. Since the stimulus hexagons are scaled to provide approximately equal response amplitudes, responses are approximately constant across rings. Responses within a ring can further be calculated as amplitude/unit area, whereby the summed responses in each ring is divided by the total area of the hexagons in the ring and plotted as nV/ deg2. Notably, the largest response is observed in the central foveal region, given the high density of cone photoreceptors and bipolar cells.
Measurements and calibration marks
All traces/graphs must include calibration marks to enable adequate data comparison within and among patients.
Each laboratory should develop its own age-adjusted, normative database. Median values rather than mean values should be reported given electrophysiologic data are not always normally distributed.
The mfERG is a non-invasive test with minimal risks. Patients may experience mild ocular discomfort during the procedure or, in sporadic cases, develop a corneal abrasion depending on the type of electrode used.
- Deviating from standardized testing conditions (i.e., lighting, flash intensity, recording environment, duration of light or dark adaptation, and pupil size)
- Electrode-based artifacts including poor contact with skin or cornea, incorrect placement, unstable position, and high electrical impedance
- Eye blinking or movement
- Defocus or uncorrected refractive error
- Reduced electrical response with aging
- Ocular media opacification
- Diurnal fluctuation
- Depressed response with anesthesia
- Variability in recordings between different device types and normative databases between laboratories
Common Artifact Types
- Line frequency interference
- Movement errors
- Eccentric fixation
- Positioning errors and head tilt
- Erroneous central peak (weak signal artifact)
- Averaging and smoothing artifacts
The mfERG is a more recent electrophysiologic test that detects and localizes distinct areas of outer retinal damage in the macula and paramacular as well as discrete peripheral areas. This precision enables electrophysiologic findings to be correlated with visual field testing. The mfERG is valuable in evaluating patients with ambiguous retinal diseases and in monitoring the progression of the disease. An abnormal mfERG generally reflects foveal cone and/or bipolar cell dysfunction along with the source of vision loss. Therefore, damage to the inner retina may have minimal effects on the mfERG waveform. In turn, the mfERG is most applicable to patients with focal deficits in visual function and an otherwise normal-appearing fundus, as commonly seen in macular dystrophies.
Retinitis pigmentosa (RP), a generalized retinal degenerative disease, is the most commonly inherited disease of the outer retinal rod-cone photoreceptors. The mfERG will show strong central responses with weak or flat signals in the peripheral rings. The mfERG is especially useful in the late stages of retinitis RP, when standard ERG tests may not be recordable given the severity of the disease. In these cases, the mfERG exhibits a generalized diminished response. Intriguingly, the mfERG is also affected in asymptomatic RP carriers, demonstrating patchy areas of retinal dysfunction.
Hydroxychloroquine Retinopathy and Bull’s Eye Maculopathy
Hydroxychloroquine (Plaquenil) is a commonly prescribed anti-inflammatory medication for treating rheumatologic and dermatologic conditions. More recently, hydroxychloroquine has been suggested as a potential off-label therapy for coronavirus, COVID-19. Among its side effects, this medication has been associated with macular retinal toxicity. Specifically, the macular rod and cone cells are damaged, while the foveal cones are spared. In advanced disease stages, this pattern of retinal loss results in a bullseye appearance, also known as bull’s eye maculopathy. A pericentral loss in the mfERG response is the most characteristic of hydroxychloroquine toxicity. The mfERG has prognostic value by identifying patients who are more prone to retinal toxicity. Early cessation of medication is necessary to avoid irreversible vision loss. Hydroxychloroquine is generally continued in patients with a normal mfERG with the test repeated annually, whereas the individuals with significant mfERG loss are advised to stop the medication.
Stargardt’s Macular Dystrophy
Stargardt's macular dystrophy is a recessive retinal disease caused by a mutation in the ABCA gene. The clinical exam typically reveals an apparently normal fovea with mid-peripheral flecks along with poor central vision. However, patients may also present atypically without visibly appreciable flecks or diminished central vision. In affected patients, the central and paracentral responses in the mfERG will be significantly diminished in affected patients. Conversely, a normal mfERG excludes Stargardt's disease.
Occult Macular Dystrophy
Occult macular dystrophy (OMD) is a rare, inherited retinal degenerative disease. Patients may clinically present with unexplained, progressive central vision loss, despite a normal-appearing fundus, decreased color vision, and normal fluorescein angiogram. Affected patients will exhibit a decreased central response density on mfERG.
Branch Retinal Artery Occlusion
Branch Retinal Artery Occlusion (BRAO) is retinal ischemic damage caused by a blockage in a branch of the central retinal artery. The mfERG response is characteristically decreased in a “cookie-cutter” fashion corresponding to the pattern of the affected retinal arterial blood supply. This diminished mfERG response is notably observed despite a normal-appearing retina in chronic disease.
Multiple Evanescent White Dot Syndrome
Multiple Evanescent White Dot Syndrome (MEWDS) is an atypical retinal inflammatory disease that commonly affects otherwise healthy young to middle-aged women. Patients are clinically present with photopsia and an enlarged blind spot on visual field testing. The mfERG response is characteristically depressed in the retinal region corresponding to the blind spot. Since these ocular findings are self-limiting and frequently accompanied by flu-like symptoms, MEWDS is often referred to as the “common cold” of the retina. Similarly, these mfERG abnormalities are generally reversible, and retinal responses return to normal after a few months.
Alzheimer’s Disease (AD): the leading form of dementia pathologically hallmarked by fibrillar beta-amyloid and hyper-phosphorylated tau deposition in the central nervous system (McKhann G, Drachman D, et al., Neurology, 1984). Notably, abnormal protein deposition and degenerative changes have also been reported in the retina. Recent mfERG studies have suggested retinal electrophysiologic dysfunction in AD. In particular, mfERG responses in AD have shown significant amplitude reduction in the foveal and perifoveal outer retina. Studies have also provided evidence of retinal dysfunction in early AD. mfERG responses have shown decreased amplitudes along with implicit time prolongation.
Parkinson’s Disease (PD): a neurodegenerative disease involving abnormal accumulation of α-synuclein (α-syn) protein resulting in dopaminergic neuronal atrophy. A diminished P1 amplitude density in the mfERG response has been shown as a significant non-invasive, clinical biomarker for diagnosing PD.
Enhancing Healthcare Team Outcomes
The retina is a complex neuronal structure, and patients with retinal disease frequently present with difficult-to-diagnose causes of vision loss. Given this challenge in identifying the etiology of disease within the retinal infrastructure, an interprofessional team approach is essential for providing sufficient patient care. Patients with acute onset vision loss commonly present to the emergency department, where nurses, as the first point-of-contact, will triage the patients according to the severity and acuity of symptoms. For concerning conditions in the setting of indeterminate clinical diagnosis, medical professionals, including physicians and nursing staff, routinely order extensive and expensive testing, the majority of which return is unremarkable. In turn, patients are instructed to follow up with an outpatient ophthalmologist. Typically, this will involve a standard eye examination by a comprehensive ophthalmologist. However, these preliminary diagnostic tests largely detect structural abnormalities, which are not always consistent with clinical presentation. In turn, patient diagnoses may be mistaken for a benign condition and, in some cases, presumed to be malingering. When the cause of vision impairment remains undefined, despite extensive medical workup, neuro-ophthalmology is the field to which clinicians often turn. [Level 4]
Considering a broad differential is important to distinguish retinal dysfunction from alternative, similar appearing causes since the choice of therapy depends on the underlying etiology of the disease process. Electroretinography, in conjunction with clinical findings, provides invaluable data for patient management while avoiding unnecessary testing. Transparent communication and care coordination between nurses, physicians, ophthalmologists, including subspecialists, are essential for deriving a correct diagnosis and therapeutic decision-making with proper management thereafter. [Level 5]