Pattern Electroretinogram

Anatomy and Physiology

The structural architecture of the retina constitutes ten layers comprising various cell-types and synaptic connections involved in visual processing. The inner retina includes nerve fiber layer axons, ganglion cells as well as their dendritic synaptic connections, and amacrine cells. The outer retina consists of the rod and cone photoreceptors, which transmit visual information to second-order neurons known as bipolar cells in the central retina. Following the transduction of visual information (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.[4]

Indications

The pattern electroretinogram (PERG) is a specialized test beyond standard ophthalmologic examination. Electrophysiologic testing may be indicated in the following scenarios:

• Diagnosis and management of optic nerve and macular dysfunction
• Monitor retinal disease associated with toxic drug exposure
• Assess inflammatory, compressive, and traumatic lesions[5]

Contraindications

There are no specific contraindications for the PERG. Patients with seizure disorders can safely undergo the PERG as the stimulus frequency is outside of the range, responsible for inducing most epileptic seizures. Patients who report photosensitive seizures should be evaluated with caution if the frequency which induces their seizures is around 30 Hz or is unknown.

Equipment

The instruments required to perform pattern electroretinogram (PERG) are the following:

• Electrodes
• Amplification system
• Data recording system

Personnel

Appropriately trained technicians perform pattern electroretinogram (PERG) in large referral centers equipped with an electrophysiology laboratory. Retina specialists and neuro-ophthalmologists are typically responsible for the interpretation of the electrophysiological results.

Preparation

Electrode Placement

Recording Electrodes

• Depending on the type of electrode, the recording electrode is placed on the corneal surface, on the bulbar conjunctiva adjacent to the inferior limbus of the cornea.
• The International Society for Clinical Electrophysiology of Vision (ISCEV) recommends that skin (surface) recording electrodes should not regularly be used for recording the standard pattern electroretinogram (PERG). [2] Skin electrodes placed on the lower eyelid will record a lower PERG amplitude than those in contact with the eye. Skin recording electrodes may, however, be useful under specific circumstances when a corneal electrode is contraindicated or in pediatric practice.  Skin electrode have also successfully been used in studies looking at patients with glaucoma, but as the use of a skin electrode deviates from the PERG standard, ISCEV recommends that it should be noted in the report.[2]
• Topical anesthesia is applied to minimize ocular surface discomfort with corneal contact electrodes.[2]

Reference Electrodes

• Place separate surface electrodes on the skin close to the outer canthus of each ipsilateral eye.
• For monocular PERG recordings, the contralateral occluded eye may be used for placement of the reference electrode.
• Forehead, earlobe, or mastoid are not recommended for placement as this may contaminate the PERG with potentials generated by the fellow eye.

Ground Electrodes

Typically placed on the forehead and connected to the "ground input" of the recording system.[2]

Patient Preparation

Per the International Society for Clinical Electrophysiology of Vision (ISCEV) guidelines:

• Electrically isolated recording environment
• Record without pupil dilation to maximize image quality
• Minimum 30-minutes of recovery time in normal room lighting is required for patients exposed to strong light stimuli from alternative imaging techniques (i.e., fundus photography or fluorescein angiography)
• Instruct patients to fixate on a target within the stimulator while minimizing eye movement. Patients who are unable to see the fixation point may be advised to look straight ahead while maintaining a steady gaze.
• The binocular recording is recommended as this improves fixation stability and reduced examination time.
• A monocular recording is recommended in individuals with ocular misalignment.[2]

Technique or Treatment

The pattern electroretinogram (PERG) response can be either transient or steady-state, depending on the stimulus. The standard, transient PERG, is recorded in response to low contrast-reversal frequency stimuli (1-2 Hz), whereas the steady-state PERG is seen with a higher reversal frequency (8 Hz). Since the steady-state PERG does not allow for direct measurements of individual waveform components, it can be harder to interpret and requires appropriate knowledge and software to evaluate the recordings.  High frequency steady-state recordings negate some of the effects of poor fixation on the recording, increasing intertest reproducibility and steady-state PERG has been optimized for the early detection of glaucoma. [6] 

The standard, transient response separates the PERG into wave components, including a negative wave at about 35 msec (N35) followed by a positive wave at approximately 50 msec (P50) and a large, negative wave at around 95 msec (N95).[2]

Waveform Components

P50

The P50-wave is the initial positive deflection originating from RGCs as well as from outer retinal photoreceptor cells, namely the macular cones. This wave-component is largely a measure of outer retinal function.

N95

The N95-wave is the negative deflection following the P50-wave that originates from the inner retina. This wave-component reflects the RGC function.

Waveform Analysis (Figure 1)

Amplitude

The amplitude is the maximal light-induced electrical response (voltage) generated by the various retinal cells. PERGs can be analyzed according to the amplitudes and implicit times of the wave components. The P50 amplitude is calculated from the trough of N35 to the peak of P50. The N95 amplitude is calculated from the peak of P50 to the trough of N95. In turn, N95 amplitude includes the P50 amplitude, and P50 includes that of N35. If the N35 is poorly defined, the P50 amplitude is calculated from the average baseline, which is between time zero and the onset of P50 to its peak.

Implicit Time

Implicit time (time-to-peak) refers to the time needed for the electrical response to reach maximum amplitude. Implicit time is calculated from stimulus onset to the peak of the corresponding wave-component and reflects the rate of signal conduction.

Latency

Latency is the period from stimulus onset to response onset, as opposed to the peak of the response (i.e., implicit time).

The N95 to P50-Wave Ratio

The ratio of the N95- to P50-wave amplitudes provides an index of inner to outer retinal function.[2]

Protocols

The PERG response has a small amplitude and differs depending on the technique used. The ISCEV standards for generating the PERG response and for minimizing variability between procedures, thus enabling data to be compared among laboratories. ISCEV defines the following clinical protocols for PERG stimulus parameters and recording:

Field and Check Size

The standard PERG stimulus is a black and white reversing checkerboard. The check size for the standard PERG is a width of 0.8 degrees (± 0.2) for each individual square check (± 5% error). While a square stimulus field is not required, the aspect ratio between the width and the height of the stimulus field should be from 4:3 to 1:1. The mean width and height of the stimulus field should be 15 degrees (± 3).

Luminance

The PERG response is complicated to elicit given a low stimulus luminance. A photopic luminance higher than 80 cd/m is required for the white areas of the stimulus. Mean stimulus luminance must be constant with no transient changes in luminance during checkerboard reversals.

Contrast

The contrast between black and white square checks should be close to 100% and no less than 80%.

Background Illumination

Background illumination beyond the checkerboard stimulus and typically involves using dim or ordinary room lighting. For all recordings, ambient lighting should be the same bright lights should be kept out of a subject’s direct view.

Data Display System

Traditionally the stimulus for PERG has been displayed on a cathode-ray tube (CRT) monitor. Liquid crystal display (LCD) and light-emitting diode (LED) displays can have a flash artifact when the pattern reverses, which complicates the recorded response and no longer generates a PERG but rather a hybrid flash electroretinogram coupled with a PERG. This is hard to interpret as the waveform components no longer correspond to the anatomical areas listed above and thus should be avoided. It should be ensured that if LCD or LED displays are used, it is devoid of a flash artifact.

Reversal Rate

The reversal rate of 4.0 ± 0.8 reversals per second (rps) should be used when recording the standard PERG response.

Recording

At least 100 artifact-free sweeps should be acquired and averaged. However, more sweeps will be needed under circumstances when the PERG response is small, undetectable, or collected with significant background noise. Two trials for each stimulus condition must be acquired to confirm standard PERG reproducibility. Superimposing PERG responses can help to evaluate the quality and reproducibility of recordings.

Averaging and Signal Analysis

Given the small amplitude of the PERG response, signal averaging is required. The analysis period or sweep time should be at least 150 ms with a stimulation rate of 4 rps and 250 ms intervals between reversals.

Artifact Rejection

The limit for computerized rejection should be no higher than ± 100 microvolts.

Sampling Rate

A sampling rate minimum of 1,000 Hz (i.e., 1 ms per point) is recommended.

Complications

The pattern electroretinogram (PERG) is a non-invasive test with minimal risks. Patients may experience mild ocular discomfort during the procedure or, in very rare cases, develop a corneal abrasion depending on the type of electrode used. Other interfering factors are described below.

Interfering Factors

• 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.[2]

Clinical Significance

Electrophysiological abnormalities frequently occur early and often precede structural findings on retinal imaging. The PERG has critical clinical applications in unexplained vision loss, especially when the fundus appears normal or when there is disc pallor in the absence of visible vessel irregularities accompanied by significant macular abnormalities.[7]

Optic Nerve Versus Macular Dysfunction

The PERG response may be normal or decreased in optic nerve dysfunction. In particular, a decreased N95 amplitude is almost invariably observed in optic nerve disease with primary RGC dysfunction. P50 is typically spared in optic nerve disease. Notably, however, P50 amplitude and implicit time may be reduced in severe disease.

In contrast, an undetectable PERG or significant P50 amplitude reduction in the absence of decreased implicit time is indicative of macular dysfunction. Besides, P50 amplitude reduction in macular dysfunction may be likely be accompanied by concomitant 95, such whereby the N95: P50 ratio is unchanged, although the N95 is occasionally better preserved.[8] 

Hereditary Optic Neuropathy

Leber’s Hereditary Optic Neuropathy (LHON): A rare, maternally inherited mitochondrial disease of the RGCs characterized by bilateral loss of vision typically manifesting in the second to third decades of life. LHON exhibits significant N95 reduction with P50 preservation of the PERG response.[7][9] 

Dominant Optic Atrophy (DOA): An autosomal dominant optic neuropathy affecting the RGCs most commonly associated with an OPA1 nuclear gene mutation. In contrast to LHON, DOA is classically diagnosed during the first decade of life with a slowly progressive and symmetric loss of vision. DOA shows a preferential decrease in N95 amplitude in the early stages of disease followed by reduced P50 amplitude and implicit time in advanced stages.[7][10]   

Multiple Sclerosis

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system, including the anterior visual pathways, namely the optic nerve. Optic neuritis, inflammation of the optic nerve, characteristically presents as unilateral vision loss with pain on eye movement. Patients are typically females between 30 to 50 years of age. Demyelination of the retinal ganglion cell axons leads to optic nerve atrophy. PERG studies in MS have shown preferential N95 amplitude reduction with P50 sparing. Notably, reports have shown reductions in both N95 and P50 in acute optic neuritis, with the recovery of P50 to normal after remission.[11][12] 

Non-Arteritic Ischemic Optic Neuropathy

Nonarteritic anterior ischemic optic neuropathy (NAION) is acute onset, ischemic damage of the optic nerve most commonly affecting elderly patients greater than 50 years of age, presenting with altitudinal visual field defects. Risk factors include age, hypertension, diabetes mellitus, smoking, and crowding of the optic nerve. NAION patients typically exhibit N95 amplitude reduction with the preservation of P50 in the PERG response.[13][14] Notably, reports have also suggested P50 amplitude reduction with increased implicit time secondary to compromised blood flow to the retinal layers.[15] 

Glaucoma

Glaucoma, a leading cause of irreversible blindness worldwide, is a progressive optic neuropathy characterized by retinal ganglion cell degeneration and peripheral visual field loss. PERG has been recognized as an important test in diagnosing and managing glaucoma and diabetic edema.[16][17] Glaucoma has shown diminished N95 amplitude and prolonged implicit time in the PERG response. Notably, glaucoma suspects have similarly shown prolonged N95 implicit time, although no significant attenuation in amplitude. P50 is preserved in both glaucoma and glaucoma suspects.[18]

Neurodegenerative Diseases

Alzheimer Disease (AD): The most common form of dementia pathologically hallmarked by fibrillar beta-amyloid and hyper-phosphorylated tau accumulations in the central nervous system.[19] Several PERG studies have provided evidence of retinal electrophysiologic dysfunction in AD. In particular, PERG responses in AD have shown significant amplitude reduction and prolonged implicit time in both N95 and P50, suggesting that retinal dysfunction in AD may involve the outer retina in addition to the inner retina.[20] Studies have also provided evidence of retinal dysfunction in early AD. PERG responses have shown decreased N95 and P50 amplitudes along with P50 implicit time prolongation.[21] 

Parkinson Disease (PD): A neurodegenerative disorder involving abnormal α-synuclein (α-syn) protein deposition leading dopaminergic neuronal atrophy.[22] PERG studies have shown significant N95 amplitude reduction in addition to P50 amplitude reduction with prolonged implicit time.[23][24][25]

Enhancing Healthcare Team Outcomes

The neurosensory retina is a complex structure, and patients with ophthalmologic disease often present with unexplained vision loss. Identifying the etiology of disease within the retinal infrastructure is a diagnostic challenge. Therefore, an interprofessional team approach is crucial for providing adequate patient care. Patients with acute onset vision loss frequently present in the emergency department. Nurses triage the patient as the first point of contact based on symptom severity and onset.

Given the concern for emergent conditions and an indeterminate clinical diagnosis and, providers and nursing staff routinely order a costly workup involving a battery of tests, and the majority of which return negative. 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 mostly 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. Neuro-ophthalmology is the field that is commonly acknowledged when the etiology of vision impairment remains unknown, despite extensive medical workup.[9] [Level 4]

Considering a broad differential is essential to distinguish retinal dysfunction from alternative, similar appearing causes since the option 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, providers, ophthalmologists, including subspecialists, are essential for deriving a correct diagnosis and therapeutic decision-making. [Level 5]