This article provides a brief overview of the tests used in clinical ocular electrophysiology to understand their diagnostic value in optometry and ophthalmology.
As optometrists we may be confronted with a patient with unexplained vision loss or unusual retinal pathology. In these cases it is useful to know that some objective tests can reveal many things about the function of the visual system. Ocular electrophysiology is the measurement of bioelectrical activity within the eye and visual cortex. Resting potentials exist over the membranes of inactive cells and when a stimulus occurs, the cellular movement of charged ions generate action potentials, which send signals along nerve fibres. These bioelectric currents are very small. However, as the cells in the visual system are highly ordered, these potentials can be summed and measured. Clinical electrophysiology of vision assesses these summed responses using several types of test. The International Society for Clinical Electrophysiology of Vision (ISCEV) sets clinical standards for the recording of each of these tests ensuring that results from clinical investigations are directly comparable. The electro-oculogram (EOG) assesses the function of the retinal pigment epithelium (RPE).1 The electroretinogram (ERG) is used to ascertain either a full field or focal summed response from the retina and can be particularly useful when testing for rod-cone dystrophies.2 The pattern electroretinogram (PERG),3 and multifocal electroretinogram (mfERG),4 assess smaller, more localised areas of the retina using patterned stimuli and may be used to evaluate ganglion cell function or macular dystrophies. Visual evoked potentials (VEPs),5 assess cortical potentials, and are, therefore, useful in testing multiple aspects of vision.
The difference in resting electrical potential between the anterior and posterior eye is known as the standing potential. The change in this signal during light and dark adaptation is recorded by the EOG. The majority of this standing potential (around 10mV) comes from the RPE and is sustained by tight junctions between the cells here.1,6 Thus by measuring the standing potential of the eye we are measuring the function of the RPE.
When undergoing an EOG, skin electrodes are positioned as shown in Figure 1 (The skin electrode positioning for EOGs. In this instance, non-disposable electrodes have been used. Another set of electrodes is set up identically adjacent to the patient’s other eye). An earth electrode is placed at a point away from the ocular structures, such as the earlobe. As good fixation is required some patients, such as young children or those with a learning disability, may be unable to undertake the test.1 After dark adapting for 20 minutes the patient is asked to look alternately at targets 15o either side of the centre of a Ganzfeld bowl for 15 minutes, with an EOG recorded every 10 seconds; this is then repeated in light adapted conditions. The waveform recorded has a negative trough during the dark phase followed by a positive rise in the light phase. The light response of the EOG is affected in RPE disorders and those affecting the photoreceptor layer.7 The ratio of the amplitude of the dark trough and light rise is called the Arden ratio,1 and has been found to be above normal in patients suffering from digoxin toxicity.
The summed electrical responses from retinal cells can be recorded by full field and focal ERGs,2 resulting in repeatable waveforms such as the one seen in Figure 2 (A waveform produced by the ISCEV standard photopic 3 ERG in a normal patient: 1 indicates the crest of the a-wave trough; 2 indicates the crest of the b-wave peak; 3 indicates the baseline ERG measurement; and 4 indicates the second of two small ripples on the rising edge of the b-wave, known as oscillatory potentials. Amplitude (μV) and implicit time (ms) of the a-wave are measured from position 3 to position 1. Amplitude (μV) and implicit (mS) time of the b-wave are measured from position 1 to position 2). Flash ERGs record the electrical responses to flashes of light of a uniform luminance. These flashes either occupy the entire visual field (a full field flash ERG) or are restricted to a desired part of the retina (a focal flash ERG). They may be performed on a dilated patient in scotopic conditions (with 20 minutes dark adaptation before testing) or photopic conditions (with 10 minutes light adaptation before testing). Photopic ERGs elicit responses from cones only whereas rods and cones may contribute to scotopic ERGs. In some cases the stimulus is set to flash at a faster rate, resulting in a flickering light being presented, and are known as a flicker ERGs. The ERGs are recorded with reference, earth and active electrodes. Active electrodes should be positioned as close as possible to the patient’s cornea as this is where signal strength is strongest (see Figure 3: Electrode positioning for ERGs. An earth electrode is placed at the forehead and a reference electrode is placed at the temple. The active electrode here is a conductive fibre draped along the lower fornix and held in place with adhesive pads). Contact may be made through the cornea itself using a contact lens electrode, the bulbar conjunctiva via gold foil electrodes or conductive fibres, or on the lower eyelid using a skin electrode.
The peaks and troughs of the waveforms produced reveal the activity of particular parts of the retina and are measured in terms of their amplitude, and implicit time, that is to say, the length of time it takes the waveform to appear after the stimulus. Full field ERG amplitudes rapidly increase in early infancy then have a tendency to decrease with age.
Reduction in amplitude or increase in implicit time of any particular part of the waveform outside that of the normal age range can highlight damage to the group of cells responsible for its generation. The amplitudes and implicit times of the a- and b-wave are measured for all flash ERGs.
Photoreceptors are responsible for generating the a- wave,10 whereas b- waves are generated by on- and off-bipolar cells.11 Oscillatory potentials may arise on top of the b-wave with increasing stimulus intensity9 (see Figure 2); these occur due to amacrine cell activity.13 Flicker ERGs result in a sinusoidal response, and in this case it is the amplitude and implicit time of the first peak which is measured (see Figure 4: A waveform produced from a 41Hz flicker ERG in a normal patient. Time to first peak is measured (instead of implicit time) from the midpoint of the flash stimulus to one. In this instance amplitude is measured from 2 to 3). Another aspect of the waveform produced is the photopic negative response (PhNR), which is shown in Figure 5 (A waveform containing the photopic negative response component of a normal patient, indicated at 3. The a- and b- components of this waveform are indicated at 1 and 2, respectively); this is produced due to ganglion cell activity but is not routinely assessed as part of the ISCEV standardised protocol.14
The standardised full field ERG protocol can be particularly useful in the diagnosis of rod-cone deficiencies such as congenital stationary night blindness (CSNB), which can affect the photoreceptors and on-bipolar cells. In this case it is helpful to compare results elicited by different protocols. In CSNB a dim stimulus full field ERG, which only stimulates rod photoreceptors, will display a small or entirely absent response. But, an ERG that elicits a response from the rods and cones – for example, the scotopic bright flash ERG2 – will produce a large a-wave and a reduced b-wave (see Figure 6: A scotopic ERG recorded to a dim flash (left column) and a bright flash (right column) in the right eye and left eye of a patient with CSNB. An example ERG waveform produced by a healthy patient during each of these tests is also shown. It can be seen that there is no rod response in the right eye and left eye of the affected individual to a dim flash and that there is an a- wave and a reduced b-wave in response to a bright flash. Image courtesy of Dr Chris Hogg, Moorfields Eye Hospital ).12 As such, patients who are suspected of CSNB may benefit from a referral for electrophysiological testing.
Other aspects of waveforms produced by the standardised protocol may be used to assess additional aspects of retinal function. For example, oscillatory potentials are sensitive to localised retinal ischaemia and so may be used to assess diabetic retinopathy. It is also permissible to deviate from the ISCEV recommended guidelines by altering test parameters such as the stimulus intensity or colour in order to assess a particular aspect of visual function in more detail. An example of this is the PhNR, which is thought to reflect ganglion cell activity.14 A reduction in amplitude or increase of implicit time of the PhNR trough indicates reduced ganglion cell activity which may be useful in the objective assessment of functional loss in glaucoma. In some cases, comparing the results of different types of electrophysiological tests can aid diagnosis. For example, in Best’s vitelliform macular dystrophy, the ERG may be within normal range while the EOG may show a reduced light rise.7
The PERG records a response from the central 10-16o of the retina.3 The stimulus is a high contrast checkerboard pattern which changes while the overall luminance remains constant. The pattern reverses at a frequency of 1–3Hz in front of the patient who remains undilated, wearing their optimal optical correction in order to resolve the pattern displayed. Contact lens electrodes are not permitted for use with PERGs as they may obstruct the clear retinal image.
The waveform produced contains retinal responses to both the pattern and the luminance of the stimulus (see Figure 7: An example of a PERG waveform containing the luminance driven component (P50) and pattern driven component (N95). Image courtesy of Dr Chris Hogg, Moorfields Eye Hospital).3 The waveform consists of a positive peak at roughly 50ms (P50), which is dominated by responses to local luminance changes (30% of the response originates from the inner retina; 70% from the ganglion cells). The subsequent negative trough occurs at approximately 95ms (N95), which is entirely dependent on the ganglion cells and is dominated by the pattern response. As the PERG is largely dependent on ganglion cell function, a shortening of the P50 implicit time and/ or reduced amplitude of the N95 suggests ganglion cell dysfunction. Thus, PERG has diagnostic potential in glaucomatous patients. The P50 can also be used to assess macular function. For example, an abnormal P50 amplitude combined with a normal full field ERG would suggest the presence of a maculopathy such as age-related macular degeneration (AMD),15 whereas an abnormal P50 amplitude combined with abnormal responses in full field light adapted ERGs would suggest generalised cone dysfunction. PERGs can also be used to determine visual prognoses in patients about to undergo surgical treatment for pituitary tumours.16 Poor postoperative recovery is associated with poor preoperative PERGs.
Multifocal electroretinogram (mfERG)
The mfERG is used to assess the cone function of the posterior pole.4 The stimulus is a collection of either 61 or 103 hexagons over a field diameter of 40–50o (see Figure 8: Multifocal electroretinogram plots: A) An example of the hexagonal array stimulus shown during mfERGs; B) Plot of waveforms generated by each hexagon in a healthy patient. It can be seen that each waveform consists of a negative trough (N1) followed by a positive peak (P1) and then a second negative trough (N2); C) An example trace array from a patient with AMD. Waveform amplitudes are notably reduced in the central field where retinal disease is active; D) An example trace array from a patient with retinitis pigmentosa. Waveform amplitudes are notably reduced where this disease is active, the peripheral field. Image courtesy of Dr Samantha Strong). The hexagons are scaled so that each retinal region produces an equivalent response. An algorithm ensures that each hexagon is either black or white at any one time and that the mean retinal luminance remains the same throughout. Various types of active electrodes may be used for this type of test and as in full field ERGs the patient should be dilated. If necessary, trial lenses may be positioned in front of the patient to correct refractive error but care must be taken to avoid inducing an artificial scotoma with the lens casing.
In mfERGs a waveform is produced for each hexagon consisting of a negative trough (N1) followed by a positive peak (P1) followed by another negative trough (N2) (see Figure 8);17 this denotes the global retinal response in the outer retinal area encompassed by that particular hexagon and as such is useful for monitoring diseases that affect the posterior pole. When compared to age-matched controls the amplitude and implicit time of the central mfERG’s P1 have been shown to be significantly reduced in both the affected and unaffected fellow eyes of patients with pre and early AMD.18 It has also been shown that reduction of amplitudes and implicit times have been found only in areas local to disease activity in patients suffering from central serious retinopathy.19 Patients with Stargardt macular dystrophy display a reduction in amplitude of the macular mfERG which can even be seen when patients’ visual fields and acuities remain normal.20 When tested with an mfERG, responses from patients with retinitis pigmentosa will show a delay in implicit time and reduction in amplitude of the affected areas;21 these responses correlate well with functional loss shown during Humphrey visual field analysis.
Visually evoked potential (VEP)
VEPs are the collective response of the neurons in the visual cortex. It is possible to use a single flash stimulus (flash-VEP) or a pattern reversal checkerboard stimulus (pattern-VEP) for these tests. Pattern reversal stimuli are preferred in a clinical setting and are particularly useful in assessing patients with nystagmus or those who are suspected to be malingering.5 However, when the patient’s vision, optics or cooperation is poor, flash VEPs may be more suitable. It is not necessary to dilate patients for VEPs but, unlike the PERG, the pattern-VEP may be performed at high or low contrast. It is also possible to vary the colours of the checkerboard pattern to red-green and blue-yellow or to introduce motion stimuli to assess higher cortical functions with the VEP.
VEPs are performed on the occipital lobe with one electrode on the midline and one either side of this;5 these are referenced to a frontal electrode on the midline. Pattern-VEPs have similar waveforms to PERGs and mfERGs with a negative trough (N75) followed by a positive peak (P100) and a final negative peak (N135).
VEPs can be incredibly useful in a clinical setting for many reasons. For example, varying the size of the checkerboard pattern until cortical response is lost can give the clinician an idea of the patient’s visual acuity,5 which is very useful with non-verbal patients. Flash-VEPs can be used in paediatric ophthalmology to ascertain if there is any functioning visual cortex, and can also be used in patients with significant media opacities such as a mature cataract or vitreous haemorrhage.5 In optic neuropathy and maculopathy, VEPs may be used in conjunction with PERGs to exclude macular involvement.15 Additionally, VEP traces can also signify the presence of a lesion or indicate misrouting of optical fibres such as that seen in the incomplete desiccation of temporal fibres of albino individuals.
Systemic conditions may also be highlighted by VEPs – for example, a brain tumour may cause a reduction in VEP amplitude or an increase in implicit time, and demyelinating diseases such as multiple sclerosis can cause an increase in VEP implicit time.22
Electrophysiological tests can be very useful to clinicians. Whether a patient has unexplained vision loss, an undiagnosed pathology or suspected of malingering, these electrophysiological tests can objectively determine the source of any disease and help differentially diagnose many conditions. Assessing the function of the whole retina may be obtained with EOG (rod and RPE responses) and ERG (photoreceptors, bipolar cells, amacrine cells, ganglion cells). The macular area and central posterior pole can be assessed on a more localised level by the PERG and mfERG, respectively. VEPs can also provide vital information from the visual cortex ranging from lesion assessment to an objective assessment of a patients visual acuity.
About the authors
Angharad Hobby MCOptom graduated from Cardiff University and qualified as an optometrist in July 2014. She is currently studying for her PhD at City, University of London, and continues to work in hospital practice in Sussex and is a faculty member at the Johnson & Johnson Vision Care Institute. Tamsin Callaghan MCOPtom, PhD is an optometrist and a lecturer at City, University of London and recently completed her PhD at Cardiff University.
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- Hood DC, Holopigian K, Greenstein V, et al. (1998) Assessment of local retinal function in patients with retinitis pigmentosa using the multi-focal ERG technique. Vision Research 38(1): 163-179
- Fuhr P, Borggrefe-Chappuis A, Schindler C, et al. (2001) Visual and motor evoked potentials in the course of multiple sclerosis. Brain 124(11): 2162-2168.