|Year : 2013 | Volume
| Issue : 1 | Page : 45-54
Electrophysiology for ophthalmologist (A practical approach)
UBM Institute, Dadar, Mumbai, Maharashtra, India
|Date of Submission||19-Sep-2012|
|Date of Acceptance||08-Nov-2012|
|Date of Web Publication||22-Jan-2013|
UBM Institute, A/1 Ganesh Baug, 214 Bhalchandra Road, Dadar, Mumbai, Maharashtra
Source of Support: None, Conflict of Interest: None
The article deals with the basic understanding of electrophysiological tests in clinical practice. Electrophysiological tests involves assessing the function of the rod-cone system and proximal visual pathway. Visual evoked potential (VEP) is performed to assess the function of the proximal optic nerve. Electro-oculogram (EOG) is used to study the photoreceptor-RPE junction. Electroretinogram (ERG) is used to assess the function of photoreceptor (a-wave) and the inner retina (b-wave). Pattern ERG is helpful to study the macular cone function and ganglion cell function. The a-wave in ERG is diagnostic of rod-cone or cone-rod dystrophy. The amplitude of b-wave in ERG helps us to distinguish inner retinal dysfunction from photoreceptor dysfunction. Hence ERG is not only helpful in making a diagnosis but is also helpful in studying the prognosis of the disease which eventually helps in counseling the patient. Pattern ERG when used in conjunction with pattern VEP helps to pinpoint the cause of an unexplained loss of vision. Multifocal ERG studies the focal responses at the posterior pole within the arcades. The most important use of a multifocal ERG is in the early detection of hydroxychlroquine toxicity. Visual acuity assessments with sweep VEP, focal ERG and multifocal VEP are the newer developments in electrophysiology.
Keywords: Electronegative ERG, electrophysiology, ERG, mfERG, Visual evoked potential
|How to cite this article:|
Bhatt D. Electrophysiology for ophthalmologist (A practical approach). J Clin Ophthalmol Res 2013;1:45-54
Electrophysiological tests involve assessing the function of the rod-cone system and proximal visual pathway. There are multiple tests performed to arrive at a diagnosis. Electroretinography (ERG) and visual evoked potential (VEP) are the most common of them. The waveforms in these tests not only help to make a diagnosis but also are helpful to give prognosis to the patient. The tests also help in diagnosing an unexplained loss of vision. There are newer developments in electrophysiology in form of multifocal electroretinography (mfERG) and multifocal visual evoked potential (mfVEP). The purpose of writing this article is to have a basic understanding of the waveform in electrophysiology and their interpretations.
Electrophysiology is a retinal electric potential obtained by visual stimulation.
| Aims of Electrophysiology|| |
- Diagnose a known retinal dysfunction
- Diagnose unexplained loss of vision
Following investigations are done in electrophysiology testing 
- Visual Evoked Potential
- Complete Electroretinogram
- Pattern Electroretinogram (PERG)
- Multifocal Electroretinogram (mfERG)
- Electro-oculogram (EOG)
| Visual Evoked Potential|| |
VEP is a cortical response that is time-locked to a visual stimulus event, such as the contrast-reversal of the checkerboard pattern or a flash of light. The amplitude is typically 4-15 μv, and the latency is 90-100 ms [Figure 1].  The channels mentioned on the left side of the figure are the protocols in VEP for right and left eye at 1,0 degree viewing angles. The first protocol in each eye is taken at large check size and the second at small check size. The N75 is the first negative response, and its latency time from the beginning of the response is noted. The P100 is the first positive response with its latency time noted in the third column. This wave is the most important wave in VEP. The N135 is a large negative response noted but is of not much importance clinically. The amplitude of N75-P100 is noted in the fifth column. Hence, the column third and fifth are the most important.
Pattern VEP is used in patients who can concentrate on a pattern stimulus. It is extremely helpful to study optic nerve function. A delay in the latency of P100 is suggestive of optic nerve dysfunction mainly involving the axon i.e. optic neuritis or demyelination [Figure 2]. Please note the delay in latency P100 on the left side (more than 110, which is considered normal). A reduction in the amplitude of P100 is usually suggestive of visual disturbance proximal to the optic nerve or neuronal tissue (mainly atrophic changes) [Figure 3]. Hence, pattern VEP is used to rule out optic nerve dysfunction and acts as a guide to locate the level of visual loss. If patient cannot fixate or has a dense opaque, media then a flash VEP is done to rule out the presence or absence of visual response from the optic nerve. The results of flash VEP is very variable and hence, it is used only as a marker for presence or absence of response from the optic nerve. [Figure 4]. The N1, N2, and N3 are negative responses, and P1, P2, and P3 are positive responses. In flash VEP, it's only the amplitude of P1 and P2, which are of importance.
|Figure 2: Shows normal right latency and increased latency on the left side|
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|Figure 3: Shows normal left amplitude and reduced amplitude on the right side suggestive of atrophic changes|
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| Visual Acuity (Sweep VEP)|| |
Sweep VEP is performed to obtain a rapid, objective estimate of visual acuity. Sweep VEP response is not obtained with checkerboard pattern but with a continuous sweep of linear light and dark responses. Sweep VEP acuity estimates have provided important information in the diagnosis and treatment of childhood amblyopia. In adult, it is used to assess visual acuity in uncooperative patents or patients who are malingering [Figure 5]. The graphs at the bottom of [Figure 5] show responses from multiple sweep stimulus. The graph at the top shows a regression analysis with the slope from the graph determining the visual acuity in (frequency, logMAR, decimal, and Snellen chart)
| Electro-Oculogram (EOG)|| |
The EOG is a measure of the function of the retinal pigment epithelium (RPE) and the interaction between the RPE and the photoreceptor. Any disorder which affects rod photoreceptor will affect the EOG, and the light rise is typically reduced in retinitis pigmentosa (RP) and related photoreceptor dysfunction. Arden ratio represents ratio between the light and dark trough and peaks. The Arden ratio above 1.8 is normal [Figure 6] and below 1.6 is abnormal. Between 1.8 and 1.6, it is suspicious. The [Figure 6] shows light and dark trough responses in graphic form and numerical form, but the most important is the Arden ratio. The principal use of the EOG in clinical practice is in the diagnosis of best vitelliform macular dystrophy, where there is severely reduced or absent EOG rise accompanied by a normal ERG.
| Pattern ERG|| |
The pattern electroretinogram (PERG) is the retinal response to a structured stimulus, such as a reversing black and white checkerboard or grating. It is a powerful clinical tool, allowing both the objective clinical evaluation of macular function (P50) and a direct assessment of retinal ganglion cell function (N95). [Figure 7] shows a normal PERG]. [Figure 7] shows the first negative waveform (N35) than a positive waveform (P50), the photoreceptor response, and the last negative waveform (N95), which is ganglion cell response. PERG is considered an intermediate step between full-field ERG and VEP, hence it should not be read alone. Pattern ERG is affected by diseases confined to the macula and, conversely a patient with generalized retinal dysfunction sparing the macula will have an abnormal ERG but a normal PERG. Hence, patients with severe form of rod-cone dystrophy with good visual acuity have markedly abnormal ERG but fairly good PERG due to sparring of the macula at this stage of the disease. Thus, PERG can be used as a marker for staging and progression of rod-cone dystrophy.
Pattern ERG is also used in assessment of early changes of glaucoma. Since the N95 component represents the ganglion cell function, pattern ERG is the first waveform to be affected in early changes of glaucoma. 
| Unexplained Loss of Vision|| |
Electrophysiology is gold standard in assessing patients with unexplained loss of vision. Pattern VEP and pattern ERG is done as a simultaneous recording for evaluation of these patients. The algorithm in [Figure 8] helps us to evaluate unexplained loss of vision better.
|Figure 8: Algorithm for role of pattern ERG in unexplained loss of vision|
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If pattern ERG and pattern VEP are normal, then the cause of unexplained loss of vision is usually non-organic. If pattern ERG is normal and pattern VEP is abnormal, then the visual loss is due to optic nerve dysfunction. If the P50 component of pattern ERG is normal and the N95 is abnormal, then we should rely on pattern VEP, which is usually abnormal and represents optic nerve dysfunction. If P50 is abnormal, then we are probably dealing with photoreceptor dysfunction. The next step is to do full field ERG, which will determine if there is a pure macular dystrophy of a generalized retinal dysfunction.
| Full-Field Electroretinogram|| |
Full-field ERG is a mass electrical response of the retina to luminance stimulation. It has two main components the scotopic (dark-adapted response) and the photopic (light-adapted response).
The scotopic response has two main waveforms, namely the rod-response b-wave and the maximal response a and b wave [Figure 9]. The first only positive b-wave noted in [Figure 9] is a pure rod response and is the first response obtained after scotopic adaptation. This wave is mainly used to study the rod photoreceptors. The second and third response in [Figure 9] is maximal response noted to first light stimulation. This response is the most important waveform used for differentiation between photoreceptor and inner retinal dysfunction. The a-wave (which is negative) represents the function of the photoreceptor, pre-photo transduction, and the b-wave (which is positive) represents the function of the inner retina (bipolar cells, post-photo transduction). 
The photopic response is obtained after bright photopic adaptation [Figure 10]. The first wave is obtained after a single photopic flash. This wave also has a negative a-wave and a positive b-wave. The negative a-wave represents the cone photoreceptor function, and the positive b-wave represents the inner retinal function. [Figure 10] shows N1-P1 value in 30 Mhz ERG, which states the extent of cone response.
The next response in photopic condition is the 30 Hz flicker response. At 30 Hz, the poor temporal resolution of the rod system, in addition to the presence of rod-suppressing background, enables a pure cone-specific waveform to be recorded.
There are three main groups of diagnosis established on ERG.
- Rod-cone dystrophy
- Cone-rod dystrophy
- Inner retinal dysfunction
| Rod-Cone Dystrophy|| |
Rod-cone dystrophy is a group of diseases involving dysfunction of the rod and cone photoreceptor layer, but mainly involving the rod photoreceptor. Patients have symptoms of night blindness. [Figure 11],[Figure 12] and [Figure 13] shows a typical ERG of rod-cone dystrophy. The rod responses are markedly reduced (a-wave), and this leads to secondary reduction of the b-wave also [Figure 11]. Both the waves are reduced evenly. RP is the most common condition in this group. Along with the rod response, the cone response is also reduced but to a lesser extend [Figure 12]. [Figure 13] shows a reduced pattern ERG, but with fairly good response, suggestive of macular sparring at this stage of the disease.
[Figure 14],[Figure 15] and [Figure 16] shows a severe case of RP leading to marked scotopic, photopic, and macular pattern ERG reduction. Thus, ERG not only helps in diagnosis of rod-cone dystrophy but also helps in staging of the disease.
| Cone-Rod Dystrophy|| |
Cone-rod dystrophy is a group of diseases involving reduced function of the cone photoreceptor. [Figure 17] shows a scotopic response in cone dystrophy, the rod b-wave and the a-b wave in maximal response is reduced but not markedly as in rod-cone dystrophy. [Figure 18] shows marked reduction of the photopic a-wave and 30 Hz flicker response. The 30 Hz flicker reduced response is highly suggestive of reduced cone function. Pattern ERG is also reduced in cone dystrophy and also acts as a point of differentiation between pure macular dystrophy and cone dystrophy [Figure 19].
| Inner Retinal Dysfunction|| |
The b-wave of the bright flash ERG response arises in inner retina, principally the bipolar cells, and thus relates to activity occurring post-phototransduction. This is also called electronegative ERG. The wave pattern shows a near normal a-wave and absent or reduced b-wave. The negative b-wave is appreciated better in the scotopic ERG than in the photopic ERG.
The most common conditions noted in this group are congenital stationary night blindness (CSNB), X-linked retinoschisis, melanoma-associated retinopathy (MAR), birdshot chorioretinopathy, and Batten disease More Details. The classical clinical disorder commonly associated with a negative ERG is central retinal artery occlusion (CRAO). This finding reflects the duality of the retinal blood supply i.e. the photoreceptors are supplied by choroidal circulation and inner retina by the central retinal artery. Hence, ERG is also used in ruling out ischemic retinopathy, especially in diabetics. Differentiation between photoreceptor layer and inner retinal dysfunction is one of the most important uses of ERG. 
[Figure 20] shows a classic case of CSNB with a negative b-wave in scotopic condition with a good a-wave. [Figure 20] shows that the b-wave response in the second and third graph does not reach even the starting point of the graph and hence, it is negative. [Figure 21] shows a photopic response in CSNB.
|Figure 21: Photopic response in CSNB shows reduced cone response (note the negative b-wave is appreciated better in scotopic response)|
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| Multifocal ERG (mfERG)|| |
Multifocal electroretinography is an investigation that can simultaneously measure multiple electroretinographic responses at different retinal locations by cross-correlation techniques. mfERG, therefore, allows topographic mapping of retinal function in the central 40-50 degrees of the macula. The display can be in the form of trace array [Figure 22] of a 3D representation [Figure 23]. [Figure 23] shows a scale at the bottom right, the vertical scale of 0 to 22 nV is the amplitude, and the horizontal scale of 0 to 80 ms is the latency. The trace array shows multiple responses over small areas, especially from arcade to arcade. The 3D map represents pseudo color response divided into multiple hexagons.  The foveal peak which has highest response is seen in the center in white color (maximum response). The small black arrow represents the blind spot (minimum response). All modes of display in mfERG can be presented in a single page [Figure 24]. [Figure 24] shows a tip tracing on the right side, which elaborates the scale of the graph in nV at each degree of response.
|Figure 23: 3D map of multifocal ERG shows high central foveal response and blind spot|
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Clinically, mfERG is used to study the function of the macula when in doubt or to study the function over a localized area of macula [Figure 25]. In ARMD, mfERG can be used to rule out presence of CNVM in dry ARMD or in follow-up of ARMD treatment. In diabetic retinopathy, mfERG is mainly used to rule out ischemic changes in the macular region. It is used to study the macular function in follow-up of surgeries (macular hole, epiretinal membrane, or diabetic retinopathy). 
The most important clinical use of mfERG is to assess hydroxychloroquine (HCQ) toxicity. There is circumferential suppression of the responses in ring 2 (immediate para-foveal region) noted in HCQ toxicity [Figure 26]. [Figure 26] shows the reduced response in second ring, which is just outside the large central conical response from fovea. This pattern is not noted in any other macular condition. This reduced response in ring 2 is noted even when the patient is asymptomatic and does not show any macula changes due to HCQ toxicity. This reduced response is known to reverse when the drug is stopped. Hence, mfERG forms an important investigation in the evaluation of HCQ toxicity. 
| Electroretinogram in Children and Infants|| |
In children, electrophysiological testing procedures provide an objective and non-invasive method for visual pathway evaluation. In children and infants, it is very challenging to perform ERG and VEP test. As children are not co-operative, it is difficult to obtain a good response, hence two reproducible responses are a must to make a diagnosis. Anesthesia should be avoided as far as possible as it impairs with the recording of ERG and especially in VEP. A mild form of sedation can be used if necessary. Usually, skin electrodes are used, and dilatation of pupil is avoided. A single pattern ERG and VEP response can be obtained together. The protocol in infants and small children is modified to obtain a reasonable good waveform in first photopic condition and after a small scotopic adaptation. In older children, a complete protocol is performed.
The questions that are asked to an electrophysiologist in children areIs the child blind?
- Why has this child got nystagmus?
Even with a restricted protocol, the following questions can be answered when performing electrophysiological test in children, and hence they are modified according to the age and symptoms of the patients. 
- Is there an ERG response or a functioning visual pathway?
- Is there a generalized cone dysfunction?
- Is there a functioning scotopic system?
- Is there a negative ERG?
The disease can be broadly classified in these groups, and further evaluation can be done at a later date. Visual causes of nystagmus can also be excluded. Almost all the dystophic conditions have a similar waveform as in adults. One of the common causes of congenital blindness is Leber congenital amaurosis (LCA). The ERG waveform in LCA is typically severely reduced or undetectable [Figure 27] and [Figure 28] show scotopic and photopic responses in LCA].
| Recent Advances in Electrophysiology|| |
The newer machines in electrophysiology can now do focal ERG at any spot over the posterior pole. This is done by combining a fundus camera with an ERG machine. [Figure 29] shows a Fundus image, and [Figure 30] shows an ERG response at a particular designated area. This method may be used in future to study the effects of drugs on the macula. Color-specific ERG can be obtained by flicker stimulation at temporal frequencies around 12 Mhz. Another major new development is the mfVEP [Figure 31]. This is the response obtained over the optic disc. This holds promise of obtained objective perimetry in future. 
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 17], [Figure 18], [Figure 19], [Figure 20], [Figure 21], [Figure 22], [Figure 23], [Figure 24], [Figure 25], [Figure 26], [Figure 27], [Figure 28], [Figure 29], [Figure 30], [Figure 31]