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Visual evoked potentials and their use in an optometric setting.

This article considers the clinical applications of visual evoked potentials for objective assessment of visual function in practice.

1 CET POINT

Introduction

Nowadays the optometrist has at their disposal a host of imaging and functional assessment tools. Fundus photography, OCT, perimetry, and tonometry, all improve the rates of early detection and diagnosis of optic nerve pathology such as glaucoma. (1) However, the aetiology of optic nerve pathology encompasses a variety of conditions: raised intracranial pressure (papilledema), compression (optic nerve glioma), inflammatory (optic neuritis) as well as relatively benign congenital anomalies, to name but a few. (2-5) The use of electrodiagnostics provides a valuable objective assessment of optic nerve function and although it is not routinely available outside of a hospital setting it is important to understand the value of the visual evoked potential (VEP) and how it can complement other optometric measures to provide a fuller diagnosis.

The visual evoked potential (VEP)

A visual evoked potential (VEP) is the electrical response recorded from the scalp over the visual cortex following a visual stimulus. (6) The response is recorded by electrodes placed at set positons using a universal placement method called the 10-20 system (see Figure J). This system was originally used for electroencephalographic (EEG) electrode placement. For the VEP, only a small amount of the positions are utilised. These positions are determined by measuring the total distance between anatomical bony landmarks, such as the nasion and the inion (small indentation at the back of the head), as well as the circumference. The electrodes O2, Oz and 02 are placed in a line at the back of the head over the occipital lobe, the most prominent portion of the visual cortex, separated laterally by 10% of the total circumference and positioned 10% (of the inion to nasion distance) vertically up from the inion (see Figure 1). Because the amplifiers read the difference between two inputs, a reference electrode is placed at the front of the head in a less visually active position, usually at Fz (see Figure J). Frequently, just the single Oz electrode on the midline is used but more detailed studies require an extensive assessment of the occipital region and hence a wider electrode array.

The origins of the VEP

The majority of the VEP signal is from neurons in the striate cortex at the posterior tip of the occipital lobe known as VI. Visual information is relayed to this region via the optic radiations from the lateral geniculate nucleus (LGN). The decussation or separation of optic nerve fibres at the optic chiasm means that the right hemifield is projected to the left striate cortex and the left hemifield is projected to the right striate cortex (see Figure 2). (7,8)

The cortical projections from the fovea are represented by a relatively large region of cortex compared to the peripheral retina; (9) these projections are located at the outer central region of the calcarine sulcus whereas projections from the peripheral retina are located in the folds of the sulcus and further away from the scalp. (9) Therefore, the VEP is dominated by the input from the central 10 degrees of the retina with little electrical input from the periphery.

One of the challenges in recording the VEP is that the response is a cortical potential and is, therefore, immersed amongst the intrinsic EEG activity of the cortex and is not easily distinguished. To separate it to the point where we can measure its components, a technique called signal averaging is used. In short, the intrinsic EEG activity is somewhat random whereas the VEP is an elicited potential that has a certain morphology and occurs after a certain time following the presentation of a stimulus. By recording the average of multiple presentations, the elicited potential will sum together whereas the random background EEG will cancel out and theoretically leave just the VEP. Stroboscopic or strobe light was used to elicit the potential for the first clinical VEPs but work by Regan et al showed that the visual cortex responds better to more structured stimuli with edges rather than diffuse stimuli and paved the way for the use of patterned stimuli. (10) The pattern reversal stimulus remains the most reliable and universally used today in clinical visual electrophysiology but the diffuse flash stimuli also has its place in paediatrics for example.

Types of VEP

Pattern reversal

An alternating checkerboard stimulus is used to elicit the cortical response (see Figure 3, page 78--top). It uses a field of high contrast (>80%) black and white squares and reverses at 1Hz.11 The cortical neurons at VI respond strongly to the high contrast stimuli. (10) In normal participants this produces a robust and highly reliable response with little variation between individuals. The waveform produced is triphasic with an initial negative deflection followed by a positive peak and another negative deflection. The peaks of the three waves are termed N75, P100 and N135, respectively. The letter denotes the orientation of the deflection (positive or negative) and the number is the approximate time of the peak latency from stimulus onset. (11) It is important to note that although the P100 suggests a normal time to peak of 100ms, this figure will vary slightly depending on the machine being used and the protocol in place at that particular site. Therefore, there is a need for each laboratory to establish its own normative data.

Pattern onset

Pattern onset is similar to the reversing checkerboard stimulus except the checkerboard stimulus is alternated with a uniform grey background with the same mean luminance as the checkerboard (see Figure 3--middle). (11) It is less dependent on fixation and defocus so it is often used when testing small children, patients with nystagmus, or for assessing patients with nonorganic visual loss; (12,13) however, it is more variable than the pattern reversal stimulus. The waveform consists of a positive peak followed by a negative deflection and a positive peak, termed C1, C2 and C3, respectively.

Flash VEP

The flash VEP uses a strobe light to elicit a response. This method is most frequently used in very young children who are unable to focus on a pattern stimulus. The waveform is highly variable and is more of a crude measure of optic nerve function compared to output obtained with patterned stimuli. The waveform (see Figure 3--bottom) is made of a number of positive and negative deflections but the second positive peak (P2) is considered the most reliable and the time to peak from the flash onset is typically used as an indicator of optic nerve conduction. (11)

Steady-state VEP

This method involves presenting the pattern reversal or pattern onset stimulus at a known high frequency to achieve a steady state response or sinusoidal response. By using a mathematical method called Fourier analysis it is possible to measure the amplitude of the known frequency; this can provide information on whether the patient can see the stimulus. For example, if the patient is unable to make out the stimulus then the amplitude of the measured response will be reduced or absent. By recording multiple trials and decreasing the size of the checks until a signal is no longer observed it is possible to obtain an objective measure of visual function; this is a very effective method for assessing visual acuity in nonverbal patients or infants. (14)

Factors influencing the VEP

To correctly record and interpret the pattern reversal VEP in a clinical setting, knowledge of how changes to the stimulus parameters can affect the recorded waveform is crucial as often these factors can be simulated by pathology.

Contrast

Typically the contrast of the pattern is >80%. Reducing the contrast of the checkerboard will cause VEP latency to increase for all check sizes. The lower the contrast the more prolonged the P100 latency. (15) This is especially important when testing patients where contrast may be affected, such as those with amblyopia.

Blur or defocus

When recording a pattern reversal VEP, standards set out by the International Society for Clinical Electrophysiology of Vision (ISCEV) stipulate that the patient must be optimally refracted. (11) The latency of the VEP can increase significantly with myopic or hyperopic changes. Studies have shown that this is more marked in hyperopia and that smaller check sizes are more affected than larger ones. (16,17)

Luminance

The mean luminance of the monitor is typically 60cd/ m2. Changes to the mean luminance can also affect the latency and amplitude of the VEP. The lower the luminance the more prolonged the P100 latency will be and the greater the amplitude loss; this will typically affect all check sizes equally. (18) It is important to bear luminance in mind when testing patients with any form of media opacity or patients with tinted lenses.

Clinical uses of the VEP

Amblyopia

In amblyopic eyes, the latency of the P100 is slightly prolonged and of lower amplitude as a result of reduced contrast sensitivity (see Figure 4). The second positive component of the VEP (not routinely measured) can have a peak latency that is shortened compared to the same response in the non-amblyopic eye. It is suspected that this is in part down to a loss of specific contrast mechanisms.19

Albinism

This genetically heterogeneous condition can be subdivided into either oculocutaneous or ocular albinism with the former characterised by hypopigmentation of the hair, eyes and skin. (20) Typical findings upon examination of the eye are: nystagmus, iris transillumination, foveal hypoplasia, and hypopigmentation of the retinal pigment epithelium, although not all of these features may be present. Abnormal crossing of the optic nerve fibres at the optic chiasm is also a common feature in albinism. In albinism most of the fibres from the temporal and nasal region cross to the contralateral side. Because of this unusual feature the VEP can be a useful tool in diagnosing albinism. (20) By placing electrodes over the left occipital and the right occipital (O1 and 02--see Figure 1) and stimulating each eye individually with the fellow eye patched, the misrouting of the optic nerve fibres will present as a large VEP on the left when stimulating the right eye and vice versa. Pattern onset or flash stimuli are usually used as they are less affected by nystagmus. (20)

Non-organic visual loss

Non-organic visual loss or functional visual loss may be suspected when the symptoms described do not agree with the clinical exam. This usually presents in children but also can occur in adults for a variety of reasons. Some of the causes may be a conscious effort to deceive the optometrist for financial or other gain, it may be the case that the individual is over anxious and believes that there is a problem where there isn't one or other stressors such as bullying or family issues. (21)

Although certain patterns on automated perimetry may provide clues to malingering, objective electrophysiological testing is often recommended to completely exclude an organic cause. The electroretinogram (ERG) can provide a full assessment of retinal function including the macula. The VEP can then be used to assess the function of the optic nerve and the visual cortex. (21) Given that the pattern reversal VEP is susceptible to changes related to inaccurate fixation and defocus it is recommended that the patient is closely observed during the recording to prevent a false positive result. (22) A more robust measure such as flash or pattern onset stimulus can also be used that are less susceptible to poor fixation or defocus.

Optic neuritis

Optic neuritis is a symptom of a variety of diseases but most notably multiple sclerosis (MS). Clinically, the patient will experience an acute decrease in visual acuity, reduced visual fields, pain and compromised colour vision as a result of demyelination of the optic nerve. The VEP will show an increased latency of the P100 peak even in subclinical cases (see Figure 6).

Halliday studied a group of patients with MS and showed that over 90% of the patients had abnormally prolonged VEP latency with many of them never experiencing any visual symptoms. (23) The VEP may improve in some patients after a period of time but may remain abnormal even after there has been some recovery of vision indicating remnant subclinical damage. (24) Magnetic resonance imaging (MRI) has taken over as the gold standard in the diagnosis of optic neuritis and MS, but the VEP still remains as a sensitive indicator of dysfunction and particularly has a use in monitoring cases with suspected subclinical lesions. (25)

Conclusion

The VEP has been in use as a clinical tool for many years and, despite advances in imaging technology, remains one of the best objective assessments of optic nerve function as well as being a relatively cheap, quick and non-invasive test. As the VEP can be affected by lesions anywhere along the retino-cortical pathway, it is wise to include other methods of electrophysiology such as the pattern and full-field ERG to rule out problems in the proximal retina. In an optometric setting with experienced practitioners, the VEP not only complements the many tests used in the diagnosis of ocular disease and optic nerve dysfunction but also aids in the categorisation, prognostic outlook and monitoring of patients both during and post treatment.

Exam questions and references

Under the enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk. Please complete online by midnight on 22 September 2017. You will be unable to submit exams after this date. Please note that when taking an exam, the MCQs may require practitioners to apply additional knowledge that has not been covered in the related CET article.

CET points will be uploaded to the GOC within 10 working days. You will then need to log into your CET portfolio by clicking on 'MyGOC' on the GOC website (www.optical.org) to confirm your points.

Visit www.optometry.co.uk, and click on the 'Related CET article' title to view the article and accompanying 'references' in full.

John Maguire BSc (Hons), MSc

* John Maguire is a clinical measurement scientist and has over 10 years' experience working in hospital based neuro and visual electrophysiology. He holds a BSc in Clinical Measurement from Dublin Institute of Technology and a Masters in Investigative Ophthalmology and Vision Sciences from the University of Manchester. Mr Maguire is currently doing a PhD based in the School of Optometry and Vision Science at the University of Bradford and his research interests involve the use of experimental electroretinography to investigate the response from the sub types of photoreceptors in healthy and diseased retina.

Course code: C-56586 Deadline: 22 September 2017

Learning objectives

* Be able to explain to patients about the use of visual evoked potentials for ocular investigation (Group 1.2.4)

* Understand the use of visual evoked potentials for assessing retinal function (Group 3.1.3)

Caption: Optometrists

Caption: Figure 1 The 10-20 universal system used for electrode placement. The recording electrodes for the VEP are shown in red. The total distance between the nasion and the inion and the total circumference of the head are shown as dashed blue lines

Caption: Figure 2 The normal visual pathway showing the nasal optic nerve fibres crossing at the optic chiasm, projecting to the contralateral side of the visual cortex. The temporal fibres do not cross at the optic chiasm and project to the ipsilateral side of the visual cortex

Caption: Figure 3 The different stimuli used and the typical normal waveforms recorded with markers. The top image shows the pattern reversal stimulus and the pattern VEP waveform. The middle image shows the pattern onset stimulus and the pattern onset VEP. The bottom image shows the strobe stimulus and the flash VEP

Caption: Figure 4 The pattern VEP from a normal eye (top) and an amblyopic eye (bottom). The P100 component is reduced and prolonged in the amblyopic eye. The second positive component (P2) is also noticeably shorter in the amblyopic eye

Caption: Figure 5 The normal visual pathway (left) and the visual pathway in ocular albinism (right). In both cases the left eye is being stimulated while the fellow eye is patched. The VEP from the normal eyes (bottom left) shows a symmetric electrical response at each electrode. The VEP in ocular albinism (bottom right) shows a response in the right occipital when testing the left eye and almost no response on the left and vice versa when testing the right eye. The thin blue and red lines on the albinism diagram indicate the small amount of temporal fibres (~10%) that do not cross over

Caption: Figure 6 Pattern VEP from a patient diagnosed with optic neuritis. The top trace is from the unaffected eye and shows a normal P100 component. The lower trace shows the response from the affected eye. The amplitude of the response is reduced and the latency of the P100 component is very prolonged in comparison to the top trace
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Title Annotation:Visual evoked potentials
Author:Maguire, John
Publication:Optometry Today
Date:Aug 1, 2017
Words:2815
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