Ординатура / Офтальмология / Английские материалы / Ophthalmology Investigation and Examination Techniques_James, Benjamin_2006

Fig. 15.7 Normal-pattern electroretinogram. The key features are: (1) N35; (2) P50; and (3) N95.

The components of the PERG are conventionally identified by their normal implicit time in milliseconds with a suffix to indicate positive or negative polarity. There is a small negative deflection (N35) followed by a larger, broader positive deflection (P50) and another broad negative deflection (N95) (Fig. 15.7).

The N95 component seems to be closely related to ganglion cell activity and is sensitive to early glaucomatous optic nerve damage. The PERG response is an early casualty of Stargardt’s maculopathy and may be profoundly reduced or extinguished before the foveal changes are evident clinically. The PERG can therefore be a useful test in a child who presents with an unexplained reduction in visual acuity.

Multifocal electroretinography

As stated above, the standard flash ERG is a global response from the retina and gives no information about whether some areas of the retina are more involved in a disease process than others. It is not possible to obtain a meaningful focal ERG simply by projecting a beam of light on to a small area of retina, because scattering and reflection of light in the eye result in illumination of a much wider area of retina.

Multifocal electroretinography (mfERG) is an elegant technique by which local ERGs can be extracted simultaneously from a number of areas of the retina. The stimulus consists of an array of scaled hexagons (smaller around the central fixation target, larger in the peripheral field), each element of

which flashes in a pseudo-random sequence known as an m-sequence.

Responses are recorded under light-adapted conditions with a similar electrode configuration to the standard ERG. Even with good recording conditions and a high level of patient cooperation, it is necessary to record for several minutes to obtain stable recordings. The raw mfERG response is undecipherable to casual analysis, but because each hexagon follows an m-sequence, a computer can be used to extract an ERG trace from each hexagon by a mathematical process known as kernel analysis. Each focal ERG obtained in this way is a composite wave which contains responses to single flashes (firstorder responses), on top of which is superimposed the effects of earlier flashes in the sequence (secondand higher-order responses, i.e. adaptation effects; Fig. 15.8).

Superficially, the first-order kernel of the mfERG resembles a conventional photopic ERG with a negative deflection followed by a positive deflection. Clinical evidence suggests that these components are equivalent to the a-wave and b-wave of the full-field ERG, but it must be remembered that mfERG traces are mathematically derived responses rather than directly recorded ERGs. The amplitude of the mfERG response is also very small – a few hundred nanovolts at best – and great care has to be taken to achieve a good signal-to-noise ratio.

Although used extensively as a research tool, the mfERG is generally less useful than the full-field ERG in routine clinical practice. It distinguishes between healthy and poorly functioning areas of retina quite well, but gives little information about the function of different retinal cell types. It is degraded by defocus, media opacity and patient movement.

Electro-oculography

When the retina is illuminated by a sustained light source following a period of dark adaptation, the ERG a-wave and b-wave are followed by three distinct slower potentials known as the c-wave, fast oscillation and slow oscillation (or light rise). The c-wave is a cornea-positive potential which peaks at about 2 s from the light onset and represents the sum of a retinal component and a retinal pigment epithelial component. The fast oscillation is a cornea-negative potential which reaches a trough at about 1 min from light onset and the light rise is a

cornea-positive potential which peaks at 10–15 min from light onset. The fast oscillation and light rise are both due to changes in ion concentrations across the retinal pigment epithelium cell membrane.

The c-wave and fast oscillation are technically difficult to record in conscious subjects and are of limited clinical usefulness. However, the light rise can be measured more easily by an indirect technique known as electro-oculography (EOG), developed by Arden (Arden, 1962).

If electrodes placed at the lateral canthus and medial canthus are connected to a galvanometer, adduction of the eye will cause a deflection of the galvanometer in one direction while abduction of the eye will result in a deflection in the opposite direction. This is because, even in the dark, the living eye has a potential difference of about 6 mV between the cornea and the retina (DuBois-Reymond, 1849). The light rise is superimposed on this standing potential, and, for the same degree of adduction or abduction of the eye, will result in an increased deflection of the galvanometer.

In the clinical EOG protocol, the subject is seated in front of a diffuser, behind which is a bank of fluorescent light tubes (Fig. 15.9). In the centre of the diffuser, there is an LED to act as a fixation target. Another pair of LEDs either side of the central fixation target can be illuminated alternately to help the

Fig. 15.8 Normal multifocal ERG trace array. (Courtesy of Moorfields Eye Hospital.)

subject make excursions of the eyes to left and right of a constant amplitude. For most of the time during the test, the subject looks at the central LED, but every minute, the left and right LEDs illuminate alternately to prompt the subject to make a series of excursions of the eyes to left and right. The potential difference between the lateral and medial canthal electrodes is measured on a slow time base and the periods of eye movement appear as deflections above and below the baseline. During dark adaptation, the amplitude of the deflections on the trace gradually diminish to a constant level, referred to as the dark trough. The lights are then turned on and the amplitude of the deflections on the trace start to increase, reaching a maximum after about 10 min of light adaptation. The ratio of the maximum amplitude in the light to the minimum amplitude in the dark is an indirect measurement of the retinal pigment epithelium light rise and is commonly known as the Arden index (Fig. 15.10).

The lower limit of the Arden index in normal subjects is about 1.7, but it can be as high as 4.0, and may vary in the same individual between tests performed on different occasions. Any condition resulting in a substantial reduction in the ERG will also reduce the EOG light rise. The EOG is influenced by a number of pharmacological agents, notably acetazolamide and ethanol. The variability

Clinical Visual Electrophysiology

a

b

Fig. 15.9 (a) Electro-oculogram recording and

(b) electrode positions. The two red fixation lightemitting diodes can just be seen on the light box.

Fig. 15.10 Normal electro-oculogram response.

of the EOG and the fact that it is a tedious test to perform limits its clinical utility, but it has proved useful in Best’s vitelliform dystrophy, where the EOG light rise is consistently subnormal, while the ERG is usually unaffected. VMD2 (bestrophin), the product

of the gene commonly implicated in Best’s vitelliform dystrophy, is a chloride channel protein found in the basal cell membrane of the RPE. Mutations in this gene result in reduced conductivity and hence an impaired EOG light rise.

The visually evoked cortical potential (VECP or VEP)

It is possible to record electrical responses from the primary and secondary visual cortex in response to visual stimuli of various types. This is facilitated by the fact that several areas responsible for the processing of visual information, notably the primary cortex serving the macula, lie on the outer surface of the brain close to the skull. The brain is, of course, an electrically busy organ and the VEP has to be extracted from a cacophony of electrical noise, particularly the alpha rhythm of the electroencephalogram (EEG).

To record the VEP, a midline occipital skin electrode is placed and is referenced to a mid frontal electrode. Additional electrodes may also be placed over the occipital and parietal areas of both hemispheres. Recordings are time-locked to a repetitive stimulus which allows alpha rhythm and other unwanted noise to be averaged out, leaving a response that is attributable to the visual stimulus. The primary visual cortex gives particularly strong and repeatable responses to a reversing checkerboard pattern, as described for the PERG. Another commonly used stimulus is referred to as ‘pattern onset’, where a checkerboard pattern alternates with a uniform grey screen of the same mean luminance.

VEPs originating from the secondary visual areas have been recorded to many other types of stimulus including motion, stereopsis and colour. It is also possible to record multifocal VEPs with a similar apparatus to that used for the multifocal ERG. These specialised VEPs are used in research studies but have not yet found their place in routine clinical practice.

The waveform of the VEP depends on the type of visual stimulus, the position of the electrodes used to record it, the maturity of the visual cortex and the level of alertness of the subject. Even where the stimulus characteristics and placement of recording electrodes are carefully standardised, considerable interindividual variability in the waveform is common because the calcarine cortex is highly and variably convoluted, and because the relationship between

anatomical landmarks on the skull and surface features of the brain is imprecise. It is not possible to assign functional significance to particular components of the VEP waveform; instead, the VEP can be analysed in terms of implicit time of components, vector (direction of spread of components through the brain, where multiple electrodes are used) and tuning characteristics (optimum stimulus conditions for producing the strongest possible response).

A typical adult pattern reversal VEP trace has three main components, which are named according to their polarity and implicit time as N75, P100 and N135 (Fig. 15.11). Of these, the P100 component is usually the largest and most reproducible response when recorded with a midline occipital electrode. Because the macula projects to the most posterior part of the occipital cortex, the P100 is a maculadominated response. Both retinas project to both hemispheres, so one eye must be tested at a time in order to compare responses between the two eyes .

The pattern reversal VEP can be used as an approximate objective measure of visual acuity. When the size of the checks is reduced to the point where the contrast borders can no longer be resolved, the cortical response disappears. At a check size which subtends a visual angle of 15 min arc, a visual acuity of approximately 6/18–6/24 is required for a clear cortical response. At a check size of 60 min arc, a visual acuity of 3/60–6/60 is required for a clear cortical response.

Demyelinating optic neuritis impairs conduction in the optic nerve. In the acute phase, where visual acuity is significantly reduced, the VEP is often undetectable. When the visual acuity has returned to normal, the VEP from the affected eye usually recovers, but remains permanently delayed, typically by 30 ms or more (Fig. 15.12).

Ocular albinism is associated with a variable degree of misrouting of optic nerve fibres. Fibres from the temporal retina which would normally project to the ipsilateral hemisphere instead decussate in the chiasm and project to the contralateral hemisphere. There is not a close relationship between the extent of misrouting and the visual acuity or amplitude of nystagmus, but misrouting can often be detected using the pattern VEP. As well as a midline occipital electrode, electrodes are placed over the left and right occipital lobes and the responses resulting from stimulation of either eye are compared at each electrode position. In the most obvious examples of misrouting, stimulation of the right eye will result in a response predominantly over the left occipital lobe and vice versa. Sometimes the misrouting is more subtle and is only evident on careful measurement of the traces (Fig. 15.13).

Abnormalities in the visual pathway at or posterior to the chiasm can be detected with the pattern VEP using hemifield stimulation. The patient views a fixation target in the centre of the screen and the checkerboard stimulus occupies either the left or right half of the screen. Hemifield responses must be recorded with an array of electrodes (normally five) over the occipital lobes.

The pattern VEP can be used to monitor the maturation of the visual system in infants. Its development fairly closely parallels the development of visual acuity recorded by other tests, such as preferential looking.

The pattern VEP is degraded by uncorrected refractive error, unsteady fixation and drowsiness. It is possible to record a VEP to simple flashes of light. This is useful where the visual acuity is too poor to perceive even large checks or where fixation or concentration is poor. The flash VEP is much less

Clinical Visual Electrophysiology

Fig. 15.12 Pattern visually evoked potential in right optic neuritis. Note the delayed P100 in the right eye compared to the normal trace in the left.

Fig. 15.13 Visually evoked potential responses showing misrouting of the visual pathway.

(a) The result of subtracting the response from the right hemisphere from that of the left, with the right eye being stimulated. (b) The lower trace is similarly obtained while stimulating the left eye. The asymmetry is obvious.

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Fig. 15.14 Normal flash visually evoked potential. The key feature is: (1) P2.

macula-dominated than the pattern VEP and can be recorded through cataracts or corneal scars. Unfortunately, it is much more variable in appearance than the pattern VEP and, in general, it is used as a basic indication of the integrity of the visual pathway from the eye to the occipital cortex. The most consistent feature is a positive component, designated P2, which usually occurs at around 125 ms after the flash, but identification of even this component is not always easy (Fig. 15.14).

With the exception of demyelination, the VEP gives little help in differentiating the type of lesion affecting the optic nerves or other parts of the visual pathway. It will not, for instance, distinguish between an ischaemic lesion and a compressive lesion. It can be useful, however, for monitoring the progress of a known condition affecting the visual pathways.

Dark adaptometry

Dark adaptometry is a psychophysical measurement rather than an electrophysiological test, but it is usually performed in electrophysiology departments. The human eye has a remarkable ability to detect

(1 million times) of light intensity. This is achieved to a small extent by altering pupil diameter and to a larger extent by bringing rods or cones into or out of play, but the most important mechanism for adjusting the gain of the visual system is the balance between bleach and regeneration of visual pigment.

Dark adaptation is measured by bleaching the retina with an intense light, turning out the light, then periodically recording the brightness at which a test light projected a few degrees away from fixation can just be perceived, until the eye is fully dark-adapted. A typical dark adaptation threshold curve is shown in Chapter 1. The curve is biphasic, with the first part due to the cones and the second part to the rods. The step between the cone and rod parts of the curve is referred to as the ‘rod–cone break’.

In achromatopsia, the cone component of the dark adaptation curve is absent and recovery from bleach occurs rapidly. In rod–cone dystrophies, such as autosomal dominant retinitis pigmentosa, and in the various types of congenital stationary night blindness, the rod part of the dark adaptation curve is absent, resulting in an elevated dark adaptation threshold even after prolonged dark adaptation.

Oguchi’s disease is a rare form of night blindness due to mutations in the genes which encode either arrestin or rhodopsin kinase. It is characterised by a metallic sheen of the retina on fundoscopy which disappears on prolonged dark adaptation (Mizuo’s phenomenon). The dark adaptation curve is greatly prolonged, but eventually reaches a normal threshold level after several hours of dark adaptation.

Summary and suggestions for further reading

This chapter has given an overview of visual electrophysiological investigations and their application in a clinical setting, with a number of specific examples of diseases of the visual system which show characteristic electrophysiological features. Visual electrophysiology is a rapidly developing field of visual science and the scientific literature can be daunting for the trainee ophthalmologist. The ISCEV website (www.iscev.org) gives details of the electrophysiological standards referred to in this text and contains other educational resources. There is a superb online textbook on the organisation of the human retina by Helga Kolb et al. at http:// webvision.med.utah.edu. For up-to-date references on specific inherited ocular diseases, consult the Online Mendelian Inheritance in Man (OMIM) website at www.ncbi.nlm.nih.gov.

Clinical Visual Electrophysiology

■Despite the sophistication of the recording equipment available, a critical aspect of the process of electrophysiology is the training and experience of the electrophysiologist. Meticulous attention to detail is required for repeatable and reliable results

■Electrophysiology is a part of the diagnostic process. It does not, often, provide the diagnosis per se and clinical guidance as to the tests required is critical

■The International Society for Clinical Electrophysiology of Vision (ISCEV) website (www.iscev.org) provides standards as referred to in this chapter and other useful information and resources for the trainee

References

Arden GB, Barrada A, Kelsey JH. New clinical test of retinal function based upon the standing potential of the eye. Br J Ophthalmol 1962; 46: 449–467.

DuBois-Reymond E. Untersuchungen uber die thierische Elektrizitat, Vol. 2. Berlin: Reimer; 1849: 256–257.

Einthoven W, Jolly W. The form and magnitude of the electrical response of the eye to stimulation at various intensities. Q J Exp Physiol 1908;

1: 337–416.

Granit R. Sensory mechanisms of the retina with an appendix on electroretinography. London: Oxford University Press; 1947.

Introduction

Fluorescein angiography is used to examine vascular structures in the eye such as the iris, retina and choroid. Injected fluorescein dye remains within normal blood vessels, thus leakage into surrounding tissue indicates vascular pathology. The time taken for the dye to pass from the arterial to the venous circulation (and, indeed, the time taken just to reach the eye) may also provide some functional information about the circulatory system. It is most commonly used to investigate retinal diseases such as diabetic retinopathy.

Principles

The principles of this technique were first described by MacLean and Maumenee and later developed by Novotny and Alvis. Fluorescein has the property of absorbing light in the blue wavelength and emitting it in the green wavelength (fluorescing). Fluorescein is a vital dye: its characteristics are shown in Box 16.1.

Dye injected into a peripheral vein passes into and through the choroidal and retinal circulation. A blue light projected into the eye causes the fluorescein to emit a green light. This is photographed through a yellow barrier filter. This removes any reflected blue light but allows the fluorescent green wavelength through. The picture is captured on to black and white film or the charge-coupled device (CCD) of a digital camera. Fluoroscopy is the direct viewing of the fluorescence without recording it. This can be done with an indirect ophthalmoscope fitted with a blue filter. The principles of fluorescein angiography are shown in Figure 16.1.

Light source

Cobalt blue filter (excitation filter)

Eye: fluorescein stimulated in retina

Box 16.1

Properties of sodium fluorescein

Nature

Crystalline hydrocarbon dye

Colour

Orange/red

Molecular weight

376 kDa

Wavelength of excitation

465–470 nm

Emission wavelength

520–530 nm

Yellow filter (barrier filter)

Film or CCD plate

Fig. 16.1 The principles of fluorescein angiography. CCD, charge-coupled device.

Table 16.1 Side-effects and risk factors of fluorescein injection

The technique

There is both a morbidity (Table 16.1) and a mortality (1 in 200 000) associated with this technique. Its use should be limited to diagnosis in patients whose management may be altered by the results or subjects included in an ethically approved research study. Some of the indications are shown in Box 16.2. Full resuscitation facilities must be available close at hand.

The patient is comfortably seated at the retinal camera (Fig. 16.2). The blue exciter filter and the yellow barrier filter are appropriately positioned in the camera. Injection of dye (5 ml of 10% fluorescein) is best given in the antecubital fossa, ensuring that the patient feels no discomfort as this may signify extravasation of dye into the tissues, which can be very painful. A test injection of 1–2 ml of normal

Box 16.2

Common indications for fluorescein angiography

■To diagnose and demarcate choroidal neovascular membranes

■To differentiate between collateral vessels (nonleaky) and new vessels (leaky) in the retina

■To diagnose subclinical cystoid macula oedema

■To define the extent of macular leakage after branch retinal vein occlusion

■To define the site of subretinal leakage in central serous retinopathy before laser treatment

■To identify ischaemia in diabetic maculopathy

Fig. 16.2 The patient seated comfortably at the fundus camera.

saline is useful to confirm correct placement of the cannula before fluorescein is given.

The dye can cause nausea as it reaches the cerebral circulation and patients should be warned of this. In addition it colours the skin yellow for 12–24 h and is excreted in the urine, turning it a bright yellow colour. Sublingual administration of Stemetil 3–5 mg 1 h before the administration of dye may abolish the nausea.

The sequence of the angiogram

The sequence shown in Figure 16.3 demonstrates the different phases of an angiogram. Abnormalities may occur at different stages, and the stage at which an abnormality appears may be important diagnostically.

The protocol starts with a normal colour image (Fig. 16.3a) and a red-free picture, taken with only the green filter in place to give a black-and-white image of the fundus prior to fluorescein injection (Fig. 16.3b). The dye is then injected. Image capture begins again at the time it is anticipated that the dye will reach the eye. This will vary in patients according to their age and the state of their cardiovascular system; it is typically 8–12 s.

The earliest fluorescence is seen in the choroidal circulation and vessels arising from it, for example, a cilioretinal artery or a subretinal neovascular membrane. Choroidal fluorescence may be patchy initially but in health should be uniform at the start of the venous phase. The xanthophil pigment of the

central macular region masks the fluorescence from the choroid; this region therefore remains dark.

The arterial phase of the retinal circulation is seen next (Fig. 16.3c), followed by the arteriovenous or capillary phase and subsequently the venous

phase which initially demonstrates laminar flow (Fig. 16.3d). Late-phase pictures (after 5 min) are also taken (Fig. 16.3e). In this particular sequence a subfoveal neovascular membrane lights up early in the sequence and remains brightly fluorescent into