Ординатура / Офтальмология / Английские материалы / Moorfields Manual of Ophthalmology_Jackson_2007

Indocyanine green angiography

Indocyanine Green Angiography

Background Similar to fluorescein angiography except that indocyanine green (ICG) absorbs and emits in near infrared, and is albumin bound such that it does not leak freely from the choroidal capillaries. The use of infrared increases transmittance through the RPE, haemorrhage, and exudates, relative to fluorescein angiography. ICG can therefore image choroidal disease and haemorrhagic neovascularization. It is particularly helpful for idiopathic polypoidal choroidal vasculopathy (IPCV), acute multifocal placoid pigment epitheliopathy (AMPPE) (Fig. 10.10), and multiple evanescent white dot syndrome (MEWDS).

Method

■ICG angiography can be performed just before or after fluorescein angiography but ask if it will alter management or if other tests would suffice.

■Exclude : uraemia; liver disease; pregnancy; allergy to ICG, iodine, and seafood.

■Obtain written consent.

■Indicate to the photographer the area of interest and presumed diagnosis.

■Dissolve 50 mg of ICG in the solvent provided.

■Place an intravenous cannula in the antecubital fossa. Some clinicians use a 23-guage cannula, others prefer a Venflon for more secure access.

■Inject as a rapid bolus but watch for extravasation.

■Request late photographs, as some abnormalities are not evident for 20–30 minutes.

■Retain i.v. access in case of anaphylaxis.

A

B

C

Fig. 10.10: AMPPE. Multiple areas of choriocapillary hypoperfusion are visible on the ICG (B) but not the fluorescein angiogram (C).

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Autofluorescence imaging

Autofluorescence Imaging

Background Autofluorescence imaging detects the lipofuscin in the RPE as a measure of its metabolic activity. This in turn is driven by photoreceptor outer segment renewal and is a balance between accumulation and clearance. Lipofuscin increases progressively with age and is greatest at 10 degrees of eccentricity. The level of autofluorescence is raised if there is RPE dysfunction or increased metabolic load, and is decreased with photoreceptor cell loss. Various disorders give rise to characteristic changes, and this technique may be the only method of defining abnormalities such as those in bull’s-eye dystrophies. It can also be used to verify the integrity of the RPE.

Methods Images can be obtained with an argon blue laser (488 nm) for excitation and a wide band-pass barrier filter with a wavelength cut off at 500–521 nm extending beyond 650 nm for the longer wavelengths. In most centres, a confocal laser ophthalmoscope is used which ensures that the autofluorescence recorded is derived from the ocular fundus (Fig. 10.11).

A

B

C

Fig. 10.11: Autofluorescence image of age-related macular degeneration with normal (C) for comparison.

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Electrophysiology

Electrophysiology

Background Electrophysiology is an important diagnostic tool for many retinal, optic nerve, and functional eye diseases. Full electrodiagnostic testing is expensive, so ensure it is clinically indicated. Provide staff with a clinical summary including VA, refraction, history, the results of fundus examination and differential diagnosis.

■Pattern electroretinogram (PERG)

1.Indications : disease of the macula or optic nerve.

2.Method : a recording electrode is placed on the cornea or conjunctiva (if using gold foil, anaesthetic is usually not required) and a reference electrode at the outer canthus. The standard stimulus is a high-contrast reversing checkerboard of low temporal frequency, and size 10 to 16 degrees. Defocus produces a poor waveform, so good refractive correction is important.

3.Key points : main measurements are P50 and N95. N95 arises in the retinal ganglion cells. P50 arises in part from the retinal ganglion cells and in part from other inner retinal neurones, but is driven by the macular photoreceptors and acts as an objective index of macular function. In the presence of an abnormal VEP (see below), an abnormal P50 localizes the disease to the macula and an abnormality confined to N95 localizes the disease to the optic nerve/retinal ganglion cells. The PERG is used in association with the (full-field) ERG to better characterize retinal function.

■Full-field electroretinogram (ERG)

1.Indications : generalized retinal dysfunction.

2.Method : a recording electrode is placed on the ocular surface, a reference at the outer canthus, and ground on the forehead. Stimulus protocols are designed to discriminate rod and cone function: the rod-specific response is obtained under dark adaptation using a dim white flash and consists of a positive-going b-wave arising in the inner nuclear layer. A bright white flash then gives a mixed rod–cone response (dominated by rods) with a large negative-going a-wave, the first 10–12 ms of which reflects photoreceptor function; the oscillatory potentials (small wavelets superimposed on the ascending limb of the b-wave) arise from the amacrine cells but have limited

clinical value. After restoration of photopic conditions, cone function is recorded using both a 30 Hz flicker stimulus and single white flashes, both superimposed upon a rod-saturating background.

3.Key points : rod–cone dystrophies (e.g. retinitis pigmentosa) show severely decreased rod b-wave amplitude with reduction in the mixed-response a-wave confirming photoreceptor disease. Cone ERGs are less severely affected but characteristically show both delay and amplitude reduction. Cone–rod dystrophies show the cone ERGs to be more affected than the rod ERGs. Inner retinal dysfunction, such as X-linked retinoschisis or X- linked congenital stationary night blindness shows an electronegative ERG in which the rod-specific ERG is subnormal, but there is preservation of the photoreceptorderived mixed response a-wave with selective b-wave reduction such that the a-wave is larger than the b-wave. This is known as a ‘negative’ ERG because the waveform is dominated by the negative a-wave, and indicates dysfunction post-transduction (Fig. 10.12).

■Multifocal electroretinogram (mfERG)

1.Indications : assessing spatial function of central retinal cones.

2.Method : multiple hexagons are presented on a monitor which flash on and off according to a pseudorandom binary sequence. The waveforms are produced by mathematical calculation of the individual responses to each of the individual hexagons.

3.Key points : newer technique, highly dependent upon accurate fixation by the patient. May have some use in focal macular disease.

■Electro-oculogram (EOG)

1.Indications : diagnosis of Best’s vitelliform maculopathy (p. 503). Also useful in the diagnosis of AZOOR (acute zonal occult outer retinopathy).

2.Method : electrodes are placed at the lateral and medial canthi with a ground lead on the forehead. The cornea is positive relative to the retina so when the patient looks left to right between two fixed targets, a potential difference is recorded between the inner and outer canthal electrodes. The amplitude of the potential reaches a minimum under

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Electrophysiology

ratio of the light peak to the dark trough is known as the Arden index, and is usually >180% in normals.

3.Key points : the response is generated by the interaction between the RPE and the retina. The degree of EOG light rise reduction is often related to the degree of rod ERG abnormality, but in Best’s disease there is a normal ERG but severe reduction in the EOG light rise.

■Visually-evoked potential (VEP)

1.Indications : these include: optic nerve disease; diagnosis of albinism; to assess visual potential in patients with media opacity; estimation of visual function in conjunction with ERG in infants and children; functional visual loss.

2.Method : active electrodes are placed in the occipital areas. The reference may be midfrontal, or the ipsilateral sylvian area. The stimulus is a flash and pattern. The waveform is recorded monocularly. The normal pattern

reversal response has a prominent major positive component at ≈100 ms, the P100.

3.Key points :

a.Demyelination : shows a delayed P100, with less effect on amplitude and waveform.

b.Albinism is associated with misrouting of fibres in the optic chiasm so that there is a contralateral predominance in pattern appearance VEP such that the largest and earliest response from both right and left eyes occurs over the contralateral hemisphere.

c.Media opacities : flash VEP is usually minimally affected, and so may be useful in detecting underlying maculopathy or optic neuropathy.

d.Cautionary note : macular disease also causes VEP delay, and a delayed VEP in itself should never be assumed to reflect optic nerve disease unless there has been a measure of macular function obtained with mfERG or preferably PERG.

Recommended reading: Fishman GA, et al. Electrophysiologic testing in disorders of the retina, optic nerve, and visual pathway, 2nd edn. Ophthalmology monograph 2. San Francisco: AAO; 2001.