SN7 SHORT NOTE 5M

Describe the investigations used in retinal disease.

Ophthalmology · 3rd Professional

Angiography

Fluorescein angiography (FFA) — sodium fluorescein is injected intravenously and photographed with blue excitation and a yellow-green barrier filter. It is largely protein-unbound and therefore leaks from abnormal retinal vessels, but does not cross the intact blood-retinal barrier — which is exactly what makes it useful
Phases — the choroidal flush at 10 to 15 seconds (a patchy background fill); the arterial phase; the arteriovenous (capillary) phase, which shows the perifoveal capillary net best; the venous phase with laminar then complete filling; and the late (recirculation) phase at 5 to 10 minutes, in which leakage and staining are judged
HYPERfluorescence has four causes, and separating them is the whole of interpretation: a window (transmission) defect, where pigment epithelial loss reveals background choroidal fluorescence — it appears early, follows the choroidal filling and does not enlarge or intensify; leakage, which increases in size and intensity with time, as in neovascularisation and oedema; pooling into an anatomical space such as a pigment epithelial detachment; and staining of tissue such as a scar or drusen

Imaging and Electrophysiology

Investigation	Principle and use
Optical coherence tomography (OCT)	Low-coherence interferometry, giving cross-sectional images of the retina at near-histological resolution. Non-invasive, quick and repeatable; it has become the single most used investigation in ophthalmology. Demonstrates macular oedema, subretinal fluid, macular holes, epiretinal membranes, and the retinal nerve fibre layer in glaucoma; and guides retreatment with anti-VEGF
OCT angiography	Detects motion contrast from flowing red cells to map the retinal and choroidal circulations without any dye. Non-invasive and repeatable, and shows the capillary layers separately; but it has a small field and does not show leakage, so it complements rather than replaces fluorescein angiography
B-scan ultrasound	Essential where the media are opaque — dense cataract, vitreous haemorrhage, corneal opacity. Demonstrates retinal detachment, tumours (with calcification in retinoblastoma and low reflectivity in melanoma), foreign bodies, and optic disc drusen
Fundus autofluorescence	Images lipofuscin in the pigment epithelium without dye; maps geographic atrophy and dystrophies, and shows metabolic stress at lesion borders
Electroretinogram (ERG)	A mass response of the whole retina. The a-wave is negative and arises from the photoreceptors; the b-wave is positive and arises from the bipolar and MULLER cells. Scotopic testing assesses rods and photopic (and flicker) the cones. Definitive in retinitis pigmentosa, in which the scotopic response is reduced before symptoms; and in siderosis, where it monitors toxicity. A normal ERG with poor vision points away from a generalised retinal dystrophy
Multifocal ERG	Maps localised retinal function, particularly at the macula.
Electro-oculogram (EOG)	Measures the standing potential across the pigment epithelium, expressed as the ARDEN ratio of light peak to dark trough. Characteristically abnormal with a normal ERG in best vitelliform dystrophy.
Visually evoked potential (VEP)	Assesses the whole visual pathway from retina to occipital cortex. Used in optic neuritis (delayed latency), in demyelination, in albinism (showing chiasmal misrouting), and to estimate acuity in infants and in suspected non-organic visual loss

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