Fundus Imaging & Peripheral Retinal Exam Technique

Fundus Fluorescein Angiography

Fundus fluorescein angiography (FFA) images the retinal and choroidal circulation after an intravenous injection of sodium fluorescein. A blue excitation filter, roughly 490 nm, illuminates the fundus; a yellow-green barrier filter, roughly 530 nm, passes only the light the dye itself emits and blocks the reflected excitation light. That filter pair is what gives FFA its high-contrast vascular images. Leakage, staining, blockage, and non-perfusion are all read the same way, as a deviation from the dye's normal transit pattern.

Ultra-Widefield Scanning Laser Imaging

Ultra-widefield systems such as Optos build a scanning laser ophthalmoscope around an ellipsoidal mirror, which places a virtual pivot point inside the patient's pupil. As the laser beam sweeps it pivots through that virtual point, capturing up to roughly 200° of the fundus in a single image, often without pupil dilation. A single widefield frame can document peripheral retinal disease that would otherwise take several overlapping BIO exams to piece together.

Scleral Depression & the Peripheral Exam

Scleral indentation, performed during binocular indirect ophthalmoscopy, pushes the peripheral retina centrally into the visible field. That is what brings the vitreous base and ora serrata into view; neither structure is reliably visible without it. The technique is essential when working up retinal tears, lattice degeneration, and other far-peripheral lesions, since the unaided BIO view stops short of that zone.

Panretinal Photocoagulation Delivery

Panretinal photocoagulation (PRP) ablates ischaemic peripheral retina to treat proliferative diabetic retinopathy, and it can be delivered through a slit-lamp contact lens or through an indirect-ophthalmoscope-mounted laser. The indirect system's advantage is reach: it treats extreme peripheral retina, including zones that need scleral indentation just to see, that a slit-lamp contact lens cannot access at all. That reach is why BIO laser delivery is preferred for treating retinopathy of prematurity (ROP) and for peripheral PRP through small, poorly dilating pupils.

Optic Disc Cup-to-Disc Assessment

The vertical cup-to-disc (C:D) ratio is checked on every fundus exam. No single number defines abnormal, since the normal range varies widely across the population, but a vertical C:D ratio above roughly 0.6, or an inter-eye asymmetry greater than 0.2, raises suspicion for glaucomatous optic nerve damage. Either finding should prompt IOP measurement, disc photography, and formal visual field testing.

Macular Contour & the Cherry-Red Spot

Two findings show why stereoscopic viewing matters at the fundus. The foveal light reflex, normally a bright central glint off the foveal pit, dims or disappears when macular oedema or an epiretinal membrane distorts the foveal contour. Picking up that loss reliably takes a stereoscopic view, a non-contact slit-lamp lens or a contact fundus lens, not a flat monocular one.

The cherry-red spot of central retinal artery occlusion (CRAO) works by anatomy rather than contour. The inner retina at the fovea is normally very thin and has no ganglion cells, so the choroidal circulation underneath shows through as a red glow even in an otherwise ischemic eye. The surrounding retina turns pale and opaque as its ganglion cell axons swell from ischemia. That contrast, a red fovea against a whitened retina, is the cherry-red spot, characteristic of CRAO, though lipid storage diseases such as Tay-Sachs can produce a similar spot. CRAO is an ophthalmic emergency. The inner retina tolerates ischemia for only about 90–100 minutes before damage becomes irreversible, and current guidance places the window for reperfusion or thrombolytic measures at roughly 4.5 hours from symptom onset, the same threshold used in acute stroke management. Atherosclerotic plaque from the ipsilateral carotid artery is the most common embolic source, so a CRAO diagnosis should prompt urgent carotid and cardiovascular workup, not ocular management alone.

High-Yield Pearls

• FFA: blue excitation in (~490 nm), yellow-green emission out (~530 nm); the barrier filter is what makes the image high-contrast.
• Ultra-widefield SLO imaging (Optos) reaches ~200° via an ellipsoidal mirror with a virtual pupil-plane pivot, often without dilation.
• BIO laser delivery reaches periphery a slit-lamp contact lens cannot: the key reason it's used for ROP and peripheral PRP.
• Vertical C:D >0.6 or >0.2 inter-eye asymmetry warrants a glaucoma workup, even though there's no single universal "abnormal" number.
• CRAO: ~90–100 minutes of retinal ischemic tolerance, ~4.5-hour treatment window, carotid atherosclerosis is the most common embolic source.

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