Introduction

Fluorescein angiography is a fundal photography, performed in rapid sequence following intravenous injection of fluorescein dye. It provides following information’s:

1.     The blood flow characteristics in the vessels as the dye reaches  and circulates through the retina and choroid

2.     It provides fine details of the retinal circulation and pigment epithelium that may not otherwise be visible.

3.     It records functional integrity retinal vessels, so their assessment can be done.

Fluorescein

• Sodium fluorescein is an organic dye.

• Sodium fluorescein (C20H10O5Na2) has molecular weight of 376 Daltons.

• It is 80% bound to plasma albumin. The remaining 20% is seen during angiography

• The Sodium fluorescein absorbs light in the blue range of the visible spectrum, with absorption peaking at 490nm (blue). It emits light at 530nm (yellow).

• It is metabolized by the liver and excreted by the kidneys. Most dye is cleared with 24 hours and patients should be warned that their urine will appear orange during this time.

Physiology

There are two circulations within the fundus:

a.      Choroidal circulation -
The fluorescein freely leaks out of the fenestrated choroidal capillaries, and from there through Bruch's membrane. However, tight junctions between retinal pigment epithelium (RPE) cells prevent dye reaching the retina.

b.     Retinal circulation - 
The retinal blood vessel endothelial cells are joined by tight junctions which prevent leakage of fluorescein into the retina. This constitutes the blood retina barrier. Any leakage from the retinal vessels is considered as an abnormal.

Fluorescein cannot diffuse through tight cellular junctions. These are present at two sites within the fundus:

1. Retinal blood vessel endothelium

2. Retinal pigment epithelium.

Phases Of Fluorescein Angiography

n     Choroidal phase - Choroidal filling via the short ciliary arteries results in initial patching filing of lobules, very quickly followed by a diffuse (blush) as dye leaks out of the choroidocapillaris. Cilioretinal vessels and prelaminar optic disc capillaries fill during this phase

n      Arterial phase - The central retinal artery fills about 1 second later than choroidal filling. So dye is only visible in central retinal artery but not in retinal veins.

n      Capillary phase- The capillaries quickly fill following the arterial phase. The perifoveal capillary network is particular prominent as the underlying choroidal circulation is masked by luteal pigment in the retina and melanin pigment in the RPE. At the centre of this capillary ring is the foveal avascular zone 500um in diameter

n     Venous phase - Early filling of the veins is from tributaries joining their margins, resulting in a tramline effect. Later the whole diameter of the veins is filled.

n     Late phase - After 10 to 15 minutes little dye remains within the blood circulation. Dye which has left the blood to ocular structures is particularly visible during this phase.

Arteial Phase

Early Venous Phase

Late Venous Phase

Pooling of dye in CSR(Central Serous Choroidoretinopathy).

Hyper-fluorescence and Hypo-fluorescence (Courtsey: Dr Sudhir Singh)

Window defect due to RPE atrophy (Courtsey: Dr Sudhir Singh)

Dye leaking neo vascularization and aneurysms,Venous beading(Courtsey: Dr Sudhir Singh)

Procedure

n     The patient is advice to sit in front of fundus camera comfortably and take explained written consent.

n     The central retina is focused with fundus camera.

n     One coloured fundus photograph of each eye taken before injecting dye.

n     One photograph of each fundus also taken with exciter filter in place to record pseudo fluorescence and auto fluorescence.

n     5ml of 10% sodium fluorescein dye is injected as a bolus into the vein (preferably antecubital) of the patient's arm or in any other vein of forearm with the help of scalp vein set.

n     The blue filter (excitation filter) of fundus camera turned on. The fundus is viewed through a yellow filter (barrier filter). As blue light cannot pass through a yellow filter in normal circumstances nothing can be seen. However, fluorescein dye within retinal and choroidal blood vessels absorbs blue light and emits yellow light; this yellow light passes through the filter and is photographed. Only tissues that contain fluorescein are visualized.

n     An initial exposure rate of 1 frame (snap) every few seconds (5 seconds) documents the arteriolar and early venous filling phase of the study. Exposures are then made at less frequent intervals until 20 to 25 frames are exposed. Late phase photograph are taken after a pause of 10to 30 minutes.