Principles of fluorescein angiography

1. What is fluorescein angiography?

2. What kind of substance is sodium fluorescein?

3. How is fluorescein angiography performed?

4. What are the side effects of fluorescein angiography?

5. How do ocular structures determine the distribution of fluorescein 

6. What are the phases of a normal fluorescein angiography.

7. List some of the main indications for fluorescein angiography.

1. What is fluorescein angiography?
It is a fundal photography, performed in rapid sequence following intravenous injection of fluorescein dye.
It provides three main information:
a. the flow characteristics in the blood vessels as the dye reaches 
    and circulates through the retina and choroid
b. it records fine details of the pigment epithelium and retinal 
    circulation that may not otherwise be visible
c. give a clear picture of the retinal vessels and assessment of their 
    functional integrity.
2. What kind of substance is sodium fluorescein?
Sodium fluorescein (C20H10O5Na2) is an organic dye. It has a molecular weight of 376 daltons, and is 80% bound to plasma albumin. The remaining 20% is seen during angiography. The dye 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.
3. How is fluorescein angiography performed?
5ml of 10% sodium fluorescein dye is injected as a bolus into the vein (preferably antecubital) of the patient's arm.
The eye is illuminated using blue light produced by a blue filter (excitation filter). 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 contains fluorescein are visualized.

4. What are the side effects of fluorescein angiography?

    The side effects include:
    • Those which are unavoidable such as a temporary tan skin colour form the dye, red after image from the photoflash and discoloration of the urine

    • Nausea and vomiting (10%). Usually transient and no treatment is usually needed.
    • Vasovagal synocope (1%) and no treatment is needed. But in extreme bradycardia, IV atropine may be needed.
    • Anaphylaxis such as bronchospasm, urticarial skin rash and hypotension (<1%). Treatment is with chlorpheniramine (piriton) 10mg IV, hydrocortisone 100mg IV and give oxygen and adrenaline 1ml of 1:1000 IM for hypotension and bronchospasm.
    • Cardiac and respiratory arrest (<0.01%). Treatment would involve cardiopulmonary resuscitation.

5. How do ocular structures determine the distribution of fluorescein angiography?

Fluorescein cannot diffuse through tight cellular junctions. These are present at two sites within the fundus:
    • retinal blood vessel endothelium
    • retinal pigment epithelium.
There are two circulation within the fundus:
    • 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 prevents dye reaching the retina
    • 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 abnormal
Capillaries in the ciliary process re permeable to fluorescein, so dye rapidly appears in the aqueous following intravenous injection. Fluorescein in the aqueous and vitreous emits yellow light which reflects off white structures within the eye causing these structures to falsely appear fluorescent. The optic disc, myelinated fibres and hard exudates appear progressively more pseudofluorescent through the course of an angiogram for this reason.

6. What are the phases of a normal fluorescein angiography?

Normally 10-15 seconds elapse between dye injection and arrival of dye in the short ciliary arteries. Choroidal circulation precedes retinal circulation by 1 second. Transit of dye through the retinal circulation takes approximately 15 to 20 seconds.

Normal angiogram can be divided into five phases (see the pictures below)

1. 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.
2. Arterial phase - 
the central retinal artery fills about 1 second later than choroidal filling
3. 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.
4. 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.
5. 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.

Arterial phase

Early venous phase

Venous phase

Late phase
7. List some of the main indications for fluorescein angiography.
    Fluorescein angiography is used mainly for the study of abnormal ocular vasculature. The following are the main indications for fluorescein angiography:
      Diabetic mellitus: 
      • detecting any significant macular oedema which is not clinically obvious;
      • locating the area of oedema for laser treatment;
      • differentiating ischaemic from exudative diabetic maculoplathy;
      • differentiating between IRMA and new blood vessels if clinical differentiation is difficult.

      Retinal vein occlusion: 

      • determining the integrity of the foveal capillary bed and the extent of macular oedema following branch retinal vein occlusion
      • differentiating collaterals from neovascularization (see pictures below)
      • less commonly it is used purely to determine the extent of retinal ischaemia (as this can be done clinically)

FFA is used to determine if the abnormal blood vessels are collaterals or neovascularization following branch retinal vein occlusion. The absence 
of dye leakage is in favour of collaterals and sectoiral photocoagulation
is therefore not indicated.
      Age-related macular degeneration
      • locate the subretinal neovascularization and determine its suitability for laser treatment.

      Other indications:

      • Locating  subretinal neovascular membrane in various conditions (high myopia, angioid streaks, choroidal rupture and chorioretinitis)
      • Locating abnormal blood vessels (for example idiopathic retinal telangietasia, retinal retinopathy etc)
      • Looking for break down of RPE tight junctions (central serous retinal retinopathy) or the blood retinal barrier (cystoid macular oedema)
      • Help with diagnosis of retinal conditions (for example Stargardt's disease gives a characteristic dark choroid).
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