What is OCT?
It stands for optical coherence tomography and is based on the imaging of reflected light. Unlike a camera, it can resolve depth, at a very fine level. The resolution can be down to 0.01mm and provides us with a very nice cross-sectional view of the retina.
How does it work?
A beam of infrared light is scanned across the retina. The machine collects the reflected light and measures its “time of flight” delay. That is, light coming from deeper layers takes longer to return compared to light from superficial layers. The light does not interfere with or affect the retina. The various reflections are then built up into an image with computer generated colours. It is not a photography nor an x-ray.
What does it tell us?
OCT reveals structural or anatomical changes in the retina. It does not assess the function of the retina although conclusions can be made based on the OCT. There are many conditions that cause anatomical changes in the retina, particularly macular degeneration, but also macular holes and macular fibrosis. OCT is very useful in monitoring treatment of macular degeneration in those patients having injections of Lucentis or Avastin. OCT also can assess the health of the nerves of the eye and this is useful in managing patients with glaucoma. Scans can be done to look specifically at the nerve fibres in the retina and also at the structure of the optic nerve. The optic nerve is the cable that links the eye to the brain.
How is OCT done?
This part is really easy. All the patient has to do is look at a yellow-green spot inside the machine and technology does the rest.
It is possible to takes photos of various parts of the eye including the cornea, lens and retina. A special camera is needed to photograph the retina inside the eye. The photos are taken using a bright flash and processed digitally by computer. You can often view the images on the computer screen.
These images allow the doctor to assess macular degeneration, diabetes, glaucoma, vein blockages and other conditions. The photos also make a good record against which future changes can be calibrated.
FFA (Fluorescein Angiography)
What is FFA?
This is a diagnostic test whereby special photographs are taken of the eye to give the doctors more information about the condition of the back of the eye and to decide on the best form of treatment.
Fluorescein is a yellow dye injected into a vein in the arm or hand. The dye travels in the blood stream to the eye and photographs are taken during its passage through the eye’s circulation. The test demonstrates abnormalities in retinal blood vessels and aids in diagnosis and treatment of many eye conditions, especially macular degeneration, diabetic retinopathy, retinal vein blockages.
Photos are taken with a special camera and processed digitally onto computer. Results are available immediately.
Associated risks with FFA
Fluorescein angiography may be associated with any of the following risks:
- Yellow colour change of skin and urine – this affects everyone and lasts 24 hours.
- Nausea – may develop 30 seconds or so after injection, but often passes quite quickly.
- Vomiting – rarely follows nausea and again resolves quickly.
- Fainting or dizziness
- Temporary dazzling
- Temporary rose-coloured vision
- Allergy reactions: These are very rare. The most common allergy reaction is an itchy skin rash known as hives. In very exceptional circumstances severe and life-threatening allergy reactions can occur resulting in hospitalisation or even rarely in death. (Risk less than 1 in 50,000).
You must not have fluorescein angiography if you are pregnant – please tell your Doctor.
Please inform your Doctor if you have had any reaction to fluorescein angiography on a previous occasion.
Visual field analysis is a very sensitive way of assessing your side vision. This procedure is commonly used to monitor glaucoma but is also useful in a wide variety of eye disorders, particularly those affecting the optic nerve or retina. The procedure takes about 7 minutes per eye, is painless, and merely requires the patient to press a button when they see a flash of light. You do not require eyedrops for this test. Glaucoma sufferers are typically tested in this way every 6-12 months.
If you are having cataract surgery you will need to have the eyes measured. The reason is that when the cataract is removed, a new acrylic lens will be placed inside your eye. The lens is customised for each eye so the doctor needs to know all about your eye.
Measurements of the outside shape of the eye are taken at first. This determines how much astigmatism (eye out of round) is present. Next the diameter of the eyeball is assessed using ultrasound. This requires the use of a small eye bath and some water. It’s a bit wet! With these numbers, a computer will estimate what lens is suitable for your eyes. In the majority of cases the doctor is able to implant a lens to very accurately correct for long distance. Equally, corrections to give reading vision can be done in some cases. Astigmatism can also be corrected.
Electroretinography is a highly sophisticated method of objectively assessing the function of the retina. Using standardised light stimuli, tiny electrical currents generated by your retina can be measured using powerful computers. These currents are amplified and recorded onto the screen, giving a waveform, which is then analysed. There are two types of ERG, the full field and the multifocal. Full field ERG gives a measure of the combined response of rod and cone cells. Multifocal ERG examines only the cone cells. ERG testing can take up to 1-2 hours and often requires a period of dark adaptation in which the patient must remain in a darkened room for 20 minutes. Eyedrops are required. The procedure is painless.