The OCT (Optical Coherence Tomography) is a non-invasive imaging technique which uses light waves to take cross-section images of the Retina.
In practice we assess the Macula, Optic Nerve and Cornea with the OCT.
Part 1: The Macula- Our central vision.
The Macula is a pigmented area at the centre of the retina. This is the area that we use to read and see fine detail. The Macula is responsible for clear central vision.
The OCT scan provides a view of the Retinal layers using a non-invasive technique. In practice we have found this tool invaluable for the monitoring of dry Macula degeneration and the diagnosis of Wet Macula degeneration.
The benefits of macula OCT stretch beyond Macula degeneration detection and monitoring. Here are a few examples:
- We have seen vitreomacular traction and epiretinal membranes which can affect the level of vision.
- Central serous retinopathy (CSR) can cause temporary vision impairment. This is caused by a build-up of fluid under the retina which can accumulate in the Macula region. This will show up on OCT imaging.
- Macula Oedema post cataract surgery. The OCT is part of our screening process.
- Diabetic changes to the Macula can also be diagnosed and monitored with OCT imaging.
How will changes to the macula affect our vision?
- Difficulty reading
- Difficulty recognising faces
- Faded colours
- Straight lines appear distorted
- In the early stages some patients may be asymptomatic
Vitreomacular traction/Epiretinal membrane:
-Distortion of central vision
-May be asymptomatic
-Difficulty with daily vision related tasks
Macula Oedema/Central Serous Retinopathy:
- Distorted central vision
- Blind spot in central vision
- Early stages some of our patients are asymptomatic
- White objects may appear dull
Regular eye checks are important to ensure any new conditions are picked up early and if deemed necessary referred onwards for investigation.
Part 2: Optic nerve changes and Glaucoma.
Changes in the optic nerve appearance have been found to correlate with vision and visual field loss in glaucoma.
The optic nerve is made of nerve cells. It consists of over one million nerve fibres.
We have a natural blind spot due to the absence of photoreceptor (light-sensitive) cells, in the part of the retina where the optic nerve exits the eye.
A number of conditions can occur which affect the Optic nerve, however this article will focus on Glaucoma.
Glaucoma is one of the most common conditions affecting the optic nerve. Glaucoma can be caused by high intraocular pressure, or high pressure in the fluid that is inside the eye (vitreous humor). This high pressure compresses the optic nerve and damages the cells.
Glaucoma can also occur when the pressure of the eye is not elevated, but changes to the Optic nerve fibres are noted.
Optical Coherence Tomography (OCT) can directly measure the thickness of the nerve fibre layer and create a three dimensional representation of the optic nerve. This image allows us to analyse and monitor changes to the Retinal Nerve fibre layer, detecting any areas of atrophy.
There are 4 main types of Glaucoma:
- Acute glaucoma
This happens quickly following a sudden blockage to the flow of aqueous fluid in the eye. It can be painful and cause permanent damage to your sight if not treated properly.
- Chronic glaucoma
Chronic glaucoma is the more common type of glaucoma. It can develop over many years. Eye pressure rises very slowly and there is no pain but the field of vision gradually becomes impaired. Visual fields are important in the detection of Glaucoma.
- Developmental glaucoma
This is caused by defects in the drainage system in the eye. If parents notice their child has either a cloudy, white, enlarged or protruding eye they should consult a doctor.
- Secondary Glaucoma
Secondary to another eye condition. There may be symptoms of secondary glaucoma following an eye injury, an infection, inflammation, a tumour or an enlarged cataract.
Part 3: The Cornea- Our window
OCT imaging is also used to view then layers of the cornea.
This image can aid in the detection of corneal dystrophies,
The tissues of the cornea are arranged in three basic layers, with two thinner layers, or membranes, between them. Each of these five layers has an important function.
What is the purpose of the Cornea?
The cornea is the eye’s outermost layer. It is the clear and an important role in protecting then eye and focusing your vision.
The cornea and lens of the eye are built to focus light on the retina, which is the light-sensitive tissue at the back of the eye. When light hits the cornea, it bends the incoming light onto the lens. The lens refocuses that light onto the retina, which begins the translation of light into vision. The retina converts light into electrical impulses that travel through the optic nerve to the brain, which interprets them as images.
The cornea also serves as a filter that screens out damaging ultraviolet (UV) light from the sun.
OCT Imaging and the Cornea:
The OCT image allows us to view a cross section of the Cornea.
This can aid in then diagnosis of conditions which may permanently affect the Corneal structure and in turn Visual Acuity. Here are a few examples:
- Corneal Dystrophy
A corneal dystrophy is a condition where the Cornea loses clarity due to a buildup of material.
- Iridocorneal Endothelial Syndrome (ICE)
This is caused by the movement of cells from the cornea to the iris. This loss of cells from the cornea causes corneal swelling and distortion of the iris and pupil. This cell movement also blocks the fluid outflow channels of the eye, which causes secondary glaucoma.
- Dry Eye
The OCT can be used to measure tear volume. This can be used in the diagnosis and monitoring of dry eye.
Other areas we may use corneal thickness measurements:
The OCT is also useful for screening prior to refractive surgery.
During Glaucoma diagnosis Corneal thickness is important because it can mask an accurate reading of eye pressure. Intra Ocular pressure may be underestimated in patients with a thinner Cornea, and overestimated in patients with a thicker Cornea.
The OCT examination is painless and the images are obtained and analysed during the appointment.