Optical coherence tomography

Before we get into what it is and why a optical coherence tomography We are going to study the organ responsible for vision, which is the The images of our environment penetrate our eyes and are projected on the retina. The retina is a light sensitive tissue; When light falls on its surface, a series of chemical and electrical phenomena are triggered that are translated into nerve impulses that are sent to the brain through the optic nerve. Its thickness is 0.8 mm in its thickest portion and 0.1-0.2 mm in its thinnest. It is made up of several layers of specialized neurons interconnected with each other.

Various areas can be seen on the surface of the retina:

• Papilla or optic disc: it is the point where the optic nerve, arteries and veins enter the eyeball. It is a pink disc that is located at the back of the eyeball. In the papilla there are no photoreceptors, so being a point that does not provide vision is also known as a blind spot.
• Macula: yellowish area in the center of the retina. Its function is to act as a filter for light radiation. At its center is the fovea.
• Fovea: a shallow, depressed area on the surface of the retina. blood vessels surround the fovea, while within it there are only small capillaries. In the very center of the fovea there is an area called the foveolus, an area about 0.5 mm in diameter without capillaries to promote vision.

The only cells that are directly sensitive to light are found in the most superficial layer of the retina and are the rods and cones. The rods work mainly in low light conditions and provide black and white vision, the cones however are adapted to high light situations and provide color vision.

Its main advance consists of the possibility of obtaining high resolution images, both of the surface of the retina and of its different layers. Computerized image processing allows three-dimensional images of the retina to be obtained.

The optical coherence tomography It is based on a complex optical principle called interferometry, which uses an infrared light source that penetrates the eye tissues and splits into several light beams. One of them penetrates the retina and another is captured by a reference mirror. On their return trajectory, both beams collide with each other, generating “interferences” that, when captured by a detector, are translated into a color image that represents and indicates the thickness of those of the tissues studied. Cool colors, such as blue or black, correlate with thinner fabrics and warm colors, such as red or white, with thicker fabrics.

The study of the thickness of the different layers of the retina can be useful for the diagnosis and follow-up of different ones.

How is the optical coherence tomography study performed?

The examination is performed with the patient seated, without requiring anesthesia or sedative medication. No flashes or anesthetics are used. Unlike other ophthalmological examinations, prior dilation of the pupil is not usually necessary. The examination lasts a few minutes.

Preparation for the study

Despite the high technical complexity, it is a simple test to perform, with hardly any discomfort for the patient. It does not require fasting or any other type of preparation. The study is completely painless with no associated adverse effects. Exceptionally, during the exam, discrete eye discomfort accompanied by tearing and conjunctival burning may be perceived, which remit minutes after the study. In those cases in which the use of mydriatric eye drops is required, the most common side effects are blurred vision and glare. Rarely, elevation of intraocular pressure occurs.

Study risks and contraindications

By not using flashes or requiring anesthetics, optical coherence tomography becomes a safe exploratory technique. There are no contraindications to the practice of Optical coherence tomography. Although the use of eye drops to dilate the pupil is not usually required, the use of eye drops should be avoided especially in patients with angle closure.

Reasons why the study is carried out

It is vitally important to diagnose a new vision problem. Sometimes a simple examination with an ophthalmoscope will allow a diagnosis to be made, but in those cases in which said examination is insufficient, it will be necessary to resort to more sensitive techniques. The optical coherence tomography allows measurements of the different areas of the retina and each of its layers. The finding of any abnormality in the observed measurements can be correlated with different eye problems. Once the diagnosis is made, it can be useful to control its evolution and the response to the different treatments applied.

Its use is accepted in various ophthalmological pathologies, such as: age-related macular degeneration, glaucoma, macular edema, diabetic retinopathy, central serous chorioretinopathy and epiretinal membranes.