Prism correction

In normal conditions, when there is parallelism of the visual axes and normal binocular vision, the images perceived by each eye merge in the visual brain, giving way to an image with three-dimensional characteristics.

When there is a deviation, the image of the object of interest falls in different points of the retina of each eye, leading to diplopia or double vision.

A prism is a "special" triangle-shaped eyeglass (it is formed by two non-parallel sides that together form an angle) that deviates or redirects the image of the object towards its vertex, making it "fall" correctly on the retina of each eye. This means the brain perceives just one image and the patient will not have double vision.  

Due to the fact that diplopia or double vision may be a sign of a base pathology, it is fundamental to undergo a full exploration by a specialist to determine the cause of double vision.

Once the cause has been determined, when the case is stable (for more than 6 months) it is possible to prescribe prism correction. The prisms are used in the treatment of minor, stable deviaions causing double vision in the primary eye position (looking straight ahead).

Likewise, the prisms are also useful for some patients with nystagmus (involuntary shaking of the eyes) who have mild torticollis, to block the shaking, and in some cases of phoria (latent strabismuses) that cause symptoms of asthenopia (visual fatigue, headache and blurred vision). 

It is important to remember that the aim of the prism is not to correct the deviation, but rather deviate the image to prevent double vision. Therefore, the prism will not correct the base problem or the aesthetic component that leads to the strabismus.

The prism is measured and prescribed using a measuring unit called prism dioptres. Depending on the magnitude and type of deviation and the double vision of the patient (vertical, horizontal or mixed), the prism is placed in one direction or another and in one or both lenses. 

It is not always easy to determine the power of the prism. Some patients will need the prism to be put in trial frames and they will be asked to keep them on for half an hour to see if they really do alleviate their symptoms. 

Generally, the prism is cut out as part of its manufacture. In this way, the lenses incorporated into the prism correction have the same appearance although sometimes the glass is slightly thicker. 

The prism is placed in the eye where the deviation is detected, as long as it is small. If the prism required is of a higher power, it is generally recommended that it is shared between the two eyes. 

On occasions, the patients will need different prism correction for nearsight and farsight. In these cases, two different glasses should be used instead of multifocal lenses.

In some cases, a Fresnel prism is prescribed. They are adhesive strips that are placed on the glasses lens. Their main characteristic is that they correct deviations of a larger magnitude than conventional prisms.

They are not an ideal solution for long-term use, but they are useful in certain cases to assess the risk of postoperative diplopia (double vision) in patients that are to undergo strabismus surgery or even to determine degree of compensatory torticollis that we will be able to correct with the operation. 

They are also an option for patients with large deviations who do not want to undergo surgical treatment and in cases where the deviation is still not stable and needs changed over a short time. The main disadvantage is that they slightly distort the eyesight, plus they do not fix the base problem, which is the strabismus.