Of the five senses, our eyes are the most important. Our eyes are responsible for providing nearly 80% of information we receive on a daily basis. Sight is such a strong sense that it can inform the interpretation of the other senses as well (i.e. a meal seems to taste better when it is visually presented in an appealing way, a material looking “rough” has us making assumptions on what it will feel like before we touch it). Our sight helps us socialize, recognize verbal tones by observing facial cues, recognize patterns, and helps us with movement and speed. We evolved with two eyes looking forward for a reason.
When working as a pair, our eyes give us the ability to see great visual detail and a wider field of view, allowing us to see and interpret more of our surroundings. Having two eyes allows us to see objects with accurate “3D” vision, as we are able to see more of the front and sides of object in question and subconsciously determine depth and volume. With two eyes we enjoy precise depth perception and the ability to determine the distance of objects in relation to us and each other.
It is easier to think of our eyes as two cameras, each placed side by side but looking at the same point in space. Because the cameras cannot be in the exact same place, they will have slightly different views of what they are looking at. The cameras will record and send their images out, and if we think of our brain as an “editor” then our brain is responsible for merging the two images together into one clear, highly detailed three-dimensional image. This is also a simplified explanation for how those 3D blockbusters we all enjoy are made.
What Is Diplopia?
But what happens when one camera is slightly misaligned, looking in the wrong place, or unable to adjust to look in the correct direction? What happens when the editor receives two images that are too different from each other to merge properly?
What commonly happens is a breakdown of that detailed vision we have evolved to rely so heavily on. One of the most common complaints from this breakdown is double vision, also known as diplopia. Depending on the severity of the diplopia it may be harder to determine the exact size, shape and location of the object you are looking at in relation to you. Diplopia can make it difficult or near impossible to read words on a page, but differs from the more common experience of blurry vision that can be corrected with near vision glasses.
There can be several reasons why the eyes may not be able to line up properly with one another, or for the brain to struggle with merging the two images together, from problems of focusing light on the right part of the retina to problems with the muscles that control eye movement. Determining the root cause of diplopia is key to prescribing the optimal solution, but one of the most common ways of correcting it is through the use of prism lenses.
What Are Prism Lenses?
Prisms are commonly recognized as clear objects shaped like a triangle or wedge that can refract light in specific ways. One of the most popular learning experiences involving prisms is observing how they can split white light into all the colours of the visual spectrum. But instead of scattering light, other prisms can bend and focus light to a specific point, altering the path light travels so that it hits a different spot instead. These are the types of prisms that can be used for ocular correction and assistance with diplopia.
Prism lenses made for glasses will appear with one side on either the vertical or horizontal plane thinner than normal, and the opposite side thicker than normal, creating the impression of that typical triangle shape. By adjusting the angle at which the thinner and thicker sides are, prism lenses can bend all light passing through them in different directions: upwards (superior), downwards (inferior), in towards the nose (nasally) or out towards your ears (temporally). Light can also be bent towards the diagonal directions, although this is less commonly required.
When noted on a prescription, prism is indicated with a power in diopters, much like in the power (SPH) or cylinder (CLY) columns, although will lack a “+” or “-” indicator. The direction of the base (that thicker side of the lens) would be indicated after the numerical strength of the prism. For example, a right eye prescription with “2D BO” means the right eye requires 2 diopters of base out prism. Prism can be required in just one eye, or both eyes, and can be made with a single vision prescription or multifocal prescription.
But how do prism lenses help with diplopia?
What Causes Diplopia In The First Place?
There can be many underlying causes for persistent diplopia. Refractive prescriptions presenting with aniseikonia (where differences in the refractive power required for each eye makes perceived objects larger or smaller in comparison) and anisometropia (where the refractive power required for each eye differs greatly, including instances where one eye requires “plus” correction and the other requires “minus” correction) can induce instances of diplopia, and usually requires an adjusted prescription to accommodate for it or for prism lenses to be prescribed as well.
Ocular conditions such as cataracts, keratoconus and strabismus can in some cases also benefit from the use of prism lenses, but often require other therapies and treatment options such as surgery to treat the underlying cause. In patients diagnosed with strabismus, prism lenses may still be required post surgery.
Other conditions that are not necessarily related to the ocular system may still have an impact on the quality of your vision, and may induce instances of diplopia. Diabetes is a well known condition that can affect the eyes greatly if blood glucose levels are not properly managed on a day to day basis. While a diabetic patient experiencing diplopia will benefit from prism lenses, until their blood glucose levels are under control their prescriptions will be unstable and can change rapidly in a short amount of time. The preferred treatment in this case would aim to lower and control the blood glucose levels. If the diplopia is still present after the prescription has stabilized, then options such as prism lenses can be explored.
What Are Fresnel Prisms?
Another condition that can cause diplopia is physical trauma to the head or upper body. Injuries such as concussions can have a large and lasting impact on the brain, and given how our sight is a result of our brain interpreting signals and images received by the eyes, then it is no surprise a concussion can impact how well we can see. Whether it is a new difficulty in controlling the muscles that control the movements of the eyes, or if it is confusion in merging those two aforementioned images together, trauma-induced diplopia requires correction just as refractive diplopia does.
Diplopia caused by trauma is not typically permanent, but can be hard by doctors to predict when the diplopia symptoms will resolve themselves as the brain heals. To this end, temporary prisms are utilized as the prescription or power requirement might fluctuate greatly in a short amount of time and it would be too costly for the patient to purchase new prescription lenses for each change.
A Fresnel prism is a plastic sheet designed to stick to the patient’s lenses, with ridges cut into the surface to induce the refracting prismatic effect. The sheets come premade with different strengths, and need only be cut to the shape of the patient’s glasses. They are attached to the back of the patient’s lenses in the direction required to shift the direction of the image (base up, base down, base in, base out as mentioned before). While a Fresnel prism has less aesthetic qualities when compared to regular prism lenses, as the lined ridges cut in the plastic are visible if looking at a patient wearing them, they excel as a temporary measure to correct diplopia because they can be removed and interchanged easily should the prismatic power need adjusting or the diplopia resolves altogether.
I Have Prism, Who Should I Speak To?
If you have been experiencing frequent periods of diplopia, it is always best to speak with your general practitioner and optometrist in order to determine the root cause. Once the root cause is discovered, options to treat the symptoms or the underlying condition can be explored. This may result in prism lenses being prescribed as a part of your glasses prescription.
Prism lenses are more specialized than normal refractive lenses, and take a great deal of care to fit properly. It is always important to discuss prism lenses with a licensed professional should you find it noted down on your prescription. In order to work effectively and not exasperate existing diplopia symptoms, the lenses must be fitted precisely and the frames adjusted carefully to sit in the optimal spot centered in front of the patient’s eyes. This requires more than just a pupilairy distance measurement, other specific measurements can include noting the height of the pupil centre from the bottom of the frame, the distance the frame sits from the patient’s eyes and the amount of forward or pantoscopic tilt the frame has when it rests on the patient’s face. It is often suggested that the frames themselves be chosen with care and consideration for the type of lenses that will be fitted in them. On average prism lenses are more costly compared to regular single vision or multifocal lenses, so it is important to go over all of your options with your eye care team.