GLASSES & rehabilitation

Often, Optometrists may not prescribe glasses for small amounts of far-sightedness, near-sightedness, or astigmatism, especially if the patient views the world clear enough for their daily function. However, these small amounts are often crucial in a patient who has suffered neurological insult, such as a head injury. For one, the refractive error allows for improved contrast sensitivity by increasing the clarity and boldness of the letters.

Additionally, patients with head injury often experience sensory overload. Therefore, they can experience increased sensitivity to refractive error. Put another way, the smallest amount of refractive error can allow for improvements in subjective symptoms. This can include correcting mild amounts of far-sightedness, near-sightedness, or astigmatism, or a small change in glasses prescription that is already worn.

More importantly, correcting the refractive error can allow for improved visual balance which can lead to improved functioning of the binocular system – that is, the ability for both eyes to work cooperatively together at the (automatic) behests of the brain. Visual balance allows for balance of the accommodative system, which reduces disruption in binocular system.

It is common for more than one pair of glasses to be prescribed in these instances. For younger patients, this can include a pair of glasses for full-time wear indoors, and sunglasses for outdoor use. For older patient, multifocals are usually avoided. The lower portion of the lens in multifocals are allocated for near vision and can further impact balance while the patient is walking, therefore increasing risk of falling. For this reason, separate distance and reading glasses are often prescribed for indoor use in addition to sunglasses for outdoor use.

Prisms work by deviating the path of light. It is often used for four different reasons.

1. Injury to the nerves that are responsible for controlling eye muscles can be impacted during head injury. This can cause muscle imbalance, leading to an eye being deviated inwards, outwards, upwards, or downwards. A prism can be used to compensate for these misalignments. The placement of the prism allows the light to be deviated and fall onto the retina in such a way that both eyes receive the appropriate image of the object they would have otherwise seen if it were not for misalignment. Therefore, the eye muscles are no longer straining to correct the skewed vision.
2. Head injury can also impact convergence and divergence ability, leading to a myriad of symptoms. A prism can be used in these instances to reduce the demand to either converge or diverge, and therefore enhance performance. For example, in convergence insufficiency, there is difficulty with ability to converge adequately at near. A prism in this case can be used to reduce the demand of converging at near, therefore improving ability to complete near tasks (i.e., tasks you do that require to look nearby) – such as reading or writing – and reduce associated symptoms.
3. Head injury can impact our ability to process visual space and can often lead to what is known as visual collapse. In a previous article, we had discussed the focal and ambient processes of vision. Due to sensory overload, our visual system may have not process both the focal and ambient systems as per usual. Prisms can be used in these cases to reduce visual collapse, improving peripheral awareness, and therefore increase interaction between the focal and ambient processes of vision.
4. Head injury can also cause a visual midline shift, a concept that we discussed in our article titled Post-Concussion Vision Syndrome. Prism can be used for these patients to improve their sense of position in space, allowing for improved gait and perception of their visual environment.

The following is a video showing the impact that can occur from glasses containing prism in a patient with mTBI: https://www.youtube.com/watch?v=WxMdwlxr5vM

Head injury can impact the function of the eye’s lens, and therefore cause difficulty with accommodation. Low powered plus lenses (glasses that magnify print) can be prescribed in glasses to reduce the demand to accommodate (flex the eye’s lens). This can ultimately lead to improved ability to complete near-associated tasks, such as read or writing, and also improve the ability to change visual focus between distance and near.

Due to sensory overload following head injury, patients can often experience light sensitivity. Often, a mild tint can be prescribed for indoor use to relieve symptoms experienced from fluorescent lighting and background lighting from technology (such as phones, computers, and TV). Sunglasses can be prescribed for outdoor use to relieve light sensitivity experienced from outdoor lighting.

Sometimes, placing tape on the inner portion of lenses can help reduce visual detail that worsens sensory overload, kind of like turning down the noise, by separating the area of vision where the images from our two eyes overlap. By separating the visual worlds of both eyes, they can now work independently instead of struggling to work together. This can also help the visual system focus more on the peripheral, or ambient, visual system which helps us organize the “where” of space, rather than focusing on our central visual system, or “what” pathway. For more information on the ambient and focal pathways, please refer to our VVM article.

The short answer is yes. This depends on a case-by-case basis, however as the visual system improves, any “crutches” that were previously prescribed, such as prism, low plus, or tint, will likely be removed. In some cases, multiple prescription changes are required through the course of rehabilitation, whereas others may require one simple change at the midway or at the end of their rehabilitation.