Double vision, medically termed diplopia, is the perception of two separate images from a single object. This experience can range from a slight shadow or ghost image to two fully distinct, separated images. Whether glasses can correct diplopia depends entirely on its underlying cause. Specialized corrective lenses are effective for certain types by managing how light enters the eyes, but other instances require medical or surgical intervention beyond a simple prescription.
Distinguishing the Types of Double Vision
Understanding whether double vision originates in one eye or is a result of both eyes failing to work together is the first step in determining treatment. The two primary categories are monocular diplopia and binocular diplopia, which are easily differentiated with a simple test. If the double vision persists even when the unaffected eye is covered, the condition is classified as monocular, meaning the problem lies within the structure of that single eye.
Monocular diplopia is typically caused by structural issues that distort the light pathway before it reaches the retina. Common causes include cataracts, where the eye’s lens has become cloudy, or irregularities in the cornea’s shape, such as keratoconus. Uncorrected refractive errors like astigmatism, which causes light to focus unevenly, can also lead to this type of double vision. Because the issue is within the eye itself, standard glasses or contact lenses that correct the refractive error can sometimes resolve the problem.
Conversely, binocular diplopia is diagnosed when the double image disappears immediately upon covering either eye. This form of double vision is not an issue of light distortion but rather a problem of eye misalignment. It occurs because the two eyes are not pointing at the same target simultaneously, causing each eye to send a slightly different image to the brain. This misalignment is often due to issues affecting the extraocular muscles or the nerves that control them.
How Specialized Lenses Correct Diplopia
Eyeglasses can be effective in treating binocular diplopia, provided the misalignment is stable and measurable. This correction is achieved through specialized eyewear known as prism lenses. A prism is a wedge-shaped component incorporated into the lens that bends the light before it enters the eye. The prism shifts the image seen by the misaligned eye, redirecting the light so it falls on the corresponding spot on the retina in both eyes.
By realigning the images on the retinas, the prism lens allows the brain to successfully fuse the two signals into a single image. The prescription is measured in prism diopters and includes a “base” direction, which indicates the orientation of the prism’s thickest edge. For small, stable amounts of misalignment, the prism is ground directly into the permanent eyeglass lens.
When eye misalignment is fluctuating or temporary, a temporary solution called a Fresnel prism may be used. This is a thin, flexible vinyl sheet covered in tiny ridges that functions like a prism and is pressed onto the surface of a standard lens. Fresnel prisms are often used as a diagnostic tool or a short-term treatment because they are easier to adjust or remove. Standard corrective lenses can also address monocular diplopia when the cause is a basic refractive error by focusing the light correctly onto the retina.
When Medical Intervention is Necessary
While glasses offer a solution for many cases of stable binocular diplopia, they cannot treat the root cause, which may be a serious medical condition. Binocular double vision can signal a problem affecting the cranial nerves that control the eye muscles. Conditions like cranial nerve palsies, often caused by poor circulation due to diabetes or hypertension, can weaken or paralyze eye muscles, leading to misalignment.
The sudden onset of double vision requires immediate medical evaluation, as it can signal a severe neurological event, such as a stroke, aneurysm, or tumor pressing on a nerve. Systemic diseases, including Graves’ disease (an autoimmune condition affecting the thyroid), can cause inflammation and thickening of the eye muscles, leading to chronic misalignment. Myasthenia gravis, a neuromuscular disease, can also produce double vision that fluctuates throughout the day.
If glasses are unsuccessful, or the misalignment is too large or unstable for a prism, non-optical treatments are considered. Vision therapy involves a structured program of eye exercises designed to improve eye coordination. Alternatively, eye muscle surgery may be performed to physically adjust the length or position of the eye muscles, bringing the eyes into better alignment. Botulinum toxin injections can also be used to temporarily weaken an overactive eye muscle, helping to balance muscle tension.