Double vision, medically known as diplopia, occurs when the eyes fail to align perfectly, causing a person to see two images of a single object. This misalignment in the visual system, often called a binocular vision issue, can lead to severe eye strain, headaches, and a significant reduction in quality of life. To manage this problem, eye care professionals often turn to corrective prisms, which manipulate the path of light entering the eye to fuse the two separate images into one. The core question for many individuals who prefer contact lenses is whether this precise prism correction can be housed within a discreet lens worn directly on the eye.
The Direct Answer: Prism Contact Lenses
Prism contact lenses are not a standard, readily available product like typical vision correction lenses, but they do exist for specialized cases. They are highly customized devices, and their use is substantially less common than prism spectacles. Optical prism intended for double vision correction must be distinguished from the small amounts of prism used only for lens stabilization. Custom soft contact lenses may correct very mild cases of vertical misalignment. Scleral lenses, which are large-diameter rigid gas permeable lenses, offer a more stable platform for moderate correction in any direction. Scleral lenses can correct up to about 10 prism diopters, whereas soft lenses are limited to around 4 diopters of vertical prism.
The Optical Principle of Prism Correction
A prism is an optical wedge that works by bending light before it reaches the eye, effectively shifting the perceived location of an object. This light-bending action differs from the simple focusing power found in standard prescriptions. The prism is oriented with a thick end, known as the base, and a thin end, called the apex. Light passing through the prism always bends toward the base, but the brain perceives the image as shifting toward the apex. This intentional shift allows the eyes to relax and align the two images, achieving comfortable binocular fusion. Prism correction is defined by its power, measured in prism diopters, and the direction of its base.
Horizontal correction uses a base-in or base-out orientation to correct eyes that drift inward or outward. Vertical correction uses a base-up or base-down orientation to address one image appearing higher than the other.
Why Contact Lenses Are Challenging for Prism
The primary difficulty in manufacturing effective prism contact lenses is maintaining rotational stability on the eye. For a prism to work correctly, its base must remain in a precise, fixed position to direct the light path accurately. Unlike spectacle lenses, which are held rigidly in a frame, contact lenses naturally rotate on the eye with blinks and eye movements. If the prism rotates even slightly, the correction will be misaligned, potentially making the double vision worse.
To combat this rotation, contact lenses often use a design called prism-ballast, which incorporates a thicker, heavier area at the bottom of the lens. This weighting uses gravity and eyelid pressure to stabilize the lens. However, this method makes the lens significantly thicker in one area, which can reduce oxygen permeability and cause discomfort, sometimes leading to contact lens intolerance. Furthermore, incorporating a high-power prism correction makes the lens too thick to be comfortably worn, limiting prism contacts to low-power corrections.
Standard Treatments That Replace Prism Contacts
Since prism contact lenses are not a universal solution, the established and most common treatment for binocular double vision is prism spectacles. The prism correction is ground directly into the eyeglass lens, which ensures the base remains perfectly stable and eliminates the rotation problem inherent to contact lenses. Spectacles are the gold standard because they can incorporate a higher degree of prism power with a consistent and reliable result.
Vision Therapy
For some patients, a non-invasive approach called vision therapy may be recommended. This involves a structured program of eye exercises designed to retrain the eye muscles. Vision therapy improves the brain’s ability to coordinate the two eyes and is often used for issues like convergence insufficiency.
Surgical and Injection Options
For severe or constant eye misalignment, known as strabismus, surgery on the extraocular muscles may be necessary to physically realign the eyes. Botulinum toxin injections can also be used as a temporary measure to relax an overactive eye muscle. This can allow for potential recovery or serve as an alternative to surgery.