Binocular diplopia is a visual disturbance where a person perceives two images of a single object. It occurs when the eyes are misaligned and unable to work together, causing each eye to send a slightly different image to the brain. Unable to fuse these two distinct images into a single, three-dimensional picture, the brain perceives double vision. This condition is present only when both eyes are open and resolves when one eye is closed.
Identifying Binocular Diplopia
The double images may appear side-by-side, one on top of the other, or diagonally separated. Depending on the cause and which eye muscles are affected, the distance or orientation of the double image can change as you move your eyes. This can lead to associated symptoms like dizziness, eye strain, and difficulty with balance or reading.
A straightforward way to determine if double vision is binocular is to cover one eye. If the second image disappears, it confirms the issue stems from a misalignment between both eyes. This test helps differentiate it from monocular diplopia, where the double vision persists in one eye even when the other is closed, often due to a structural problem within the eye itself, such as a cataract.
Underlying Causes of Misaligned Eyes
Misalignment resulting in binocular diplopia can originate from problems with the eye muscles, the nerves that control them, or the brain. Weakness in the extraocular muscles responsible for eye movement is a common cause. Thyroid eye disease, an autoimmune condition, can cause inflammation and swelling of these muscles, restricting their movement. Myasthenia gravis, another autoimmune disorder, disrupts nerve signals to muscles, causing weakness that often affects the eyes first.
Damage to the cranial nerves that supply signals to the eye muscles can also lead to diplopia. The third, fourth, and sixth cranial nerves control eye movement, and palsy of any of these can weaken the corresponding muscles. This nerve damage can be a complication of diabetes, which affects blood flow to the nerves. Multiple sclerosis can also damage the protective sheath around these nerves, interrupting signals from the brain.
Issues within the brain can interfere with its ability to coordinate eye movements. A stroke can damage the brainstem or cerebellum, areas that manage eye alignment. A brain tumor or an aneurysm can exert pressure on cranial nerves or brain structures involved in vision. Increased pressure inside the skull from trauma can also impact nerve function.
A pre-existing misalignment of the eyes known as strabismus can also be a cause. A minor, latent misalignment called a phoria is often controlled unconsciously by the brain’s fusion mechanisms. With age, fatigue, or illness, the brain’s ability to compensate can weaken, allowing the underlying deviation to become apparent. The sudden onset of this condition requires prompt medical evaluation, especially with other neurological symptoms.
The Diagnostic Process
Diagnosing the source of binocular diplopia involves a detailed evaluation by a healthcare provider, often an ophthalmologist or neurologist. The process includes several steps to pinpoint the cause and measure the misalignment.
- A comprehensive medical history and a thorough eye examination to assess visual acuity, pupil function, and the overall health of the eyes.
- An evaluation of ocular motility, the ability of the eyes to move together in all directions, which helps pinpoint which specific muscles or nerves may be affected.
- The cover-uncover test, where the examiner has the patient focus on an object while covering and then uncovering each eye to observe any corrective movements.
- Prism testing, which involves placing prisms of varying strengths in front of the eyes to provide a quantitative measurement of the deviation.
- Blood tests to screen for systemic conditions like diabetes or thyroid disease, which are known causes of diplopia.
- Imaging studies like an MRI or CT scan of the brain and eye sockets to reveal evidence of a stroke, tumor, or nerve inflammation.
Medical and Therapeutic Interventions
Treatment for binocular diplopia is directed at alleviating the symptom of double vision and addressing the root medical problem. The approach depends on the underlying cause and can include several interventions.
- Corrective lenses with special prisms can be ground into eyeglasses or applied as a film. These prisms bend light to help realign the two images, allowing the brain to fuse them into a single picture.
- Vision therapy involves a structured program of eye exercises designed to strengthen the eye muscles and improve the brain’s ability to control eye alignment.
- Patching one eye or using an opaque contact lens can eliminate the second image, providing immediate relief while waiting for a definitive treatment to take effect.
- Botulinum toxin (Botox) injections can be administered to relax an overactive eye muscle, allowing its opposing muscle to function more effectively and improve alignment.
- Medications that improve nerve-to-muscle signal transmission can resolve double vision if it is a symptom of a systemic condition like myasthenia gravis.
- Eye muscle surgery may be an option for persistent cases of misalignment. This procedure involves physically repositioning one or more eye muscles to correct their pulling action.