Seeing double, medically termed diplopia, is a common visual phenomenon when you relax your gaze or let your eyes drift out of focus. This intermittent double vision occurs because the finely tuned coordination between your two eyes has momentarily broken down. Normally, the brain works continuously to keep the eyes perfectly aligned to create a single, unified image. When you stop exerting this effort, a slight, underlying misalignment becomes visible, resulting in the temporary perception of two separate images.
The Difference Between Monocular and Binocular Double Vision
Double vision is classified into two main types. Monocular diplopia is present even when you cover the unaffected eye, meaning the problem originates within the optical system of the single open eye. Causes often relate to issues like astigmatism, cataracts, or dry eyes, which distort light entering the eye.
Seeing double only when both eyes are open and upon relaxation is almost always binocular diplopia. This type disappears immediately when either eye is covered, indicating an issue of eye alignment or coordination. Binocular diplopia occurs when the image falls on different points of the retina in each eye, and the brain cannot merge these disparate images into one.
Understanding the Role of Eye Focus and Fusion
Maintaining single vision requires the continuous, coordinated action of two systems: vergence and fusion. Vergence is the simultaneous movement of both eyes in opposite directions (inward for convergence, outward for divergence). Fusion is the neurological process where the brain merges the slightly different images from each eye into one three-dimensional perception.
The eyes’ ability to maintain fusion is a constant effort managed by the extraocular muscles. When you relax your eyes, you release the motor command maintaining precise alignment. This momentary relaxation allows a latent deviation, known as a phoria, to manifest.
A phoria is the natural tendency for the eyes to drift out of alignment when the brain’s fusion effort is removed. For example, this drift might be a slight tendency outward (exophoria) or inward (esophoria). When you relax the muscles holding the eyes straight, they momentarily drift to this resting position, and the single image splits into two before the brain can realign them.
The system of accommodation (the eye’s ability to focus the lens) is closely linked to vergence. Focusing on a near object causes the eyes to automatically converge inward, a relationship known as the near triad. This coupling means that changes in focus effort influence eye alignment, and a breakdown in this link contributes to intermittent double vision upon relaxation.
Common Underlying Causes
The most common causes for this intermittent double vision are subtle binocular vision dysfunctions. Latent deviations (phorias) are the root cause, representing the eye position when the fusion mechanism is not actively engaged. These phorias are well-compensated by the visual system until the eyes are intentionally relaxed or fatigued.
Mild muscle imbalances or general eye fatigue (asthenopia) can also cause the temporary breakdown of fusion. Prolonged visual tasks, such as reading or screen use, can exhaust the muscles that maintain alignment. When the effort ceases, the eyes deviate, and double vision occurs.
Uncorrected refractive errors, such as hyperopia (farsightedness), can contribute to the issue. The constant accommodative effort required to maintain a clear image inadvertently affects the vergence system. If this effort is suddenly released, the resulting misalignment leads to the double image.
While most cases of intermittent diplopia are benign, they can sometimes signal conditions like intermittent strabismus, where eye misalignment is more significant. Less common, more serious causes involve neurological or muscular issues, such as myasthenia gravis, where the intermittent nature is due to waxing and waning muscle weakness.
Diagnosis and Treatment Options
Diagnosis
A comprehensive eye examination by an eye care specialist is necessary to diagnose the cause of intermittent diplopia. The specialist will use tests, like the cover test, to confirm the double vision is binocular and measure the size and direction of the phoria. This test quantifies the latent misalignment by observing eye movement when one eye is momentarily covered.
Diagnostic testing includes assessing eye movements (motility testing) and evaluating the full range of fusional vergence. This measures the eyes’ capacity to converge and diverge while maintaining a single image. The results determine if the phoria is “decompensated,” meaning the visual system struggles to overcome the misalignment.
Treatment
Treatment often begins with corrective lenses, sometimes incorporating prism lenses. These prisms bend light to compensate for the slight misalignment, reducing the muscular effort needed for fusion. Vision therapy, a program of targeted eye exercises, is another common approach designed to strengthen the vergence system and improve the brain’s ability to maintain fusion.
Sudden-onset or constant double vision, especially if accompanied by pain, severe headache, or weakness, requires immediate medical attention, as it may indicate a serious neurological or systemic condition.