Horizontal diplopia is double vision where you see two images side by side rather than stacked on top of each other. It happens when your eyes are misaligned in the horizontal plane, meaning one eye drifts inward or outward relative to the other. The condition is almost always binocular, which means the doubling disappears when you close either eye.
How Your Eyes Stay Aligned
Each eye has six small muscles that control its movement. For horizontal alignment specifically, two muscles do most of the work: one pulls the eye outward (toward the ear) and one pulls it inward (toward the nose). These muscles are controlled by cranial nerves that run from the brainstem to each eye. When both eyes point at the same target, the brain fuses the two images into one. When one eye drifts horizontally, even slightly, you see two images sitting next to each other.
The sixth cranial nerve is the most common culprit in horizontal diplopia. It controls the muscle that moves your eye outward. When this nerve is damaged or compressed, the affected eye can’t turn outward properly, and images split apart horizontally. Because this nerve takes the longest path from the brainstem to the eye, it’s especially vulnerable to injury, swelling, or pressure changes inside the skull. When only this nerve is involved, the double vision stays purely horizontal, with no vertical or rotational component.
What It Feels Like
The two images you see sit next to each other, and the gap between them typically changes depending on where you look. If the muscle that moves your eye outward is weak, the doubling gets worse when you try to look toward the affected side. It may be mild or even absent when you look in other directions.
Where the doubling shows up also matters. Some people notice it only at distance, some only up close, and some at both. Double vision that appears mainly when looking at distant objects can point to a different problem than double vision that worsens at near. For example, convergence insufficiency, a condition where the eyes struggle to turn inward together, typically causes horizontal diplopia when reading or doing close work but not when looking across a room.
The eyes can drift in two directions. When an eye turns inward (called esotropia), the two images overlap differently than when an eye turns outward (called exotropia). Your eye care provider will note which direction the drift goes because it narrows down the possible causes significantly.
Common Causes
Sixth cranial nerve palsy is the leading cause of isolated horizontal diplopia. In adults, the most frequent triggers are vascular, particularly diabetes and other conditions that damage small blood vessels supplying the nerve. These cases often resolve on their own within two to three months as blood flow recovers. Non-vascular causes include head trauma, inflammation, and compression from tumors. Skull base tumors, for instance, can press on the sixth cranial nerve as it passes along the base of the brain.
In children, the picture is different. The most common risk factors for sixth nerve palsy include increased pressure inside the skull, blood vessel abnormalities, and tumors. Any child with new horizontal double vision needs prompt evaluation.
Thyroid eye disease is another important cause. The condition enlarges the eye muscles, physically tethering them and restricting movement. The muscles are affected in a predictable order, starting with those that control vertical movement, but the inner and outer muscles can also be involved, producing horizontal diplopia. Unlike nerve damage, where the muscle simply can’t contract, thyroid eye disease creates a mechanical restriction, so the eye gets stuck rather than going weak.
Myasthenia gravis can also produce horizontal double vision. This autoimmune condition disrupts communication between nerves and muscles, causing weakness that worsens with use. A key distinguishing feature is fatigability: the double vision tends to get worse throughout the day or after sustained visual effort, and it’s often accompanied by drooping eyelids. Thyroid eye disease, by contrast, more commonly causes eyelid retraction (wide-open eyes), bulging, and redness.
In older adults, a condition called sagging eye syndrome has emerged as the single most common diagnosis. A large study at a university eye center found that among 945 patients over age 40 who came in with binocular diplopia, sagging eye syndrome accounted for 31.4% of cases. It results from age-related weakening of the connective tissue bands that hold the eye muscles in position. The percentage of patients with this diagnosis increased sharply with age, from about 4.7% in those under 50 to over 60% in those over 90.
How It’s Diagnosed
The first and most important test is the cover test. Your provider covers one eye, then the other, watching for a shift in the uncovered eye. If the eye moves horizontally when the other is covered, there’s a measurable misalignment. This simple test confirms the diplopia is binocular, meaning it comes from the eyes not working together rather than a problem within one eye itself. If you still see double with one eye closed, that’s monocular diplopia, which has entirely different causes like cataracts or corneal irregularities.
Beyond the cover test, your provider will check how well each eye moves in all directions, looking for specific patterns that map to particular nerve or muscle problems. A visual acuity test checks the sharpness of each eye independently. If binocular diplopia is confirmed, imaging with MRI or CT scans may be ordered to look at the muscles, nerves, brain, and surrounding bone structures. Blood tests can help identify systemic causes like thyroid disease or diabetes.
Treatment Options
Treatment depends entirely on the underlying cause. When the cause is vascular, like a sixth nerve palsy from diabetes, the standard approach is observation for several months because many of these cases resolve spontaneously. During the waiting period, the main goal is managing the double vision so you can function day to day.
Prism lenses are the most common non-surgical treatment. A prism bends light before it enters your eye, shifting the image so it lands in the right spot despite the misalignment. For smaller deviations, the prism can be ground directly into your glasses, and lenses up to about 10 prism diopters are generally well tolerated this way. For larger deviations, a stick-on Fresnel prism can be applied to one lens of your glasses. These are thinner and lighter than ground-in prisms, making them practical for corrections in the range of 12 to 25 prism diopters. Most patients need a prism that corrects at least 65% of their total deviation to eliminate the double vision.
Prisms work well as a temporary solution or for stable, smaller misalignments. They don’t fix the underlying problem, but they can make the double vision disappear while you’re wearing them.
When Surgery Is Considered
If the misalignment is large, stable, and not improving on its own, surgery on the eye muscles can realign the eyes. The procedure involves loosening or tightening one or more of the horizontal muscles to bring both eyes back into alignment. Surgery for horizontal misalignment has a success rate of 60% to 80% overall, with success defined as reducing the remaining deviation to within 10 prism diopters of perfect alignment.
Certain factors predict better outcomes. Eyes that drift inward (esotropia) tend to do better surgically than eyes that drift outward (exotropia), partly because outward-drifting eyes have a higher tendency to gradually drift back after surgery. One study found that esotropia carried more than four times the odds of surgical success compared to exotropia. The absence of significant vision loss in the weaker eye also strongly predicted a good result, with nearly six times higher odds of success. Using adjustable sutures, which allow fine-tuning of the muscle position shortly after surgery, has also been shown to improve outcomes significantly compared to fixed sutures (75.7% vs. 54% success).
For thyroid eye disease, surgery is typically delayed until the active inflammatory phase has passed and the eye position has been stable for at least six months. Myasthenia gravis is generally managed with medications that improve nerve-to-muscle signaling, and surgery is rarely the first-line approach.