What is double vision?
Double vision is the perception of two images of a single object seen adjacent to each other (horizontally, vertically, or obliquely) or overlapping. Diplopia is the medical term for double vision. Polyplopia is the perception of three or more images of a single object overlapping each other.
Double vision is called "monocular" when the double image is perceived by an eye that is tested alone. In "binocular" double vision, each eye sees a single image when tested alone, but a double image is present when both eyes are open.
What causes double vision?
There are dozens of causes of double vision, ranging from benign to life-threatening. Therefore, it is important for the doctor to carefully review the history and perform an examination to determine the cause and initiate appropriate treatment when necessary. Sometimes, emergency treatment is needed.
Most causes of monocular diplopia stem from poor focusing of light by the eye. Refractive errors (myopia, hyperopia, astigmatism) are causes. Dry eye (from a variety of causes such as meibomitis, Sjögren's syndrome, and decreased tear production following refractive surgery) can produce diplopia that varies with blinking. Cataracts (clouding of the natural lens) and posterior capsule opacification (which can occur after cataract surgery) are common in people over 60 years of age and can produce diplopia along with other vision problems. Other conditions that interfere with proper focusing of light include corneal irregularity from keratoconus, swelling or scars, and retinal conditions such as epiretinal membranes. Rarely is the underlying cause a medical emergency in cases of monocular diplopia.
Binocular diplopia on the other hand is produced by a misalignment of the two eyes, which in some instances can be caused by life-threatening conditions. For example, aneurysms, strokes, trauma (head injury), and increased intracranial pressure (for example, from a growth or mass such as a brain tumor) can cause eye misalignment by affecting the nerves that control eye movement and alignment. . These nerves control the extraocular muscles that move the eyes in different direction of gaze. If these nerves lose blood supply or are compressed, the muscles they control become weak or paralyzed. This is called a cranial nerve palsy.
Other diseases affecting the nervous system can lead to binocular double vision. In multiple sclerosis, nervous system inflammatory lesions that affect various parts of the visual system may result in double vision that varies over time. Guillain-Barré syndrome can also produce double vision from nerve damage. Migraine headaches can cause a sudden but temporary strabismus (eye misalignment). In myasthenia gravis, the communication between the nerves and the eye muscles is abnormal, resulting in diplopia that varies by the time of day, typically worsening with fatigue.
Binocular double vision can also occur with damage to the eye muscles themselves. Graves' disease (often associated with thyroid disease), orbital inflammations, vascular disease (as seen with diabetes and high blood pressure), and others are examples of diseases that directly affect the extraocular muscles through compression, poor blood supply, or local eye inflammation. Eye movement and alignment can also be abnormal if muscles and orbital tissue become trapped in a skull bone fracture from trauma, leading to restriction of eye movement in certain directions of gaze.
Childhood strabismus (eye misalignment), such as crossed eyes (esotropia) or out-turned eyes (exotropia), occasionally produces double vision, and often children develop ways to suppress the double vision. However, this suppression may result in amblyopia (a weakening of the eye's visual development). For this reason, children whose eyes appear misaligned should be evaluated by an eye doctor for treatment that may range from glasses, vision therapy, and patching to surgery.
Sometimes the cause of binocular double vision is relatively harmless, such as when fatigue or illness results in a temporary misalignment of the eyes. This is called a phoria, which is a variable, intermittent type of strabismus (eye misalignment). Another fatigue-related misalignment is convergence insufficiency, which is the inability to keep the eyes converged (slightly crossed) for long periods while reading. This can often be alleviated with eye exercises ("pencil pushups" vision therapy) or prism glasses.