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Double vision (diplopia) is seeing two images of one object. Double vision may occur when only one eye is open (monocular diplopia) or, more commonly, when both eyes are open (binocular diplopia). Binocular double vision disappears when either eye is closed. Other symptoms, such as eye pain, bulging eye, or muscle weakness, can be present depending on the cause of double vision.
Monocular double vision can occur when something distorts light transmission through the eye to the retina (the light-sensing structure at the back of the eye). There may be more than two images. One of the images is of normal quality (for example, in brightness, contrast, and clarity). The other image or images are of inferior quality.
The most common causes of monocular double vision are
Refractive error is imperfect focusing of light rays on the retina. Astigmatism (see What Is Astigmatism?) is refractive error caused by abnormal curvature of the cornea (the clear layer in front of the iris and pupil).
Other causes of monocular double vision include corneal scarring and a dislocated lens.
Binocular double vision suggests the eyes are not pointing at the same object. People normally see an object as a single image even though each eye receives its own separate image of that object. To perceive single images, the eyes must be aligned so that both point to the same object at the same time (called conjugate alignment). When the eyes are not properly aligned, people see two images, both of equal quality. Sometimes binocular double vision becomes apparent only when people move their eyes to an extreme in a certain direction (for example, to the far right or left, or up or down).
There are many possible causes of binocular double vision. The most common are
Most commonly, the eyes are misaligned because of a disorder affecting the cranial nerves supplying the muscles that move the eyes, called extraocular muscles (see Palsies of Cranial Nerves That Control Eye Movement). The paralysis may be isolated and the cause may be unknown. Known causes include disorders that typically interfere with the ability of the nerves to control muscles. For example, myasthenia gravis (see Myasthenia Gravis), botulism (see Botulism), and Guillain-Barré syndrome (see Guillain-Barré Syndrome) can affect muscles throughout the body, including the muscles that move the eyes.
Anything that mechanically interferes with eye motion can keep the eyes from aligning properly and cause double vision. Examples include entrapment of an eye muscle in a fracture of the eye socket and deposition of abnormal tissue in the eye socket as can occur with the form of hyperthyroidism called Graves disease.
Some causes of double vision are minor, but some can be very serious. The following information can help people know when to see a doctor and what to expect during the evaluation.
In people with double vision, certain symptoms and characteristics are cause for concern. They include
Any symptoms besides double vision that could represent nervous system dysfunction (for example, weakness or paralysis, numbness, speech or language problems, trouble swallowing or walking, vertigo, headache, incontinence, or clumsiness)
Bulging of the eye (proptosis)
Recent injury to the eye or head
Double vision should always be evaluated by a doctor even if it is temporary. People who have warning signs should be evaluated by a doctor right away, usually in an emergency department. All people who have double vision, even if it has resolved, should see a doctor as soon as convenient, usually within a few days.
Doctors first ask questions about the person's symptoms and medical history. Doctors then do a physical examination. What they find during the medical history and physical examination helps suggest a cause of the double vision and any tests that need to be done (see Some Causes of Double Vision When Both Eyes Are Open).
Doctors want to know whether double vision involves one or both eyes and whether it is constant or comes and goes. They also ask whether the images are side by side or on top of one another and whether double vision tends to occur only when the person is gazing in a particular direction. Doctors ask about any pain, numbness of the forehead or cheek, facial weakness, vertigo, and swallowing or speech problems because these symptoms may indicate a cranial nerve problem. Doctors also ask about symptoms of other nervous system problems and symptoms of other disorders.
The most important part of the physical examination is the eye examination. Doctors check the person's vision. They also carefully look for bulging of one or both eyes and a drooping eyelid and check how the pupils respond to light. They check the eyes' movements by asking the person to follow their finger as it moves up and down and far to the right and to the left. Doctors then use a slit lamp (an instrument that enables a doctor to examine the eye under high magnification—see Figure: What Is a Slit Lamp?) and ophthalmoscopy (see Figure: What Is an Ophthalmoscope?) to examine the internal structures of the eyes.
Symptoms and examination findings can provide helpful information about which causes are most likely. For example, if double vision comes and goes and there are other symptoms of possible nervous system dysfunction, myasthenia gravis and multiple sclerosis are among the likely causes. If the eyes do not point in the same direction, the direction of gaze in which double vision occurs sometimes indicates which cranial nerve is dysfunctional.
Some Causes of Double Vision When Both Eyes Are Open
People with double vision in one eye usually are referred to an ophthalmologist (a medical doctor who specializes in the evaluation and treatment—surgical and nonsurgical—of eye disorders). Testing is not needed before the person is referred. The ophthalmologist examines the person's eyes carefully for eye disorders.
In people with double vision affecting both eyes, more testing is often needed because many disorders may cause binocular double vision. Tests depend on what doctors find during the history and physical examination.
Most people require imaging with magnetic resonance imaging (MRI) or computed tomography (CT) to detect abnormalities of the eye socket (orbit), skull, brain or spinal cord. Imaging may need to be done right away if doctors think an infection, an aneurysm, or a stroke is the cause of double vision.
In people with symptoms of Graves disease (such as bulging of the eyes, eye pain, watering, and an enlarged thyroid gland), thyroid tests (serum thyroxine [T 4 ] and thyroid-stimulating hormone [TSH] levels) are done. Testing for myasthenia gravis and multiple sclerosis may be needed, particularly if double vision comes and goes.
Not all people require testing. Some cases of double vision clear up without treatment. If symptoms and examination findings suggest no serious cause, doctors may recommend that the person's eyes be checked regularly for a few weeks to see whether the vision clears up before they recommend any testing.
People with double vision plus sudden or severe pain, injury, or symptoms of nervous system dysfunction should usually go to an emergency department.
Double vision may go away on its own, but people should still see a doctor.
The most important part of the examination is the eye examination, but usually imaging is needed.
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