context: When you open your eyes and see a single, clear image, you probably take it for granted. But it's the result of a complex process that requires the many parts of your vision to work together smoothly. Double vision can occur with one eye or both. If it's in one eye when the other is closed, it is less worrisome, but still serious. If it occurs when both eyes are open, it could signal a major disorder. To understand what can go wrong, it helps know the parts of your eye and how they work together. Double vision when the injured eye is open: Cornea: The clear window into your eye. Its main job is to focus light. If your double vision goes away when you cover one eye, you might have cornea damage in the uncovered eye. If only one cornea is warped, you may only see double in that eye. Glasses can probably fix the problem. Damage can be from: Keratoconus, when your cornea becomes cone-shaped Infections, like shingles or herpes Scars Dryness Lens: It sits behind your pupil and helps focus light onto your retina. Cataracts are the most common lens problem. Surgery almost always fixes them. Double vision when both eyes are open: Muscles: They control eye movement and keep the eyes aligned with each other. If a muscle in one eye is weak, then it won't move in sync with the healthy eye. When you look in a direction controlled by the weak muscle, you see double. Eye muscle problems can be from: A problem with the nerves that control them. Myasthenia gravis, an autoimmune illness that stops nerves from telling the muscles what to do. Early signs include double vision and drooping eyelids. Graves' disease, a thyroid condition that affects eye muscles. It can cause vertical diplopia, where one image is on top of the other. Nerves. They carry information from your brain to your eyes. Problems with them can lead to double vision: Multiple sclerosis can affect nerves anywhere in your brain or spinal cord. If it damages the nerves that control your eyes, you may see double. Guillain- Barre syndrome is a nerve condition that causes progressive weakness. Sometimes, the first symptoms are in your eyes and include double vision. Diabetes can cause nerve damage to the muscles that move your eyes. That can lead to double vision. Brain : The nerves that control your eyes connect directly to your brain, where images are processed. Many causes of double vision start in the brain. They include: Strokes Aneurysms Increased pressure inside the brain from trauma, bleeding, or infection Brain tumors Migraine headaches Double vision can happen with no other symptoms. Depending on the cause, you may also notice: Misalignment of one or both eyes (a "wandering eye" or "cross-eyed" appearance) Pain when you move your eye Pain around your eyes, like the temples or eyebrows Headache Nausea Weakness in your eyes or anywhere else Droopy eyelids Double vision that's new or unexplained needs medical care right away. With so many potential causes, it's important to discover the reason without delay. Your doctor will most likely use more than one method to find out what's causing your problem. He might try blood tests, a physical exam, and maybe an imaging test like computed tomography ( CT) or magnetic resonance imaging ( MRI). One of the most effective tools for your doctor is the information you provide. Think about these questions before your appointment. When did the double vision start? Have you hit your head, fallen, or been unconscious? Were you in a car accident? Is the double vision worse at the end of the day or when you're tired? Have you had any other symptoms besides double vision? Do you tend to tilt your head to one side? Look at old pictures, or ask family -- you may not even be aware of the habit. Is the double vision only present with both eyes open or only present when one eye is open? Now, focus on something in your field of vision that isn't moving, like a window or a tree. Are the two objects side by side, or is one on top of the other? Or are they slightly slanted? Which one is higher or lower? Are both images clear but not in line? Or is one blurry and the other clear? Cover one eye, then switch. Does the problem go away when either eye is covered? Pretend your field of vision is a clock face. Move your eyes around the clock, from noon to six and around to 12 again. Is your vision worse at any clock position? Does any position make it better? Tilt your head to the right, then to the left. Do any of these positions improve your eyesight or make it worse? The most important step is to identify and treat the root cause. If weak eye muscles are to blame, or if a muscle has been pinched as a result of injury, surgery may help. Medications can treat myasthenia gravis. Surgery or medicine is available for Graves' disease. Medicines and insulin can control blood sugar in diabetes. If double vision can't be reversed, treatments can help you live with it. Special eyewear, like an eye patch or prism glasses, can ease the effects.