What Is Diplopia (Double Vision)? What Causes It?

Diplopia, or double vision, is the perception of a single object as two separate images. It is divided into monocular diplopia and binocular diplopia. Depending on its cause, it may be a serious sign affecting visual health.

Monocular diplopia is generally associated with disorders of the cornea or lens. Conditions such as cataracts, astigmatism, and keratoconus may cause it. Binocular diplopia results from problems involving the eye muscles, nerves, or brain.

Neurological evaluation is important when diagnosing diplopia. Stroke, tumors, and vascular diseases may cause double vision. Diplopia may therefore be a finding that requires urgent evaluation.

Treatment is planned according to the underlying cause. Glasses, prism lenses, medication, or surgical procedures may be used. Early diagnosis is critical for preserving visual function.

• Monocular Diplopia: Double vision that occurs when looking with one eye.
• Binocular Diplopia: Double vision that occurs when both eyes are open.

• Eye Muscle Problems: Strabismus, myasthenia gravis.
• Nerve Damage: Diabetic neuropathy, multiple sclerosis.
• Brain Diseases: Stroke, tumor.

• Perceiving two images of the same object.
• Blurred vision.
• Balance problems and dizziness.

• Eye examination.
• Evaluation of eye muscle movements.
• Magnetic resonance imaging (MRI) or computed tomography (CT).

• Treatment of the underlying cause, such as treating infections with medication.
• Prism glasses.
• Surgical correction in certain cases.

• Permanent vision problems.
• Balance problems.
• Difficulty performing daily activities.

• Having regular eye examinations.
• Keeping underlying conditions such as diabetes and hypertension under control.

• Diabetes.
• Advanced age.
• History of neurological disease.

Diplopia means that a person perceives one object as two separate images. It may fundamentally result from the brain's inability to combine the images received from both eyes, known as impaired fusion, or from problems in the optical system of the eye. To use a more concrete analogy, each eye functions like a separate camera, while the brain contains an editor who combines these two recordings into a single image. If the editor cannot align the recordings correctly because of a technical or synchronization problem, two separate images appear on the screen. This can seriously affect the person's visual quality and daily life.

What Is Monocular Diplopia?

Monocular diplopia occurs when double vision persists with one eye open and continues after the other eye has been closed. If you continue to see double with your left eye while your right eye is closed, or vice versa, this may indicate monocular diplopia. The problem is generally caused by structural or optical abnormalities within the eye itself. If a refractive error such as astigmatism is present, for example, light entering the eye may create more than one focal point on the retina. This may result in double vision described as blurred vision or a ghost image.

Monocular diplopia may also occur because of incorrect light refraction in conditions such as cataracts, where the lens loses its transparency, or keratoconus, where the cornea becomes thinner and cone-shaped. These examples can be compared to a camera with a defective lens. If the lens is scratched or distorted, a single lens creates two different focal points, making one photographed object appear doubled.

What Is Binocular Diplopia?

Binocular diplopia occurs when double vision is present while both eyes are open but disappears when either eye is closed. Both cameras continue recording, but the brain cannot combine the two images correctly. It is generally associated with a problem affecting the eye muscles or the coordination of eye movements. Strabismus, meaning loss of parallel alignment between the eyes, is one of the best-known causes of binocular diplopia. One eye may turn inward, outward, upward, or downward. Because the visual axes are no longer parallel, two different viewing angles arise and the brain cannot superimpose them.

Neurological causes such as cranial nerve palsies involving the third, fourth, or sixth cranial nerves may also cause binocular diplopia. When one of the nerves controlling the eye muscles is damaged, the affected eye cannot turn sufficiently or maintain the correct position. Depending on the direction of gaze, the person may describe horizontal, vertical, or oblique double images. This can be compared to viewing films of the same scene recorded from two different angles that cannot be properly superimposed.

Factors contributing to monocular diplopia generally involve disruption of the eye's optical axis, meaning the path through which light enters and reaches the retina. These factors can be summarized as follows:

Refractive Errors:

• Astigmatism: Asymmetrical curvature of the cornea or natural lens distributes light rays across different points instead of focusing them at a single point.
• High Myopia or Hyperopia: Very high refractive errors may cause focusing abnormalities that produce ghost images or a sensation of double vision.

Corneal Problems:

• Keratoconus: The cornea becomes thinner, steeper, and cone-shaped. It causes significant astigmatism and may consequently lead to monocular diplopia.
• Corneal Scars: Opacities or surface irregularities caused by infection, trauma, or surgery prevent light from being refracted correctly.

Lens Disorders:

• Cataract: A lens that has lost its transparency scatters light and may cause a single object to appear doubled.
• Lens Subluxation: Trauma or connective tissue disorders may displace the lens from its normal position and cause irregular refraction.

Retinal and Macular Diseases:

• Macular Degeneration: When the formation of a clear central image is disrupted, the brain may sometimes interpret light signals as duplicated.
• Epiretinal Membrane: Thin membranes that form over the retina may affect the fovea, the area responsible for sharp vision, and cause distorted sight.

When these conditions are suspected, the first step is to examine all components of the eye's optical system, including the cornea, lens, and retina. Detailed biomicroscopy, optical coherence tomography (OCT), and corneal topography are highly useful for identifying the location of the abnormality. The key feature of monocular diplopia is that double vision continues when the other eye is closed. A simple procedure known as the cover test is therefore very helpful during the initial diagnosis.

The fundamental mechanism of binocular diplopia is the inability of the extraocular muscles to work together or a disruption of the parallel alignment of the eyes. A neurological or musculoskeletal imbalance is frequently present. The most common causes include:

• Strabismus:

Misalignment may be present from childhood as a congenital condition. At a later age, trauma or neurological disorders may cause weakening of the eye muscles, nerve palsy, or loosening of the supporting tissues.

• Cranial Nerve Palsies:

Third Nerve Palsy (Oculomotor Palsy): Drooping of the eyelid, known as ptosis, may develop and the eye may assume a downward and outward position. The patient frequently experiences double vision.

1. Trochlear Nerve Palsy: Vertical diplopia is particularly noticeable while using stairs or reading a book. The head is frequently tilted to compensate.
2. Abducens Nerve Palsy: When the eye cannot move outward, the patient experiences horizontal double vision.

• Myasthenia Gravis:

In this autoimmune disease, the strength of muscle contraction decreases throughout the day. Vision may be better in the morning, while eyelid drooping and double vision increase toward the evening. This can be imagined as the body's energy reserves gradually decreasing and the eye muscles becoming exhausted.

• Goiter or Graves' Disease (Thyroid Eye Disease):

Overactivity of the thyroid gland may cause swelling of the tissues and muscles around the eyes. The eyes may be pushed forward, producing a condition similar to strabismus. Diplopia develops when the visual axes become misaligned.

• Orbital Masses or Trauma:

Tumors, infections, or fractures within the orbit may mechanically restrict free movement of the eye or push the eyeball in another direction, causing double vision.

• Neurological Diseases (Multiple Sclerosis, Stroke, and Others):

Lesions in the brain centers controlling eye movements or in the neural connections, such as brainstem lesions and medial longitudinal fasciculus syndrome, may cause diplopia.

Each of these conditions may cause double vision of varying severity and in different directions. Neurological examination, visual field testing, and cranial MRI may be requested to clarify the diagnosis. In binocular diplopia, the image becomes single when one eye is closed. This is its most distinguishing characteristic.

A person experiencing diplopia may not describe the complaint only as seeing two images. The following symptoms may also accompany it:

• Headaches and Eye Strain: When the brain makes excessive effort to combine two misaligned images, the person may experience severe headaches and pressure around the eyes.
• Visual Confusion and Difficulty: The positions of objects become confusing, and the perception of distance and depth may deteriorate. This can create safety risks, particularly while using stairs or driving.
• Lines Shifting While Reading: Letters may appear to overlap. Reading becomes extremely tiring and concentration is disrupted.
• Changes in Head Position: People with vertical diplopia may unconsciously tilt their head sideways or forward to reduce double vision.

If you see what appears to be two lamps, two car headlights, or two faces, and this affects your quality of life, you should consult a specialist.

Diplopia may be only a symptom, but it can also act as a warning sign of a more serious underlying disease. Examples include:

• Diabetes: Small-vessel disease may cause temporary palsy of the third, fourth, or sixth cranial nerve.
• Hypertension: High blood pressure may damage the small vessels supplying the nerves and affect the nerves controlling eye movement through a similar mechanism.
• Aneurysm or Brain Tumor: These may compress structures in the brainstem or areas through which the cranial nerves pass, causing diplopia.
• Multiple Sclerosis (MS): Specific conditions such as internuclear ophthalmoplegia may cause diplopia in MS, a demyelinating disease of the central nervous system.
• Thyroid Diseases: Thyroid disorders such as Graves' disease may affect and swell the eye muscles and connective tissues, resulting in double vision.

Like a device hidden within the foundation of a building, diplopia may initially provide only a small warning signal. Ignoring this signal may result in missing complications that develop later. If diplopia is accompanied by other symptoms such as sudden vision loss, speech difficulties, or loss of balance, a healthcare facility must be consulted.

Many methods are used to diagnose diplopia, but the first and most important step is obtaining a detailed medical history. The ophthalmologist or neurologist asks when the diplopia began, the circumstances in which it increases or decreases, and whether there is a history of head trauma or systemic disease.

• Cover Test: This easily distinguishes monocular from binocular diplopia.
• Eye Movement Examination: Eye performance is evaluated in every direction of gaze to determine whether there is a muscle-related or nerve-related restriction.
• Cover-Uncover Test and Alternate Cover Test: These detect latent misalignment between the eyes, known as heterophoria, or manifest misalignment, known as heterotropia.
• Prism Tests: Prism lenses are used to measure the severity and direction of the deviation causing double vision.
• Biomicroscopy and Fundus Examination: The anterior segment, including the cornea, iris, and lens, and the posterior segment, including the retina and optic nerve, are examined in detail.
• Radiological Tests (MRI, CT): Brain imaging is requested when a condition involving the nervous system or orbital region is suspected.
• Blood Tests: Indicators of systemic conditions such as diabetes and thyroid disorders are investigated.
• Electrophysiological Tests: Examinations such as electromyography may provide guidance in conditions such as myasthenia gravis.

Just as an engineer requires various measurements to understand the structural balance of a building, the ophthalmologist needs multidimensional data to identify the source of diplopia.

Treatment of diplopia depends entirely on the underlying cause. Common approaches include:

Correctable Refractive Errors:

• Glasses or contact lenses may correct conditions such as astigmatism, myopia, and hyperopia. Some patients with monocular diplopia may obtain relief simply by using an appropriate pair of glasses.
• Toric or specially designed lenses may minimize visual distortion caused by corneal diseases such as keratoconus.

Prism Glasses (Prismatic Correction):

• In binocular diplopia, prism glasses may be effective when the ocular axes are only slightly misaligned and the angle is small enough not to require surgery.
• Temporary solutions such as Fresnel prisms are frequently used during the early stages of nerve palsy because the palsy may recover over several months.

Occlusion Treatment:

• Temporarily covering one eye is the most definite way to prevent double vision, but it eliminates depth perception during daily activities.
• If a child is at risk of amblyopia, the physician applies this method according to a specific treatment plan.

Surgical Procedures:

• Strabismus surgery can reposition the eye muscles and restore coordinated alignment.
• Orbital surgery aims to eliminate mechanical or space-occupying problems caused by conditions such as tumors, trauma, and thyroid eye disease.

Botulinum Toxin Applications:

• Botulinum toxin temporarily blocks excessive muscle activity. In sixth nerve palsy, for example, botulinum toxin may be injected into the lateral rectus muscle to compensate for inward turning of the eye.
• Its effects last for several months. During this time, the nerve may partially recover.

Treatment of Underlying Systemic Diseases:

• Conditions such as diabetes and hypertension must be brought under control.
• Medications for myasthenia gravis, such as pyridostigmine, and immunosuppressive treatments may reduce the muscle weakness causing diplopia.
• In thyroid diseases, treatments that balance hormone levels and medications such as corticosteroids or surgery to reduce orbital pressure may be considered.

Rehabilitation and Orthoptic Devices:

• Vision therapy or orthoptic exercises may benefit patients with conditions such as convergence insufficiency.
• Computer programs and specialized exercise cards may be used to improve coordination between the eye muscles.

The treatment process can be compared to carpentry. After identifying which component is bent, which screw is loose, or which board has decayed, the appropriate correction must be made. Sometimes only one screw is loose, and tightening it resolves the problem. In other cases, the entire structure must be repaired.

Diplopia is not always an emergency. However, if any of the following symptoms are present, it is important to seek medical care immediately:

• Sudden-Onset Double Vision: If double vision is present upon waking one morning or develops very quickly, it may result from a condition requiring urgent treatment, such as cerebrovascular disease or nerve compression.
• Severe Headache Accompanied by Speech Difficulties or Weakness: A neurological event such as a stroke should be suspected.
• Severe Eye Pain and Redness: Conditions such as infection or an acute glaucoma attack may be present.
• Double Vision After Trauma: A blow to the head, an orbital fracture, or a concussion may require urgent evaluation.

When double vision develops suddenly or after trauma, deciding to wait and see whether it resolves may result in damage that is difficult to reverse.

Double vision can significantly impair quality of life and daily functioning. Even simple everyday activities may become difficult:

• Driving: Seeing two images of oncoming headlights or traffic signals may increase the risk of an accident.
• Using Stairs: People with vertical or oblique diplopia may experience serious difficulty because of impaired depth perception.
• Reading and Computer Use: When lines appear to overlap, eye strain and headaches develop quickly.
• Social Interaction: Seeing two images of another person's face or having difficulty maintaining eye contact may interfere with social communication.
• Psychological Effects: People with persistent diplopia may become more susceptible to depression and anxiety and may withdraw from social environments.

Many patients develop their own methods for coping with diplopia, such as tilting the head, partially closing one eye, or avoiding bright light. These behaviors may cause neck pain and permanent postural problems.

An ophthalmologist should generally be the first specialist consulted. The ophthalmologist performs an initial assessment to determine whether the source of diplopia is ocular, neurological, or systemic. If a neurological problem is suspected, a neurologist becomes involved and brain imaging and neurological tests are performed. When necessary, specialists in endocrinology, internal medicine, or other fields collaborate to identify the underlying cause.

Diplopia can be an important sign of both ocular and general health. It may result from a simple astigmatic error that can be corrected within minutes, or from a serious condition such as a cerebral aneurysm. Diplopia should therefore not be underestimated, and specialist advice should be obtained.

• Have Regular Eye Examinations: If you have systemic conditions such as diabetes, hypertension, or thyroid disease, seeing an ophthalmologist at least once a year is beneficial.
• Early Diagnosis Can Save Lives: If sudden double vision is accompanied by a severe headache, speech difficulties, or weakness, seeking immediate medical help saves critical time in managing life-threatening conditions such as stroke.
• Follow the Treatment Plan: Recommendations involving prism glasses, botulinum toxin, or surgery should be followed carefully. Each treatment has its own likelihood of success and possible side effects.
• Rest and Exercise Your Eyes: When diplopia is mild, eye exercises or regular breaks during work and reading may be beneficial.
• Do Not Miss Follow-Up Examinations: Diplopia may worsen over time, but spontaneous recovery is also possible in conditions such as minor nerve palsies. Regular follow-up with your physician is important so treatment can be updated when necessary.

The eyes are among the body's most important means of communicating with the outside world. Any problem affecting them directly influences quality of life. If you notice an unusual change such as double vision, consult a specialist. With an informed approach, early diagnosis, and appropriate treatment, the difficulties caused by diplopia can be reduced considerably.

If you experience diplopia, your eyes should not be neglected, just as a weak or decayed tooth should not be left untreated. The condition should be managed carefully and the physician's recommendations followed precisely. Ultimately, providing the brain, which processes the eyes like cameras, with the highest-quality visual signal is an essential part of a healthy life.