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Diplopia - Double vision

The diplopia, or double vision, is a visual symptom that is manifested by the simultaneous perception of two images related to a single object. Double vision can be transient, constant or intermittent.

The diagram shows the horizontal and vertical muscles of the eyeball and their insertions. Often diplopia is the result of a dysfunction of these muscles. Image taken from: www.mstrust.org.uk

Single binocular view

The ability to perceive images correctly depends on the coordination of the visual system. Many structures interact to elaborate and interpret a luminous stimulus, including:

  • Cornea and crystalline, in the front part of the eye, act as a photographic lens helping to focus the light that enters the eye;
  • The retina is the photosensitive layer of tissue that lines the back of the eye and converts the light signals perceived into nervous stimuli;
  • The optic nerve transmits electrical signals from the retina to the brain, where focused images are processed.

Each eye sees an object independently and vaguely different from the other eye, in relation to the different position on the front field. However, only one three-dimensional image is perceived as the brain is able to control the eye muscles so that they accurately bring the observed object into focus, and to process in a single image the information captured by each eye. Structural or functional problems at any component of the visual system may involve double vision.

Causes

Double vision can be caused by different conditions. Some causes of diplopia are relatively minor, while others require urgent medical attention.

Monocular diploma

The monocular diplopia occurs in only one eye, while the other is unaffected. Therefore, when the eye manifesting the symptom is covered, usually double vision disappears and the subject should be able to see normally. Often, the two images perceived are only slightly separated ("ghosting"): one appears normal (for brightness, contrast and sharpness), while the other is of lower quality.

Usually, monocular diplopia is due to a structural problem, within the eye itself, which distorts the transmission of visual information, such as a scar or other irregularities on the cornea. Astigmatism and cataract are the most common causes.

Monocular diplopia can be determined by:

  • Dry eye syndrome : can cause double images, due to a qualitative or quantitative defect of lacrimation.
  • Crystalline luxation : the ligaments (zonular fibers) that maintain the crystalline lens in the correct position are damaged, due to trauma or due to a systemic pathology such as Marfan syndrome. The crystalline lens, therefore, can move to the front or back of the eye causing diplopia.
  • Cysts and eyelid bulges : both of these conditions can exert pressure on the front part of the eyeball, causing the temporary separation of the image. The presence of a cyst or an edema of the eyelid can alter the shape of the anterior part of the eye, causing a slight shift in the path of the light rays entering the eye, therefore a variation in the points where they are focused on the retina.
  • Astigmatism : refractive error caused by an irregular curvature of the cornea.
  • Keratoconus : a disease that causes the deformation of the cornea, which becomes progressively thin and conical in shape.
  • Pterygium : thickening of the conjunctiva, which extends over the cornea.
  • Other corneal anomalies (corneal dystrophies, infections, scars, etc.) and crystalline anomalies, such as cataract (opacity of the lens).
  • Retinal abnormalities, such as macular degeneration.

Double monocular vision is less common than binocular diplopia.

Binocular diploma

The binocular diplopia occurs when both eyes, although functional, fail to converge to focus on the desired object; in these cases, double vision therefore represents the result of an ocular misalignment. The eyes may be facing in slightly different directions, causing different visual information to be sent. In such conditions, the images from each eye are not similar enough for the brain to create a clear and single image: the result is the perception of double images (2 figures of equal quality). The vision usually returns to normal if one of the two eyes is covered.

Often, binocular diplopia is the result of dysfunction of the extrinsic muscles. Other causes involve mechanical interference with eye movement, a generalized disorder in neuromuscular transmission or a disease that affects the cranial nerves that innervate the eye muscles (example: paralysis of the 3rd, 4th or 6th cranial nerve). The symptom is often caused by a strabismus, but if it occurs suddenly it could be the sign of a more serious medical condition.

  • Strabismus: it is a convergence defect determined by the lack of coordination between the extrinsic muscles, responsible for deviation or misalignment of the two eyes; this prevents us from directing the gaze of each eye to the same objective, hindering proper binocular vision. Not all cases of strabismus cause diplopia.
  • D years to the nerves that control the extrinsic muscles : the nerves can be damaged by brain damage caused by infections, multiple sclerosis, stroke, head injury or brain tumor, particularly if located in the lower back of the brain. Direct traumas can damage a nerve at any point in its course.
  • Cerebrovascular diseases : they affect the blood vessels that supply blood to the ocular structures or the brain (for example: aneurysm, stroke or transient ischemic attack).
  • Diabetes : it can damage the blood vessels that supply the eye and cause nerve problems that control the eye's muscular movements.
  • Myasthenia gravis : is an autoimmune disease that blocks nerve stimulation of the muscles. Often the first signs of this neuromuscular disorder are double vision and drooping eyelids.
  • Endocrine-related exophthalmos : the protrusion of the eyeballs is the result of a basic hormonal disorder. Graves' disease is one of the most common causes of thyroid hyperactivity (hyperthyroidism) and diplopia (typically vertical) can result from edema and fibrosis involving the extrinsic muscles (infiltrative ophthalmopathy).
  • Compression injury: double vision can also be caused by a brain tumor or a blood clot behind the eye, which prevents the normal movement of the eyeball. Even a neoplastic mass near the base of the skull, in the area of ​​the breasts or within the eye socket can cause the same effect.
  • Inflammatory or infectious lesions : for example, orbital myositis, sinusitis, abscesses, cavernous sinus thrombosis etc.
  • Trauma (eg fracture, hematoma etc.): a head injury can cause damage to the muscles or nerves that coordinate eye movements, especially in the event of a fracture of the bones of the eye orbit.

Temporary Diploma

Temporary diplopia can occur due to a traumatic event (example: concussion), excessive physical tiredness or intoxication from a substance, such as alcohol or some drugs. A transient episode of diplopia is generally of no clinical significance, indicating only a brief "relaxation" of the mechanism of fusion of the visual stimuli of the central nervous system.

Symptoms

Diplopia can be constant, intermittent or occur only when the eyes are looking for an image in a particular direction (left or right, more likely in case of weakness of the eye muscles).

In addition, double vision can be:

  • Horizontal: the two images are side by side;
  • Vertical: the images are displayed one above the other;
  • Diagonal: an oblique separation occurs, where the images are both vertically and horizontally displaced from each other.

Sometimes, diplopia is confused with blurred vision. In this second case, a single image appears less sharp, fuzzy and not defined in detail. In double vision, however, two images are perceived simultaneously (one for each eye).

Double vision can occur alone or may be accompanied by other symptoms. These may include:

  • Misalignment of one or both eyes (strabismus);
  • Pain without or with eye movements, in one or both eyes;
  • Pain around the eyes;
  • Headache;
  • Nausea;
  • Drooping eyelids (ptosis);
  • Dizziness.

In adults, if diplopia develops suddenly it can be a sign of a serious medical condition that produces effects on the eyes, muscles, nerves or brain. The eyes of patients with muscle paralysis may look cross-eyed or stray.

Diplopia in children

How to recognize it

In most cases, double vision is easy to spot in adults, as they can describe what they are seeing. The symptoms are more difficult to detect in children, which may not clearly explain a possible change in vision.

If double vision affects one or both eyes, it affects the interpretation of visual information. During childhood, however, the brain can quickly adapt to the problem, ignoring or "suppressing" one of the two images and increasingly relying on the signals received from the dominant eye (amblyopia). Because of this, neglected amblyopia can lead to permanent vision loss in the affected area.

If a child has diplopia, he may squint in an attempt to see better, turn his head in an unusual way or look sideways instead of forward. Most children are successfully treated if the condition is detected and treated early.

Diagnosis

The first step is to determine whether double vision involves one or both eyes and whether the images are separated from the vertical, horizontal or diagonal plane.

A complete evaluation of diplopia starts with a detailed medical history, collecting elements on: onset (gradual or sudden), duration, frequency (intermittent or constant), variability with the position of the head or the gaze of the eyes, associated symptoms (such as pain, headache and weight loss), past and current medical conditions (hypertension, diabetes, atherosclerosis, alcohol abuse) and ongoing drug therapies. The assessment should look for neurological symptoms and other cranial nerve dysfunctions, such as changes in vision, numbness of the forehead and cheek, facial weakness, dizziness, hearing loss, speech difficulties and other sensory abnormalities. Non-neurological symptoms related to potential causes must also be ascertained: nausea, vomiting and diarrhea (intoxication), palpitations, heat sensitivity and weight loss (Graves' disease) and difficulty in controlling the bladder (multiple sclerosis).

Physical examination begins with a review of vital signs for fever and the general appearance of signs of toxicity. The eye examination detects the initial position of the eyes and evaluates visual acuity (with possible correction of refraction defects). The ophthalmologist should note the presence of eye swelling, drooping eyelids or pupillary abnormalities. In the diagnosis of diplopia, the doctor pays particular attention to the way in which the eyes concentrate and move together to focus a visual stimulus (alignment, convergence and focus). The examination should measure ocular motility (full or limited), possible misaligned eye movement and nystagmus. This assessment is important, as any deficits found suggest an orbital or retrobulbar disorder.

Any injury to the eye or eyelid can be assessed using a slit lamp. Ophthalmoscopy should be performed, in particular, to detect any abnormalities of the lens and retina.

The general examination should include further investigations to confirm a specific causal condition. For example, if hyperthyroidism is suspected, the patient undergoes tests to measure the functionality of the thyroid gland (levels of serum thyroxine and thyroid stimulating hormone). For patients with intermittent diplopia, tests for myasthenia gravis and multiple sclerosis should be considered. A magnetic resonance imaging (MRI) or computerized tomography (CT) of the head allows to verify any signs of trauma, hemorrhage, tumors and other pathological changes (vascular, orbital, cranial or central nervous system).

Treatment

The treatment consists in the management of the basic disorder. Treatment options include eye exercises, correction with eyeglasses, and, in more extreme situations, surgery or botulinum toxin injections. In some cases, double vision can be improved by correcting the underlying cause. If the visual symptom cannot be reversed, some treatments can help people live with diplopia. Sometimes, this requires wearing an eye patch or special prismatic glasses to minimize the effect of double vision.

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