Double Vision, Seeing Double – Causes of Diplopia

What is diplopia?

Diplopia, or double vision, occurs when an individual sees two or more images of the same object. It should not be confused with blurred vision where a single object appears unclear.

Vision in humans is single binocular vision. This means that the images from both eyes are recorded and processed by the nervous system but perceived as a single image. A disruption in this processing mechanism can result in double vision. It may be present in one eye (monocular) or as a result of a problem bringing the two images from each eye together (binocular).

Diplopia can also be a normal phenomenon, for example, when focusing on an object that is close up, another object in the distance can appear double. This is known as physiological diplopia and generally goes unnoticed as the brain has a way of suppressing these double images.

Normal Vision

The eye is a complex unit comprising visual receptors, a lens system to focus light on these receptors and nerves to relay the visual information to the brain. At the front of the eye, the cornea has evolved into a clear dome to allow light through to the retina. The retina, a layered structure covering the back of the eye is composed of cells called rods and cones and nerve cells. These rods and cones are the visual receptors for the eye. They contain photosensitive compounds and directly communicate with nerve cells. Between the cornea and retina is a the lens, which focuses the light coming through the cornea as an image on the retina (refraction).

When light falls on the retina, the compounds in the rods and cones react to light and generate action potentials along the nerve cells. These electrical impulses travel via the optic nerve to the visual cortex in the brain. Here these impulses from each eye (two images) are translated and fused into one single image.

Humans have evolved to be able to move each eye by using six different muscles that sit on the outside of the eyeball. These muscles are controlled by impulses from three cranial nerves (CN III, IV and VI). Together the nerves and muscles coordinate eye movements to keep both eyes pointing accurately at the same object. A bony casing in the skull called the orbit protects each eye. This casing has an aperture at the back to allow the cranial and visual nerves to pass information between the eyes and brain.

Pathophysiology of Diplopia

Damage to any element of the system, the eye, its muscles, nerves, surrounding structure and parts of the brain that process the visual information could lead to diplopia.
Any damage to the eye muscles or nerves that supply the eye muscle, result in poor co-ordination of eye movements, meaning the visual axis (where the eye is pointing) will not be the same for each eye and diplopia may result. Any damage to the eye structure itself through trauma, disease or systemic illness will also cause double vision.

Causes of Diplopia

Diplopia can occasionally be an indication of serious illness and as such it is advisable to consult with a doctor as soon as possible. Some of the causes like alcohol or drug intoxication are not related to disease processes and subside once the substance is eliminated from the body.

Monocular Diplopia

In monocular diplopia, the symptoms tend to originate from a problem within the eye itself and the double vision persists even when the unaffected eye is covered. The two images appear close together, a phenomenon known as ghosting. The most likely cause of monocular diplopia is a refractive error. This happens when light rays are not focused on the retina by the lens and symptoms can be resolved by looking through a pinhole.

Other causes of monocular diplopia include the following :

  • cataract
  • corneal problems, like keratoconus (affects the corneal shape) and corneal scarring
  • degeneration of the macular part of the retina
  • dislocation of the lens
  • astigmatism
  • disorders of the visual cortex (rarely)

Binocular Diplopia

Binocular diplopia may be related to abnormal alignment of the eyes. In this case, the double vision symptoms resolve when one eye is covered.

Strabismus

In strabismus or squint, the alignment of the visual axis is impaired thereby resulting in double vision. In children, it may be a result of prenatal infections, genetic factors, brain disorders or injury as well as certain degenerative conditions. In adults, strabismus may arise due to a stroke, trauma, poisoning, and the various causes of neuropathy (nerve damage and degeneration).

Trauma

A blow to the face can fracture the orbit causing swelling around the eye and entrapment of the ocular muscles results diplopia. Double vision may also occur with a hematoma following trauma.

Systemic illness

  • Multiple sclerosis
  • Associated signs and symptoms : loss of concentration and memory, decrease in balance and coordination and sensations or numbness or tingling (paresthesia).
  • Myasthenia gravis
  • Associated signs and symptoms : drooping of the eyelid, slurred speech and muscle weakness. Symptoms can resolve after rest.
  • Diabetes mellitus (sugar diabetes)
  • Blurred vision as a result of osmotic changes in the lens needs to be differentiated from diplopia which may be a result of nerve damage (neuropathy) or microvascular damage.
  • Hyperthyroidism (Grave’s disease)
  • Associated symptoms : protrusion of the eyeball (exopthalmus), eye pain, light sensitivity (photophobia), enlarged thyroid gland and unintentional weight loss.

Inflammatory

Infectious or non-infectious inflammation in and around the eye, and related structures, which is not result of  trauma (mentioned above), may include :

  • sinusitis
  • orbital cellulitis
  • exposure to chemical irritants (both eyes affected)
  • optic neuritis
  • cavernous sinus thrombosis
  • certain autoimmune diseases

Vascular

Disruptions in the vascular supply either of the brain (as is seen in a stroke) or within the retina of the eye (diabetes, hypertension) may also result in diplopia. Occasionally, a distended blood vessel (posterior communicating aneurysm) in the brain can compress one of the cranial nerves that controls eye movements thereby causing double vision.

Tumours

Tumours, benign or malignant, can cause diplopia either by affecting the movemet of the eye, compressing nerves or applying pressure on the visual cortex of the brain. This is more frequently seen with paranasal sinus tumors, and those in the orbit or near the base of the skull.

Nerve Palsy

Any disruption of the nerve impulses of cranial nerves III (3), IV (4) and VI (6) will affect the movement of the eye as these nerves supply the eye muscles. This can affect the normal alignment of the eyes and result in double vision.

References

  1. Diplopia. Merck