The eye is shaped as a slightly irregular, hollow sphere that receives light and translates it into images. Light first passes through the cornea, the clear, front “window” of the eye. The cornea’s refractive power bends (refracts) the light rays that move freely through the pupil, the opening in the center of the iris through which light enters the eye. The iris (the colored part of the eye) works like the shutter of a camera, by opening and closing. It has the ability to control the diameter and size of the pupil, thus controlling the amount of light entering the eye. After passing through the iris, the light rays pass through the natural crystalline lens. This clear, flexible structure operates like a camera lens, shortening and lengthening its width . Light rays pass through a dense, transparent gel-like substance, called the vitreous, that fills the globe of the eyeball and helps the eye maintain its spherical shape. In a normal eye, the light rays come to a sharp focusing point on the retina. The retina functions much like the film in a camera. It is responsible for capturing all of the light rays, processing them into light impulses, then sending them to the optic nerve.
The movements of the eyes are controlled by six pairs of muscles located on the outer part of each eye. Under normal circumstances, where both eyes look towards the same direction, one pair of muscles contracts while another relaxes. These pairs of muscles function separately with the intention to keep the eyes in parallel positions and to allow them to move towards all directions.
Normal Functioning Eye Coordination
For normal vision, both eyes must focus on the same target simultaneously. Each eye perceives the same object from a slightly different angle, thus sending the brain two different images, which are then combined into a unique three-dimensional image. This process is important for depth perception, namely the ability to identify the relative space between objects.
Strabismus – Diplopia – Amblyopia
Strabismus occurs when one eye looks directly towards an object while the other eye aligns to a slightly different direction, inwards or outwards. A clear image and a blurry one are sent to the brain from each eye respectively. As a result, these images cannot be combined into one (diplopia) and the brain chooses to ignore the image received from the weaker eye. Thus, the greatest portion of vision is taken on by the stronger eye leaving the weaker one to be used much less (amblyopia).
The eyes function fully from the age of six months old, while the brain function that perceives and processes visual stimuli grows until the age of six to seven years old.
Diagnosing visual impairment
Stereoscopic vision can be defined as the visual ability to distinguish depth; good stereoscopic vision implies that both eyes coordinate and function properly making it unlikely for strabismus or amblyopia to develop. In general, the stereoscopic vision exam is the way to diagnose and medicate visual impairment in children.
What is diplopia?
Diplopia may occur in one or both eyes by various conditions and it is the visual irregularity when the patient sees an item double and one of the two images is more faded than the other.
Causes of diplopia
- Irregularly shaped cornea (refractive error)
- Dry eye due to deficient tear production
- Cornea anomalies
- Anomalies of the natural lens of the eye, such as cataract
- Retina anomalies, such as macular degeneration
How often does diplopia occur?
Diplopia is a crucial ophthalmological disease that may also develop into a neurological issue.
In diplopia suddenly appears at an adult age and it does not derive from childhood strabismus, it is indicative of the existence of other conditions that affect the free movement of the eye, the muscles, the nerves or the brain.
- Thyroid condition that affects the muscles of the outer eye. The disorder most commonly obstructs the eyes from fully looking up, leading to diplopia usually when looking up or down.
- Conditions of the blood vessels that supply blood to the brain or of the nerves that control the muscles of the eyes (craniac nerves IV & VI). For instance, a stroke or an ischaemic incident (for the ages of 50, 60, 70) may seriously affect the muscles that move the eye.
- Diabetes that affects the blood vessels of the eyes. People suffering from unregulated hypertension or even diabetes, are likely to develop diplopia after a vessel stroke. In such cases diplopia occurs towards the right or the left side of vision, forcing the patient to tilt their head in order to clear their vision.
- Myasthenia gravis that weakens the body muscles, inducing diplopia towards the end of each day due to severe fatigue.
- Multiple sclerosis that influences the nerves of the eye. The intervention of an ophthalmologist is essential to perform a brain MRI in case of an optical nerve inflammation (ages 20-50). Afterwards, a therapy will be prescribed in consultation with a neurologist.
- Aneurism, a blood vessel lump, due to weak blood vessel walls.
- Brain tumor or cancer behind the eye, obstructing the free flow or affecting the nerves of the eye muscles.
- Head injury that either impairs the brain or the nerves responsible for muscle eye movement or damages the eye socket and limits the muscle movement.
- Past strabismus cases that had been overcome during infancy may occur again around the age of 50 or 60; it is however a non-severe case of diplopia.
How is diplopia diagnosed?
Initially, it is crucial to establish whether diplopia is monocular or binocular.
Monocular diplopia: A rare type of diplopia that is caused by astigmatism and cataract. It is present when only the diseased eye is uncovered and not when it is covered.
Binocular diplopia: This type of diplopia occurs when both eyes fail to coordinate and it is not hereditary. Vision is normal if one of the eyes is covered, since when both eyes are uncovered, the brain is not able to process the two received images.
How is diplopia treated?
The ophthalmologist will examine and treat monocular diplopia with:
- Glasses prescription
- Contact lenses
- Refractive surgery, if it is about astigmatism
- Cataract operation, if it addresses people between 60-70 years old.
Due to its neurological pathology derivation, an ophthalmologist, general practitioner or neurologist can treat binocular diplopia.