The eye (from the Latin word oculus, eye) is the organ of vision. By capturing the light, it allows us to interact with the environment. The human eye can distinguish shapes and colors.

The adult eye, or eyeball, is a sphere about 2,5 cm in diameter. It is protected in front by the eyelids, each of which has eyelashes. They are lined with a membrane, the conjunctiva, which secretes a lubricating mucus to prevent drying of the eye. The lacrimal gland, located above the eye, continuously releases tears (about 1mL / day / eye) helping to clean and protect the eye.

The bulb of the eye

The eye itself is called the bulb of the eye. It is a hollow sphere filled with liquids, the wall of which is made up of three superimposed tunics (envelopes). The transparent lens is the “lens” of the eye, it focuses light on the retina. It is protected by a liquid called aqueous humor.

The outer covering of the eye is a fibrous coat. It is a protective membrane which is made up of two elements: the sclera and the cornea. The sclera is the “white of the eye”. The cornea is transparent, it is the most exposed part of the eye. It very often suffers lesions but its capacity for regeneration and healing is extraordinary. The cornea can also be transplanted without risk of rejection because it does not contain any blood vessels.

The intermediate envelope is a vascular tunic which is made up of three elements:

• The choroid, behind the eye, is a membrane rich in blood vessels that nourishes the three ocular tunics. It contains a dark brown pigment which absorbs light preventing its reflection inside the eye.
• The ciliary body is located in front of the eye, in the continuity of the choroid. Composed of ciliary muscles, it acts on the shape of the lens by adapting its curvature to the distance from objects.
• The iris is located between the cornea and the lens. It contains pigment cells which determine the color of the eyes. In the center is the pupil, a round orifice that allows light to enter the eye.

The inner coat of the eye is the retina. It extends from back to front, to the ciliary body. Half a millimeter thick, it is composed on its outer side of a pigment layer, stuck to the choroid, which absorbs excess light thanks to melanin granules. Its inner side contains millions of visual cells: rods and cones. At its center is a small area called macula : this is where visual acuity is at its maximum.

Visual cells

Visual cells are so-called photoreceptor neurons because they react to light.

The rods (120 million in each retina) are widely located on the periphery of the retina. They are responsible for peripheral vision and make it possible to distinguish shades of gray in semi-darkness.

The cones (6 million) are located mainly in the center of the retina. They are involved in the vision of colors and details in bright light.

There are three types that each detect different wavelengths of visible light:

• Cones of the first type react mainly to blue light
• Cones of the second type in green light
• Cones of the third type react to red light

When excited by light, photoreceptors translate light energy into an electrical signal. These visual messages leave the retina through the optic nerve to reach the brain where they will be interpreted to produce visual sensations.

Light path through the eye

When the light enters the eye, it passes successively through the cornea, the aqueous humor, the lens and the vitreous body, then reaches the retina in order to stimulate the photoreceptors. The image projected on the retina is inverted and smaller than the observed object.

The main global cause of visual impairment is cataracts with almost 47,9% of cases. Other major causes of visual impairment include glaucoma (12,3%), age-related macular degeneration (8,7%), diabetic retinopathy (4,8%) and trachoma (3,6% ) ³.

Retina

Achromatopsia total : insufficiency of the three types of cones which results in a total absence of color vision. The vision is in shades of gray.

Daltonisme : insufficiency of one of the types of cones which makes it impossible to distinguish certain colors, more frequently red or green.

Age-related macular degeneration (AMD) : degeneration of the cells of the macula which results in a loss of central vision (peripheral vision is not affected).

Retinal detachment : due to a small pocket of fluid located under the retina, it mainly results from a tear in the retina which allows intraocular fluid to infiltrate and lift it. The affected area no longer works.

Diabetic retinopathy : retinal damage resulting from retinal vascular disorders which may occur in people who have had diabetes mellitus for several years.

Vision disturbances

Nearsightedness: Clear vision up close but blurry vision from afar.

Hyperopia: blurred near vision.

Presbyopia: decrease in near visual acuity.

Astigmatism: blurred vision both near and far.

crystalline

Cataract: clouding of the lens interfering with the passage of light which confuses near vision. Leading cause of blindness in the world.

Eyelids

Stye: infection of the base of the eyelid after infection with bacteria.

Trachoma: infectious disease caused by the bacteria Chlamydia trachomatis, initially affecting the eyelid and may cause blindness after repeated infections. Occurrence of the disease in crowded places with limited access to water and health care. Spread rapidly on contact with the hands or clothing of an infected person.

Eye disease

Glaucoma: damage to the optic nerve resulting in loss of the visual field.

Dry eye syndrome: due to a decrease in the secretion of tears or an imbalance in its composition.

In children

Visually impaired : represents 1% of births. Babies don’t stare at their toys and don’t react to smiles. A delay in movement and awakening is noted. Low vision can also result in white pupils, jerky eye movements and a small eye.

Strabismus : 4% of children are affected. Present from the first days of life or late onset (up to 3 years). Serious disease which requires rapid treatment: one eye quickly becomes “dominant” and its vision will develop normally while the “lazy” eye remains deviated, which stops its development.

Eye and vision examination

At any age, it is important to regularly check the health of the eyes during a medical examination at the ophthalmologist. This monitoring is all the more important if you have risk factors (family history or recurrence of eye diseases, diabetes, taking medications such as cortisone).

Sunglasses

Sunglasses protect the eyes from solar radiation, especially ultra violet (UVA and UVB) which can damage the eye. It is strongly recommended to wear them both at sea and in the mountains where the light is particularly important in order to prevent cataracts for example (8).

Diet and eye health

Vitamins, fatty acids… Certain foods are beneficial for our eyes.

Treatments

Viagra

An American study (6) from 2005 showed that taking Viagra could cause non-arteritic ischemic optic neuropathy in people at risk: the blood circulation is interrupted at the level of the optic nerve which causes a sudden drop in vision. In 2014, Australian researchers (7) also showed that Viagra can destroy cells in the mouse retina causing adverse effects on vision.

Cortisone

Corticosteroids increase the risk of glaucoma and cataracts. The short dose of treatment exposes little to these risks while people receiving prolonged or chronic doses are more at risk (9).

Fundus examination : painless examination carried out by an ophthalmologist in order to observe the structures behind the eye, in particular the retina. It makes it possible to detect certain pathologies (diabetic retinopathy, AMD, glaucoma) or to follow their evolution.

Eye pressure measurement : it is the pressure of a liquid inside the eyes, not to be confused with arterial pressure, which is the pressure of the blood inside the arteries. It is measured in millimeters of mercury (mm Hg) using a tonometer. On average, normal blood pressure is considered to be between 10 and 21 mm Hg.

Visual acuity measurement : is verified with an examination of the vision thanks to visual test tables, without correction then with.

Amplitude of visual fields : study of the space perceived by the gaze, while the eyes remain motionless. Can be measured with the finger or with instruments.

Optical coherence tomography (OCT) : non-invasive imaging technique that allows you to visualize the thickness of the retina at the level of the macula and to check the condition of the optic nerve. Allows screening for glaucoma for example.

In 2007 (12), Swedish researchers showed a correlation between the iris and personality traits. The study of characteristic iris structures (crypts, pigments and contraction grooves) in 428 participants revealed certain patterns: people with more crypts were found to be warmer, tender and confident. While the furrows have been associated with impulsiveness. According to the researchers, the gene sequences responsible for the structure of the iris also contribute to the development of the frontal lobe of the brain (the seat of the personality).

According to a 13 study (2014), eye color and the feeling of pain could be linked: women with light eyes (blue and green) would experience less pain during childbirth than women with dark eyes. In addition, women with light eyes are less anxious and less prone to negative thoughts after birth. A genetic link would be considered but it remains to be determined.