Myopia: definition, symptoms, and treatment

General overview

When light rays enter the eye, their direction is altered by a process known as refraction. In a healthy eye, images are focused precisely on the surface of the retina, located at the back of the eye, before being transmitted to the brain. This requires a perfect balance between two factors: the eye’s refractive power (provided by the cornea and the crystalline lens, the eye’s natural optical lenses) and the axial length of the eye (the distance light must travel after refraction to reach the retina).

Visual disorders arise when this complex mechanism is imbalanced. Myopia (nearsightedness) is a prime example. Myopia has two main causes:

• An eye that is slightly too long (the most common cause)
• Excessive refractive power, due to an overly curved cornea and/or crystalline lens (so-called “index myopia”)

In both cases, images are focused in front of the retinal plane, rather than directly on it. This results in impaired distance vision, while near vision remains clear. Myopia is a widespread refractive error, affecting approximately 40% of the French population.

Degrees of myopia: mild, moderate, high, severe

Myopia presents with varying degrees of severity, quantified using an optical unit called the diopter (symbol: D). The higher the negative diopter value, the more severe the myopia. Mild myopia is defined as between -0.5 and -3D, although patients with -2.5D often already experience significant dependence on glasses or contact lenses. More advanced cases are classified as moderate myopia (from -3 to -6D) or high myopia (greater than -6D). Except in index myopia, there is a direct relationship between the axial length of the eye and the diopter value: the average eye length is 23.5 mm, and each additional millimeter corresponds to approximately -2.5 diopters.

Glasses and contact lenses

Optical correction with glasses or contact lenses remains the most common treatment for myopia and is partially reimbursed by the French national health insurance. The process begins with a diagnosis by an ophthalmologist, who quantifies the degree of myopia and prescribes the appropriate correction. Glasses or contact lenses allow distant images to be focused on the retina. Overcorrection is relatively common and may cause symptoms such as headaches due to constant accommodative effort; in such cases, the prescription should be adjusted by the specialist.

Laser refractive surgery

Myopia can also be corrected surgically, primarily to eliminate dependence on glasses or contact lenses. These procedures are not reimbursed by national health insurance, as they are considered elective, but some private insurance may provide coverage.

The principle involves using a laser to modify the curvature of the cornea and thus the refractive power of the eye. Several techniques are available:

• LASIK: The laser is applied to the deeper layers of the cornea.
• PRK (Photorefractive Keratectomy): The intervention targets the superficial corneal layers.
• SMILE: A more recent technique involving the removal of a corneal lenticule to achieve the desired correction.

The choice of technique depends on individual patient characteristics, particularly corneal thickness. For thin corneas (less than 500 micrometers), PRK is preferred over LASIK or SMILE.

What is glaucoma, a possible complication of myopia?

Glaucoma is a serious eye disease that, if untreated, can lead to blindness. It involves progressive damage to the nerve fibers connecting the retina to the optic nerve, often due to increased intraocular pressure. High myopia is a risk factor for glaucoma, among others such as family history, previous eye diseases, trauma, or prolonged corticosteroid use. Treatment may involve laser procedures or eye drops to lower intraocular pressure; surgery is required if these measures fail to halt disease progression.

Does ethnicity influence myopia?

Certain studies indicate that ethnicity affects myopia prevalence. For example, in Singapore, individuals of Chinese descent are more frequently affected than those of Indian or Malay origin. In Western countries, myopia prevalence is 25–40%, but in some Asian populations, it can reach up to 80% among schoolchildren. The prevalence of high myopia also varies: about 4.5% in Europe and the US, compared to 20% in some Asian groups. These differences highlight the importance of genetic factors, but environmental influences such as screen use and lack of natural light exposure also play a significant role.