Myopia

What is myopia?

Myopia is a spherical refractive defect in which the light rays coming from the infinite (parallel) are brought into focus in front of the retina, since the refractive power of the ocular dioptre is excessive with respect to the length of the bulb.

We will therefore have an excess of convergence (greater dioptric power). Therefore, nearsighted people see well from near and far.

The extent of myopia is measured with the divergent lens, which corrects the vice of refraction.

Some examples:

• focus at 10 centimeters refers to a myopia of 10 diopters
• the focus at 33 centimeters refers to a myopia of 3 diopters
• focusing at 1 meter refers to a myopia of 1 diopter.

Myopia can be classified into three stages:

• slight, if it goes from 0 to 3 diopters
• moderate, if it goes from 3 to 6 diopters
• high, if it is greater than 6 diopters.

Symptoms

See details: Symptoms Myopia

Causes

The causes of myopia can be multiple:

• The most frequent form in the clinic is axial or axial myopia, due to an excessive length of the eyeball compared to the norm. For this reason the fire does not fall on the retina but in front of it.
• In index myopia, however, the eyeball is normal, but the dioptric power is greater because the refractive index of the crystalline lens is higher than the norm. This occurs mainly in the cataract, a disease that affects the crystalline nucleus making it thicken and thus increasing its refractive index.
• In keratoconus myopia, however, we have a conical-shaped cornea. Keratoconus is a form of corneal degeneration in which there is a progressive thinning and flattening of the cornea that assumes the conical shape with an increase in its curvature, which causes myopia. There may also be irregularities in the corneal surface which causes astigmatism. The two can be present at the same time. Corneal transplantation is required for advanced keratoconus cases.
• Accommodative spasm myopia is due to a curvature of the anterior surface of the crystalline lens above the norm.

Correction of myopia

• traditional glasses
• contact lenses
• refractive corneal surgery or refractive photokeratectomy.

For glasses, lenses are used that make the parallel rays diverge.

Contact lenses have the same divergent principle as glasses; they require more care but offer a more complete view of the entire visual field, being spherical and resting directly on the cornea.

Laser Surgery

In recent years the use of laser surgery on the cornea has become widespread.

As described above, the most frequent form of myopia is that in which the bulb is elongated. Not being able logically to act on the bulb, it will act on the cornea and will perform a laser surgery with removal of corneal layers in order to vary the refraction of the cornea extended. For the correction of myopia, an excimer laser is used, a term that derives from English and which means excited dimers (excited means are noble gases). This type of laser emits light at a pulsating frequency (of the order of nano seconds), of very short duration and of very high frequency.

The laser performs different "passes" on the cornea and each one of them is removed one micron thick, which corresponds to a tenth of a millimeter, taking into account that the total thickness of the cornea is about 500 microns. The ablation rate is in proportion to the extent of the defect; for example, for a myopia of 3 diopters, 30 microns of cornea are removed. The final effect that is obtained is a reduction of the corneal surface with a "flattening" of it, so that the light rays, if they are focused in front of the retina in myopia, are less refracted after the operation and can fall more far away, directly on the retina itself ..

Excimer laser surgery is not risky, but in any case it cannot be applied to all subjects. It is good to wait until the patients to be subjected to the operation are at least 20-21 years old and a stable myopia, that is, it does not continue to get worse, for at least 2 years. The intervention is indicated especially in patients who have intolerance to contact lenses or who, psychologically, cannot stand glasses.

The contraindications to laser surgery are:

• Diabetes
• Connective tissue diseases (connective tissue diseases) such as rheumatoid arthritis and Syogren's syndrome. This is because the cornea is also made of connective tissue.
• Keloids (thick scars caused by excessive growth of fibrous tissue).
• Hormonal therapies: some hormones, even those contained in the contraceptive pill, can cause fluids to retain. In these cases, the thickness of the cornea is increased and therefore there is the risk of removing an excessive amount. Or, again, hormones can cause a reduction in tear secretions, therefore putting the patient at risk of developing serious eye infections after surgery, because the function of lubrication and protection of the cornea by the tears is lost.
• Some medications, especially modern antidepressants, can cause corneal opacities.

There are two different surgical techniques to solve the problem of myopia:

• PRK (refractive photokeratectomy) : the excimer laser is capable of removing extremely precise tissue (fractions of thousandths of a millimeter), unreachable by any other means. The correction of myopia is achieved by removing the superficial layers of the cornea (superficial keratectomy) in order to modify its curvature, and acts directly on the cornea itself. Astigmatism and hypermetropia are for the moment treatable with less chance of success if this technique is used. Vision recovery is achieved after a certain period of time, ranging from one to three months (depending on the extent of the correct myopia). As a secondary effect to laser treatment, usually transient corneal opacities may arise in the healing area. They can cause a reduction in visual efficiency, but usually subside in a time frame that fluctuates around six months. Furthermore, in the first post-operative phase, irregularities in the corneal surface may occur, which may cause reductions in visual acuity, but they are also temporary and tend to regress with time. Other typical manifestations of the immediate post-operative period may be pain, which usually lasts up to 48 hours after surgery and is treatable with analgesics, the sensation of a foreign body in the eye, tearing and the feeling of glare, especially overnight. In a small percentage of cases a new laser treatment may be necessary or to clean the cornea of ​​residual scars or to improve the refractive result.
• LASIK : is a method that allows you to correct defects such as myopia but also astigmatism and hypermetropia with equal success. It is performed not directly on the surface of the cornea but in the corneal stroma, which is its intermediate portion, which is accessed by a previous incision using an instrument called a microkeratocone, which creates a kind of "door" in the cornea. This technique allows a complete visual functional recovery already in the second or third day and the refractive result is optimal. Also in this case, it is possible that after some time a new intervention is indicated for the same reasons listed above. The post-operative symptomatology, also in this case, is the same.

Both operations are performed in the clinic, with local anesthesia administered by drops of eye drops, and are very rapid: the PRK technique lasts about 1-2 minutes, the LASIK technique takes about 10-20 minutes. They allow you to correct myopia up to 12-14 diopters and astigmatism and hyperopia up to 5-6 diopters.