Shortsightedness

PATHOLOGY

In a normal (emmetrope) eye, the light coming from an infinite distance focuses exactly on the surface of the retina, generating a clear image. In the shortsighted eye, conversely, light focuses at a point in front of the retina and the greater is the visual defect, the shorter is the distance at which the patient is able to see well.

Based on its extent, shortsightedness can be defined mild (up to -3 D), moderate (from -3 to -6 D) or high (more than -6 D).

The factors that can cause shortsightedness are:

– An excessive length of the eyeball: shortsightedness, in particular if it is high, often derives from an imperfect geometry of the eyeball, which appears to be too long. This defect is referred to as axial myopia. In the presence of axial myopia, the light that crossed the cornea and the lens, even if there are no deviations by these lenses, will focus in front of the retina and also in this case there will be a difficulty in focusing on distant objects.

– The cornea and the surface of the lens are too steep: if the cornea and/or the lens present a focusing power higher than normal, the eye will focus the image of an object in front of the retina; in this situation the eye cannot focus distinctly on distant objects and there will be a refractive error referred to as refractive myopia.

– The refraction index of the lens nucleus is higher than normal (index myopia).

SYMPTOMS

A blurred vision when looking at distant objects and a clear vision when looking at close objects. Headache when trying to focus.

DIAGNOSIS

It is possible to make a diagnosis for myopia during an eye examination by an ophthalmologist, both with the simple reading of the characters of the eye chart and with an in-depth instrumental investigation.

The instrumental tests that have a more widespread use are:

– Autorefractometry, the detective system for ocular refractive error measurement which provides corneal and refractive parameters;

– IOL Master, which measures the length of the eyeball (the principal cause of myopia) and the corneal curvature;

– Topography, which enables to quantify the extent of astigmatism with the relevant axis and a map of the corneal curvature in each point;

– Pachymetry, which calculates the complete thickness of the cornea;

– B-Scan Ultrasonography, in order to assess the presence of alterations having a morphological and structural character and of degenerations of the peripheral retina due to an excessive length of the eyeball, in very shortsighted patients.

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TREATMENTS

CORRECTION

Nearsightedness typically is easily corrected with prescription eyeglasses or contact lenses.

Lenses used to correct nearsightedness are concave in shape, in other words they are thinnest at the center and thicker at the edge.

These lenses are called diverging lenses or “minus power lenses” (or “minus lenses“) because they reduce the focusing power of the eye. They must have a power adequate to the visual defect. The power of lenses that correct nearsightedness is measured in units called diopters (D). The lens powers on an eyeglass prescription for myopia always begin with a minus sign. The higher the power number of the lens, the more myopia it corrects.

For example, a -6.00 D lens corrects twice the amount of nearsightedness as a -3.00 D lens.

In alternative to the prescription glasses, in particular in patients having a high degree of shortsightedness, contact lenses can be used.

 

SURGICAL THERAPY

Today myopia can be permanently corrected (or significantly decreased) thanks to refractive surgery. There exist several refractive surgery techniques, but they are all based on a modification of the curvature and, therefore, of the dioptric power, of the cornea in such a way as to compensate the refractive error. Depending on the thickness of the cornea of the patient, it is possible today to correct myopias reaching up to -10 diopters.

Through refractive surgery with excimer laser it is possible to compensate the refractive error, avoiding the correction with glasses or contact lenses.

Refractive surgery is performed under local anesthesia, is painless both during and after surgery, it only lasts a few minutes and consists in modifying the curvature of the corneal surface. The “excimer laser” in a PRK (Photo Refractive Keratectomy) surgery makes it possible to obtain a disintegration or “vaporization” of the more superficial corneal structure, i.e. the corneal epithelium, the Bowman membrane and a part of the stroma. Thanks to this surgery, the eye can focus again the images on the retinal plan and in particular in the macular area, and the patient can see again distinctly in a very short time without the need of glasses or contact lenses.

In a surgery with I-LASIK (Intra-Laser in Situ Keratomileusis), the “excimer laser” is used in combination with a “femtosecond laser” which issues a sequence of consecutive impulses with intervals of one millionth of billionth of a second. In the SMILE (Small Incision Lenticule Extraction) surgery, the entire surgical procedure is made using exclusively a “femtolaser” which makes it possible to create an intrastromal corneal lenticule that is then extracted modifying in an alternative manner the corneal dioptric power.

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