Astigmatism is a condition in which the surface of the eye, the cornea, does not have a normal curvature. A normal cornea has a perfectly spherical shape, similar to the surface of a ball, in the presence of astigmatism the surface of the eye takes on the shape of a rugby ball (ovoid). The eye has two main radii of curvature (meridians) perpendicular to each other, in the case of astigmatism one is flatter and one is more curved, this anomaly leads to a incorrect focus and an elongated and blurred image.
Types of astigmatism
Depending on the structure involved, one can have:
- corneal astigmatism, caused by an abnormal curvature of the cornea;
- internal astigmatism or lenticular astigmatismgiven by alterations in the internal structures of the eye, e.g. the crystalline lens.
Astigmatism can be classified according to its symmetry into:
- regular, the cornea has an ovoid or toric shape, with the two principal meridians forming a 90° angle between them;
- irregular, the cornea does not show symmetry, the curvature changes along one meridian and the two principal meridians are not orthogonal to each other. Typically, irregular astigmatism results in a variety of asymmetrical aberrations.
Regular astigmatism can be:
- simple myopicone meridian focuses light in front of the retina, the other on the retina;
- simple hypermetropicone meridian focuses light on the retina, the other theoretically behind the retina;
- compound myopicboth meridians focus light in front of the retina;
- compound hypermetropicboth meridians focus light theoretically behind the retina;
- mixedone meridian focuses light in front of the retina, the other behind.
Non-surgical management of astigmatism
Glasses. They represent the simplest and mainly chosen form of correction. Corrective lenses deflect incoming light rays in order to compensate for the error caused by the curvature of the cornea, the images are thus projected correctly onto the retina.
Contact lenses. Some astigmatics prefer them because they can provide clearer vision and a wider visual field. In this case, soft toric contact lenses, which can correct many forms of astigmatism, are the most suitable. The maintenance of contact lenses, however, requires regular cleaning and care to safeguard eye health.
Orthokeratology or (ortho-k) involves the fitting of rigid, gas-permeable contact lenses that maintain their regular shape while lying on the cornea, thus compensating for the irregular shape of the eye surface. The patient wears the contact lenses for limited periods, e.g. overnight, and then removes them. Orthokeratology does not permanently improve vision; if the patient no longer wears corrective lenses, they may return to their original condition.
Laser correction
Laser microsurgery acts on the refractive defect by varying the thickness of the cornea and thus allowing correct focusing on the retina. The main types of laser refractive surgery are:
- LASIK or laser-assisted in situ keratomileusisis the most common form of laser. The doctor uses a device, called a microkeratome, to remove a thin layer of epithelial tissue from the innermost layer of the cornea. This procedure allows vision to be recovered within a few days.
- PRK or photorefractive keratectomy, the laser in this case removes microscopic fractions of tissue from the superficial and inner layers of the cornea.
- LASEK or epithelial laser keratomileusis, the epithelium is not removed mechanically, but is lifted and after the laser acts in changing the shape of the cornea, the corneal tissue is repositioned. This approach involves a much thinner layer than PRK, and is used in the case of thin corneas.
Toric Intraocular Lenses (IOLs)
Toric IOLs have been developed for correcting astigmatism during cataract surgery. The surgeon performs phacoemulsification surgery according to the standard procedure, but replaces the natural cloudy lens of the eye with the toric IOL, which functions like the corresponding soft contact lenses used for astigmatism. The placement of a toric IOL following cataract surgery offers the advantage of a correction of astigmatism more stable compared to a toric contact lens, the IOL being implanted inside the eye.
Today, ophthalmologists enjoy numerous strategies for correcting astigmatism, so they can choose the most suitable one for the individual patient and improve his or her quality of vision and quality of life. Moreover, even irregular astigmatism, whose correction is more problematic, can with new technologies and a customised approach be reduced and made more regular.
Sources:
-Astigmatism - Definition, Etiology, Classification, Diagnosis and Non-Surgical Treatment. Dieudonne Kaimbo Wa Kaimbo. Book: Astigmatism - Optics, Physiology and Management, 2012.
-Modern Management of Astigmatism. Li et al. International Ophthalmology Clinics, 2013.
Dr. Carmelo Chines
Direttore responsabile
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