For the ocular system to function under perfect conditions, the rays of light must travel through it correctly, passing through the cornea and the lens to focus on the macula, a specific area of the retina. This ensures clear vision. Also, thanks to the pupil, the eye is equipped to control the amount of light which enters based on requirements: when there is little light, the pupil dilates, and when there is a lot of light, the pupil contracts.
When this balance is achieved, the eye is considered “emmetropic”. If not, the images become blurry and the eye is referred to as “a
mmetropic”. There are different types of a mmetropia and refractive defects.
Diagram of an EMMETROPIC eye, with normal axial length, with rays which enter in parallel and focus on a single point on the retina. Example:
Myopia is a refractive defect which occurs when images focus in front of the retina instead of on it. As a result, distance vision is affected because the images come out blurry. The reasons for which this lack of focus can occur are either because the eyeball is too long (long axial length) or because the surface of the ocular systems (cornea or lens) are too curved.
Today, one of the most commonly used options to permanently resolve myopia is Laser surgery, although the specific technique used depends on the eye of each patient. Through this procedure highly satisfactory and immediate results are achieved. Another solution is phakic intraocular lenses
Diagram of a MYOPIC eye, with long axial length, with rays which enter in parallel and focus in front of the retina.
Hypermetropia occurs when the rays of light which enter the eye cross at a point behind the retina, which causes blurriness especially with near objects. The causes of this lack of focus are either because the eye is too short (short axial length) or because the ocular surfaces (cornea and lens) have too flat a curve. This refractive defect is often not detected because, depending on the age and severity of the hypermetropia, the eye can compensate using the accommodation of the lens, although this ability is lost over time. Therefore in some cases, blurriness is not the only indication of Hypermetropia. Other symptoms include headaches, eye irritation or tiredness; due to the eye’s constant effort during accommodation to achieve clear vision.
Laser surgery is a technique which works well for less severe cases of hypermetropia. There are other options such as phakic intraocular lenses
s when the refractive defect is more severe, or even pseudophakic lenses which replace the crystalline lens, recommended for patients aged 45 and over.
Diagram of a HYPERMETROPIC eye, with short axial length, with rays which enter in parallel and focus behind the retina.
Astigmatism is a refractive defect which causes a distortion of images, affecting vision at all distances. It can occur on its own or combined with myopia or hypermetropia. It is caused by the different curvatures of the cornea. The latter tends to be oval in shape, and this irregularity causes a lack of focus to a greater or lesser extent. The rays of light which pass through the eyeball do not focus on a specific place but on different points, causing distortion of the image. In most cases, the origin is genetic although it can also appear after a trauma, illness or a surgical operation such as a cornea transplant for example.
Depending on the degree of astigmatism, it can manifest itself in different ways. If it is a minor case and is not accompanied by another refractive defect, it is highly likely that the patient will not suffer blurriness but will experience headaches, eye pain or dizziness after performing up-close tasks over a long period of time. In more severe cases however, vision will be reduced and distorted.
Today there are surgical techniques to correct astigmatism. As is the case with myopia and hypermetropia, it can be corrected through Laser surgery provided that the degree of astigmatism is not very severe, and depending on whether it is or is not associated to myopia or hypermetropia. Also available are toric intraocular lenses, which can be phakic (implanted between the cornea and the crystalline lens) or pseudophakic (which replace the crystalline lens). These are often used to correct severe astigmatism or astigmatism in conjunction with another refractive defect.
Diagram of the eye with ASTIGMATISM, normal axial length, with rays which enter in parallel and focus on different points (for example, one in front of the retina and another behind the retina).
In normal conditions, to be able to focus on objects in near vision, we subconsciously use what is called “accommodation”. This is the lens’ ability to focus and defocus on objects according to requirements. Presbyopia is the loss of this ability to focus on near objects and is related to age. The lens deteriorates over time and hardens, losing the ability to change its shape to achieve clarity in objects. This begins to appear from the age of 40-45. The lens loses its accommodative capacity over time which ultimately causes this blurriness. This is a gradual process which usually stops at around 60 years of age and happens naturally. Common symptoms are a loss of the ability to see very small letters, headaches, letters which “join together” or the inability to focus up close.