Astigmatism

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Symptoms

A typical symptom of astigmatism is lack of sharpness in both near and distant vision. The affected people cannot see clearly the contrasts among horizontal, vertical, or slanted lines – e.g. they exchange similar characters such as numerals 0, 3 or 8 or letters H, M, or N. Other symptoms are eyestrain or headache.

Causes

People suffering from astigmatism do not have a regular, round shaped cornea (or lens) – it is flat in some axes or conversely more curved (it has in fact an egg-like shape). Segments of light rays passing through the optical train meet outside the retina and therefore the image falling on the retina is blurry, foggy, and deformed. The exact cause of the differences in corneal curvature is unknown. However, astigmatism tends to be hereditary. Other causes might be eye injuries, corneal diseases or a history of eye surgery. Astigmatism can occur alone or in combination with short- or long-sightedness.

Examination

A routine examination of visual acuity is done by a device called an autorefractometer. The doctor will seat you in front of it and let your chin and forehead lean against a comfortably formed rest. The doctor himself will be seated in front of you. In front of one eye, you will see a defocused fixation point – typically used images are a picture of a house in front of a landscape, a balloon in the sky or a boat sailing in the sea. The point gradually sharpens and blurs again. The device measures the amount of optical power necessary for dioptre corrections. Examining both eyes takes about 5 seconds and is completely painless.

A more precise examination of distant visual acuity is done by optotypes, most often the so-called Snellen charts. These are charts or illuminated boards on the wall in the examination room containing lines with letters of different size. Every line is labelled by a number that equals to the distance from which you should be able to read the letters without difficulties. The doctor will usually seat you or place you 5 meters from the optotypes. You will be asked to cover one eye with your hand – only slightly, without pressure, otherwise you can decrease its ability to distinguish. Afterwards you will be asked to read the chart in lines, or only the letters indicated. The same will be done with the other eye.

To measure eye defects the doctor will use special glasses and a set of corrective lenses. Wearing glasses you will read again from the optotypes and the doctor will add or remove lenses until you see clearly, acutely and sharply.

You will be also examined by two other optical devices – a keratometer and a keratotopograph. These examinations are also completely painless and should not cause the patient anxiety. You will be again asked to sit in front of the device, rest your head and chin and look straight ahead. The keratometer will measure the refraction on the eye cornea and its curvature. The keratotopograph will display the color map of the cornea, showing its steepest and flattest part as well as its curvature in each zone. It is primarily used to map astigmatism precisely and to diagnose corneal diseases. No special preparation is needed for this examination. We only recommend not going partying or staying up late the night before the exam– tiredness can distort visual acuity and influence the results of the examination.

Conservative treatment

Glasses

Dioptre glasses serve as a visual aid to correct vision. They usually consist of a fixed frame into which optical lenses are attached. The frame should not narrow the visual field and its shape should be adapted to the face. The optical lens is an optical set of two centered areas, in most cases spherical. The lenses of the glasses were initially made of glass, nowadays they are made also from different plastic materials to reduce the risk of breaking and because of their lighter weight compared to glass lenses. The disadvantage is the softness of the material – plastic lenses are exposed to scratches. Nevertheless, the majority of plastic lenses can undergo the so-called “hardening” treatment and can thus provide greater resistance to mechanical damage. Coating of the hydrophobic layer protects the lens against the concentration of the moisture and due to a very smooth surface provides easier cleaning. Lipo-phobia coating means higher resistance against grease, e.g. fingerprints. Anti-glare coating might be also applied on lenses, which prevents creation of glares, reflections on the surface of the glasses. Advantage is also smaller eye tiredness in artificial lighting, while working on a computer or watching TV, as well as better vision while driving (particularly at night) etc.

Astigmatism can be corrected by toric lenses. These are cylindrical lenses with different levels of correction in various surfaces; it means they refract the light in one axis differently than in another. They compensate for the cylindrical shape of the astigmatic cornea. The cylindrical glass must be located exactly on the axis corresponding to the measured axis of astigmatism.

Contact lenses

Contact lenses are a modern optical device intended to be used directly on the cornea. Its curvature replaces the curvature of the anterior corneal surface. It serves mainly to correct short-sightedness, long-sightedness, astigmatism, or presbyopia. They also have a therapeutic or protective function in certain eye diseases, for instance, and are used to moisture the cornea when damaged. They can also be used as bandages after eye surgery or as a drug carrier etc.

The technology and production of contact lenses was developed by the Czech scientist Otto Wichterle. Since then contact lenses have experienced significant changes due to modern technologies. According to the material used, they can be divided into two main groups – hard and soft lenses. On one hand, due to their non-porosity, hard contact lenses do not adsorb chemicals and other evaporations. They also have the ability to replace the natural shape of the cornea by a new reflecting surface. On the other hand, they are not oxygen permeable and prevent the eye from “breathing.” When applied for the first time, they are usually uncomfortable and it takes some time to get used to them. Correct usage must be checked regularly. They are mostly recommended for long-term wearing, such as yearly. On the contrary, soft contact lenses are highly permeable to oxygen. When applied for the first time, they are immediately comfortable due to their soft material. However, they are made of less durable material, which wears out quickly. It is necessary to change them regularly as sediments from tear film are formed. They are mainly designed for short-term wearing, such as daily, monthly, etc.

Astigmatism can be corrected by toric contact lenses, which correct the irregular curvature of the cornea. The lens must be curved conversely in two perpendicular axes in order to “balance” the different curvature. Compared to the common contact lenses, they have two dioptre values. Toric contact lens must fit precisely onto the eye and if any detachment occurs, it must be returned to the correct position. Only then is the maximal visual acuity provided.

Surgery

Radical keratotomy

Radical keratotomy is a surgical method to treat dioptric defects. Its objective is to correct vision in such a way so that the patient no longer needs to wear glasses after surgery or at least does not need such “thick” glasses. The doctor will perform (radial) incisions into  healthy corneal tissue with a delicate microsurgical knife – the surgery is in lay terms called “the sun” as the incisions are concentrated around the small circle as rays are concentrated around the sun During the healing process, the cornea flattens, its curvature is reduced and this results in lower dioptres. The number, length, and depth of the radial incisions depend on the initial curvature, the thickness of the cornea and the number of dioptres on the eye. This surgery is performed on an outpatient basis under local anesthesia and takes about 15-30 minutes.

Refractive lensectomy and intraocular lens implantation

To treat dioptric defects a method known as refractive lensectomy can be used. Its objective is to correct vision in such a way so that patients no longer need to wear glasses after surgery or at least do not need such “thick” glasses. By means of ultrasound, the original clear lens is emulsified and its remains are aspirated from the eye. An artificial intraocular lens with a pre-calculated dioptric value is implanted into a preserved pouch. Compared to procedures carried out on the cornea only, this procedure requires more intervention.

Initially, artificial, hard (inflexible) intraocular lenses made of plastic (polymethylmethacrylate) were used. The development of delicate surgical methods led to the demand for soft, flexible intraocular lenses, made of biomaterials (silicone, acrylic, collamer etc.) A soft, folded lens is inserted into the eye through a small incision and, once inside the eye, the lens unfolds into the required shape. According to the visual correction needs, it is possible to select monofocal or multifocal lenses. The first provides clear vision of a single focal point in the lens that is either near or far. The latter allows sharp vision at multiple distances. The surgery is performed on an outpatient basis under local anesthesia and takes about 15-30 minutes.

Implantation of phakic lenses

Patients with a higher dioptre defect or with a thin cornea, where laser surgery is not recommended, might have the special, so-called phakic intraocular lenses implanted, which compensate the dioptre defect inside the eye. Through a small incision in the cornea, the lens is inserted into the anterior chamber of the eye and attached to the iris at a safe distance from the human lens or to the posterior chamber, and placed directly on the human lens. After the surgery, there are two lenses present. The advantage of this method is its reversibility into the original condition – if the patient is not satisfied with the phakic lens, it may be removed without problems and replaced with another. The surgery is performed on an outpatient basis under local anesthesia and takes about 15-30 minutes.

Equipment/devices used

Projection optotype

This device projects onto a wall tables with letters, numerals, signs or hooks of various size that help determine visual acuity from various distances. It further allows color vision to be examined – the eye’s ability to recognize colors and lights of different wavelengths. It also contains many tests to determine visual functions. Our clinic uses the projection optotype of the Nidek Company.

LCD optotype

This device uses LCD screens with high-resolution to display letters, numbers, symbols or hooks of various size which function to determine visual acuity from various distances, even under normal day-light conditions. It further allows color vision to be examined – the eye’s ability to recognize colors and lights of different wavelengths. It also contains many tests to determine visual functions. Our clinic uses the projection optotype of the Nidek Company.

Autorefractometer

This device measures dioptric eye defects (short-sightedness, long-sightedness, astigmatism). It performs automatic, fast, and very accurate measurements of eye refraction by the image projected on the retina and then it analyses the number of dioptres necessary to obtain a sharp image.

Keratometer (opthalmometer)

This device uses an optical method for measuring the diameter or the degree of curvature of the cornea (in millimeters or dioptres) and for determining corneal astigmatism.

Contactless pachymeter

This device is designed for ultrasound measurement of the corneal thickness in its entirety without the necessity of attaching (touching) the test probe to the eye. Local eye anesthesia is therefore not necessary and the risk of transmitting infection is reduced. Our clinic uses the contactless pachymeter of the Nidek Company.

Keratotopograph

This device uses an optical method for imaging the color map of the cornea’s curvature. It is mainly used to diagnose keratoconus – a corneal disease characterized by its conical camber.

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