Contents
To understand what astigmatism is and what its causes are, it is necessary to understand the structure of the eye. Our eye consists of three elements: the optical system (lens and cornea), the retina and the optic nerve.
The optical system is located at the front of the eye and refracts the light rays passing through to form an image on the retina. The retina is located at the back of the eye and is a layer of cells that convert colour and light into electrical signals.
The optic nerve serves as the “cable” that carries these signals from the retina to the brain. Astigmatism usually develops when there are problems with the lens or cornea. For the cornea to function properly, it must be perfectly curved and hemispherical in all directions. Astigmatism impairs vision because the surface of the cornea is uneven or curved more to one side than the other.
When light hits the irregularly shaped cornea, it cannot focus properly at a single point on the retina. This results in a significant decrease in visual acuity and blurred vision. With this condition, objects in the surrounding environment may also appear distorted.
Vision and perception is reduced at all distances, no matter how close or far away the object is.
Types and causes of the disease
Specialists classify astigmatism into three main types.
Myopia (myopia).
It can be simple or complex. With myopic astigmatism, the cornea becomes irregularly spherical in shape, as a result of which distant objects in some places seem blurred, and in other places – the contours are clearly visible.
Hypermetropia.
Symptoms are similar to hyperopia and are of two types: combined and simple. The only difference is that the clarity of what is visible changes: close objects seem only partially blurred.
Mixed type.
One eye is nearsighted and the other is farsighted. Depending on the cause of occurrence and the disturbance of peripheral vision, astigmatism can be divided into congenital and acquired. People with congenital astigmatism inherit the irregular shape of the cornea from their parents. It should be noted that very few people have perfect corneas, and each person has a minimal degree of this condition.
If the degree of astigmatism does not exceed 0.5 diopters, astigmatism does not affect vision (functional or physiological astigmatism); at 0.75 diopters and above, the condition can seriously affect vision (especially in children).
Otherwise, it can lead to amblyopia, the so-called “lazy eye”, which requires long-term treatment and monitoring. Acquired astigmatism is caused by pathological effects on the cornea:
- Trauma and scarring;
- Burns to the conjunctiva;
- Inflammation (keratitis);
- Diseases of the cornea (dystrophy, keratoconus);
- Diseases of the eyelids;
- Surgery and postoperative suturing of the sclera and corneal surface.
When the normal structure and integrity of the cornea is disturbed, its optical properties are altered and vision is impaired.
The main symptoms of the condition
The symptoms and degree of astigmatism vary. Some people may not notice any change in their vision. If the symptoms cause no discomfort, treatment may not be necessary. The degree of the condition often changes over time, but the severity of the symptoms – the ability to see details and objects – does not.
The main symptoms of astigmatism include:
Blurred images caused by a disturbance in the refraction of light rays by the cornea and lens. This causes circles, ovals, lines or “figure eights” to form on the retina instead of dots, resulting in blurred or double images in vertical, horizontal or oblique directions. In milder forms of the disease, vision and this symptom are virtually indistinguishable from normal vision.
The condition is accompanied by headaches (often throbbing) and eye pain (burning, tearing or watering). It is very difficult to recognise astigmatism from this symptom – the headache does not always occur with intense visual strain and can occur after a few hours. Many people are therefore unaware of the link between headaches and astigmatism. Deterioration of vision at twilight due to astigmatism. Deterioration of vision at twilight is often referred to as “hen’s eye”.
People with this condition have difficulty finding their way in the dark. Driving in the dark is particularly dangerous. Increased sensitivity to light, pressure and pain in the eyes can affect daily life. Rapid tiredness and eye strain. This can interfere with both near tasks (such as reading) and tasks that require distance vision (such as driving).
Frequent head tilting and squinting. Eyes with astigmatism and uncorrected vision see a distorted image. Tilting and turning the head or squinting when looking at objects is one of the pathological symptoms. When you squint, the vertical meridians of the eye flatten and the circle of light emitted by the light source becomes smaller. This treatment improves vision for a short time.
In addition, astigmatism causes the head to tilt sideways when the main meridians are shifted. As a result, objects are pulled in one direction and take on a recognisable shape. In children, who are often unaware of their visual impairment, special attention should be paid to the above symptoms, regular vision tests should be carried out and corrective measures should be taken in time to prevent the disease from developing into more complex forms.
It should be noted that astigmatism is not usually associated with redness or tearing of the eyes. Nor should it cause darkening, halos or other changes in the appearance of the eye. Such symptoms often indicate other problems such as glaucoma, cataracts, conjunctivitis or diabetic retinopathy.
Diagnosis
Consultation with an ophthalmologist for further correction of corneal astigmatism includes a series of measurements to assess visual function, examination of all ocular structures, refractive analysis and indirect imaging studies.
Visual acuity (ophthalmoscopy) may be measured with or without correction. In the latter case, the doctor places a trial frame on the patient’s eye. One eye is covered with an opaque screen and a cylindrical lens with a different refractive power is placed in front of the other eye. The aim of this procedure is to maximise visual acuity.
The degree of refraction is determined using a shadow test with cylindrical and spherical lenses. The most complete information about the refractive error is obtained by performing the refraction test with the pupils open.
To effectively correct astigmatism, it is also important to determine the cause of the astigmatism (inflammatory or degenerative corneal disease). This is done by biomicroscopic examination of the eye and ophthalmoscopy. To determine the presence and degree of keratoconus and corneal astigmatism, computerised keratotopography is performed – it is a non-invasive method of obtaining topographical maps of the anterior surface of the cornea.
Sight correction and pathology treatment
The best non-surgical method of treating astigmatism is to choose and wear special glasses or contact lenses (soft, toric or ordinary rigid). Toric lenses can correct up to 6.0D in the posterior segment and up to 4.5D in the anterior segment.
Nocturnal rigid lenses redistribute the epithelial cells of the cornea and change their shape (orthokeratology). They are worn only at night.
Laser surgery for astigmatism. This is the most effective and popular method of vision correction. A special device corrects the shape of the cornea to the required parameters. The laser separates a thin layer from the surface of the cornea and removes the unwanted part. The remaining tissue is returned to its original place. As a result of the operation, the shape of the cornea is changed, allowing the patient to focus the image clearly on the retina. The parameters of the change are always set individually. Laser treatment of astigmatism is always accompanied by a full diagnostic examination of the visual system to identify possible contraindications (e.g. inflammation, traumatic scarring of the eye, diabetes mellitus).