Mixed astigmatism is a refractive pathology in which parallel rays of light penetrating through the optical media of the eye do not form the correct focal line on the retina. Clinical symptoms are represented by a decrease in visual acuity, blurring and distortion of the image under consideration, visual discomfort at dusk. Diagnostics is reduced to performing visometry, keratotopography, biomicroscopy, retinoscopy, keratometry, computer autorefractometry. To eliminate the symptoms, the use of glasses or contact lenses is indicated. An alternative is an excimer laser correction method.
Mixed astigmatism is a refractive error in which light rays do not converge at one focal point on the inner shell of the eye. According to statistics, the correct type of astigmatism occurs in 60% of cases. In the overall structure of all refractive pathologies, the mixed form accounts for about 13%. In some ethnic groups, this figure reaches 30%. Among males, the prevalence of the disease is 20% higher than among women.
Despite extensive research, the etiology remains completely unexplored. Mixed astigmatism is one of the genetically determined pathologies inherited by autosomal dominant type. At the same time, the disease may occur a second time in persons with an unburdened family history. The following causes of astigmatism are known:
- Keratitis. Inflammation of the cornea entails the formation of superficial subepithelial or stromal opacities. Their presence distorts the refraction of light rays and leads to a change in refractive indices, mainly due to the cylinder.
- Saltzman nodular degeneration. This is a non-inflammatory process that progresses slowly and is characterized by the formation of single or multiple whitish-gray subepithelial nodules. As a result, the corneal surface becomes uneven.
- Eye injuries. The main cause of post–traumatic astigmatism is a penetrating corneal injury. Erosive defects are more favorable in prognostic terms. As the erosion heals, the degree of cylindrical refraction component decreases.
- Microsurgical interventions. The following operations can induce the formation of defects on the anterior pole of the eye: removal of the pterygium, layered or through keratoplasty. Mixed corneal astigmatism increases with the incorrect location of the paracentesis.
The development of astigmatic refraction is based on a change in the surface (less often the shape) of the cornea. At the same time, its curvature becomes uneven along the two main meridians (vertical and horizontal). In the occurrence of pathology, an important role is assigned not only to the anterior, but also to the posterior surface of the cornea.
Mixed astigmatism is also observed with uneven curvature of the lens surfaces. With this type, the severity of clinical symptoms worsens with age. The appearance of opacities leads to incorrect refraction of light rays through the lens. As a result, the process of their focusing on the retina is disrupted. The decentration of the intraocular lens leads to such disorders.
In practical ophthalmology, there are congenital (hereditary) and acquired forms of a mixed form of the disease. With proper astigmatism, the main meridians are located at right angles to each other. With the wrong (irregular) type, this relationship is broken, and refraction changes its strength in one direction. Correct astigmatism in clinical practice is classified into:
- Direct. The refractive force is much greater vertically than horizontally, so a meridian located at an angle of 90° (+/-15°) is considered stronger than a horizontal one.
- Reverse. It is characterized by a greater horizontal refractive power. The meridian located at an angle of 180° (+/-15°) is stronger than the vertical one and refracts light rays with greater force.
- With oblique axes. The main meridians in this type of mixed astigmatism occupy an inclined position. The strong axis lies within 16°-74°, and the weak axis lies between 106° and 164°. At the same time, the angle between the meridians remains 90 °.
The main clinical manifestations are distortion and blurring of the image in front of the eyes, visual impairment. The degree of distortion of the object in question is greater at a far distance than at a near distance. With a large spherical-cylindrical component, the quality of near vision is also reduced. Patients may note that the distortion is most noticeable in one of the directions (vertically, horizontally or diagonally). Objects lose clarity, the circle becomes like an oval.
In the absence of optimal correction, patients complain of headache, increased fatigue with visual loads. Symptoms increase when reading small print. Children with astigmatic refraction get tired faster at school. There are difficulties in mastering the material. Adults note that driving at night is accompanied by discomfort and the appearance of glare.
Mixed astigmatism in children leads to the development of amblyopia. Also, refractive anomaly predisposes to the appearance of strabismus. With a unilateral lesion, monocular diplopia is possible. When choosing contact lenses, as well as in the postoperative period, there is a high risk of dry eye syndrome. In 14.3% of patients, it is not possible to achieve the desired corrected visual acuity after laser keratomylosis.
The initial examination of the patient includes the study of anamnestic information, clinical symptoms and the performance of functional tests. An ophthalmologist evaluates the position and mobility of the eyes, the ability to fusion. However, the results of instrumental examinations play a decisive role in the diagnosis. Mixed astigmatism is diagnosed using the following methods:
- Visometry. The degree of visual acuity reduction correlates with objective refraction data and the severity of the cylindrical component. It is necessary to check visual functions in the distance, near and at a working distance. Also, visometry is carried out with correction and a special diaphragm.
- Retinoscopy. It is used to study the objective refraction of the patient and to select the appropriate method of treatment. The main advantage is the informative use in early childhood. The results of the study do not depend on the patient’s reaction.
- Keratometry. Allows you to measure the radius of curvature of the anterior surface of the patient’s cornea in individual meridians. The result is affected by the condition of the tear film of the patient’s eye. Makes it possible to quantify the corneal component of astigmatism.
- Keratotopography. The method is included in the examination program for an irregular form of the disease. The results indicate a change in the topographic characteristics of the corneal surface, as well as related causes (keratoconus, complicated dystrophy).
- Biomicroscopy. It is a review of the anterior segment of the eyeball using a slit lamp. Changes are visualized (post-traumatic defects, dystrophy, opacities) that affect keratometric parameters and can lead to mixed astigmatism.
- Computer autorefractometry. It is performed at the stage of the initial examination of the patient to obtain information about subjective refraction. Based on the indicators, a trial correction is selected, with which visual functions are checked at the required distances.
Mixed astigmatism requires constant correction. In childhood, with concomitant amblyopia, the complex of therapeutic measures includes the use of temporary occlusion of the better-seeing eye. Pleoptoorthoptic treatment is indicated with a decrease in visual acuity and a violation of the position of the eyes. For the purpose of non – surgical treatment:
- Glasses. Full correction is necessary with a regular form of pathology. If it is intolerant, the strength of the lenses is increased gradually, in several stages (as the patient adapts). Spherocylindric glasses for children under 6 years old are prescribed taking into account age norms.
- Contact lenses. The use of glasses in the irregular variant of the disease allows to improve visual functions, but does not neutralize aberrations in the proper volume. Therefore, after a detailed examination and the absence of contraindications, soft or hard toric contact lenses are recommended. Their use in early childhood is justified in the intolerance of complete eyeglass correction and the progression of amblyopia.
With mixed astigmatism, it is possible to perform excimer laser vision correction, which is one of the directions of refractive surgery. The most common technique is considered to be LASIK (laser-assisted in situ keratomileusis). With this variant of pathology, it is possible to achieve the expected result in 85.7% of cases. Postoperative hypocorrection requires repeated surgery to obtain maximum corrected visual acuity.
Surgical intervention is carried out mainly in adulthood. However, research results show that laser correction is a priority in the complex treatment of refractive and anisometropic amblyopia in children and adolescents. The use of femtolaser keratomylosis reduces the degree of amblyopia and the magnitude of anisometropia, which ensures optimal social adaptation of patients.
Prognosis and prevention
The prognosis for mixed astigmatism is favorable for life and work capacity. However, the absence of permanent correction can lead to an irreversible decrease in visual functions. Timely treatment of amblyopia in childhood makes it possible to restore visual functions. Patients under the age of 18 are recommended to undergo a routine examination by an ophthalmologist twice a year, after 18 – 1 time a year.
Specific preventive measures have not been developed. Non-specific prevention includes a set of recommendations on visual hygiene that children should follow. These include daily walks in the fresh air, working at close range with sufficient lighting, visual gymnastics.