Myopic astigmatism is a pathology of refraction in which parallel rays of light do not gather into a single focal line on the inner shell of the eye. Clinical manifestations: decreased vision, distortion and blurring of the image in front of the eyes, “squinting” of the eyes. The main diagnostic methods include visometry, computer autorefractometry, biomicroscopy, keratotopography, retinoscopy, ophthalmoscopy. Conservative therapy is reduced to the use of optical correction (glasses, toric contact lenses). Surgical treatment involves performing laser keratomylosis (LASIK).
Myopic astigmatism is a refractive anomaly accompanied by the formation of a defocus zone, in which all or part of the light rays are refracted in front of the retina. In the general structure of astigmatism, the share of myopic species accounts for about 54%. According to statistics, the regular variant occurs in 67% of cases. The complex form of the disease prevails over the simple one. Men get sick more often than women. Pathology is widespread everywhere, but it is more often detected in residents of Asian countries.
The etiology of myopic astigmatism has not been fully studied. Despite contradictory studies, it has been proven that the progression of axial myopia is one of the predisposing factors for the occurrence of refractive anomaly. Congenital myopic astigmatism develops with embryonic disorders of the formation of the eyeball. The main reasons for the acquired form:
- Eye injuries. A penetrating wound leads to a post-traumatic change in refraction. The resulting erosion changes the surface of the cornea. As the edema decreases and the tissue regenerates, the indicators reach reference values or become less noticeable.
- Keratoconus. This is a degenerative non-inflammatory disease in which there is an uneven thinning of the cornea. At the same time, patients have an irregular variant of the disease, often combined with myopia.
- Keratitis. Myopic astigmatism is formed in the presence of defects on the surface or in the thickness of the cornea. Deep turbidity reaching the stroma occurs as a result of severe inflammation of herpetic, adenoviral or bacterial nature.
- Iatrogenia. Microsurgical interventions on the anterior segment of the eye become a frequent cause of refraction changes. The cylindrical component appears in the postoperative period after the removal of the pterygium, keratoplasty, and paracentesis.
More often, myopic astigmatism is a genetically determined pathology inherited by an autosomal dominant type. However, the occurrence of the disease in persons with an unencumbered family history is not excluded. The mechanism of development is based on the lesion of optical media involved in the process of refraction of light. At the same time, the anterior-posterior size of the eyeball often exceeds the reference values.
In the presence of pathological changes on the surface of the cornea or transformation of its shape, part of the light rays can focus in front of the retina. Similar disorders are noted in violation of the transparency of the lens or lenticonus. With an average and high degree of myopia, there is a risk of formation of sclera staphyloma, which contributes to the appearance of a retinal defocus zone.
Myopic astigmatism can be congenital or acquired. With the correct (regular) form of the disease, an angle of 90 ° is formed between the leading meridians. The irregular variant is characterized by a change in the refractive power mainly in one direction. In clinical ophthalmology , the disease is classified into the following types:
- Plain. One of the main meridians is located in front of the retina, the other is on it. In most cases, the degree of pathology is weak or average. Optical correction is more easily tolerated by the patient.
- Complicated. The leading meridians focus in front of the retina, but at a distance from each other. The severity varies widely, in some cases reaching a high degree. More often it requires a step-by-step selection of correction.
This disease is characterized by a decrease in vision, which is most noticeable when looking into the distance (inability to see the number of public transport, street names, the inscription on the blackboard at school, etc.). Patients complain of blurring and distortion of the image. In order to neutralize the symptoms, it is necessary to get closer to the object. “Squinting” of the eyes refers to the number of adaptive reactions.
With a large cylindrical component, discomfort occurs even when trying to distinguish small details at arm’s length. Myopic astigmatism is accompanied by headache, increased fatigue with prolonged visual load. Working near (no more than 40 cm from the eyes) with mild to moderate severity of the disease does not cause difficulties.
With the development of pathology in early childhood, there is a possibility of amblyopia along with strabismus, which appears a second time. With concomitant myopia of a high degree, patients are at risk of the formation of a choroidal neovascular membrane, myopic macular retinoschisis, peripheral posterior vitreous detachment. Prolonged use of contact lenses causes dry eye disease.
During the initial examination, an ophthalmologist collects anamnestic information, complaints and conducts functional studies. Myopic astigmatism is often accompanied by a violation of the position of the eyes and accommodation disorders, so it is important to assess accommodation reserves and make a test with overlap (cover-uncover test). To make a diagnosis , the following examinations are indicated:
- Visometry. The decrease in visual acuity is most pronounced in the distance. Visual dysfunction correlates with the index of the spherical-cylindrical component. With a high degree of astigmatism, it is important to assess the vision up close in the glasses and lenses of the patient.
- Computer autorefractometry. It is carried out at the stage of primary examination. Makes it possible to judge subjective refraction. Based on the data obtained, visometry is repeated with a trial point correction.
- Retinoscopy. This method is considered to be the “gold standard” in the diagnosis of myopic astigmatism. It is used to obtain objective information about refraction. It is performed after achieving complete cycloplegia. An alternative is skiascopy.
- Biomicroscopy of the eye. Allows a detailed examination of the anterior segment of the eyeball. With the help of a slit lamp, it is possible to visualize pathological changes in the cornea (stromal opacities, dystrophy, ulcerative defects), which could serve as an etiological factor.
- Computer keratotopography. It is used to identify topographic corneal features (post-traumatic disorders, keratoconus, keratoglobus). The radius of curvature in the main meridians is measured automatically.
- Ophthalmoscopy. Visualization of the fundus is performed in order to identify changes accompanying astigmatism (myopic cone, staphyloma). With high degree of myopia, it is necessary to examine the peripheral parts of the retina in detail.
With myopic astigmatism, constant optical correction is shown. Glasses are prescribed based on the indicators of objective refraction, taking into account individual tolerance. The power of the lenses should be the closest to full. However, in case of intolerance, the cylindrical component is increased in stages, taking into account the adaptation of the patient.
Toric contact lenses allow you to achieve better visual acuity and eliminate aberrations. The use of pleoptoorthoptic treatment is justified with a decrease in corrected visual acuity, accommodation disorders or incorrect eye position. Temporary occlusion of the better-seeing eye is used in concomitant amblyopia.
Vision correction using an excimer laser (LASIK, laser keratomylosis) makes it possible to eliminate the need for glasses and soft contact lenses or reduce their strength. The more pronounced the cylindrical index, the higher the probability of repeated intervention. In severe corneal lesions, preference is given to LASIK with a wavefront.
Laser ablation improves visual functions at dusk, which is important for drivers. At the same time, surgical correction of vision is contraindicated in case of sharp thinning of the cornea (according to the results of pachymetry), progression of the disease or a high degree of concomitant myopia. Refractive surgery is performed after the age of 21.
Prognosis and prevention
The prognosis for life and working capacity is favorable. Timely diagnosis of myopic astigmatism and the use of optical correction can prevent the development of amblyopia. Specific prevention has not been developed. It is important to follow the recommendations on visual hygiene, which include visual gymnastics, daily walks in the fresh air, working near with sufficient lighting. In childhood, a planned ophthalmological examination is indicated twice a year, after 18 years – 1 time a year.