Anatomy, Physiology, and Pathology of the Auditory, Visual, and Speech Organs - Shvetsov A.G. 2006

Anatomy, Physiology, and Pathology of the Visual Analyzer
Pathology of the Visual Analyzer
Pathology of the Optical System of the Eye

Pathology of the eye's optical system (cornea, lens, vitreous body) results in the loss or impairment of one of the primary components of Visual Perception—refraction, which focuses images on the retina. Refractive errors manifest as hyperopia, myopia, and astigmatism.

Hyperopia is a type of refractive error characterized by light rays from close objects focusing behind the retina, causing the retinal image to appear blurred. The cause of hyperopia can be either the relative weakness of the eye's refractive system (known as refractive hyperopia) or the relative shortness of the eye's anteroposterior axis (known as axial hyperopia).

Refractive hyperopia is almost always acquired and develops As a result of various pathological processes (corneal flattening, absence of the lens, etc.). Axial hyperopia, as a rule, is congenital. 90% of children are born with mild hyperopia (1—3 diopters), but by 8—12 years of age, the eyes of most children become proportional (emmetropic), and myopia may even develop.

Most cases of hyperopia, however, are classified as combinational, where a relative shortness of the eye's anteroposterior axis coexists with a relative insufficiency of its refractive power. In this case, both the axis of THE EYE AND the refractive power of its optical system remain within the normal range, but these two decisive factors combine in such a way that a mild degree of hyperopia typically develops. This occurs with normal visual acuity and is not accompanied by any pathological Changes in the eye.

By the time physical development is complete, about 50% of people are hyperopic, while the remaining 50% are emmetropes and myopes.

Due to its Structure, a hyperopic eye cannot naturally see clearly at a distance (where parallel rays originate) or up close (where diverging rays originate). However, most hyperopic individuals see well at a distance and often quite satisfactorily up close. This is explained by ocular accommodation, which, through additional tension of the lens's suspensory apparatus, allows the eye to refract incoming light rays more strongly and bring them to a focus on the retina. Consequently, individuals with hyperopia are forced to constantly strain their accommodation for both near and distance Vision, establishing a habitual, constant accommodative effort. Without corrective lenses, this constant accommodative strain easily leads to eye strain in hyperopes, manifesting as headaches, dull pain in the forehead and around the eyes, a feeling of pressure in the eyes, and difficulty reading (letters run together and become blurry). A break from visual work usually temporarily relieves these symptoms, but they recur once work is resumed.

With moderate to severe hyperopia, concomitant convergent strabismus often develops. In addition, hyperopic eyes are considered more predisposed to developing glaucoma.

Myopia (nearsightedness). In a significant proportion of children (30—40%), an excessive increase in the anteroposterior dimension of the Eyeball, which naturally distances the retina from the eye's refractive media (lens, cornea), leads to The Development of so-called axial myopia: in this case, the posterior focal point of The Optical System lies in front of the retina. Sometimes, the refractive power of the eye is relatively strong, leading to refractive myopia, but axial myopia is clinically the most common. As with hyperopia, cases of combinational myopia are quite frequent, typically characterized by a relatively low degree of this refractive error.

Myopic individuals typically hold books close to their eyes, tilt their heads significantly while writing or drawing, and squint when looking at distant objects. A very common symptom of myopia is the presence of so-called 'floaters', which the patient perceives as tiny grayish specks that drift with the movement of the eyeball.

When an object is brought very close to the eyes, the eyes must converge strongly (aligning the visual axes of both eyes on the object of fixation), which facilitates ocular accommodation. Prolonged convergence in myopic individuals can lead to a specific state of involuntary, sustained accommodative strain (known as 'spasm of accommodation'), which further increases the refractive power of the eye, making it appear even more myopic than it actually is. Strong, prolonged convergence increases the load on the extraocular Muscles; this can lead to eye strain and Muscle fatigue, accompanied by an aching sensation in the eyes, pain in the orbital region, and headaches. The muscular aching disappears if one eye is closed, as convergence is eliminated when using only one eye. Prolonged, intense convergence also exhausts the muscular convergence apparatus, disrupting binocular vision—initially temporarily, and later permanently. Monocular vision becomes established, causing the eye excluded from binocular fixation to deviate outward, resulting in concomitant divergent strabismus.

In the vast majority of cases, myopia begins to develop during school years, especially in high school, and continues to progress with age if preventive measures are not taken ('progressive myopia'). Progressive myopia is divided into two stages: pseudomyopia ('spasm of accommodation') and true myopia. Spasm of accommodation is a functional state of refractive strain that can be relieved by special exercises, after which vision returns to normal. True myopia is a pathological condition that requires corrective lenses.

The primary causes of progressive myopia lie in the excessive strain on ocular accommodation caused by heavy visual workloads. Therefore, it is mainly detected during school age: in primary school as a spasm of accommodation, and in high school as true myopia. The causes of progressive myopia also exhibit regional patterns. For example, the number of myopic individuals is higher in northern regions than in southern ones; in some countries (such as Japan), the prevalence of myopia is significantly higher. These variations are linked to levels of insolation and dietary habits. Myopia is more common in urban areas than in rural ones, and in specialized schools compared to regular ones; it is less common among children who play sports than among physically inactive children. It should also be noted that a predisposition to myopia is hereditary (specifically, insufficient scleral rigidity is inherited). However, the genetic factors that determine the onset and progression of myopia are not absolute. One cannot ignore environmental influences and use genetics to justify inaction. Excessive enlargement and subsequent elongation of the eyeball can occur due to increased Blood flow to the eye and elevated intraocular pressure during prolonged reading while lying down, sitting with a steep HEAD tilt, or under accommodative strain caused by poor lighting and prolonged viewing of small objects. Therefore, to prevent myopia in children, they must be taught to keep reading Materials at a distance of 35—40 cm from their eyes, and the other mentioned risk factors must be eliminated.

Progressive myopia gradually leads to irreversible morphological changes in the eyes and a pronounced decrease in visual acuity that is poorly or not at all correctable with lenses. The more severe the myopia, the more the eyeball is enlarged, which stretches the thin retinal layer covering the posterior surface of the eye, risking retinal detachment and degeneration. The sclera and cornea also suffer (stretching and thinning), as do the vitreous humor (syneresis and liquefaction) and the Optic nerve (papilledema and degeneration).

Degenerative (pathological) myopia is usually congenital (of hereditary or prenatal origin), beginning at birth or in early childhood. In this case, refractive errors progress rapidly and continue into middle age. This form of myopia can lead to frequent and serious complications that may result in blindness. It is poorly corrected by glasses.

Astigmatism is characterized by a combination of Different types of refraction or different degrees of the same refractive type in a single eye, due to the uneven curvature of the cornea, or in some cases, an irregular shape of the lens. In an astigmatic eye, light rays originating from any single point do not form a sharp point focus on the retina.

Although this refractive error is quite common, our understanding of its causes and effective correction remains far from complete. Astigmatism not only drastically reduces and complicates the correction of visual acuity, but also distorts the retinal image. Correcting astigmatism requires custom-made lenses that account for the corneal topography of areas with altered refractive power, as well as any accompanying myopic or hyperopic refractive errors.

Theoretically, the refractive powers of the right and left eyes could be absolutely identical, but in practice, this is extremely rare. Usually, these characteristics are close to each other, and differences in the images projected onto the retina are neutralized by the visual cortex. If the refractive differences between the right and left eyes are significant and left uncorrected, it can lead to a difference in the perceived size of the same object's image on each retina—aniseikonia—or retinal image distortions (aberrations), manifesting as reading difficulties and impaired spatial perception.

Amblyopia—'lazy eye'. Literally translated from Greek, amblyopia means 'dim sight'. Today, this term is applied to a specific Class of visual impairments characterized, on one hand, by the absence of any specific disease that could explain its cause, and on the other hand, by the inability to correct vision with lenses to a level exceeding 0,5.

Amblyopia can be either toxic in origin, associated with alcohol or tobacco abuse, or functional, related to visual deprivation or underuse.

At first glance, it seems absurd that an eye is 'not used.' After all, if it is open, it should see. And indeed it does, but because of insufficient resolving power of central vision, the signal sent to the visual cortex from this eye is simply ignored. This visual pattern can become so habitual that normal binocular vision fails to develop or, if already established, becomes suppressed. Furthermore, even after the underlying cause of amblyopia is resolved, the Brain cannot restore binocular vision on its own because the cortical Cells responsible for stereoscopic vision have physically degenerated. Amblyopia can be caused by differences in the size of the eyeballs, which is a fairly common occurrence. It has been established that an increase in eyeball size of just 1 mm can reduce visual acuity in that eye from 1,0 to 0,05. Often, amblyopia is the result of significant differences in the refractive power of the right and left eyes, or a high degree of astigmatism.

In about a third of cases, amblyopia is accompanied by convergent or divergent strabismus, though it is difficult to determine which is primary.

Strabismus (crossed eyes). In addition to convergent or divergent strabismus, alternating strabismus—where one eye deviates and then the other—occurs rarely but does happen. The eyes may deviate constantly or intermittently (intermittent strabismus), but in all cases of strabismus, only one eye participates in the act of vision at any given time, while the other is turned away.

For the most part, strabismus is considered a result of abnormal binocular vision development during childhood. In this case, both eyes are functional but lose The ability to work in coordination.

Quite often, strabismus is associated with severe hyperopia. This is a classic case of strabismus, in which one of the eyes is turned inward toward the bridge of the Nose. Other types of strabismus are caused by different factors, not all of which are known to doctors. A hereditary predisposition to strabismus is noted in some families.

Nystagmus is the involuntary, oscillatory movement of the eyeballs. Pathological nystagmus, which frequently accompanies visual impairments in children, is caused by pathologies during prenatal development, labor, or the early postnatal period. The presence of nystagmus significantly complicates the processes of ocular accommodation and convergence.

Leukoma is an opacity of the cornea caused by cicatricial changes following a perforating ulcer (resulting from a purulent inflammatory process) or a penetrating corneal injury. In the presence of dense and extensive cicatricial changes covering all or most of the corneal surface, complete blindness or a significant reduction in vision usually develops. Localized corneal opacities significantly reduce visual acuity when centrally located, partially or completely covering the pupillary area. Minor impairments of corneal transparency, sometimes barely discernible—known as a "nebula"—lead to irregular refraction of light rays in the cornea and often to distortion and blurring of the visual images received by the eye. Congenital leukoma in children who have undergone an intrauterine inflammatory process of the cornea is characterized by a flat, smooth, and shiny corneal surface. Post-traumatic leukoma is characterized by an uneven corneal surface and thinning in the area of the opacity, while retaining its luster. Chemical (especially alkaline) eye Burns often result in dense, heavily vascularized, total leukomas that reduce visual acuity to mere light perception.

Following a perforating ulcer or a penetrating injury, during the scarring process, the cornea becomes fused to the scar, resulting in an adherent leukoma (leukoma adherens) fused with the iris. This is accompanied by pupillary distortion and increased intraocular pressure (secondary glaucoma).

A cataract is an opacity of the crystalline lens that reduces its transparency.

Congenital cataracts occur in 1 out of 200 newborns, yet they account for 10% of all blindness cases among preschool children. Congenital cataracts can be hereditary, transmitted in a dominant pattern, or arise from intrauterine pathology. Various infectious and toxic Factors affecting the embryo or fetus can lead to the development of congenital cataracts. These primarily include maternal viral infections (rubella, Influenza, Toxoplasmosis), as well as metabolic and endocrine disorders in women during Pregnancy (such as hypocalcemia due to parathyroid insufficiency).

Traumatic cataracts develop when the lens capsule is damaged as a result of mechanical penetrating or contusion trauma. Lens changes in traumatic cataracts are typically localized at the sites of capsule damage and, depending on their Location, can determine the degree of visual acuity loss.

Exposure of the eye to any type of ionizing radiation causes a radiation cataract, while exposure to certain chemicals (naphthalene, mercury, ergot, etc.) results in a toxic cataract. Cataracts that develop due to high doses of sulfonamides, as complications of Inflammatory Diseases of the cornea (iritis) and the uveal tract (uveitis), Metabolic Disorders (diabetes, hypoparathyroidism), various infectious diseases, myopia, glaucoma, retinal detachment, etc., are called consecutive cataracts. In all these cases, the cataract is diffuse, and its severity determines the degree of visual acuity loss. Risk factors for functional impairment in various PARTS OF THE eye's optical system include corneal developmental anomalies, which are most commonly characterized by changes in its size or radius of curvature:

§ microcornea (small cornea) and megacornea (large cornea)—a congenital decrease or increase in the size (diameter) of the cornea. Changes in corneal size lead to alterations in its curvature, which significantly impairs clinical refraction and visual Functions. Additionally, they may be accompanied by increased intraocular pressure (glaucoma);

§ keratoconus and keratoglobus—corneal disorders in which its shape is significantly altered. In keratoconus, the central part of the cornea thins and bulges forward like a cone, whereas in keratoglobus, the corneal surface is convex not only in the center but throughout its entire extent. This always leads to visual impairment in the form of astigmatism;

§ congenital primary corneal opacities are rare and usually result from impaired Embryogenesis due to maternal illness (Syphilis, Gonorrhea, tuberculosis, toxoplasmosis, etc.), as well as nutritional and neurogenic factors. Many opacities are diffuse, located deep and centrally, while the overlying epithelium remains shiny and smooth. Another type of corneal opacity is congenital pigmentation;

§ congenital corneal degenerations are also rare and have a hereditary (familial) pattern. They are characterized by a non-inflammatory, progressive course. In these cases, the opacity involves the superficial and middle layers of the cornea, reducing vision to light perception. Over time, as the eye grows, the cornea stretches and thins, which is accompanied by a significant reduction or almost complete disappearance of the opacity. Stable and pronounced corneal opacities in children over 3 years of age, when corneal growth is largely complete, require Surgical Treatment—keratoplasty or keratoprosthesis;

§ keratitis refers to inflammatory corneal diseases of various etiologies (bacterial, viral, metabolic, hypovitaminosis-related, etc.); they are relatively rare, but their most common outcome is residual corneal opacity. This is caused not so much by vascularization as by Connective Tissue degeneration (scarring) of its deep, non-regenerating structures, and as a rule, it does not undergo complete resolution. Consequently, a persistent reduction in visual acuity occurs;

§ corneal vascularization—the ingrowth of Blood Vessels into the cornea, which is normally avascular. It can be caused by systemic diseases or traumatic inflammatory processes, leading to a reduction in corneal light transmission.

Developmental anomalies of the lens are most commonly represented by:

§ microphakia (small lens) and macrophakia (large lens)—congenital anomalies of lens development, accompanied by a more or less pronounced reduction in vision due to refractive errors and weakened accommodative capacity. Macrophakia is often accompanied by glaucoma;

§ aphakia—the condition following the extraction of a cataractous lens. It is characterized by a sharp decrease in visual acuity due to the absence of accommodation. Vision correction is achieved using soft or rigid (for astigmatism) contact lenses.