Pathology of the Oculomotor Apparatus

Pathology of the oculomotor apparatus includes strabismus and nystagmus.

Strabismus (squint) occurs in 1.5-3.5% of children, making about 3% of the eye pathology. There are at least 100 mln people with squint in the world. Besides a cosmetic defect, strabismus is accom­panied by serious monocular and binocular dysfunctions. It makes visual activity of the patients with strabismus difficult and restricts a selection of profession, in this connection the detection and timely treatment of this pathology is a social problem, the ophthalmolo­gists, pediatricians, doctors of a sanitary-hygienic profile are in­terested in its solution.

Strabismus

It is necessary to distinguish true strabismus from pseudo- and latent squint.

Pseudo-strabismus or gamma angle arises because the optic axis of the eye going through the centre of the cornea and a nodal point of the eye does not coincide with a visual axis connecting the cen­tral retinal fossa through a nodal point of the eye with the exa­mined object. The angle may be positive (deviation of the eye out­ward) and negative (eye is deviated inward).

Unlike true strabismus, there is binocular vision, there are no adjusting movements, and both eyes are usually deviated symmet­rically.

Latent squint (heterophoria) is impairment of the muscular equi­librium that is manifested in impairment of binocular vision. There is no evident strabismus, binocular vision by colour-test; however there is an adjusting movement, fatiguability in visual work.

Treatment. Orthoptic exercises on sinaptophor and with prisms for restoration of physical reserves, wear of glasses with prisms, exercises of the eyes, operation on the oculomotor muscles in rare cases.

True strabismus may be concomitant and paralytic.

The differential diagnosis of these forms is based on 3 signs:

1. Mobility of the eye is not impaired in concomitant strabis­mus, and limited in a paralytic form.

2. Diplopia is absent in concomitant and present in paralytic strabismus.

3. Equality of the primary (angle of deviation of the squinting eye) and secondary (deviation of the healthy eye in object fixa­tion by the squinting eye) squint angles is present in concomitant strabismus, and in paralytic squint the secondary angle is bigger than the primary one.

The data of anamnesis (terms of disease development, accom­panying pathology) are also important.

Concomitant strabismus is characterized by a deviation of one eye from a fixation point and impairment of binocular vision.

The etiological factors of concomitant strabismus are various and it is presented in the following kind. Concomitant strabis­mus is related to diseases with a genetic predisposition, in which the heredity is presented as a pathogenetic or conditionally eti-ologic factor, that is not strabismus itself is inherited, but com­plex of the factors promoting its occurrence. The immediate cause of concomitant strabismus is the impairment of the bifix-ation mechanism, i.e. ability of the oculomotor system to direct and to keep visual axes of both eyes on the fixation object si­multaneously.

Various causes lead to impairment of bifixation mechanism in presence of favorable conditions. One of them is low vision or blind­ness of one eye. The worse seeing eye ceases to fix the object seen by the other eye, and deviates in this or that direction. Children of younger age more often have convergent strabismus; divergent squint is observed in senior children and adults.

The cause of concomitant strabismus can also be aniseikonia i.e. different sizes of the image on the retina of both eyes in different eye refraction (anisometropia).

Accommodation-refractive factor is of primary importance in etiology of concomitant strabismus. As it is known, accommoda­tion and convergence are closely connected and together with a pupillary reflex make one functional system of eye fixation to fi­nal distances. In hypermetropia each distance needs stronger ac­commodation, than in emetropia. Due to it there is increased im­pulse to convergence in hypermetropia. On the contrary, in myo­pia the need for accommodation is either considerably reduced, or absolutely absent. It weakens stimulus to convergence. Thus, in uncorrected hypermetropia there is a tendency to esotropia, and in uncorrected myopia — to exotropia. The marked tendency is usually easily overcome thanks to fusion and does not result in a deviation of one of the eyes from a fixation point. If the ability to merge is impaired, there is a convergent or divergent concomitant strabismus, usually periodical at first, and then constant. The ac­commodation-refractive factor is less important in origin of diver­gent strabismus than convergent. It is explained by the fact that myopic refraction is more often formed during school years, when the mechanism of binocular vision is already well developed and stable enough.

Thus, there is a form of concomitant strabismus, in which the main role of its development belongs to the accommodation-refrac­tive factor. It is the so-called accommodation strabismus.

Of great importance in the etiology of concomitant strabismus is affection of the central nervous system, especially in the period of intrauterine development or at the early stage of postnatal on­togenesis.

It is more often nuclear, radicular and truncal paresis of the nerves responsible for the eye movements. In children of younger age paretic strabismus, as a rule, takes a character of concomi­tant.

Thus, under the influence of the listed above etiological factors discoordination of activity of sensory and motor systems of both eyes occurs, i.e. the impairment of binocular vision and bifixation mechanism, which is manifested by deviation of the visual axis of one eye from the fixation point.

Classification of concomitant strabismus. According to the di­rection of deviation there are horizontal strabismus: convergent (es­otropia) and divergent (exotropia); vertical strabismus upward (hypertropia) and downward (hypotropia), as well as horizontal strabismus with a vertical component.

If both eyes squint alternately, strabismus is called alternating, if one eye squints, it is called unilateral.

Concomitant strabismus is called accommodative, if under the influence of atropinization or correction deviation is eliminated, and nonaccommodative, when the optic correction does not exert in­fluence on a position of the squinting eye. If the angle of deviation in wearing glasses is not eliminated completely, we speak about partially accommodative strabismus.

Peculiarities of visual and oculomotor systems in concomitant strabismus. In alternating strabismus each monocular system is high-grade by itself, but the possibility of simultaneous active par­ticipation of both systems in the act of vision is excluded. The al­ternate functioning of any monocular system is carried out at the expense of suppression (inhibition) of the other one. The clinical manifestation of inhibition is functional scotoma which arises in vi­sual field of the deviated eye.

In a number of cases in small squint angles there is observed fusion of the images falling on the central fossa of the retina of one eye and a paracentral site of the retina or the other one. Such condition is called the abnormal correspondence of the retina.

In unilateral strabismus only one, strictly determined monocu­lar system may participate in the act of vision in two open eyes, and the other one (on the side of the deviated eye) is in a suppressed condition. It results in stable reduction of visual acuity of the con­stantly squinting eye — amblyopia. Amblyopia in concomitant strabismus arises due to disturbance of binocular vision, therefore it is called disbinocular.

The clinical classification of disbinocular amblyopia is the fol­lowing: by degree of reduction of visual acuity: 0.8-0.4 (low); 0.3-0.2 (average); 0.1-0.05 (high); 0.04 and lower (very high); accor­ding to a condition of visual fixation: with a central fixation; with alternating fixation; with noncentral, wrong fixation: parafovea, macular, peripheral; with absence of fixation (fixation is a site of the retina to which the object is fixed).

Peculiarities of divergent strabismus. Divergent strabismus oc­curs much rarer than convergent; it arises in later age and is char­acterized by smaller rate of sensory impairments. The causes of occurrence of divergent strabismus can be failure of convergence associated with affection of its nervous apparatus or reduction of accommodation stimulus, weakening of fusion, excessive impulse to divergence. A combination of these causes is marked quite of­ten. The secondary divergent strabismus sometimes results from hypereffect of the operation performed for convergent strabismus.

Excessive divergence is amplified impulse to a divergence of vis­ual axes. In fixation of the closely situated object binocular vision is present and the tendency to a deviation of one of the eyes is over­come by fusion. In vision afar there is an amplified divergence and eye deviates outward. The operation of a recession of external straight muscles is indicated. The orthoptic exercises have no effect.

Principles of treatment of concomitant strabismus

1. Determination of refraction after 4-5-day atropinization.

2. Administration of the appropriate glasses for constant wear.

3. Treatment of amblyopia (pleoptics). An old and traditional method is occlusion. It may be straight (a better seeing eye is "switched off), opposite (a squinting eye is switched off in wrong fixation) or alternate in alternating strabismus.

Penalisation, i.e. creation of artificial anisometropia in the pa­tient, vision of a leading eye is worsened and the amblyopic eye becomes fixed. It is used in children under 3 during 3-4 months.

Apparatus treatment of amblyopia is used in children over 3.5-4 years old. A classical method of pleoptics is the Cuppers' meth- od of successive images, when the whole retina is lightened except for the area of the central fossa. After lightening the optotypes (drawings, letters) are shown before the eye for 2-3 minutes, while the successive image is kept. Children can see optotypes only by central part of the retina. Thus the central fixation develops. The method is especially effective in wrong fixation. Lately a method of local light irritation of the center of the retina "by blinding light" has also been applied with the help of the helium-neon laser of low power. Now many new methods of treatment of amblyopia are being developed, in which the light or structured stimuli are used for irritation of the retina. Our department devised the method of treatment of amblyopia with the help of impulse low power mon-ochromic light. The method has no contraindications, can be used not only in eye medical establishments, but also in home conditions, that increases efficiency of treatment.

After the increase of visual acuity up to 0.3 the next stage of treatment begins, i.e. orthoptics, a complex of apparatus methods, which enables to remove scotoma of inhibition, develop fusion and fusion reserves in conditions of separation of visual fields. Treat­ment is carried out on synaptofor, "bivisiotrainer", etc.

In high visual acuity and correct position of the eye diploptics is used, it is a method of restoration of binocular vision by artifi­cial causing diplopia and trainings for fusion of the double images into one.

Accommodation form of strabismus is treated conservatively. In nonaccommodation forms operative methods are included in a complex of treatment. Basically, 2 types of such operations are per­formed — elongation (recession) of the muscle weakening its func­tion and shortening (tenorraphia) of the muscle intensifying it. In unilateral strabismus one eye is operated, in alternating strabismus — both. Orthoptic and diploptic treatment should be carried out after the operation. Treatment of strabismus is completed by ex­ercises for development of stereoscopic vision on the stereotrain-ers. Strabismus is considered cured only when binocular stereosco­pic vision is present in a correct position of the eye.

Prophylaxis of strabismus is of great importance. In preventive examinations of the eye in children of early age it is necessary to reveal not only obvious strabismus, but also defects of binocular vision, and also conditions favouring concomitant strabismus (ametropia, anisometropia, diseases of the eye accompanying by unilateral reduction of visual acuity, restriction of mobility of the eye, marked adjusting movements of the eyes in tests with clos­ing). In case of their detection more detailed examination of the child is made and necessary therapeutic and prophylactic meas­ures (correction of ametropia, measures on increasing visual acui­ty, orthoptic exercises) are administered.

Paralytic strabismus arises in affection of nuclei or trunks of the oculomotor, trochlear and abducent nerves, and also as a result of affection of these nerves in muscles or muscles themselves.

In affection of one of the muscles the affected eye deviates in the opposite direction. The squint angle increases while moving the gaze to the side of the affected muscle. The angle of the second­ary deviation is bigger than the angle of the primary deviation. The movements of the eye to the side of the affected muscle are absent or sharply limited. There are marked diplopia (usually in fresh affections) and giddiness disappearing during closing of one eye. The ability to estimate correctly the object place by the af­fected eye is quite often impaired (false monocular projection or localization). The compelled position of the head — turn or its bend to this or that side can be observed.

The diagnosis of paralytic strabismus is based on characteristic symptoms. It is important to establish what muscle or a group of muscles is affected. Investigation of double images is mainly used for this purpose. Thorough neurological examination and electro­myography are made to determine localization of the centre of the affection focus.

Treatment of paralytic strabismus consists, first of all, in thera­py of the basic disease. Electrical stimulation of the affected mus­cle is made and exercises directed at development of the eye mo­bility. In mild paresis the orthoptic exercises are useful. For elimi­nation of diplopia eyeglasses with prisms are used and occlusion of the affected eye or incomplete occlusion with the help of dim glass spectacle. In stable paralyses and paresis the operation is in­dicated. It is performed not earlier than 6-12 months after active treatment and stabilization of the process. In inherent paralytic stra­bismus it is expedient to make operation at the age of 3-4 years. In paralysis of the abducent nerve the operation suggested by O'Connor is usually performed. It consists in formation of tendon-muscular flaps of the superior and inferior muscles and their sew­ing to the external straight muscle at the site of attachment. In the large angles of deviation a resection of the external straight and recession of the internal straight muscles are also made.

Nystagmus

Nystagmus is a syndrome manifested by spontaneous oscillatory movements of eyeballs. Two basic forms of pathological nystagmus are distinguished — ocular or fixation, and neurogenic or central.

Nystagmus usually arises in congenital or early acquired weak­ness of vision due to various diseases of the eye, congenital pa­thology of the oculomotor apparatus, and it is also an inherited pathology.

The reduction of visual acuity in nystagmus is caused by or­ganic changes in the apparatus of the central vision and dysfunc­tions due to insufficient activity of the visual analyzer (relative amblyopia). Potential possibilities of vision improvement in nys­tagmus basically depend on a degree of dysfunctions.

There are distinguished congenital and acquired nystagmus caused by blindness or sharp reduction of vision as a result of var­ious diseases of the eye. There is congenital nystagmus without ev­ident changes of the eye and nystagmus with accompanying visi­ble changes (congenital cataract, albinism, atrophy of the visual nerves, degeneration of the retina, coloboma, aniridia etc.).

According to the character of oscillatory movements the follow­ing forms of nystagmus are distinguished:

1) oscillating nystagmus with equal in magnitude phases of fluc­tuations;

2) jerky nystagmus with different phases of fluctuations — slow in one side and fast in another;

3) mixed nystagmus when pendular or jerky nystagmus is ob­served. There is also distinguished horizontal, vertical, rotatory and diagonal nystagmus. Horizontal nystagmus is observed more often.

Treatment of nystagmus includes selection of optic correction, influence on the apparatus of accommodation, pleoptic treatment and operations on the oculomotor muscles.

Date: 2015-01-11; view: 1434