Biological significance of the pupillary reflex. Syndrome of impaired pupillary reflexes. Features of the structure of reflex arcs

Pupillary reflexes are examined using a number of tests: pupillary reaction to light, pupillary reaction to convergence, accommodation, pain. The pupil of a healthy person has a regular round shape with a diameter of 3-3.5 mm. Normally, the pupils are the same in diameter. Pathological changes in the pupils include miosis - narrowing of the pupils, mydriasis - their dilation, anisocoria (pupillary inequality), deformation, disorder of the pupils' reaction to light, convergence and accommodation. The study of pupillary reflexes is indicated when selecting for classes in sports sections, when conducting an in-depth medical examination (IME) of athletes, as well as for head injuries in boxers, hockey players, wrestlers, bobsledders, acrobats and in other sports where frequent head injuries occur.

Pupillary reactions are examined in bright diffuse lighting. The lack of reaction of the pupils to light is confirmed by examining them through a magnifying glass. When the pupil diameter is less than 2 mm, the reaction to light is difficult to assess, so too bright lighting makes diagnosis difficult. Pupils with a diameter of 2.5-5 mm that react equally to light usually indicate the preservation of the midbrain. Unilateral dilatation of the pupil (more than 5 mm) with the absence or decrease in its reaction to light occurs with damage to the midbrain on the same side or, more often, with secondary compression or tension of the oculomotor nerve as a result of herniation.

Usually the pupil dilates on the same side where the space-occupying lesion is located in the hemisphere, less often on the opposite side due to compression of the midbrain or compression of the oculomotor nerve by the opposite edge of the tentorium cerebellum. Oval and eccentrically located pupils are observed in the early stages of compression of the midbrain and oculomotor nerve. Equally dilated pupils that do not respond to light indicate severe damage to the midbrain (usually as a result of compression during temporotentorial herniation) or poisoning with M-anticholinergic drugs.

Unilateral constriction of the pupil in Horner's syndrome is accompanied by a lack of pupil dilation in the dark. This coma syndrome is rare and indicates extensive hemorrhage into the ipsilateral thalamus. The tone of the eyelid, assessed by raising the upper eyelid and the speed of closing the eye, decreases as the coma deepens.

Methodology for studying the reaction of pupils to light. The doctor tightly covers both eyes of the patient with his palms, which should be wide open at all times. Then, one by one, the doctor quickly moves his palm away from each eye, noting the reaction of each pupil.

Another option for studying this reaction is to turn on and off an electric lamp or a portable flashlight, brought to the patient’s eye, the patient tightly covers the other eye with his palm.

The study of pupillary reactions should be carried out with the utmost care using a sufficiently intense light source (poor illumination of the pupil may either not constrict at all or cause a sluggish reaction).

Methodology for studying the reaction to accommodation with convergence. The doctor asks the patient to look into the distance for a while, and then quickly move his gaze to fixate an object (finger or hammer) brought close to the eyes. The study is carried out separately for each eye. In some patients, this method of studying convergence is difficult and the doctor may have a false opinion about convergence paresis. For such cases, there is a “testing” version of the study. After looking into the distance, the patient is asked to read a small written phrase (for example, a label on a matchbox) held close to the eyes.

Most often, changes in pupillary reactions are symptoms of syphilitic damage to the nervous system, epidemic encephalitis, less often - alcoholism and organic pathologies such as damage to the stem region, cracks in the base of the skull.

Study of the position and movements of the eyeballs. With pathology of the oculomotor nerves (III, IV and VI pairs), convergent or divergent strabismus, diplopia, limited movements of the eyeball to the sides, up or down, and drooping of the upper eyelid (ptosis) are observed.

It should be remembered that strabismus can be a congenital or acquired visual defect, but the patient does not experience double vision. When one of the oculomotor nerves is paralyzed, the patient experiences diplopia when looking towards the affected muscle.

More valuable for diagnosis is the fact that when clarifying complaints, the patient himself declared double vision when looking in any direction. During the survey, the doctor should avoid leading questions about double vision, because a certain contingent of patients will answer in the affirmative even in the absence of data for diplopia.

To find out the causes of diplopia, it is necessary to determine the visual or oculomotor disorders present in a given patient.

The method used for the differential diagnosis of true diplopia is extremely simple. If there are complaints of double vision in a certain direction of gaze, the patient should close one eye with the palm of his hand - true diplopia disappears, but in the case of hysterical diplopia, the complaints remain.

The technique for studying eye movements is also quite simple. The doctor asks the patient to follow an object moving in different directions (up, down, to the sides). This technique allows you to detect damage to any eye muscle, gaze paresis, or the presence of nystagmus.

The most common horizontal nystagmus is detected when looking to the sides (the abduction of the eyeballs should be maximum). If nystagmus is a single identified symptom, then it cannot be called a clear sign of organic damage to the nervous system. In completely healthy people, examination may also reveal “nystagmoid” eye movements. Persistent nystagmus is often found in smokers, miners, and divers. There is also congenital nystagmus, characterized by rough (usually rotatory) twitching of the eyeballs that persists with a “static position” of the eyes.

The diagnostic technique for determining the type of nystagmus is simple. The doctor asks the patient to look up. With congenital nystagmus, its intensity and character (horizontal or rotatory) are preserved. If nystagmus is caused by an organic disease of the central nervous system, then it either weakens, becoming vertical, or completely disappears.

If the nature of the nystagmus is unclear, it is necessary to examine it by moving the patient to a horizontal position, alternately on the left and right side.

If nystagmus persists, abdominal reflexes should be examined. The presence of nystagmus and extinction of abdominal reflexes together are early signs of multiple sclerosis. The symptoms that confirm the presumptive diagnosis of multiple sclerosis should be listed:

1) complaints of periodic double vision, fatigue of the legs, urination disorders, paresthesia of the extremities;

2) detection during examination of an increase in the unevenness of tendon reflexes, the appearance of pathological reflexes, and intentional trembling.

The anatomical structure of the visual pathway is quite complex and includes a number of neural links. Within the retina of each eye, this is a layer of rods and cones (I neuron), then bipolar (II neuron) and ganglion cells with their long axons (III neuron). Together they form the peripheral part of the visual pathway, represented by the optic nerves, chiasm and optic tracts. The latter end in the cells of the external geniculate body, which plays the role of the primary visual center. From them originate the fibers of the central neuron of the visual pathway (radiatio optica), which reach the area striata of the occipital lobe of the brain. The primary cortical center of the visual analyzer is localized here.

Optic nerve (n.opticus) begins with a disc formed by the axons of retinal ganglion cells and ends in the chiasm. Its total length varies in adults from 35 to 55 mm. A significant part of the nerve is the orbital segment (25-30 mm), which in the horizontal plane has an S-shaped bend, due to which it does not experience tension during movements of the eyeball.

Over a considerable distance (from the exit from the eyeball to the entrance to the canalis opticus), the nerve, like the brain, has three membranes: hard, arachnoid and soft. Together with them, its thickness is 4-4.5 mm, without them - 3-3.5 mm. At the eyeball, the dura mater fuses with the sclera and Tenon's capsule, and at the optic canal, with the periosteum. The intracranial segment of the nerve and the chiasm, located in the subarachnoid chiasmatic cistern, are dressed only in a soft shell.

The intrathecal spaces of the orbital part of the nerve (subdural and subarachnoid) are connected to similar spaces in the brain, but are isolated from each other. They are filled with a fluid of complex composition (intraocular, tissue, cerebrospinal fluid). Since intraocular pressure is normally twice the intracranial pressure (10-12 mm Hg), the direction of its current coincides with the pressure gradient. The exception is cases when intracranial pressure increases significantly (for example, with the development of a brain tumor, hemorrhages in the cranial cavity) or, conversely, the tone of the eye sharply decreases.

All nerve fibers that make up the optic nerve are grouped into three main bundles. The axons of ganglion cells extending from the central (macular) region of the retina constitute the papillo-macular fascicle, which enters the temporal half of the optic nerve head. Fibers from the ganglion cells of the nasal half of the retina go along radial lines to the same half. Similar fibers, but from the temporal half of the retina, on the way to the optic nerve head “flow around” the papillo-macular bundle above and below.

In the orbital segment of the optic nerve near the eye, the relationships between nerve fibers remain the same as in its disc. Next, the papillo-macular bundle moves to the axial position, and the fibers from the temporal and nasal halves of the retina move to the corresponding parts of the optic nerve.

Thus, the optic nerve is clearly divided into right and left halves. Its division into upper and lower halves is less pronounced. An important clinical feature is that the nerve is devoid of sensory nerve endings.

In the cranial cavity, the optic nerves connect above the area of ​​the sella turcica, forming a chiasma (chiasma opticum), which is covered with the pia mater and has the following dimensions: length - from 4 to 10 mm, width - 9-11 mm and thickness 5 mm. It borders below with the diaphragm of the sella turcica (the preserved portion of the dura mater), above (in the posterior section) with the bottom of the third ventricle, on the sides with the internal carotid arteries, and behind with the pituitary infundibulum.

In the area of ​​the chiasm, the fibers of the optic nerves partially intersect due to portions associated with the nasal halves of the retinas. Moving to the opposite side, they connect with fibers from the temporal halves of the retinas of the other eye and form the visual tracts. The papillo-macular bundles also partially intersect here.

Visual tracts (tractus opticus) they begin at the posterior surface of the chiasm and, rounding the outer side of the cerebral peduncles, end in the external geniculate body (corpus geniculatum laterale), the posterior part of the visual thalamus (thalamus opticus) and the anterior quadrigeminum (corpus quadrigeminum anterius) of the corresponding side. However, only the external geniculate bodies are an unconditional subcortical visual center. The remaining two entities perform other functions.

In the optic tracts, the length of which in an adult reaches 30-40 mm, the papillo-macular bundle also occupies a central position, and crossed and uncrossed fibers still run in separate bundles. Moreover, the first of them are located ventro-medially, and the second - dorso-lateral.

The optic radiation (central neuron fibers) originates from the ganglion cells of the fifth and sixth layers of the lateral geniculate body. First, the axons of these cells form the so-called Wernicke's field, and then, passing through the posterior thigh of the internal capsule, they fan out in the white matter of the occipital lobe of the brain. The central neuron ends in the groove of the bird's spur (sulcus calcarinus). This area represents the sensory visual center - cortical area 17 according to Brodmann.

Pupillary reflex pathway- light and setting the eyes at close range is quite difficult. The afferent part of the reflex arc of the first of them begins from the cones and rods of the retina (according to some data, only from the cones) in the form of autonomous fibers running as part of the optic nerve. In the chiasm they intersect in the same way as the optic fibers and pass into the optic tracts. In front of the external geniculate bodies, the pupillomotor fibers leave them and, after partial decussation, continue into the brachium quadrigeminum, where they end at the cells of the so-called pretectal area (area pretectalis). Next, new interstitial neurons, after partial decussation, are sent to the corresponding nuclei (Yakubovich-Edinger-Westphal) of the oculomotor nerve. Afferent fibers from the macula of the retina of each eye are represented in both oculomotor nuclei.

The efferent pathway of innervation of the iris sphincter begins from the already mentioned nuclei and runs as a separate bundle within the n.oculomotorius. In the orbit, the sphincter fibers enter its lower branch, and then through the radix oculomotoria into the ciliary ganglion. Here the first neuron of the path in question ends and the second begins. Upon exiting the ciliary ganglion, the sphincter fibers composed of nn. The ciliares breves, having pierced the sclera, enter the perichoroidal space, where they form a nerve plexus. Its terminal branches penetrate the iris and enter the muscle in separate radial bundles, i.e. innervate it sectorally. In total, there are 70-80 such segments in the sphincter of the pupil.

The efferent pathway for the m.dilatator pupillae, which receives sympathetic innervation, begins from the ciliospinal center of Budge. The latter is located in the anterior horns of the spinal cord between the VII cervical and II thoracic vertebrae. From here connective branches depart, which through the border trunk of the sympathetic nerve (l), and then the lower and middle sympathetic cervical ganglia (ti and t2) reach the upper ganglion (level II-IV cervical vertebrae). Here the first neuron of the path ends and the second one begins, which is part of the plexus of the internal carotid artery. In the cranial cavity, the fibers innervating the pupillary dilator emerge from the mentioned plexus, enter the gangl.trigeminale, and then leave it as part of the n.ophthalmicus. Already at the apex of the orbit they pass into the n.nasociliaris and then, together with the nn.ciliares longi, penetrate the eyeball. In addition, the central sympathetic pathway(s) also departs from the Budge center, ending in the occipital cortex of the brain. From here begins the corticonuclear pathway of inhibition of the sphincter of the pupil.

This is often associated with a decrease in visual acuity, disturbances in the visual fields, and pronounced pathological changes in the fundus. The congenial reaction of the pupil of the other eye also appears to be weakened. When the healthy eye is illuminated, the friendly reaction of the pupil of the diseased eye is preserved.
Pupillary reflexes can change with various lesions and diseases of the eye, optic nerve, optic tract chiasm, i.e., with pathology in the peripheral neuron of the visual tract. It occurs with lesions within the arc of the pupillary reflex.
Pupillary reflexes (reactions) remain unchanged and normal with various lesions of the visual pathway: in the area of ​​the external geniculate body, Graziole radiance, cortical visual centers, i.e. in the central (IV) neuron of the visual pathway and the occipital lobe of the brain (fields 17, 18, 19 according to Brodmann).

Disorders of pupillary reflexes, accompanied by changes in pupil size

Pupil dilation
Unilateral dilation of the pupil without disturbances in pupillary reactions to light, accommodation with convergence. This pupillary condition is caused by irritation of the structures that provide sympathetic innervation to the iris. Clinically, this condition is manifested by Pourfur du Petit syndrome, which in its characteristics is opposite to Bernard-Horner syndrome.
Spontaneous short-term, irregular, alternating dilation and constriction of the pupils, lasting for seconds, are called “hippus”. Hippus occurs both in healthy people and in patients with certain organic diseases of the brain.
Bilateral short-term slight (1-2 mm) dilation of the pupils can occur from various painful irritations of the skin of the body: mainly the skin of the face, neck and hands. This is a ciliospinal reflex, which is carried out mainly by spinal and partly trigeminal afferent systems. The presence of this reflex indicates the normal functioning of these systems.

Pathological conditions associated with constriction of the pupils
Most often, such constriction of the pupil is one of the symptoms included in Bernard-Horner syndrome.
Miosis, anisocoria, disturbances of pupillary reactions in combination with paresis or paralysis of upward gaze, vertical nystagmus constitute Parinaud's syndrome. This syndrome is observed when the superior colliculus and pretectal nuclei are affected, which occurs most often with tumors of this localization.
Changing the shape of the pupils. Changes in the shape of the pupils are often associated with various eye diseases and are not associated with diseases of the nervous system. However, with syphilitic lesions of the nervous system, deformation of the pupils often occurs.

Reflexes are the most important function of the body. Scientists who studied reflex function mostly agreed that all conscious and unconscious acts of life are essentially reflexes.

What is a reflex

Reflex is the response of the central nervous system to irritation of the recipes, which ensures the body's response to changes in the internal or external environment. The implementation of reflexes occurs due to irritation of nerve fibers, which are collected in reflex arcs. Manifestations of the reflex are the occurrence or cessation of activity on the part of the body: contraction and relaxation of muscles, secretion of glands or its stop, constriction and dilation of blood vessels, changes in the pupil, etc.

Reflex activity allows a person to quickly react and properly adapt to changes around him and within. It should not be underestimated: vertebrate animals are so dependent on the reflex function that even its partial disruption leads to disability.

Types of reflexes

All reflex acts are usually divided into unconditional and conditional. Unconditional ones are transmitted hereditarily; they are characteristic of every biological species. Reflex arcs for unconditioned reflexes are formed before the birth of the organism and remain in this form until the end of its life (if there is no influence of negative factors and diseases).

Conditioned reflexes arise in the process of development and accumulation of certain skills. New temporary connections are developed depending on conditions. They are formed from unconditioned ones, with the participation of higher brain regions.

All reflexes are classified according to different criteria. According to their biological significance, they are divided into nutritional, sexual, defensive, orientation, locomotor (movement), postural-tonic (position). Thanks to these reflexes, a living organism is able to provide the main conditions for life.

In each reflex act, all parts of the central nervous system are involved to one degree or another, so any classification will be conditional.

Depending on the location of irritation receptors, reflexes are:

• exteroceptive (external surface of the body);
• viscero- or interoreceptive (internal organs and vessels);
• proprioceptive (skeletal muscles, joints, tendons).

Depending on the location of neurons, reflexes are:

• spinal (spinal cord);
• bulbar (medulla oblongata);
• mesencephalic (midbrain);
• diencephalic (diencephalon);
• cortical (cerebral cortex).

The reflex acts carried out by neurons of the higher parts of the central nervous system also involve fibers of the lower parts (intermediate, middle, medulla oblongata and spinal cord). In this case, the reflexes that are produced by the lower parts of the central nervous system necessarily reach the higher ones. For this reason, the presented classification should be considered conditional.

Depending on the response and the organs involved, reflexes are:

• motor, motor (muscles);
• secretory (glands);
• vasomotor (blood vessels).

However, this classification applies only to simple reflexes that combine certain functions within the body. When complex reflexes occur that irritate the neurons of the higher parts of the central nervous system, different organs are involved in the process. This changes the behavior of the organism and its relationship with the external environment.

The simplest spinal reflexes include flexion, which allows you to eliminate the stimulus. This also includes the scratching or rubbing reflex, knee and plantar reflexes. The simplest bulbar reflexes: sucking and corneal (closing of the eyelids when the cornea is irritated). Mesencephalic simple ones include the pupillary reflex (constriction of the pupil in bright light).

Features of the structure of reflex arcs

A reflex arc is the path that nerve impulses travel, carrying out unconditioned and conditioned reflexes. Accordingly, the autonomic reflex arc is the path from irritation of nerve fibers to the transmission of information to the brain, where it is converted into a guide to the action of a specific organ. The unique structure of the reflex arc includes a chain of receptor, intercalary and effector neurons. Thanks to this composition, all reflex processes in the body are carried out.

Reflex arcs as parts of the peripheral nervous system (the part of the nervous system outside the brain and spinal cord):

• arcs of the somatic nervous system, which provide nerve cells to skeletal muscles;
• arcs of the autonomic system that regulate the functionality of organs, glands and blood vessels.

Structure of the autonomic reflex arc:

1. Receptors. They serve to receive irritating factors and respond with excitation. Some receptors are presented in the form of processes, others are microscopic, but they always include nerve endings and epithelial cells. Receptors are part of not only the skin, but also all other organs (eyes, ears, heart, etc.).
2. Sensory nerve fiber. This part of the arc ensures the transmission of excitation to the nerve center. Since the nerve fiber bodies are located directly near the spinal cord and brain, they are not included in the central nervous system.
3. Nerve center. Here, switching between sensory and motor neurons is ensured (due to instantaneous excitation).
4. Motor nerve fibers. This part of the arc transmits a signal from the central nervous system to the organs. The processes of nerve fibers are located near internal and external organs.
5. Effector. In this part of the arc, signals are processed and a response to receptor stimulation is formed. The effectors are mostly muscles that contract when the center receives stimulation.

The signals of receptor and effector neurons are identical, since they interact following the same arc. The simplest reflex arc in the human body is formed by two neurons (sensory, motor). Others include three or more neurons (sensory, intercalary, motor).

Simple reflex arcs help a person involuntarily adapt to changes in the environment. Thanks to them, we withdraw our hands if we feel pain, and our pupils react to changes in lighting. Reflexes help regulate internal processes and help maintain a constant internal environment. Without reflexes, homeostasis would be impossible.

How the reflex works

A nervous process can provoke or increase the activity of an organ. When nervous tissue receives irritation, it goes into a special state. Excitation depends on differentiated concentrations of anions and cations (negatively and positively charged particles). They are located on both sides of the membrane of the nerve cell process. When excited, the electrical potential on the cell membrane changes.

When a reflex arc has two motor neurons in the spinal ganglion (nerve ganglion), the cell's dendrite will be longer (a branched process that receives information through synapses). It is directed towards the periphery, but remains part of the nervous tissue and processes.

The excitation speed of each fiber is 0.5-100 m/s. The activity of individual fibers is carried out in isolation, that is, the speed does not transfer from one to another.

Inhibition of excitation stops the functioning of the site of stimulation, slowing down and limiting movements and responses. Moreover, excitation and inhibition occur in parallel: while some centers fade away, others are excited. Thus, individual reflexes are delayed.

Inhibition and excitation are interconnected. Thanks to this mechanism, the coordinated operation of systems and organs is ensured. For example, the movements of the eyeball are carried out by alternating the work of muscles, because when looking in different directions, different muscle groups contract. When the center responsible for muscle tension on one side is excited, the center on the other slows down and relaxes.

In most cases, sensory neurons transmit information directly to the brain using a reflex arc and several interneurons. The brain not only processes sensory information, but also stores it for future use. In parallel, the brain sends impulses along the descending pathway, initiating a response from effectors (the target organ that performs the tasks of the central nervous system).

Visual path

The anatomical structure of the visual pathway is represented by a number of neural links. In the retina, these are rods and cones, then bipolar and ganglion cells, and then axons (neurites that serve as a path for impulses emanating from the cell body to the organs).

This circuit represents the peripheral portion of the visual pathway, which includes the optic nerve, chiasm, and optic tract. The latter ends in the primary visual center, where the central neuron of the visual pathway begins, which reaches the occipital lobe of the brain. The cortical center of the visual analyzer is also located here.

Components of the visual pathway:

1. The optic nerve begins at the retina and ends at the chiasm. Its length is 35-55 mm, and its thickness is 4-4.5 mm. The nerve has three sheaths and is clearly divided into halves. The nerve fibers of the optic nerve are divided into three bundles: axons of nerve cells (from the center of the retina), two fibers of ganglion cells (from the nasal half of the retina, as well as from the temporal half of the retina).
2. The chiasm begins above the area of ​​the sella turcica. It is covered with a soft shell, length is 4-10 mm, width is 9-11 mm, thickness is 5 mm. This is where fibers from both eyes connect to form the optic tracts.
3. The visual tracts originate from the posterior surface of the chiasm, go around the cerebral peduncles and enter the external geniculate body (the unconditional visual center), the visual thalamus and the quadrigeminals. The length of the optic tracts is 30-40 mm. The fibers of the central neuron begin from the geniculate body and end in the sulcus of the bird's spur - in the sensory visual analyzer.

Pupillary reflex

Let's consider the reflex arc using the example of the pupillary reflex. The path of the pupillary reflex passes along a complex reflex arc. It starts from the fibers of the rods and cones, which are part of the optic nerve. The fibers cross in the chiasm, passing into the optic tracts, stop in front of the geniculate bodies, partially twist and reach the pretectal region. From here, new neurons go to the oculomotor nerve. This is the third pair of cranial nerves, which is responsible for the movement of the eyeball, the light reaction of the pupils, and the raising of the eyelid.

The return path begins from the oculomotor nerve to the orbit and the ciliary ganglion. The second neuron of the link emerges from the ciliary ganglion, through the sclera into the perichoroidal space. A nerve plexus is formed here, the branches of which penetrate into the iris. The sphincter of the pupil has 70-80 radial neuron bundles entering it sectorally.

The signal for the muscle that dilates the pupil comes from the ciliospinal center of Budge, which is located in the spinal cord between the seventh cervical and second thoracic vertebrae. The first neuron goes through the sympathetic nerve and sympathetic cervical ganglia, the second starts from the superior ganglion, which enters the plexus of the internal carotid artery. The fiber that supplies the pupillary dilator nerves leaves the plexus in the cranial cavity and enters the optic nerve through the trigeminal ganglion. Through it, the fibers penetrate the eyeball.

The closedness of the circular work of the nerve centers makes it perfect. Thanks to the reflex function, the correction and regulation of human activity can occur voluntarily and involuntarily, protecting the body from changes and danger.

Translation from German by N.A. Ignatenko

One advantage of eye examination is that most structures are visible, so a diagnosis can be made during a clinical examination. In any case, it is very important to collect anamnesis during the clinical examination of the patient, since eye changes are often a sign of a systemic disease.

The sequence of ophthalmological examination is focused on the anatomical structure of the eye and depends on it. A strictly systematic approach is of great importance. First, an examination is necessary, and only then further measures, such as palpation, eversion of the third eyelid, staining of the cornea, dilation of the pupil for ophthalmoscopy, etc.

A detailed examination of both eyes is mandatory, even if changes are observed in one.

Anamnesis

In ophthalmology, as in all areas of veterinary medicine, a detailed history is very important. It is necessary to start with how long the animal has been with these owners, how long ago and under what circumstances changes associated with vision were noticed. Owner perceptions of a pet's eye problems can be an important factor in determining disease progression, such as the development of blindness.

With severe bilateral cataracts, fundus examination becomes impossible. If a pet owner says that their pet could see “until the pupils turned white,” then cataracts may be the sole cause of vision loss. If the owner is sure that “the pupils were normal” and the pet is already blind, then perhaps, in addition to cataracts, we can also talk about retinal degeneration. In general, questions to the owner are aimed at understanding the sequence of changes in the eyes of his pet. Regarding blindness, you can ask the following questions:

Can the patient see better in certain lighting conditions?

Does vision loss correlate with moving, rearranging furniture, or walking in unfamiliar areas (such as visiting a clinic)?

How did the owner realize that his pet could no longer see? Does the pet try to stay close to the owner’s leg all the time?

Have there been changes in the patient's general health (eg, symptoms of diabetes, etc.)?

Examination of the anterior chamber of the eye

During this test, you should try to avoid stress as much as possible. If the patient's eye is very painful and there is a risk of further damage during the examination, then it is necessary to put the animal under short-term anesthesia. First, the patient is examined in a lighted room at some distance (observation). In this case, you need to pay attention to the following points:

Are we talking about unilateral or bilateral changes?

What is the relationship of the eye to the orbit, to the eyelids, to the second eye?

Estimate the size of the eyeball: large, small, normal?

What position does the eyeball occupy: is exophthalmos or endophthalmos observed?

Are the axes of both eyes the same?

Is there a loss of the third eyelid?

Is there discharge from the eyes? Are both pupils the same size, or is there anisocoria (pupils of different sizes)? Is there dilation of the pupils (mydriasis)? (Fig. 1, 2)?

At the final stage, the auxiliary parts of the eye are examined using a focal (direct and lateral) light source. You can use an otoscope or a slit lamp for this. The slit lamp principle is based on focal illumination. It makes it possible to accurately examine the anterior and middle parts of the eye at fifteen times magnification. The assessment is carried out binocularly. Lateral illumination through the light slit makes it possible to study optical layers.

It is also necessary to pay attention to inflammation, neoplasms, anatomical abnormalities (congenital and acquired), the integrity of the cornea, the presence or absence of moisture, foreign bodies, signs of injury, pain (probable self-injury, blinking). Any changes must be documented accordingly, for example by means of a sketch (Fig. 3, 4).

To study the structures that are located behind the lens, mydriasis achieved with medication is mandatory (see Ophthalmoscopy section).

Neurological examination of the eye

Reflex test

Pupillary reflex

To assess the direct pupillary reflex, a light source is directed into the eye being examined.

Directing the light toward the temporal part of the retina can be helpful, as it is very sensitive. It is best to conduct the study in a room with normal lighting in order to immediately assess the symmetry of the pupils without complications that may arise in the dark due to changes in parasympathetic tone.

It is often difficult to assess the response to light of an unstimulated eye (indirect pupillary reflex) because room light can reflect on the cornea and complicate pupil assessment. This can be avoided using the following techniques:

Use of a direct ophthalmoscope, during which the direct response in each eye can be assessed under room light. You can darken the room or turn off the lights and move away from the patient so much that the reflection of the bottom of the eyeball is visible in both pupils using an ophthalmoscope with a “0” diopter. The assistant shines light first into one eye, then into the second eye, during which you can observe the reaction of the eye that does not receive a direct light source.

The so-called flashlight test can be performed without an assistant and without darkening the room. It is first necessary to establish with certainty that each eye exhibits a direct response. Then the light source is directed to the right eye. If the pupil responds (or if the pupil does not respond after one to two seconds), the light source is quickly directed to the left eye. If the reaction was in the left eye, then the left pupil should remain constricted (if this is not done quickly enough, the left pupil will again dilate somewhat and show a normal direct reaction to light). You need to act in the same way for the other side.

The reflex response assessment is described below.

Corneal reflex

It is controlled by the trigeminal nerve (V sensory branch) and the facial nerve (VII motor branch). Consequently, every touch or painful stimulation of the cornea leads to a reflexive closing of the eye through contraction of the orbicularis oculi muscle ( M. orbicularis oculi). A distinction is made between the direct corneal reflex (reaction of the irritated eye) and the reaction of the contralateral eye.

Threat reflex

It is also known as the blink reflex. It is controlled by the optic nerve (II afferent branch) and the facial nerve (VII motor branch). Therefore, the subcortical reflex, which is caused by sudden stimulation of the visual system (for example, a foreign body that moves towards the eye), leads to a reflexive closure of the eye and jerking of the head. The reflex may contain cortical components, since it requires intact (undamaged) photosensitive and motor areas of the cerebral cortex on the ipsilateral side. Opacity of the eye media and color deviations can lead to misdiagnosis. If the patient, for example, has a complete cataract, then testing the threat reflex will not be of practical value. The threat reflex may not directly correlate with the animal's ability to see. There are situations in which the patient sees, but the threat reflex is negative, or vice versa, the patient does not see, but the threat reflex is positive.

Reaction to light

This is an involuntary reaction of the eye to a light source. Especially if a strong light is shining directly into the eye, the reaction includes blinking, protrusion of the third eyelid (if there is a third eyelid), and sometimes moving the head in the direction opposite to the light source. Despite neuroanatomical support for this response, it is not entirely clear whether a positive response is generally a sign of unimpaired visual transmission to the brain and can be taken as a sign of preserved vision. This reflex is a more reliable indicator of vision preservation than the threat reflex, and is especially useful in those patients who have cloudy eyes for various reasons. Even complete cataracts or corneal lesions do not affect this reflex.

Visual impairment

Visual ability testing

Since we cannot ask our patients about their visual abilities, it is worthwhile to observe their behavior for a few minutes. The pupillary reflex, the threat reflex and the reaction to light rather test the integrity of the neuroanatomical structures. All these tests may be positive, and the patient is still unable to get around the obstacles or navigate his way through.

Obstacle course

You should have a simple obstacle course at your disposal, but some animals, especially cats, do not cooperate.

The obstacle course must be completed in daylight (to test photopic vision) and in the dark (to control scotopic vision) to test the visual abilities of cones and rods. Red light is useful for stimulating scotopic (rod) vision.

It is very difficult to differentiate vision loss in cats. You can sit the cat on the table and observe how confident she is when jumping and landing on her paws, how purposeful her jump was.

If there is a suspicion of one-sided blindness, then the animal must go through the obstacle course with one eye covered. In any case, both eyes should be evaluated, as some patients refuse to complete the obstacle course with one eye taped, regardless of whether they are blind or not.

Test reactions to movement

An undulating movement of the hand in front of the eye may cause the patient to blink only due to air vibrations, even if he has no ability to see. To reduce drafts, you can hold a clear plastic sheet between your hand and eye. An alternative is to use a piece of cotton wool, which is dropped in front of the patient and observed as he follows the fall. Using a test with a piece of cotton wool, you can also check the volume of the visual field, which is greatly reduced in glaucoma. To check, the cotton ball should always fly from above, from the temporal edge, down to the nasal edge.

Signs of blindness

Sudden complete blindness is usually accompanied by slower, more cautious movements, and the animal begins to bump into objects. With gradual or congenital blindness, the patient very often seems to be sighted, since he compensates for the missing vision with other senses (hearing and smell). Animals know their surroundings and move around without problems.

CAVE: An absent pupillary reflex does not indicate blindness, just as its presence does not always mean that the animal can see.

Differential diagnosis of vision loss

Vision loss (blindness) can be unilateral or bilateral, and it can also be caused by neurological and ophthalmological problems. Sometimes a thorough neurological and ophthalmological examination is necessary to find the causes. In some cases, specialized studies (electroretinography) are necessary.

1. Unilateral blindness

Loss of vision in one eye or one visual field may result from unilateral damage to the retina, optic nerve, optic tract, optic radiation, or cerebral cortex.

If the cause of vision loss lies in the optic nerve, then there is one-sided blindness and loss of pupillary response to light in both eyes. If a light source is directed into a blind eye, the pupils may be symmetrical, or the pupil in the blind eye may be slightly larger than the pupil of the healthy eye.

If the cause of blindness is in the optic tract, optic radiation or cerebral cortex, then in this case there is a loss of the visual field with a normal pupil reaction. The animal will also show other symptoms of cerebral disease associated with lesions in this area. Vision loss occurs on the side opposite the central nervous system lesion. The size of both pupils is the same.

2. Bilateral blindness

If the lesions are located in the retina, optic nerve or optic tract, then blindness is accompanied by maximally dilated pupils that do not respond to light. No other neurological symptoms are observed.

If the lesion is located in both fields radiatum or the visual cortex, then there is complete loss of vision, but the pupils are of normal size. A normal reaction to light with visual stimulation may also be seen.

Nystagmus

Nystagmus is involuntary rhythmic movements of both eyes. There are physiological and artificially induced nystagmus (provocative nystagmus), as well as pathological spontaneous nystagmus. The latter will be discussed in more detail.

Classification

Pathological nystagmus has two characteristics: by its direction and by what causes it. Both can provide information about the location of the disorder.

1. According to the direction of oscillatory movements, they are distinguished:

a) horizontal: fluctuations from one side to the other in most cases indicate a peripheral disease, rapid fluctuations go from the side of the lesion to the opposite;

b) rotational: the eye rotates clockwise or counterclockwise in the orbit, which does not indicate a specific localization of the lesion;

c) vertical: The eye rotates ventrally relative to the level of the head. This form of nystagmus is usually observed in diseases of the central nervous system;

d) direction changes: If the direction of nystagmus changes with different head positions, then this indicates a disease of the central nervous system.

2. By type of occurrence in relation to movement:

a) constant nystagmus: observed if the animal's head is in a normal position. Typically, this type of nystagmus occurs with peripheral diseases;

b) positional nystagmus: observed when the head is not parallel to the floor. It lasts more than one minute after the head has stopped moving. Positional nystagmus is observed in diseases of the central nervous system.

Causes

Pathological nystagmus is considered a symptom of peripheral or central diseases of the vestibular apparatus. The following symptoms may also be associated with it: ataxia, bowed head, circular movements and dizziness. Central vestibular disorders can be caused by damage to:

In the brain stem. Will be expressed in weakness and proprioceptive deficits;

In the cerebellum. They will be characterized by tremor, hypermetry, and absent threat reflex with normal vision. The cause of nystagmus is the asymmetry of the muscle tone of the eyeball. When the right vestibular apparatus prolapses, only the left vestibular apparatus is stimulated, this leads to a slow tonic deviation of the eyeball to the right with a rapid return to the left. In this case, the fast phase acts in the direction of the lesion. The cause of the rapid correction phase is probably located in the cerebral cortex. A characteristic feature of vestibular nystagmus is that it is in no way correlated with a vision test and can be observed in blind animals.

1. Nystagmus in peripheral vestibular disease:

a) It is very pronounced at the onset of the disease and decreases throughout the disease (rarely observed for more than a few weeks).

b) In most cases, involuntary and always independent of the position of the head.

c) It is essentially unidirectional and maintains that direction regardless of the position of the animal's head.

d) Its direction is in most cases horizontal.

e) If its appearance is caused by a lesion in the area of ​​the inner ear, then symptoms of damage to the VII pair of facial nerves and Horner's syndrome will also be detected. If the lesion is located in the area of ​​the peripheral nerves, then in this case there will be no other symptoms.

2. Nystagmus with central vestibular lesion:

a) Tends to persist. As long as the animal has the disease, nystagmus will be observed.

b) Often has a progressive course and becomes more severe over time.

c) The direction of nystagmus may change when the head is tilted.

d) It often also has vertical components.

To be continued in the next issue.