The abducens nerve refers to the apparatus that regulates eye movement. Its role there is not as significant as the oculomotor, but in the event of a loss of function, the ability to see is lost to some extent. The consensual movement of the eyeballs requires six muscles, which are innervated by three cranial nerves.
Anatomy
The abducens nerve refers to pure motor nerves. It begins in the nucleus, which is located in the midbrain. Its fibers through the bridge descend to the basal surface of the brain and move further along the groove between the pons and the pyramids located in the medulla oblongata.
The processes of the nucleus pass through the membranes of the brain and end up in the cavernous sinus. There, the fibers are on the outside of the carotid artery. After the nerve has left the sinus, it enters the superior orbital fissure and finally enters the orbit. The abducens nerve innervates only one muscle - the direct lateral.
Function
The abducens nerve provides the only function that the muscle it innervates performs, namely, it takes the eye outward. This allows you to look aroundturning heads. And also this muscle is an antagonist of the internal rectus muscle of the eye, which pulls the eyeball to the center, towards the nose. They cancel each other out.
However, if one of them is affected, convergent or divergent strabismus is observed, since a he althy muscle will dominate and, contracting, turn the eyeball in its direction. The abducens nerve is paired, so friendly eye movement and binocular vision are provided.
Research
It is not possible to check the abducens nerve and its function in isolation at the current stage of the development of medicine. Therefore, neuropathologists and ophthalmologists examine all three nerves at once: oculomotor, abducens, and trochlear. This gives a more complete picture of the defeat.
Start, as a rule, with complaints of double vision, which increases when looking at the affected side. Then comes a visual examination of the patient's face in order to determine its symmetry, the presence of swelling, redness and other manifestations of the inflammatory process. After that, the eyes are examined separately for protrusion or retraction of the eyeball, drooping of the upper eyelid.
Be sure to compare the width of the pupils and their reaction to light (friendly or not), convergence and accommodation. Convergence is the ability to focus on a nearby subject. In order to check it, a pencil or hammer is brought to the bridge of the nose. Normally, the pupils should narrow. The study of accommodation is carried out for each eye separately, butin terms of execution technique, it resembles a convergence check.
Only after all these preliminary manipulations is it checked if the patient has strabismus. And if so, which one. Then the person is asked to follow the tip of the neurological malleus with his eyes. This allows you to determine the amount of movement of the eyeballs. By moving the hammer to the extreme points of the field of view and holding it in this position, the doctor provokes the appearance of horizontal nystagmus. If the patient has a pathology of the muscular apparatus of the eye, then pathological nystagmus (small horizontal or vertical eye movements) will not take long.
Absence nerve injury
As already known, the abducens nerve of the eye is responsible for turning the eyeball outward from the bridge of the nose. Violation of the conduction of the nerve leads to a violation of the mobility of the rectus lateral muscle. This causes convergent strabismus due to the fact that the internal muscle pulls on the eyeball. Clinically, this causes double vision, or scientifically, diplopia. If the patient tries to look in the affected direction, then this symptom intensifies.
Sometimes there are other pathological phenomena. For example, dizziness, impaired gait and orientation in space. In order to see normally, patients usually cover the diseased eye. The defeat of only the abducens nerve is extremely rare, as a rule, it is a combined pathology.
Nuclear and peripheral paralysis
Afferent neuropathynerve in its peripheral part is found in meningitis, inflammation of the paranasal sinuses, thrombosis of the cavernous sinus, aneurysms of the intracranial segment of the carotid artery or posterior communicating artery, fracture of the base of the skull or orbit, tumors. In addition, the toxic effects of botulism and diphtheria can also damage brain structures, including cranial nerves. Peripheral paralysis of the abducens nerve is also possible with mastoiditis. Patients have Gradenigo's syndrome: paresis of the abducens nerve of the eye, combined with pain at the exit site of the frontal branch of the trigeminal nerve.
Most often, nuclear disorders occur against the background of encephalitis, neurosyphilis, multiple sclerosis, hemorrhages, tumors or chronic disorders of cerebral circulation. Since the abducens and facial nerves are located side by side, the defeat of one causes the pathology of the neighboring one. The so-called Fauville alternating syndrome appears (paresis of part of the muscles of the face on the affected side and reduced movements in half of the body on the other side).
Bilateral defeat
Paresis of the abducens nerve on both sides is manifested by convergent strabismus. This condition occurs most often with increased intracranial pressure. If the amount of cerebrospinal fluid is excessive, then dislocation of the brain can be observed, that is, pressing the substance of the brain to the slope at the base of the skull. With such a development of events, the abducens nerves can easily suffer. They just in this place go to the lower surface of the brain and are practically not protected by anything.
There are other brain dislocations thatare manifested by similar symptoms:
- indentation of the tonsils into the occipitocervical funnel of the dura mater;- herniation of the cerebellum into the medullary sail and others.
They are not compatible with life, so the presence of damage to the abducens nerve is a pathological finding. In addition, it must be remembered that weakness of the external rectus muscle is one of the symptoms of myasthenia gravis.
