The abducent nerve refers to an apparatus that regulates eye movement. Its role there is not as significant as that of the oculomotor, but in case of loss of function, the ability to see is lost to some extent. For the friendly movement of the eyeballs, six muscles are needed that are innervated by three cranial nerves.
Anatomy
The abduction nerve refers to pure motor nerves. It begins in the nucleus, which is located in the midbrain. Its fibers across the bridge descend to the basal surface of the brain and move further along the furrow 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 located outside the carotid artery. After the nerve has left the sinus, it enters the upper orbital fissure and finally enters the orbit. The abduction nerve innervates only one muscle - the lateral rectus.
Function
The abducent nerve provides the only function that the muscle that it innervates performs, namely, it diverts the eye outward. This allows you to look around without turning your head. 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, when one of them is affected, a converging or diverging strabismus is observed, since a healthy muscle will dominate and, contracting, turn the eyeball in its direction. The abducent nerve is paired, therefore friendly eye movement and binocular vision are provided.
Study
It is not possible to check the isolated abducent nerve and its function at the present stage of development of medicine. Therefore, neuropathologists and ophthalmologists immediately examine all three nerves: oculomotor, abduction and block. This gives a more complete picture of the defeat.
They begin, as a rule, with complaints of double vision, which intensifies when looking at the affected side. Then there is 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 the protrusion or retraction of the eyeball, the omission of the upper eyelid.
Be sure to compare the width of the pupils and their response to light (friendly or not), convergence and accommodation. Convergence is the ability to focus on a closely located 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, but according to the technique of execution, it resembles a convergence test.
Only after all these preliminary manipulations is it checked whether the patient has strabismus. And if so, which one. Then they ask the person to follow with their eyes the tip of the neurological hammer. This allows you to determine the range of motion of the eyeballs. By taking 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.
The defeat of the abduction nerve
As already known, the abducent nerve of the eye is responsible for turning the eyeball outward from the nose. Violation of nerve conduction leads to impaired mobility of the rectus lateralis muscle. This causes a convergent strabismus due to the fact that the inner muscle pulls the eyeball over itself. Clinically, this causes double vision, or scientifically, diplopia. If the patient tries to look in the affected direction, then this symptom intensifies.
Other pathological phenomena are sometimes observed. For example, dizziness, impaired gait and orientation in space. In order to see normally, patients usually cover their sore eyes. The defeat of only the abduction nerve is extremely rare, as a rule, this is a combined pathology.
Nuclear and peripheral paralysis
Neuropathy of the abduction nerve in its peripheral section occurs with meningitis, inflammation of the sinuses, thrombosis of the cavernous sinus, aneurysms of the intracranial segment of the carotid artery or posterior connecting artery, fracture of the base of the skull or orbit, tumor. In addition, the toxic effects of botulism and diphtheria can also damage brain structures, including cranial nerves. Peripheral abdominal nerve palsy is also possible with mastoiditis. Gradenigo syndrome is observed in patients: paresis of the abducent nerve of the eye in combination with pain at the exit 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 cerebrovascular disorders. Since the abduction and facial nerve are located nearby, the defeat of one causes pathology of the neighboring one as well. The so-called alternating Foville syndrome appears (paresis of part of the muscles of the face on the affected side and decreased movement in half of the body on the other side).
Bilateral defeat
Paresis of the abducent nerve from two sides is manifested by a convergent squint. This condition occurs most often with increased intracranial pressure. If the amount of cerebrospinal fluid is excessive, then a dislocation of the brain can be observed, that is, pressing the brain substance to the slope at the base of the skull. With this development of events, the abducent 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, which are manifested by similar symptoms:
- indentation of tonsils into the occipital-cervical funnel of the dura mater;
- the insertion of the cerebellum into the brain sail and others.
They are not compatible with life, therefore, the presence of damage to the abduction nerve is a pathoanatomical statement. In addition, it must be remembered that the weakness of the external rectus muscle is one of the symptoms of myasthenia gravis.