Abducens Nerve

ABDUCENS NERVE (abducens nerve), belongs to the sixth pair of cranial nerves; it originates in a nucleus located in the pons on the floor of the fourth ventricle, in the lower part of the eminentia teretis; around the nucleus, the root of the facial nerve forms a loop. The nucleus of the abducens nerve is connected via the posterior longitudinal bundle with the nuclei of the oculomotorius nerves of its own and the opposite side (which explains the synergistic activity of the lateral rectus muscle with the medial rectus of the other eye), with the vestibular nuclei, and with the superior olive. The cells of the nucleus are not particularly large, the axons extending from them form the root of the abducens nerve, which pierces the entire thickness of the pons and emerges on its anterior surface above the pyramids of the medulla oblongata. The extramedullary part of the abducens nerve extends upward along the anterior surface of the pons, penetrates through the porus abducentis into the sinus cavernosus, where, surrounded by the dura mater, it is located on the outer side of the internal carotid artery; upon exiting the sinus, the nerve enters the orbit through the superior orbital fissure, located above the oculomotorius nerve. The abducens nerve innervates only one muscle—the lateral rectus—thus its function is reduced to abduction of the eyeball outward.

Pathology of the abducens nerve. When the abducens nerve is affected, the outward mobility of the eyeball is completely lost (paralysis) or only weakened (paresis). The normal limit of mobility is considered achieved if the outer edge of the cornea touches the outer commissure of the eyelids. If this is not the case, then the lesion is present, and the clinical picture consists of the following disorders: 1) limitation of mobility, 2) diplopia, 3) secondary deviation of the eye, 4) involuntary position of the head, 5) visual vertigo, unsteady gait, incorrect orientation. Thus, if we have paralysis of the right abducens nerve, the eye cannot be displaced from the median position to the right toward the temple; diplopia appears, with both images being identical, arranged parallel to each other and at some distance from each other in the horizontal direction; the distance between them increases the further the object is moved to the right. The patient holds the head to the right. Usually, there is visual vertigo. In mild pareses, all these phenomena may be expressed so slightly that they hardly bother the patient. Isolated lesions of the abducens nerve, as of other oculomotor nerves, occur relatively rarely; more often, it is the beginning and partial manifestation of a disease of the central nervous system in general intoxications and infections. Among the infections affecting the abducens nerve along with other oculomotor nerves, syphilis should be placed in first place, as it causes almost half of the paralyses of these nerves. Lesions of these nerves are often found in epidemic encephalitis, diphtheria, disseminated sclerosis, in Heine-Medin disease, influenza, and other diseases. Among intoxications, alcohol poisoning, sausage and fish poisoning, lead poisoning, carbon monoxide poisoning are most often noted. This should probably also include sometimes occurring paralyses of the abducens nerve after spinal anesthesia with novocaine. As the beginning or partial disease in diseases of the central nervous system, paralysis of the abducens nerve occurs in meningitis (especially basilar), in tumors, hemorrhages, thromboses, softening, in lesions of the bones of the skull by tumors, caries (pyramid), in suppurations of the sphenoid and nasal sinuses, in injuries (fractures of the base, operations on Gasser's ganglion, on the orbit) and contusions of the temple. Congenital aplasia of the abducens nucleus is also often encountered; such paralyses rarely lead to secondary contracture of the antagonist. Thus, congenital paralysis is usually nuclear. All lesions of the abducens nerve can be divided according to the site of damage into cortical, conductive, nuclear, root, and peripheral. The latter, in turn, into intradural, intracranial, and orbital. Cortical and conductive lesions are entirely diseases of the brain and brainstem. Nuclear lesions are usually combined with paralysis of gaze (see). Root lesions within the stem give a picture of alternating paralysis of Foville, in which the abducens and facial nerves are affected on one side, and the limbs on the other. Peripheral paralyses of the abducens nerve have diverse origins: thus, the abducens nerve is usually affected in tumors of the cerebellopontine angle (see). The abducens nerve passes near the apex of the pyramid, and therefore inflammation from the pyramid easily spreads to it in purulent processes in the middle ear. The intradural path of the abducens nerve is short, the abducens nerve quickly enters the sinus cavernosus, where it lies on the internal carotid artery. With its aneurysms, it is usually affected, as well as with thrombosis of the sinus. Although the exit of the abducens nerve into the orbit is not narrow, this place is predisposed to fractures, and therefore traumatic paresis of the abducens nerve is so common. Proceeding from the fact that a lesion of the abducens nerve is usually only part of a disease of the central nervous system, it follows that the prognosis and therapy depend entirely on the underlying disease of the central nervous system, mainly on its etiology. Since a significant part of these paralyses depends on syphilis, timely started energetic specific treatment gives good results. In a number of infections and intoxications, if the body recovers from them, the paralysis of the abducens nerve also passes. So-called 'rheumatic' paralyses usually last about 2-3 months, the abducens nerve recovers even faster after lumbar anesthesia. On the other hand, in tumors, if they are surgically inoperable, in skull fractures, in severe injuries, recovery may not occur. In addition to the main treatment, sometimes local treatment in the form of ointments promoting resorption (iodine, mercury, ichthyol) or irritating (veratrine) to the temple area is used. The application of electricity in the form of galvanization is more appropriate.