The Abucent nerve
M van der Merwe
The Abucent nerve
M van der Merwe
Introduction and origin of the abducent nerve
The five segments of the abducent nerve
Curvatures of CN VI
Lesions and variations of the abducent nerve
Introduction and origin of the Abducent nerve
In humans the Abducent nerve (CN VI) arises from the basal plate of the developing pons. The nucleus of the Abducent nerve is situated within the pons, on the base of the fourth ventricle. The Abducent nucleus is located at the level of the facial colliculus and is near the midline similar to the Oculomotor and Trochlear nuclei that also control eye movements. A cluster of fibres from the Facial nerve (CN VII) circles around the nucleus of CN VI CITATION Aza13 l 7177 (Azarmina & Azarmina, 2013). The Abducent nerve typically exits the brainstem at the pontomedullary sulcus as a sole trunk. Along its entire course, the Abducent nerve courses alongside the subarachnoid space, petroclival region, cavernous sinus and the orbit CITATION Joo12 l 7177 (Joo, et al., 2012). The length of the Abducent cranial nerve ranges from 5mm to 17mm. In comparison to the length of the Trochlear nerve, the Abducent nerve is found to be three times shorter. Throughout the first year of life, the brain and cranial nerves grow speedily. After this period of rapid development, the brain and cranial nerves shows minimal growth and change. The Abducent nerve courses over the anterior surface of the pons, almost parallel to the midline, for distances varying from 0.5cm to 1.7cm (conditional of age), prior to penetrating the arachnoid membrane CITATION Han04 l 7177 (Hanson, et al., 2004).
The five segments of the Abducent nerve
CN VI has the lengthiest subarachnoid course of all cranial nerve and gives motor innervation to the ipsilateral lateral rectus muscle. The lateral rectus muscle abducts the eye CITATION Aza13 l 7177 (Azarmina & Azarmina, 2013). Along its course the Abducent nerve can be apportioned into 5 segments. The first 3 segments (cisternal segment, gulfar segment and cavernous segment) are located intracranially while the remaining two segments, the fissural and intraconal segments are orbital CITATION Lac07 l 7177 (Laconetta, et al., 2007). Other studies have been made that only divides the Abducent nerve into four segments (Subarachnoid segment, Petroclival segment, Cavernous segment and Intra-orbital segment). The petroclival and cavernous segments course within the two layers of the Dura mater and thus their anatomy is more difficult than that of the other segments CITATION Joo12 l 7177 (Joo, et al., 2012)Cisternal segment
The cisternal segment is situated within the membranes of the dura mater and passes from its exit at the pontomedullary sulcus to the dural foramen. From here the nerve becomes interdural and passes into the sphenopectroclival venous gulf. The Abducent nerve originates 4.13 ± 2.25 mm from the midline, ascending and moving laterally to arrive at the dural porus (diameter of 0.72 ± 0.27 mm) which is located 17.09 ± 1.51 mm below the posterior clinoid process and 8.05 ± 1.64 mm medial to the porus of the Facial nerve. The distance and width of this segment is 15.46 ± 1.73 mm and 0.56 ± 0.18 mm respectively. The Cisternal segment of the Abducent nerve receives its arterial supply from the perforating subdivisions of the preliminary portion of the basilar artery and is secured to its borders by light connective tissue CITATION Lac07 l 7177 (Laconetta, et al., 2007).
The Abducent nerve passes into the sphenopetroclival venous gulf which is a brokenly pyramidal-shaped venous space that is occupied by blood from the cavernous sinus anteriorly, from the basilar plexus medially and from the superior petrosal sinus laterally. The inferior petrosal sinus drains the blood inferiorly. The length and thickness of the Abducent nerve within this region is 11.50 ± 1.73 mm and 0.42 ± 0.21 mm respectively. Inside the gulf, the nerve runs supero-medially passing through Dorello’s canal (which is bounded superiorly by Gruber’s ligament and inferiorly by a bony notch formed by the petrous ridge, petroshenoidal suture and superolateral aspect of the clivus). Leaving Dorello’s canal, the Abducent nerve is strongly secured to the petrous apex by connective tissue and curves laterally and arrives at the dorsal curvature of the internal carotid artery. The dorsal meningeal artery supplies the Gulfar segment of the Abducent nerve. The posterior petroclinoid fold is the border between the gulfar and cavernous segments CITATION Lac07 l 7177 (Laconetta, et al., 2007).
Extradural segment that starts at the posterior bend of the internal carotid artery, extends in the interior of the cavernous sinus and ends at the dorsal margin of the superior orbital fissure. Within the cavernous sinus, the nerve courses lateral to the internal carotid artery and is fastened to the artery by connective tissue. The Abducent nerve also runs medially to the Ophthalmic nerve and superiorly to the petrolingual ligament. The diameter of the cavernous segment is 0.52 ± 0.15 mm while its length is 26.33 ± 3.93 mm. In this segment, the nerve is flattened adjacent to the lateral wall of the internal carotid artery and secured to it by connective fibres that stem from the tunica adventitia of the internal carotid artery and the nerve’s connective covering that envelopes the nerve end to end during its course. The inferolateral trunk of the dorsal meningeal artery (also known as the artery of the inferior cavernous sinus) supplies this segment of CN VI and nearby cranial nerves with blood CITATION Lac07 l 7177 (Laconetta, et al., 2007).
The Fissural segment comprises of the part of the nerve that passes through the superior orbital fissure and the common tendon of Zinn (or common tendinous ring). Within the superior orbital fissure, the Abducent nerve is located inferiorly and medially, in relation to the frontal and trochlear nerves and laterally, in relation to the superior and inferior branches of the oculomotor nerve. In the common tendinous ring, CN VI passes inferior to the nasociliary nerve, the superior division of the oculomotor nerve passes superiorly and more medial to the Abducent nerve while the inferior division of this nerve is aimed slightly inferiorly. The extent of the fissural segment is 2.57 ± 1.52 mm, and its thickness is 0.52 ± 0.15 mm. Blood is supplied to this segment of the nerve by branches from the anterior meningeal artery, ophthalmic artery and the lacrimal arteries CITATION Lac07 l 7177 (Laconetta, et al., 2007).
This segment consists of the section of the Abducent nerve that continues out of the common tendinous ring and bifurcates to the lateral rectus muscle. Here the nerve turns laterally to some extent to pass into a furrow on the medial aspect of the lateral rectus muscle where it divides into 3 or 4 major branches that supply the muscle. The main division of the nerve has a length of 1.2 ± 0.42 mm, and a width of 0.42 ± 0.27 mm. Branches from the anterior meningeal artery, ophthalmic artery and lacrimal arteries supply this segment CITATION Lac07 l 7177 (Laconetta, et al., 2007).
Curvatures of CN VI
The Abducent nerve has three angles within its intracranial course. The first curvature is in the same plane as the dural foramen. Here the nerve turns medially in the direction of the petrous apex. The second curve is above the petrous apex, where it turns inferiorly and laterally to get to the lateral aspect of the dorsal bend of the internal carotid artery. The third curvature is in the vicinity of the posterior bend of the internal carotid artery. In the area the Abducent nerve follows the external surface of the internal carotid artery CITATION Lac07 l 7177 (Laconetta, et al., 2007).
Lesions and variations of the Abducent nerve
The oculomotor, trochlear and abducent nerves all facilitate movements of the eyeball. Of these three nerves, the abducent nerve is most regularly influenced by intensified intracranial pressure and herniation. Additionally, the abducent nerve is the cranial nerve that is most commonly involved in disorders concerning the brain and eyes. However, only the intracranial part of the abducent nerve can be affected, as the part in Dorello’s canal and the cavernous sinus is extradural and not exposed to the consequences of increased pressure within the brain CITATION Han04 l 7177 (Hanson, et al., 2004).
From a clinical point of view, the two most significant mechanisms accountable for injuries to the abducent nerve are the stretching method and the direct compression of the nerve CITATION Lac07 l 7177 (Laconetta, et al., 2007). Impairment in the operation of the abducent nerve can be the result of lesions appearing at any point along the course of the nerve, from its nucleus to its termination at the lateral rectus muscle in the orbit. Lesions within the different parts of the nerve are often identifiable by involvement of neighbouring structures CITATION Aza13 l 7177 (Azarmina ; Azarmina, 2013). Peripheral lesions
Lesions to the peripheral segment of the sixth nerve will produce diplopia (double-vision), as a result of the unimpeded muscle tone of the medial rectus muscle. In such cases, the influenced eye is pulled more medially and can not be abducted in the direction of the temple. Partial damage to the peripheral part of the abducent nerve results in weak or partial abduction of the affected eye. Peripheral damage can be triggered by tumours, fractures or any other damage that stretches or compresses the abducent nerve. Also indirect damage can occur due to any process that applies a descending pressure on the brainstem, resulting in the nerve becoming stretched in the region of the clivus CITATION Aza13 l 7177 (Azarmina ; Azarmina, 2013).
Damage to the nucleus of the abducent nerve results in a palsy that has an effect on both eyes at the same time. The nucleus contains two cell types: motor neurons and interneurons. Motor neurons are responsible for controlling the lateral rectus muscle on the ipsilateral side while the interneurons traverse the midline and connects to the oculomotor nucleus that is responsible for controlling the medial rectus muscle of the adjacent eye. In normal vision, the lateral and medial movements of the eyes are coupled, so that both eyes can remain secured on the same object. Coupled gaze is mediated in the brainstem by the MLF (medial longitudinal fasiculus), which is a nerve band that connects the nuclei of the three nerves involved with movement of the eyeball. The most notable result of nuclear lesions is Internuclear ophthalmoplegia (disorder where one eye abducts properly and the other eye abducts minimally if at all) CITATION Aza13 l 7177 (Azarmina ; Azarmina, 2013).
The abducent nerve is one of the final routes for numerous cortical systems that are responsible for movement of the eye in general. Cortical control involves conjugate gaze rather than unilateral eye movement CITATION Aza13 l 7177 (Azarmina ; Azarmina, 2013).
BIBLIOGRAPHY Azarmina, M. ; Azarmina, H., 2013. The Six Syndromes of the Sixth Cranial Nerve. Journal of Ophthalmic and Vision Research, 8(2), pp. 160-171.
Hanson, R. et al., 2004. Abducens length and vulnerability?. Neurology, Volume 62, pp. 33-36.
Joo, W., Yoshioka, F., Funaki, T. ; Rhoton, A., 2012. Microsurgical Anatomy of the Abducens Nerve. Clinical Anatomy, pp. 1-13.
Laconetta, G. et al., 2007. The Abducens Nerve: Microanatomic and Endoscopic Study. Operative Neurosurgery, 61(1), pp. 7-14.