Abducens Nerve

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Description

Abducens nerve.png

The abducens nerve is the sixth cranial nerve (CN VI). It is one of the nerves responsible for the extraocular motor functions of the eye, along with the oculomotor nerve (CN III) and the trochlear nerve (CN IV).[1] It has a purely somatic motor function, which is innervation of the lateral rectus muscle, an extraocular muscle.

Course

Of all the cranial nerves, the abducens nerve has the longest intracranial course.[1] The abducens nerve emerges from the brainstem at the pontomedullary junction to enter the subarachnoid space, coursing upward between the pons and clivus to enter the Dorello canal. At the petrous apex, it angulates to enter the cavernous sinus and travels in close proximity to the internal carotid artery. The abducens nerve then proceeds through the superior orbital fissure and innervates the lateral rectus muscle.[2]

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Function

The abducens nerve is a purely somatic motor nerve, It has no sensory function.

Motor

Its main function is to carry general somatic efferent nerve axons to innervate the lateral rectus muscle which then abducts the eye on the ipsilateral side.[1] The lateral rectus muscle is one of the extraocular muscles of the eye. It originates in the bottom of the orbital cavity in the surrounding area of the optic canal, specifically in the lateral part of the common tendinous ring; the annulus of Zinn.[4]

The abducens nerve is also secondarily involve in innervation of the contralateral rectus muscle by way of the longitudinal fasciculus so that both eyes move laterally in a coordinated manner.[1]

Clinical relevance

Compromise of the abducens nerve results in the inability to abduct the ipsilateral eye and partial decrease in the ability to adduct the contralateral eye.[1] Patients usually present with binocular horizontal diplopia (double vision producing a side-by-side image with both eyes open), worse in the distance, an esotropia in primary gaze. Patients also may present with a head-turn to maintain binocularity and binocular fusion an to minimize diplopia.[2]

Damage to the abducens nerve can be caused by anything that compresses or stretches the nerve, such as tumors, aneurysms, fractures, or increased intracranial pressure (ICP).[1]

Abducens Nerve Palsy

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Assessment

The abducens nerve is examined in conjunction with the oculomotor and trochlear nerves by testing the movements of the eye. The patient is asked to follow a point with their eyes (commonly the tip of a pen) without moving their head. The target is moved in an ‘H-shape’ and the patient is asked to report any blurring of vision or diplopia (double vision).[6]

Treatment

Treatment of abducens nerve palsy include eye muscle exercises, prism therapy, injection of botulinium toxin, strabismus surgery.

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 Nguyen V, Reddy V, Varacallo M. Neuroanatomy, cranial nerve 6 (abducens). StatPearls [Internet]. 2020 Aug 11.
  2. 2.0 2.1 Hajee ME, MP E. Abducens nerve palsy. E_Medicine.
  3. Abducens Nerve - Anatomy Tutorial. Available from: https://youtu.be/qbrv5EcQDic [last accessed 20/11/2020]
  4. Cabrera AF, Suárez-Quintanilla J. Anatomy, head and neck, eye lateral rectus muscle. StatPearls [Internet]. 2020 Aug 10.
  5. U-M Kellogg Eye Center in Ann Arbor. Sixth (6th) Cranial Nerve Palsy. Available from:https://youtu.be/Pl2ozPSWfHs [last accessed 20/11/2020]
  6. Radhakrishnan A. The Abducens Nerve (CN VI). Available from:https://teachmeanatomy.info/head/cranial-nerves/abducens-nerve/.