Cranial Nerve III: Oculomotor Nerve

By:  Varsha Kumari

Photo Credit: www.depositphotos.com

Introduction:

The oculomotor nerve is the 3rd cranial nerve which is purely motor. It emerges from the brainstem just caudal to the mammillary bodies and enters the orbit through the superior orbital fissure after exiting the cavernous sinus to innervate the extraocular muscles of the eye (Joyce et al., 2023; Park et al., 2017).

Structure and Function:

The oculomotor nerve originates from two nuclei namely:

  1. Main Oculomotor nucleus: It gives rise to the somatic efferent fibers, which will innervate all the extraocular muscles of the eye, except the superior oblique muscle (innervated by the trochlear nerve) and the lateral rectus muscle (innervated by the abducens nerve).  

Somatic (voluntary) functions of the oculomotor nerve include elevation of the upper eyelid via innervation of the levator palpebrae superioris muscle. Other essential functions include the coordination of eye muscles for visual tracking and gaze fixation (Joyce et al., 2023).

  • Edinger Westphal nucleus: It is located in the midbrain, dorsal and medial to the main nucleus, and comprises the parasympathetic fibers of the oculomotor nerve.

There are two primary functions of the autonomic parasympathetic fibers (involuntary) in nature. It constricts the pupil (miosis) by innervating the sphincter pupillae muscle and accommodation through contraction of the ciliary muscle that leads to relaxation of the zonular fibers altering the curvature of the lens allowing individuals to focus the lens on near objects (Hogan et al., 2022; Yoo & Mihaila, 2023).

The oculomotor nerve adjusts and coordinates eye position during movement. Saccades are rapid, jerky motions of the eye that are controlled by:

Vestibule-ocular reflex:

It adjusts eye position during fast movements of the head. Head motion stimulates cells within the semicircular canals. The ipsilateral vestibular nucleus receives motion information, which is then sent to the oculomotor nucleus. To maintain a steady gaze, the eyes shift to the right while the head moves to the left (Joyce et al., 2023).

Optokinetic reflex: It aligns the eye position with regard to changes in the visual field. The vestibulo-cerebellum receives visual information via the pontine nuclei from the parieto-occipital eye field. It then passes from the vestibular nuclei to the oculomotor nucleus to assist the eyes in tracking huge objects within the visual area (Joyce et al., 2023).

Oculomotor nerve palsy:

Depending on which fibers are affected, Third nerve palsy classically presents with double vision, droopy eyelids, dilated pupil and a down-and-out gaze.

Several illnesses have been linked to third nerve palsy. Conditions that damage the inner fibers responsible for eye movement and lid elevation spare the pupil such as diabetes mellitus raised blood pressure and inflammatory diseases, whereas peripheral fibers are typically affected by aneurysm, herniation, tumor, or meningitis, leading to a dilated or blown pupil that cannot accommodate on examination (Raza et al., 2018; Third Nerve Palsy: Causes, Symptoms and Treatment, n.d.).

Diagnosis:

Patients with pupil-sparing third-nerve palsy should be examined to rule out any vascular causes. Blood pressure measurement, complete blood count (CBC), blood sugar (including Hb1AC), and erythrocyte sedimentation rate (ESR) are all part of essential lab workup.

If the palsy is not pupil-sparing, immediate neuro-ophthalmic assessment is required. Cranial nerve imaging is often performed using MRI. If a clinical examination indicates a possibility of an aneurysm, CT angiography should be done immediately (Modi & Arsiwalla, 2023).

Management:

Management primarily depends on the underlying cause and severity. It involves the use of permanent prism glasses to correct double vision, temporary eye patches, vision therapy, and surgical interventions to help realign the eyes and relieve pressure on the nerve (Your Guide to Oculomotor Nerve Palsy and Its Treatment, n.d.).

REFERENCES:

Hogan, M. B. H., Subramanian, S., & Das, J. M. (2022). Neuroanatomy, Edinger–Westphal Nucleus (Accessory Oculomotor Nucleus). StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK554555/

Joyce, C., Le, P. H., & Peterson, D. C. (2023). Neuroanatomy, Cranial Nerve 3 (Oculomotor). StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK537126/

Modi, P., & Arsiwalla, T. (2023). Cranial Nerve III Palsy. Clinical Cases in Eye Care, 453–456. https://www.ncbi.nlm.nih.gov/books/NBK526112/

Park, H. K., Rha, H. K., Lee, K. J., Chough, C. K., & Joo, W. (2017). Microsurgical Anatomy of the Oculomotor Nerve. Clinical Anatomy (New York, N.Y.), 30(1), 21–31. https://doi.org/10.1002/CA.22811

Raza, H. K., Chen, H., Chansysouphanthong, T., & Cui, G. (2018). The aetiologies of the unilateral oculomotor nerve palsy: a review of the literature. Somatosensory & Motor Research, 35(3–4), 229–239. https://doi.org/10.1080/08990220.2018.1547697

Third Nerve Palsy: Causes, Symptoms and Treatment. (n.d.). Retrieved December 18, 2024, from https://www.allaboutvision.com/conditions/related/third-nerve-palsy/

Yoo, H., & Mihaila, D. M. (2023). Neuroanatomy, Pupillary Light Reflexes and Pathway. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK553169/

Your Guide to Oculomotor Nerve Palsy and Its Treatment. (n.d.). Retrieved December 18, 2024, from https://www.healthline.com/health/neurological-health/oculomotor-nerve-palsy#treatment