Summary
In this article, we describe lesions to the 3rd, 4th, and 6th cranial nerves, which are the cranial nerves responsible for ocular movement.
Third nerve
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The oculomotor nucleus is located at the level of the superior colliculus ventral to the periaqueductal gray matter. The fibres of the oculomotor nerve pass ventrally through the ipsilateral red nucleus, exiting the ventral midbrain along the medial cerebral peduncle. The nerve also passes between the superior cerebellar and posterior cerebral arteries, enters the lateral wall of the cavernous sinus, and passes through the superior orbital fissure to enter the orbit.
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The nuclear complex
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The nuclear complex is comprised of subnuclei that each innervate an extraocular muscle. These subnuclei are either paired, meaning there are two subnuclei that separately innervate each muscle on each side, or unpaired, i.e. a single subnucleus which innervates both sides.
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Levator subnucleus: a single ‘unpaired’ midline structure that innervates both levator muscles. Lesion here = bilateral ptosis
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Superior rectus subnuclei: ‘paired’ nuclei, each innervates the contralateral superior rectus. Lesion here = contralateral superior rectus affected
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Medial rectus, inferior rectus, and inferior oblique subnuclei: ‘paired’ nuclei, each innervates the ipsilateral respective muscles. Lesions here = ipsilateral muscles affected
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WEBINO: ‘wall-eyed bilateral internuclear ophthalmoplegia’. This occurs as a result of lesions to the paired medial rectus subnuclei resulting in exotropia with defective convergence and conduction
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Fasciculus
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This is the portion of the oculomotor nerve which passes from the oculomotor nucleus, through the red nucleus and medial aspect of cerebral peduncle, before emerging from the midbrain. Lesions here can result in:
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- Benedikt syndrome: lesion to the fasciculus as it passes through the red nucleus = ispilateral third nerve palsy + contralateral extrapyramidal signs
- Weber syndrome: lesion to the fasciculus as it passes throught the cerebral peduncle = ipsilateral third nerve palsy + contralateral hemiparesis
- Nothnagel syndrome: lesion to the fasciculus + superior cerebellar peduncle = ipsilateral third nerve palsy + cerebellar ataxia
- Claude syndrome = Benedikt + Nothnagel syndromes
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Basilar
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The basilar portion of the oculomotor nerve comprises of ‘rootlets’ that leave the midbrain on the medial aspect of the cerebral peduncle, before coalescing to form the main trunk. The nerve then passes between the posterior cerebral and superior cerebellar arteries. Therefore, an aneurysm of the PCA, typically at the junction with the internal carotid artery, may result in a pupil-involving third nerve palsy.
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The nerve travels through the skull base in isolation on it’s way to the orbit and therefore, any head trauma which may compress the third nerve as it passes over the tentotrial edge (haematoma ⇒ coning ⇒ herniation of the temporal lobe) is more likely to result in isolated third nerve palsy
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Intracavernous
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The third nerve runs in the lateral wall of the cavernous sinus above the third nerve. Pathology here, including pituitary apoplexy, diabetes, and other cavernous sinus pathology, may result in a third nerve palsy (in addition to involving the fourth nerve, sixth nerve, and first division of trigeminal nerve.)
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Intraorbital
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After the passing through the superior orbital fissure, the oculomotor nerve divides into superior (innervating the levator and superior rectus muscles) and inferior (innervating the medial rectus, inferior rectus, and inferior oblique muscles). The branch to the inferior oblique also contains preganglionic parasympathetic fibres from the Edinger Westphal nucleus, which innervate the sphincter pupillae and ciliary muscle. Therefore, lesions of the inferior division are characterized by limited adduction and depression + dilated pupil.
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Fourth nerve
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The fourth nerve, also known as the trochlear, provides innervation to the superior oblique muscle, responsible for intorsion and depression in adduction.
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Clinical features:
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- Because the trochlear nerve is the only cranial nerve that crosses the midline, (aside from the optic nerve), a peripheral lesion causes ipsilateral effects whilst a nuclear lesion results in contralateral effects.
- Hypertropia of affected side
- Limitation of depression of affected side
- Extorsion of affected side
- Compensatory head posture (contralateral head tilt)
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Causes of isolated fourth nerve palsy
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- Idiopathic
- Trauma (often results in bilateral fourth nerve palsy)
- Microvascular
- Rarely, tumours or aneurysm
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Parks three step test
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This is a test to isolate the paretic muscle in individuals presenting with acute vertical diplopia.
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Step 1: identify the hypertropia with the eyes in primary position. If this is in the left eye, you know either left SO or IR is affected, or the right IO or SR.
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Step 2: See whether the hypertropia is more marked in left gaze or right gaze. The recti have their greatest action in abduction, whilst the obliques have greatest action during adduction. So, if the left eye hypertropia is worsened on left gaze, the culprit is likely to be the left IR (as the left eye abducts), or the right superior oblique (as the right eye adducts)
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Step 3: The head tilt test. The head is tilted to each side to assess cyclotorsion, and to see in which position the hypertropia is worse. When the head is tilted to the right, the eye turns inward (incyclotorsion, by the RSR and RSO) and the left eye turns outward (excyclotorsion, by LIO and LIR). Therefore, if the hypertropia is worse with the head tilted to the right, we know that these of these muscles must be involved. In our case, if the hypertropia is worse on right head tilt (but not left, we know the culprit is the left inferior rectus.
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Sixth nerve
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Clinical features
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- Diplopia
- Esotropia in the primary position (unopposed medial rectus action)
- Limitation of abduction on the affected side
- Compensatory face turn towards the affected side
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The sixth nerve nucleus lies at the mid level of the pons, ventral to the floor of the fourth ventricle. The fasciulus then leaves the brainstem at the pontomedullary junction.
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Nuclear lesion: lesion at the sixth nerve nucleus also involves the paramedian pontine reticular formation. Additionally, the facial nerve fibres wrap around the 6th nerve nucleus. Therefore, wemight get horizontal gaze palsy towards the side of the lesion, as well as some ispilateral LMB facial nerve palsy.
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Inferior pontine syndrome: Aka, Foville syndrome. This is characterized by ipislateral fifth – 8th cranial nerve involvement, Horner’s syndrome (central sympathetic fibres being affected) and a horizontal faze palsy
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Ventral pontine syndrome: Aka, Millard-Gubler syndrome. This is characteirsed by ipsilateral 6th nerve palsy, + contralateral hemiplegia, + ispilateral LMN facial nerve palsy
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Other conditions implicating the 6th nerve
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Acoustic neuroma: can damage the 6th nerve at the pontomedullary junction. Results in hearing loss + reduced corneal sensation
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Raised ICP: May stretch one (or both) of the cranial nerves over the petrous tip
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Gradenigo syndrome: mastoiditis or acute petrositis may damage the sixth nerve at the petrous tip.
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References
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- Salmon, John F., and Jack J. Kanski. Kanski’s Clinical Ophthalmology: A Systematic Approach. Ninth Edition, Elsevier, 2020.
- Link, Michael J., and Catina Y. Sloan. ‘Midbrain’. Encyclopedia of the Neurological Sciences, edited by Michael J. Aminoff and Robert B. Daroff, Academic Press, 2003, pp. 152–59. ScienceDirect, https://doi.org/10.1016/B0-12-226870-9/00787-5.
- Kumar, Annapurna. ‘Parks 3-Step Test’. Indian Journal of Ophthalmology, vol. 70, no. 8, Aug. 2022, p. 3167. journals.lww.com, https://doi.org/10.4103/ijo.IJO_714_22.
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