6th nerve, Dr M Saquib

Neuro Ophthalmology

1. Abducens Nerve 13 th August 2020 DR M SAQUIB Vice Principal MBBS,MS , FSCEH DELHI,FHVDESAI PUNE, EX REGISTRARA JNMCH,AMU CONSULTANT OPHTHALMOLOGIST HOD D/O OPHTHALMOLOGY G.S .MEDICAL COLLEGE Founder sec: MEDICS India , [email protected] , 9634123800

2. 2 The  abducens nerve  is the sixth paired cranial nerve. It has a purely somatic motor function – providing innervation to the lateral rectus muscle.

3. Anatomical Course The abducens nerve arises from the abducens nucleus in the  pons  of the brainstem. It exits the brainstem at the junction of the pons and the medulla. It then enters the subarachnoid space and pierces the dura mater to travel in an area known as  Dorello’s canal . 3

4. At the tip of petrous temporal bone, the abducens nerve leaves Dorello’s canal and enters the cavernous sinus (a dural venous sinus). It travels through the cavernous sinus and enters the bony orbit via the superior orbital fissure. 4

5. Motor Function The abducens nerve provides innervation to the  lateral rectus  muscle – one of the  extraocular muscles . The lateral rectus originates from the lateral part of the common tendinous ring, and attaches to the anterolateral aspect of the sclera. It acts to  abduct the eyeball  (i.e. to rotate the gaze away from the midline). 5

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10. Clinical Relevance 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). 10

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13. Clinical Relevance -  Abducens Nerve Palsy Can be caused by any structural pathology which leads to downwards pressure on the brainstem (e.g. space-occupying lesion). This can stretch the nerve from its origin at the junction of the pons and medulla. 13

14. Clinical features Abducens nerve palsy include  diplopia , the affected eye resting in adduction (due to unopposed activity of the medial rectus), and inability to abduct the eye. The patient may attempt to compensate by rotating their head to allow the eye to look sideways 14

15. The sixth cranial nerve is the most frequent cause of an isolated ocular motor palsy; it typically presents as horizontal diplopia that worsens on ipsilateral gaze, especially viewing at distance. The abduction deficit is typically associated with an Esodeviation that increases with gaze to the affected side . 15

16. Risk Factors Inflammatory and microvascular conditions are risk factors for abducens nerve palsy. Other risk factors include multiple sclerosis, encephalitis, meningitis, cavernous sinus thrombosis, hypertension, hypercholesterolemia, aneurysm, diabetes, arteriosclerosis, birth trauma, and neurosurgical intervention. [18] 16

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18. Lesions of the  cerebellopontine angle  (especially acoustic neuroma or meningioma ) may involve the sixth and other contiguous cranial nerves, causing decreased facial and corneal sensitivity (CN V), facial paralysis (CN VII), and decreased hearing with vestibular signs (CN VIII). 18

19. Chronic inflammation of the petrous bone may cause an ipsilateral abducens palsy and facial pain  ( Gradenigo syndrome) ,  especially in children who have experienced recurrent infections of the middle ear. 19

20. After Exiting The Pre– pontine Space, The Sixth Nerve Is Vulnerable To Meningeal Or Skull-based Processes, Such As Meningioma , Nasopharyngeal Carcinoma, Chordoma , Or Chondrosarcoma 20

21. In addition, the sixth nerve is susceptible to injury from shear forces of head trauma or elevated intracranial pressure. In such cases, injury occurs where the sixth nerve enters the cavernous sinus through the  Dorello canal  (the opening below the petroclinoid ligament). 21

22. Congenital sixth nerve palsies almost never occur in isolation. Abduction paresis present early in life usually manifests as a Duane syndrome 22

23. Isolated sixth nerve palsies in adults over the age of 50 are usually ischemic; ocular motility in these cases always improves and typically resolves within 3 months. 23

24. In general, at the onset of an isolated sixth nerve palsy in a vasculopathic patient, neuroimaging is not required. As noted with other isolated ocular motor cranial nerve palsies, medical evaluation is appropriate. 24

25. However, a cranial MRI is mandatory if obvious improvement has not occurred after 3 months. Other diagnostic studies that may be required include lumbar puncture, chest imaging, and hematologic studies to identify an underlying systemic process such as collagen vascular disease, , or syphilis 25

26. Impaired abduction in patients under age 50 requires careful scrutiny, because few such cases are due to ischemic cranial neuropathy. Younger individuals should undergo appropriate neuroimaging . 26

27. If negative, consideration should be given to neuromuscular junction disease, by obtaining acetylcholine antibodies or performing Tensilon testing; mechanical pathophysiologies , such as thyroid eye disease with medial rectus involvement; and meningeal -based disease, by obtaining a lumbar puncture. 27

28. Leukemia or brainstem glioma are important considerations in children. In adolescents and young adults, demyelination may be the cause. 28

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