Vi th cranial nerve

anatomy
Abducens nerve palsy is the most common ocular motor paralysis. The abducens (sixth) cranial nerve controls the lateral rectus muscle, which abducts the eye. Abducens nerve palsy causes an esotropia due to the unopposed action of the antagonistic medial rectus muscle. The affected eye turns in toward the nose and is unable to abduct properly. The deviation is constant and is usually greater at distance fixation than at near. The esotropia is also worse when the patient is looking toward the affected side.

1. VI th Cranial Nerve Palsy Dr. Vinit Kumar

2. Abducens ( VI th CN ) nerve palsy Abducens nerve palsy is the m/c ocular motor paralysis. The abducens (sixth) cranial nerve controls the lateral rectus muscle, which abducts the eye. Abducens nerve palsy causes an esotropia due to the unopposed action of the antagonistic medial rectus muscle. The affected eye turns in toward the nose & is unable to abduct properly. The deviation is constant & is usually greater at distance fixation than at near. The esotropia is also worse when the patient is looking toward the affected side.

3. Etiology ( Pediatrics ) Congenital. Palsies can occur rare. They can sometimes be associated with birth trauma & with neurological conditions such as hydrocephalus & cerebral palsy . Acquired. Acquired VI th CN palsies in childhood can be due to neoplasm, trauma , inflammation & idiopathic etiologies . Nontraumatic acquired sixth nerve palsies may be due to benign recurrent sixth nerve palsy , elevated or low intracranial pressure, or pontine gliomas . Benign isolated abducens nerve palsy can occur in childhood following ear or throat infections or viral illness.

4. Trauma, secondary to open or closed head injuries . Trauma causes indirect pressure on the nerve, to which it is very susceptible to trauma as it passes over the apex of the petrous temporal bone to the cavernous sinus . In addition, traumatic abducens nerve palsy can occur secondary to bruising and/or hemorrhage causing pressure on the nerve . Closed head trauma may cause elevated intracranial pressure and secondarily produce a non-localizing sixth nerve palsy.

5. Pontine glioma . Brainstem gliomas are one of the common tumors seen in the pediatric population & more than 80% arise from the pons with the peak age of onset between 5-8 years of age . Presenting symptoms include ataxia, disturbance of gait & unilateral or bilateral abducens nerve palsy.

6. Elevated or low intracranial pressure = Elevated ICP can cause stretching of the sixth cranial nerves which are tethered in Dorello’s canal . Their steep course over the petrous apex & the tethering make the sixth nerve susceptible to injury. The same mechanism can explain the reason for the non-localizing sixth nerve palsy that can be seen with either elevated or reduced ICP . Elevated ICP can occur secondary to a variety of different causes, including shunt failure, pseudotumor cerebri, posterior fossa tumors, neurosurgical trauma, venous sinus thrombosis, meningitis, or Lyme disease

7. Intracranial tumor , Posterior fossa tumors , such as pontine glioma, medulloblastoma, ependymoma, or cystic cerebellar astrocytoma can produce unilateral or bilateral abducens nerve palsies in children . Skull base tumors (meningioma, chordoma, nasopharyngeal carcinoma, metastasis) predominate in the adult population. Abducens nerve palsy can also present as a postoperative complication after resection of posterior fossa tumors in the pediatric population . Meningitis ( least common cause )

8. In Adults ( etiology ) Microvascular ischemia. Trauma. Idiopathic. Less likely: Multiple sclerosis. Neoplasm. Stroke. Sarcoidosis/vasculitis. Increased intracranial pressure.(ICT) Giant cell arteritis.(GCA / Temporal arteritis )

9. Lesions causing abducens nerve palsy are usually classified by the location of the lesion . Fascicular . Demyelination, vascular disease & metastatic tumors are likely causes of fascicular damage. Lesions in this area can cause Foville’s syndrome (damage to the pontine tegmentum) which is classified by partial sixth nerve palsy, ipsilateral facial weakness, loss of taste in the anterior portion of the tongue, ipsilateral Horner’s syndrome , ipsilateral facial sensory loss & ipsilateral peripheral deafness.

10. Lesions in the fascicular area can also cause Millard- Gubler syndro me, which is a result of damage to the ventral pons, is characterized by sixth nerve palsy and contralateral hemiplegia & may or may not also have ipsilateral facial paralysis .

11. Peripheral. Causes of peripheral nerve damage include closed head injury, compression & bacterial infection of the inner ea r. Localized compression can be caused by a primary pituitary tumor, craniopharyngioma or meningioma . Metastatic tumors & aneurysms involving the basilar artery can also cause an abducens nerve palsy.

12. Risk Factors Inflammatory & 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.

13. 6 th CN anatomy

14. 6 th CN anatomy ( Brainstem ) The nucleus of the sixth (abducens or abducent) nerve lies at the mid-level of the pons , ventral to the floor of the fourth ventricle . The fibres (fasciculus) leave the brainstem ventrally at the pontomedullary junction. • Nuclear lesion . A nuclear sixth nerve lesion also causes a failure of horizontal gaze towards the side of the damage due to involvement of the adjacent horizontal gaze centre (paramedian pontine reticular formation). Facial (seventh) nerve fibres wrap around the sixth nerve nucleus, so ipsilateral lower motor neurone (LMN) facial nerve palsy is also common. Isolated sixth nerve palsy is never nuclear in origin .

15. DIAGNOSIS & HISTORY The pattern of onset & associated symptoms can be very important in determining the etiology of an abducens nerve palsy. Sudden onset suggests a vascular etiology………… while slowly progressive onset suggests a compressive etiology. Subacute onset suggests a demyelinating process.

16. PHYSICAL EXAMINATION All patients with 6 th CN palsy need a complete ophthalmologic examination, including visual acuity, binocular function & stereopsis , motility evaluation, strabismus measurements at near, distance & in the cardinal positions of gaze, measurement of fusional amplitudes, cycloplegic refraction, & evaluation of ocular structures in the anterior &posterior segments. Precise assessment of ductions & versions , as well as precise orthoptic measurements in lateral gazes are helpful in determining incomitance associated with abducens nerve palsy.

17. SIGNS Because the greatest motility deficit occurs on attempt to abduct the palsied eye, palpebral fissure widening upon abduction may be seen with maximal abduction effort. The patient may also present with a head turn toward the affected eye , to keep their eye opposite the field of gaze of the affected lateral rectus muscle & there by avoid or minimize diplopia . It is important to differentiate isolated 6 th nerve associated abduction deficit from a gaze palsy or INO as this would localize the lesion to the nucleus/ internuclei of the 6 th and 3 rd nerve .

18. Symptoms Diplopia is the most common symptom. Patients will have horizontal uncrossed diplopia which is greater at distance than at near. The diplopia is also worse in the direction of the palsied muscle & gets better in the contralateral gaze ( incomitant ). In recent onset palsies, the deviation measures greater when the paretic eye is fixating and smaller when the nonparetic eye is fixing (primary and secondary deviations).

19. In cases of abducens nerve palsy due to raised intracranial pressure , patients may experience associated symptoms of headache, pain around the eyes, nausea, vomiting, or tinnitus. Low ICP from a CSF leak can also cause abducens palsy & can present with symptoms of headache & can hence present clinically very similarly to raised ICP. MRI brain & orbits can sometimes help distinguish between high or low ICP.

20. If a patient has a lesion causing the abducens nerve palsy which affects other structures in the brain, other neurologic signs may be observed. If the etiology of the abducens nerve palsy is a brainstem lesion affecting the sixth cranial nerve fasciculus, there may be associated ipsilateral facial weakness, contralateral hemiparesis, or sensory abnormalities. If the abducens nerve palsy presents together with other ipsilateral cranial nerve palsies , etiology could be a lesion involving the meninges, superior orbital fissure, orbital apex, or cavernous sinus. Presence of orbital signs or pulse synchronous tinnitus can suggest an arteriovenous fistula .

21. Clinical Diagnosis There is a limitation to abduction in the affected eye. Abducens nerve palsy causes an esotropia due to the unopposed action of the antagonistic medial rectus muscle. The affected eye turns in toward the nose & is unable to abduct properly. The deviation is constant and is usually greater at distance fixation than at near, & also greater when the patient is looking toward the affected side.

22. Diagnostic Procedures There is no universal consensus about diagnostic relevance of MRI in abducens nerve palsy. In general, if unilateral abducens nerve palsy presents acutely , MRI could be performed, especially if there is no evidence of vasculopathic risk factors. Some clinicians may prefer to routinely perform MRI in all patients with abducens nerve palsy, even with evidence of vasculopathy .

23. However, according to a 2017 review by Elder et al , the most recent input to the MRI debate was a 2013 prospective study of 109 patients > 50 years old with isolated ocular motor neuropathies who received MRI.  An etiology other than ischemia was found in 16.5% of patients, including brainstem infarction, petroclival meningioma, & cavernous sinus B cell lymphoma .

24. Due to the possiblility of dangerous diseases presenting with an isolated cranial motor neuropathy, the authors recommended MR imaging even if a microvascular cause is suspected.  The authors stress that high-resolution brain & orbital MRI with gadolinium, fat suppression,& thin coronal & axial cuts through the orbits are necessary , as sixth nerve palsy mimics (e.g. thyroid eye disease) may be missed by a standard MRI brain

25. Lab investigations Complete blood cell (CBC) count Glucose levels Glycosylated hemoglobin ( HbA1C) Erythrocyte sedimentation rate and/or C-reactive protein Fluorescent treponemal antibody-absorption test, VDRL or RPR Lyme titer Glucose tolerance test Antinuclear antibody test Rheumatoid Factor test

26. MRI could be performed for the following: Patients younger than 45 years Associated pain or other neurologic abnormality History of cancer B ilateral sixth nerve palsy Papillede ma If no marked improvement is seen or other nerves become involved An LP can be considered if MRI results are negative.

27. D/D of 6 th CN palsy Duane’s retraction syndrome (Types 1 and 3), congenital esotropia, thyroid eye disease (TED) spasm of the near reflex, longstanding esotropia with medial rectus contracture, ocular neuromyotonia , & lateral rectus myositis. An abduction deficit can be produced by orbital disease , such as a blowout fracture with entrapment of the medial rectus muscle , or by neuromuscular disease such as ocular myasthenia gravis or Miller-Fisher syndrome..

28. Clinical assessment for orbital, neuromuscular, & brainstem disease is the first step in evaluation for this condition, and after this, an abducens nerve palsy can be diagnosed by exclusion .

29. Diagnostic syndrome associated with 6 th CN palsy Millard Gubler ( ventral pontine ) syndrome Raymond syndrome Foville (inferior medial pontine) syndrome Acoustic neuroma (C-P angle tumor ) Nasopharyngeal tumours Raised ICT Basal skull fracture Gradenigo syndrome / mastoiditis / petrositis . Superior orbital fissure synd., orbital apex syndrome , tolasa hunt Fascicular Nucleus Basilar lesion Intercavernous & intraorbital

30. Millard– Gubler (ventral pontine) syndrome involves the fasciculus as it passes through the pyramidal tract & is most frequently caused by vascular disease, tumours or demyelination . As well as ipsilateral sixth nerve palsy, there is contralateral hemiplegia & often an ipsilateral LMN facial nerve palsy.

31. Foville (inferior medial pontine ) syndrome is most frequently caused by vascular disease or tumours involving the dorsal pons. It is characterized by ipsilateral involvement of the fifth to eighth cranial nerves, central sympathetic fibres (Horner syndrome) & horizontal gaze palsy.

32. Nuclear lesion . A nuclear sixth nerve lesion also causes a failure of horizontal gaze towards the side of the damage due to involvement of the adjacent horizontal gaze centre (paramedian pontine reticular formation). Facial (seventh) nerve fibres wrap around the sixth nerve nucleus, so ipsilateral lower motor neurone (LMN) facial nerve palsy is also common. Isolated sixth nerve palsy is never nuclear in origin .

33. Basilar part of 6 th CN The basilar part of the nerve enters the prepontine basilar cistern, passes upwards close to the base of the skull & is crossed by the anterior inferior cerebellar artery . It pierces the dura below the posterior clinoids & angles forwards over the tip of the petrous bone, passing through or around the inferior petrosal sinus, through the Dorello canal (underneath the petroclinoid ligament), to enter the cavernous sinus .

34. An acoustic neuroma may damage the sixth nerve at the pontomedullary junction the first symptom of an acoustic neuroma is hearing loss , and the first sign diminished corneal sensitivity . Hearing & corneal sensation should be checked in each patient of 6 th CN palsy .

35. Nasopharyngeal tumours may invade the skull & its foramina & damage the nerve during its basilar course. • Raised intracranial pressure may cause a downward displacement of the brainstem. This may stretch one or both sixth nerves over the petrous tip • Basal skull fracture ( may cause uni or bilateral palsy )

36. Gradenigo syndrome , most frequently caused by mastoiditis or acute petrositis , may result in damage of the sixth nerve at the petrous tip. The latter is frequently accompanied by facial weakness and pain, and hearing difficulties.

37. intercavernous & intraorbital part The intracavernous section runs below the third & fourth & the first division of the fifth nerves. The sixth nerve is the most medially situated & runs through the middle of the sinus in close relation to the internal carotid artery. Occasionally, intracavernous sixth nerve palsy is accompanied by a postganglionic Horner syndrome due to damage to the paracarotid sympathetic plexus .

38. Intraorbital part The intraorbital part enters the orbit through the superior orbital fissure within the annulus of Zinn to innervate the lateral rectus

39. Management ( general t/t) Treatment depends on etiology of the abducens nerve palsy. In general, underlying or systemic conditions are treated primarily. Most patients with a microvascular abducens nerve palsy are simply observed & usually recover within 3-6 months . Treatment for the diplopia associated with abducens nerve palsy can be managed with prisms, occlusion, botulinum toxin, or surgery .

40. Occlusion using Bangerter filter or pirate patch can eliminate diplopia and confusion , prevent amblyopia or suppression in younger patients, and decrease the possibility of ipsilateral medial rectus contracture. Base-out Fresnel prisms can be used to help the patient maintain binocular single vision in the primary position, but are not usually useful due to the incomitance of the deviation.

41. Botulinum toxin injections to the medial rectus of the affected eye are sometimes used to prevent secondary contraction of the medial rectus, or during transposition procedures to weaken the non-operative muscle. In general, surgical intervention is reserved for patients who have had stable orthoptic measurements for at least 3-6 months. 

42. Bangerter filter or pirate patch

43. Surgery Strabismus surgery can be performed for persistent abducens nerve palsies that demonstrate stable measurements over a 6 month period. Forced duction test is performed in the office or in the operating room in order to assist with surgical planning. Often, a resection of the affected lateral rectus & recession of the ipsilateral medial rectus (recess/resect or “R and R” procedure ) is performed. Alternatively, a resection of the affected lateral rectus with a recession of the contralateral medial rectus may be performed.

44. Various forms of transposition surgeries can be considered (e.g., Jensen, Hummelsheim , Augmented Hummelsheim with resections +/- Foster modifications, Knapp's procedure). Botulism toxin can also be used as a temporizing treatment .

45. post surgery follow up Patients may be managed closely postoperatively, and any residual or intractable diplopia can be managed with prisms . Complications The most likely complication following surgical correction of abducens nerve palsy is the risk of over- or under-correction, which can be managed postoperatively with prisms. The prognosis for sixth nerve palsy depends on the underlying etiology

46. in summary (management of 6 th CN palsy) Clinicoinvestigative work up & T/T of cause Careful history : antecedent infection,head trauma Child <16 yrs with no neurological sign & symptoms : no work up is required , close follow up every 2 weeks initially then monthly required 16 to 40 yrs : risk factors should be ruled out > 40 yrs – look for vascular cause like DM/HTN/Atherosclerosis etc > 55 yrs – GCA / temporal arteritis ruled out. Haematological : BSL (F/PP),ESR,CBC,ANA,FTA Titre ) Lumbar puncture for raised ICT Orbtal USG : for TAO / thyroid /fibrosis of EOMs binocular diplopia charting.

47. Neuroimaging : indication : in presence of multiple cranial nerve palsy & in younger pt. without muscular cause with neurophysician consultation . CONSERVATIVE MEASURES : wait + watch for 6-8 months for self improvement Vitamin B complex – acts as neurotonic Systemic steroids – hasten recovery in non specific inflammation Prevent diplopia & amblyopia in children & preserve BSV Patching /occlusion : in case of mild esodeviation & residual LR function intact, occlusion of unaffected eye is adviced to stimulate fixation with affected eye in Abduction & relaxation of ipsilateral MR to prevent contracture of MR

48. Fresnel press on prism useful to correct diplopia BOTOX : in to antagonist MR causes paralysis of MR & eye will be in primary gaze (effect lst for 2-3 months) Spontaneous recovery of 6 th CN palsy ocures in idiopathic cases ( so progress is determined by Hess screen ) SURGICAL INDICATION : When spontaneous recovery not ocure after 6 months /more To correct incomitant esotropia Improve abduction Provide BSV Eliminate AHP

49. Recess – resect operation - a supra maximal 12-16 mm recession of the antagonist MR with about 8-10 mm resection of LR is often successful first operation in most pt. with incomplete palsy. (eliminate AHP & maintain BSV ) . In case mild paresis still present , weaking of contralateral MR with or without Faden procedure consider as second operation. Adjustable suture surg. Help ful for final adjustment . Muscle transposition procedures : for complete LR palsy (FDT to rule out MR contracture ) Jensens procedure combined with MR recession : the SR ,IR & paralysed LR muscle are split for 8-10 mm from their insertion backward with the help of muscle hook. Then superior half of LR is united with lateral half of SR & inferior half of LR with the lateral half of IR with non absorbable suture ( 5-0 mersilene ) & knot should be tied near equator .

50. 4. Hummelscheim operation After splitting of SR & IR their lateral halves are disinserted & sutured to tendon of LR muscle ( this operation rarely done now days ) mainly of historical interest . 5 . BERENS & GIRARD PROCEDURE : in this operation the full SR & IR tendon are disinserted & sutured with tendon of LR at its insertion. This procedure combines with recession of MR is recommended by Von Noorden in children with complete paralysis of 6 th CN palsy .

51. 6. Carlson & Jampolsky transposition procedure MR muscle spared . After splitting of verical recti their temporal halves are inserted under the LR . It is essential to separate the halves of vertical recti to the deepest point towards the apex of orbit allowing the transposed parts of vertical recti to slide towards the LR, which also minimizes vertical deviations. This procedure reduces the angle of deviation below 10PD improve abduction , prevent relapses & also prevents anterior segment ischemia.

52. 7 . Superior rectus transposition with MR recession This procedure is successfully tried in LR palsy & also in abduction deficit condition like Duanes retraction syndrome & Mobius syndrome , especially if hypertropia is also associated. Limitation of this procedure is = it induces vertical deviation . 8. contralateral MR recession with or without Faden operation can be considered an option for residual LR abduction deficit or residual esotropia .