7.3 Cranial Nerve VI (Abducens) Palsy

Cranial nerve VI palsies are extremely common in examinations. Although relatively simple to diagnose, it is important that the examiner look carefully for signs of associated cranial nerve palsies, Horner’s syndrome and papilloedema.

Examination Outline

Examination Structure

1. Visual Acuity (Best Corrected)

2. Spectacles

  • BO prism - apex points towards the deviation

3. Inspection

  1. Abnormal head position = head turned to the same side
  2. Hirschberg (Pupil margin = 15°, Iris margin = 30°) - Esotropia
  3. (Lids normal)
  4. (Pupils normal)
Figure 7.3.1 Left CNVI Palsy

Figure 7.3.1
Left CNVI Palsy

4. Cover-Testing

  1. Esotropia in primary due to the unopposed action of MR
  2. CTD: ET greater on head turn to opposite side (i.e. incomitant)
  3. CTN: ET less marked than on CTD (because the eyes converge for near)

5. Ocular Rotations

  • Limitation of abduction due to paralysis of LR

6. Saccades

  • Slow horizontal abducting saccade

7. Other Cranial Nerves

  • CN V (Corneal sensation), CN VII, CN VIII

8. Lid and Pupil Exam

  • Look closely for any ptosis or miosis on the ipsilateral side, which localises the lesion to the cavernous sinus

9. Neurological Exam

Millard-Gubler syndrome

Ipsilateral CNVI, CNVII+
Contralateral hemiplegia (corticospinal tract)

Foville syndrome

Ipsilateral horizontal gaze palsy (PPRF), CNVII+
Contralateral hemiplegia (corticospinal tract), INO

10. Fundoscopy

  • Look for papilloedema if there is a bilateral CNVI

11. Measurements

  • BO prism on affected side

12. Forced Duction and Force Generation Testing

  • This would not be done in the exam but can be mentioned

Summary

  • Right or left (unilateral or bilateral)
  • Isolated or other associated CN palsies / Horner’s syndrome
  • Complete vs. partial (assess with ductions, saccades, forced generation, botox to MR)

Aetiology

1. Vascular

Microvascular

HT, Cholesterol, DM, Smoking

Macrovascular

Internal carotid artery aneurysm (cavernous sinus). Subarachnoid haemorrhage, carotico-cavernous fistula, cavernous sinus thrombosis, pituitary apoplexy

2. Neoplastic

  • Glioma, meningioma, nasopharyngeal Ca, clivus tumour, chordoma

3. Inflammatory

  • GCA, Demyelination, Vasculitis, Sarcoidosis

4. Infective

  • HZO, Syphilis, Gradenigo syndrome (mastoiditis or petrositis)

5. Trauma / Surgical

  • ↑ ICP (often bilateral), Base of skull fracture

6. Other

  • Wernicke
  • In children consider: Benign CNVI palsy (usually spontaneously improves after 4-6 weeks), tumour (glioma), ↑ ICP

Investigations

  1. ± MRA head with gadolinium
    1. <50 years
    2. Other CN palsies (or bilateral) or Horner’s syndrome
    3. Progressive
  2. BP, BSL / HbA1C, Lipids (for Vascular risk factors)
  3. ± ESR / CRP (for GCA)
  4. ± Vasculitic screen, ACE, Syphilis serology
  5. ± Tests for Myasthenia
  6. Hess (right CNVI palsy)
    1. Contraction of R, expansion of L
    2. R esotropia (fixation spot deviated nasally)
    3. R chart: Underaction of LR, overaction of MR
    4. L chart: Overaction of MR
Figure 7.3.2 Hess Chart of Right CNVI Palsy

Figure 7.3.2
Hess Chart of Right CNVI Palsy

Management

Treat the underlying cause (e.g. diabetes, hypertension).

Indications for treatment:

  1. Abnormal head posture
  2. Diplopia
  3. Cosmesis

Aims:

  1. Correct AHP
  2. Maximise field of BSV
  3. Align eyes in primary for cosmesis

A) Non-Surgical

  1. Uniocular occlusion for diplopia of the non-dominant eye
  2. (Fresnel) Prism (BO prism on the affected side)
    1. Even though incomitant, can be helpful in primary position
    2. Can only be used if deviation small (e.g. < 30Δ), diplopia in primary
  3. Botulinum toxin into ipsilateral MR
    1. Cosmetic improvement, check for complete CNVI
    2. BUT only temporary, doesn’t improve BSV
Figure 7.3.3 Base Out Prism for Esotropia in CNVI Palsy

Figure 7.3.3
Base Out Prism for Esotropia in CNVI Palsy

B) Surgical (Uncommon)

  • Wait at least 6-12 months and deviation is stable- most will spontaneously resolve
If Partial Paralysis (Aim for Full Field of BSV)
  • Recess(MR) / Resect(LR) ± Recess MR of contralateral eye if deviation is very large
If Complete Paralysis (Can’t Cure)
  • Transposition (SR / IR to LR) + Botox (recess) ipsilateral MR
  • Full tendon transfer (± Foster augmentation modification)
  • ½ tendon transfer (e.g. Hummelsheim)
  • Jenson procedure (SR / IR split, joined to split LR at the equator)
  • Nishida procedure [i] (halves of the SR and IR are sutured onto the sclera)

Nishida Y, Inatomi A, Aoki Y, et al. A muscle transposition procedure for abducens palsy, in which the halves of the vertical rectus muscle bellies are sutured onto the sclera. Jpn J Ophthalmol. 2003;47(3):281‐286. doi:10.1016 / s0021-5155(03)00021-2

Risks

  • Anterior segment ischaemia, new vertical deviation, over / undercorrection
  • 2nd operation: Contralateral MR Faden suture
Figure 7.3.4 Transposition of Superior Rectus and Inferior Rectus to Lateral Rectus for Complete CNVI Palsy

Figure 7.3.4
Transposition of Superior Rectus and Inferior Rectus to Lateral Rectus for Complete CNVI Palsy

Anatomy

A) Pons

CNVI Nucleus

An isolated CNVI palsy is never nuclear due to the close relationship of the CNVII fasciculus (wraps around it).

A lesion in the CNVI nucleus causes: Ipsilateral horizontal gaze palsy (PPRF) ± Ipsilateral LMN CN VII palsy.

Figure 7.3.5 Brainstem Relations of CNVI

Figure 7.3.5 Brainstem Relations of CNVI
Note the long tortuous course of the sixth cranial nerve and its relation to the clivus and cavernous sinus

Fasciculus (Outside Nucleus But Within Pons)

Millard-Gubler syndrome

Ipsilateral CNVI, CNVII +
Contralateral hemiplegia (corticospinal tract).

Foville syndrome

Ipsilateral horizontal gaze palsy (PPRF), CNVII +
Contralateral hemiplegia (corticospinal tract), INO.

B) Cerebello-Pontine Angle / Basilar

Ipsilateral V (1st sign), VI, VII, VIII (1st symptom)

  • Acoustic neuroma
  • Nasopharyngeal Ca
  • ↑ ICP
  • Base of skull fracture
  • Gradenigo syndrome (mastoiditis or petrositis)

C) Nerve

Travels between pons / pyramids either side of the start of the basilar artery
Through Cavernous sinus
Enters the Superior Orbital Fissure through the Common tendinous ring → LR

Figure 7.3.6 Relations of CNVI in the Superior Orbital Fissure

Figure 7.3.6
Relations of CNVI in the Superior Orbital Fissure

Figure 7.3.7 Pathways for Horizontal Gaze

Figure 7.3.7
Pathways for Horizontal Gaze

The Medial Longitudinal Fasiculus connects the CNVI nucleus in the pons with the contralateral CNIII nucleus in the midbrain

Figure 7.3.8 Clinical Outcomes of Different Lesions Affecting Horizontal Eye Movements

Figure 7.3.8
Clinical Outcomes of Different Lesions Affecting Horizontal Eye Movements

NB: Doll’s head remains intact with cortical lesions but abolished in brainstem lesions.

          

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