This patient has been instructed to look to her left (image A) and then to her right (image B).

(a) Name the phenomenon observed.
 
(b) List two possible locations for the lesion associated with this observation.
 
(c) If this patient were also hemiplegic, which side would be paralysed?
 

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College Answer

(a) Horizontal gaze palsy (right)

(b) Any two:
 Frontal eye field
 Posterior hemispheric lesion
 Pre-pontine Reticular Formation (PPRF)
 Abducens (VI) nerve nucleus

(c) Right sided paralysis.

Discussion

The college loves gaze palsies. And cranial nerve lesions in general.

To answer this specific question, I resorted to the free-ish copy of "Fundamentals of Neurology: An Illustrated Guide" by Mattle and Mumenthaler, available via Google Books. Horisontal gaze palsies are dealt with on page 188.

The specific disturbance is an abducens nerve palsy - the affected eye cannot abduct past midline. Judging by the college answer, the palsy is unilateral.  Causes of a unilateral 6th nerve palsy include the following:

  • Head injury (most common) with BOSF
  • Raised intracranial pressure
  • Localising lesion.... at any number of levels:
    • Damage to the frontal eye field of the frontal lobe, which occupies some of the middle frontal gyrus
    • Damage to the posterior hemispheres, which would be accompanied by a hemianopia
    • Brainstem (tumour, stroke) - the paramedian prepontine reticular formation mentioned in this question, which receives information from higher cortical centres and transmits them to the abducens nucleus.
    • Petrous portion of temporal bone (otitis media-associated osteomyelitis, mastoiditis)
    • Clivus (intraforaminal extension of nasopharyngeal carcinoma or similar)
    • Cavernous sinus (thrombosis)
    • Superior orbital fissue (base of skull fracture)
    • Any damn where (basal forms of meningitis, eg sarcoidosis, tuberculosis, cryptococcus)

The college then claims that the hemiplegic side is ipsilateral to the horisontal gaze palsy. This is weird. The damage to the brainstem should be at the level of the pons, and therefore the hemiplesia should be contralateral. An excellent 2013 article by Azarmina et al ("The Six Syndromes of the Sixth Cranial Nerve") discusses the various diffuse and focal ways in which the sixth nerve can be damaged. All the pyramidal signs are contralateral in these scenarios.

Abducens Nerve Palsies

CN IV: Abducens

Expected features

  • Failure to abduct the affected eye
abducens nerve lesion

Unilateral lesion:

  • Head injury (most common) with BOSF
  • Raised intracranial pressure
  • Localising lesion.... at any number of levels:
    • Damage to the frontal eye field of the frontal lobe, which occupies some of the middle frontal gyrus
    • Damage to the posterior hemispheres, which would be accompanied by a hemianopia
    • Brainstem (tumour, stroke) - the paramedian prepontine reticular formation mentioned in this question, which receives information from higher cortical centres and transmits them to the abducens nucleus.
    • Petrous portion of temporal bone (otitis media-associated osteomyelitis, mastoiditis)
    • Clivus (intraforaminal extension of nasopharyngeal carcinoma or similar)
    • Cavernous sinus (thrombosis)
    • Superior orbital fissue (base of skull fracture)
    • Any damn where (basal forms of meningitis, eg sarcoidosis, tuberculosis, cryptococcus)

Bilateral lesion:Freakishly rare

  • Diabetes-associated neuropathy
  • Myasthenia gravis
  • Generally speaking, polyneuropathies such as Guillain-Barre

The table below is a fragment of the greater Table of Conjugate Gaze Palsies. It may also be helpful in localising this lesion.

Disorder

Clinical features

Location of lesion

Possible causes

Disorders of Horisontal Gaze 
Horisontal gaze palsies
  • conjugate eye deviation to the side of the
    lesion
  • association with contralateral hemiparesis
  • contralateral frontal lobe
  • patient looks towards the affected lobe
  • Stroke
  • Tumor
  • Haemorrhage
  • Trauma
 
  • saccadic eye deviation to the side of the
    lesion
  • vestibulo-ocular reflex and response to
    caloric stimulation are normal
  • paramedian pontine reticular formation
  • patient looks away from the affected pons
  • Stroke
  • Tumour
 
  • loss of all adduction eye movements - a classical "6th nerve palsy"
  • Cannot be overcome by caloric stimulation
  • Lesion of the abducens nucleus or the abducens nerve
  • Stroke
  • Tumour
  • Trauma (eg. base of skull fracture)
  • Cavernous sinus thrombosis
  • see cranial nerve lesions

References

The LITFL summary of cranial nerve lesions is without peer in terms of useful information density.

Lapresle, J., and P. Lasjaunias. "CRANIAL NERVE ISCHAEMIC ARTERIAL SYNDROMES A REVIEW." Brain 109.1 (1986): 207-215.

Walker, H. Kenneth, W. Dallas Hall, and J. Willis Hurst. "Clinical methods." (1990).

Azarmina, Mohsen, and Hossein Azarmina. "The Six Syndromes of the Sixth Cranial Nerve." Journal of ophthalmic & vision research 8.2 (2013): 160.