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I ANATOMY                

 

II PHYSIOLOGY                               

III EXAMINATION: EYE MOVEMENTS & NYSTAGMUS 

IV FIXATION INSTABILITY   

VI VESTIBULAR SYSTEM  

 VII CEREBELLAR EYE   MOVEMENTS 

VIII CN PALSIES, VISUAL FIELDS, PUPIL & THE EYES

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The medulla houses the bulk of the vestibular nuclei, including the medial vestibular nucleus (MVN).  Other important structures for the control of eye movements in the medulla include the Nucleus Prepositus Hypoglossi, the inferior olivary nuclei, and the inferior cerebellar peduncles1.

Given that the NPH and MVN make up the neural integrator, it can be anticipated that lesions of these structures will result in gaze evoked nystagmus (GEN) on attempted holding of horizontal gaze. 

In addition, lesions of the inferior cerebellar peduncle interrupt climbing fibre inputs to the dorsal vermis of the cerebellum leading to excessive inhibition of the fastigial nucleus2.  This leads to ipsipulsion with hypermetric saccades to the side of the lesion, and hypometric saccades to the opposite side.

Figure 1. Structures of the dorsal and dorso-lateral medulla.

Lesions of this area, or of the cerebello-pontine angle, will result in Brun's nystagmus: a combination of nystagmus arising from structures making up the neural integrator (GEN) and a high frequency, small amplitude nystagmus due to vestibular nerve fibre injury, resulting in vestibular imbalance (manifest on looking away from the side on which the lesion has damaged the incoming vestibular fibres).

Lesions affecting the vestibular nucleus can mimic peripheral vestibular lesions with a positive head impulse sign. In the setting of an acute vestibular syndrome in which a patient has acute vertigo and a spontaneous nystagmus, a normal head impulse test raises suspicion of a central lesion in the brainstem or in the posterior medial cerebellum involving the nodulus and uvula. Other features to distinguish lesions in this region from a peripheral vestibular lesion are discussed further in the section on separating central from peripheral lesions.

Major features of the dorsolateral medullary syndrome (Wallenberg syndrome) include:
Lateropulsion: a tendency to fall or sensation of being pushed towards the affected side, (similar to an acute vestibular neuropathy). This is due to a lesion of the inferior cerebellar peduncle.
 
Dysfunction of Ocular Alignment
   Skew deviation
   Ocular Tilt Reaction
   Environmental Tilt: Floor on Ceiling Phenomenon
 
Nystagmus
   Horizontal, Torsional
   Mixed horizonal-torsional-vertical
   See-saw nystagmus
   Eyelid nystagmus
   Head nystagmus
 
Smooth Pursuit and Gaze-Holding abnormalities
   Ipsilateral eye deviation
   Impaired contralateral smooth pursuit
   Lateropulsion of pursuit
 
Saccadic abnormalities
   Ipsipulsion (lateropulsion)
   Torsipulsion
   Oblique saccadic trajectories on vertical gaze attempts

1. Dysfunction of ocular alignment.
An OTR may be present (ipsiversive head tilt, eye lower on the side of the lesion). Ocular torsion may be larger in the lower eye, possibly due to additional involvement of the posterior semicircular canal projections.  Spontaneous nystagmus, similar to that of peripheral vestibular lesions, may be horizontal or mixed horizontal-torsional.
Lid nystagmus may also occur with lid twitches synchronized with the fast phase of ocular nystagmus.

The "floor on ceiling" phenomenon is a sensation of environmental tilt, in which the entire room is tilted on its side or even upsidedown; possibly due to dysfunctional vestibular-otolith connection to central structures3.

See-saw nystagmus This nystagmus is a disjunctive, vertical-torsional nystagmus, one half-cycle of which consists of elevation and intorsion of one eye in conjunction with synchronous depression and extorsion of the other eye; the next half-cycle consists of reversal of these vertical and torsional movements.The usual cause is a lesion compressing the midbrain region, when the nystagmus is pendular. With lateral medullary lesions, however, a jerk see-saw nystagmus may occur3. In the case of the lateral medullary syndrome, the torsional component is conjugate and the fast component beats away from the side of the lesion.

2. Nystagmus
In the lateral medullary syndrome, nystagmus may be due to direct damage to the vestibular nuclei or their cerebellar, semicircular canal, or otolithic connections. Frequently, the medial vestibular nucleus is spared, since the infarction is dorso-lateral, and, if so, neural integrator function and the VOR should be normal.
Nystagmus in this syndrome is usually positional and can be horizontal, torsional, or mixed, with torsional, vertical, and horizontal components. 

Gaze-evoked eyelid nystagmus associated with ocular nystagmus has been described, in which a clinically obvious upward jerking of the eyelids occurred synchronously with the fast phase of a gaze-evoked horizontal nystagmus.

Head nystagmus may reflect a lesion in vestibulospinal and reticulospinal projections to cervical motoneurons5
Head-shaking-induced nystagmus can be ipsilesional (even when spontaneous nystagmus is contralesional), likely resulting from disinhibition of velocity storage connections between the unilateral vestibular nucleus and cerebellar nodulus/uvula4.

Spontaneous nystagmus can have features that are typically thought of as being “peripheral” including a unidirectional pattern; obeys Alexander’s Law (nystagmus increases in intensity in the direction of the fast phase); and suppresses with visual fixation4. Similarly, if there is involvement of the medial vestibular nucleus, an abnormal head impulse test may be present, since the ipsilateral VOR is deficient.

3. Smooth pursuit and gaze-holding abnormalities

Patients with the lateral medullary syndrome may complain of a sensation of their bodies being “pulled to one side” and may attempt to counteract this “lateropulsion of the body” by leaning toward the opposite side.
Due to the gaze-holding impairment, ocular movements may be similarly affected resulting in lateropulsion (identical to ipsipulsion), which is a conjugate deviation of the eyes toward the side of the lesion. This is best seen with closed lids, and, in particular, a corrective movement may be seen when the eyes are opened, as the eyes refixate on the midline2.

Smooth pursuit eye movements that track targets moving away from the side of the lesion are also impaired in patients with lateral medullary lesions, whereas pursuit toward the side of the lesion is relatively normal1.

4. Saccadic abnormalities

The abnormalities of gaze-holding noted in the previous paragraph may also be reflected in saccadic eye movement abnormalities.
Lateropulsion also affects saccades, so that the patient has:

 

 

Figure 2. Mechanism for lateropulsion in Wallenberg syndrome.
Inhibitory fibres travelling via the left inferior cerebellar peduncle are damaged, resulting in less inhibition of the left dorsal vermis. There is consequently a greater inhibition of the fastigial nuclei, and less activation of the right sided PPRF.  This results in deficient rightward saccades (contralesional hypometric saccades), and over-active leftward saccades (ipsilesional hypermetric saccades), and ipsilesional ocular lateropulsion.

 

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Video 1.  Patient with a left lateral medullary (Wallenberg) syndrome

Leftward ocular lateropulsion (apparent throughout the video during blinks - during eyelid closure, there is conjugate deviation to the left, and when the eyelids open, the eyes move to the right into primary gaze) is present with hypermetric saccades to the left (ipsilateral) and hypometric saccades to the right (contralateral), relating to injury of the climbing fibers traveling through the inferior cerebellar peduncle on the left side.

(vv)Wallenberg.mp4(tt)

From: Gold D. The acute vestibular syndrome with dysarthria, dysphagia, dysphonia, hemi-ataxia, and saccadic dysmetria due to the lateral medullary (Wallenberg) syndrome. Video. [Neuro-Ophthalmology Virtual Education Library: NOVEL Web Site]. 2017. Available at: https://collections.lib.utah.edu/ark:/87278/s6963fhm

 

References

 

  1. Kheradmand A, Colpak AI, Zee DS. Eye movements in vestibular disorders. Handb Clin Neurol. 2016;137:103-17.
  2. Zee DS, Jareonsettasin P, Leigh RJ. Ocular stability and set-point adaptation. Philos Trans R Soc Lond, B, Biol Sci. 2017;372(1718)
  3. Brazis PW. Ocular motor abnormalities in Wallenberg's lateral medullary syndrome. Mayo Clin Proc. 1992;67(4):365-368. doi:10.1016/s0025-6196(12)61553-5
  4. Choi WY, Gold DR. Ocular Motor and Vestibular Disorders in Brainstem Disease. J Clin Neurophysiol. 2019;36(6):396-404. doi:10.1097/WNP.0000000000000593
  5. Wong, A. M. (2008). Eye movement disorders. Oxford: Oxford University Press.