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II PHYSIOLOGY              

III CLINICAL EXAMINATION 

IV FIXATION INSTABILITY

V SUPRANUCLEAR to NUCLEAR 

VI VESTIBULAR SYSTEM  

VII CEREBELLAR EYE MOVEMENTS

VIII VISUAL FIELDS, PUPIL & THE EYE

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The same motor neurons and extraocular muscles are active for all types of eye movements (e.g. saccades, pursuit and vergence). Thus, systems generating innervation for different movement types are said to share a final common path1.

Midbrain centers:
The center for vertical saccades is the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), and the center for vertical gaze-holding function is the interstitial nucleus of Cajal (INC).
Clinically this means that an isolated vertical saccadic paresis or isolated vertical gaze deviation nystagmus would suggest a midbrain lesion. 

Pontine and pontomedullary centers:
The center for horizontal saccades is the paramedian pontine reticular formation (PPRF); the center for the horizontal gaze-holding function is the nucleus prepositus hypoglossi together with the vestibular nuclei and the vestibulocerebellum; these form the “neuronal integrator.”
Clinically this means that isolated horizontal saccadic palsy indicates a pontine lesion, and a unilateral PPRF lesion will result in saccadic disturbances on the side of the lesion. A purely horizontal gaze-evoked nystagmus originates from a pontine lesion. 

Figure 1. The structures of the neural integrator

Cerebellar centers:
Cerebellar lesions are often accompanied by clinically easily identifiable oculomotor disturbances.
For example, defects of the flocculus/paraflocculus are characterized by saccadic pursuit, downbeat nystagmus, and impairments of the visual fixation suppression of the VOR.
Paraneoplastic cerebellar disorders often lead to opsoclonus, in addition to the oculomotor disturbances mentioned above. 

 

Figure 2. Brainstem centres for gaze holding and saccade generation

References

  1. Ramat S, Leigh RJ, Zee DS, Optican LM. What clinical disorders tell us about the neural control of saccadic eye movements. Brain. 2007;130(Pt 1):10-35.