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This chapter discusses the main types of eye movement paralysis resulting from brainstem lesions, and the related pathophysiology. The abnormalities are easily detected at the bedside by studying three main types of eye movements: saccades; smooth pursuit; and the vestibular ocular reflex (VOR). The chapter reviews eye movement disturbances due to cerebellar and cerebral hemispheric lesions, resulting in relatively more subtle syndromes. The stroke-related lesions that most often involve horizontal gaze are located in the cerebral hemispheres and the pons. The hemispheral lesions are most often relatively large hemorrhages or infarcts that include the lateral aspect of the frontal lobe and/or the deep basal ganglia-capsular regions. Outside the brainstem, a number of suprareticular structures, located in the cerebellum and the cerebral hemispheres, control eye movements. Damage to these structures results in saccade and/or smooth pursuit disturbances usually much more subtle than those due to brainstem lesions.
Eye movement commands originate in diverse cerebral hemispheric areas (for saccades and smooth pursuit) or in labyrinths (for the vestibular ocular reflex). They are carried out in the brainstem by the immediate premotor structures and the motor nuclei. Conjugate lateral eye movements are largely organized in the pons, and vertical eye movements and convergence in the midbrain. In the first part of this chapter, we will see the main types of eye movement paralysis resulting from brainstem lesions, and the related physiopathology. Such types of abnormalities are easily detected at bedside by studying three main types of eye movements: saccades, i.e. rapid eye movements made towards a visual target (such as the finger of the examiner); smooth pursuit, elicited by a small visual target moving slowly in front of the subject's eyes; the vestibular ocular reflex (VOR), tested using the oculocephalic movement, by moving passively the subject's head. In the second part of this chapter, eye movement disturbances due to cerebellar and cerebral hemispheric lesions, resulting in relatively more subtle syndromes, will be reviewed.
Lateral eye movements
Final common pathway
The final common pathway of conjugate lateral eye movements begins in the abducens nucleus, which contains: (i) the motor neurones projecting onto the ipsilateral lateral rectus; and (ii) the internuclear neurones, which decussate at the level of the abducens nucleus, run through the medial longitudinal fasciculus (MLF) and project onto the medial rectus motor neurones in the contralateral oculomotor nucleus (Fig. 7.1) (for review, see Pierrot-Deseilligny, 1990 and Leigh & Zee, 1999).
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