Eye movements: pathophysiology, examination and clinical importance

The ocular motor system finds, focuses, fixates and follows objects to ensure optimal vision as we move through our environment. To see clearly, images must be held steady on the retina. If images move across the retina at more than a few degrees per second they appear blurred. Thus, one function of eye movements is to hold images of the world steady on the retina, and this is accomplished by the gaze-holding mechanisms--fixation, vestibular and optokinetic. Visual acuity is best at the fovea, and so a second function of eye movements is to change the line of sight so that the image of an object of interest is brought to and held close to this part of the retina. This second group of eye movements, the gaze-shifting mechanisms, comprise saccades, smooth pursuit and vergence. The net result of gaze-holding and gaze-shifting mechanisms working properly together is clear, binocular vision. Conversely, if these movements are not working together, our view of the world becomes compromised by double-vision, blurred vision or oscillopsia, the sensation that the seen environment is jumping. The anatomic substrate for each functional class of eye movements is now well defined. This means that specific abnormalities on the eye movement examination may provide clues to the underlying pathology, and suggest strategies for treatment of a variety of neurological disorders.