Saccades are rapid, ballistic, yoked movements of the eyes which bring the gaze to a new location in visual space. These movements may be performed voluntarily (tested clinically by asking the patient to "Look to your left, keeping your head still", etc.) or reflexively, i.e., in response to an object of potential interest within the visual field (tested clinically by asking the patient to shift gaze from one of examiner’s hands to another). Internuclear ophthalmoplegia may be revealed when testing saccadic eye movements.
A number of parameters may be observed, including latency of saccade onset, saccadic amplitude, and saccadic velocity. An antisaccadic task (i.e., suppression of saccades to a novel visual stimulus) may be used to assess ease of saccade suppression. Of these, saccadic velocity is the most important in terms of localization value, since it depends on burst neurones in the brainstem (paramedian pontine reticular formation for horizontal saccades, rostral interstitial nucleus of the medial longitudinal fasciculus for vertical saccades). Latency involves cortical and basal ganglia circuits; antisaccades involve frontal lobe structures; and amplitude involves basal ganglia and cerebellar circuits (saccadic hypometria, with a subsequent correctional saccade, may be seen in extrapyramidal disorders, such as Parkinson’s disease; saccadic hypermetria or overshoot may be seen in cerebellar disorders). Difficulty in initiating saccades may be described as ocular (motor) apraxia. Antisaccades may be poorly suppressed in Huntington’s disease. In Alzheimer’s disease, patients may make reflex saccades toward a target in an antisaccadic task (visual grasp reflex).
Assessment of saccadic velocity may be of particular diagnostic use in parkinsonian syndromes. In progressive supranuclear palsy slowing of vertical saccades is an early sign (suggesting brainstem involvement; horizontal saccades may be affected later), whereas vertical saccades are affected late (if at all) in corticobasal degeneration, in which condition increased saccade latency is the more typical finding, perhaps reflective of cortical involvement.
Carpenter R. The saccadic system: a neurological microcosm. Advances in Clinical Neuroscience & Rehabilitation 2004; 4(1): 6-8 Leigh RJ, Riley DE. Eye movements in parkinsonism: it’s saccadic speed that counts. Neurology 2000; 54: 1018-1019