Horizontal and vertical optokinetic nystagmus in normal and abnormal children and adults

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Horizontal and vertical optokinetic nystagmus in normal and abnormal infants and adults

Optokinetic nystagmus (OKN) is a reflexive visual response that can be elicited from humans of all ages. The optokinetic response consists of an alternating sequence of following movements (slow phases) in the direction of stimulus movements, interrupted by fast movements (quick phases) in the opposite direction. This thesis investigates how the slow and the quick phases of both horizontal and vertical OKN may have a role in the assessment of the oculomotor system. In normal adults, the speeds of horizontal OKN quick phases were compared to horizontal saccades. It was found that they had the same main sequence for duration but that the main sequence for peak velocity is sHghtly faster for saccades than OKN quick phases. Also, the speeds of horizontal OKN quick phases in normal infants were considered and found not to be significantly different (p>0.05) to those of adults. Further, the horizontal OKN quick phases in a group of children with neuronopathic Gaucher disease were examined. It was found that horizontal saccades and horizontal OKN quick phases were grossly slow in this condition. It is proposed that examination of OKN quick phases is a simple clinical means for approximating the saccadic main sequence and for identifying patients with brainstem abnormalities. Gaze position during full-field horizontal OKN stimulation was assessed. It was confirmed that the mean position of gaze is shifted in the direction opposite to stimulus movement. This was a robust phenomenon occurring even at very low stimulus speeds (2°/s). An explanation for this behaviour is proposed based on observations from patients with a vestibulocerebellar disorder. In this theory a leaky neural integrator is used to enhance velocity-matching. We also investigated the characteristics of vertical OKN and vertical saccades in normal adult subjects. We found that for amplitudes up to 20° there were no statistical differences between vertical centripetal and centrifugal saccades and no differences m the durations or peak velocities of upward and downward saccades. On the other hand, vertical saccades were significantly slower than horizontal saccades and vertical OKN quick phases were significantly slower than vertical saccades. We found that vertical OKN gain was asymmetrical: upward OKN gain was greater than downward OKN gain by an average of 0.08. Vertical OKN was also studied in adults with strabismus and infants and children with neurological abnormalities. The patients with strabismus often demonstrated a similar asymmetry of vertical OKN as seen in control subjects, with upward OKN stimulus motion eliciting a higher gain, although the gain was usually lower and the responses were very variable. In infants and children, abnormal vertical OKN was found in those who had either a neurometabolic disease or an abnormality of the cortex, brainstem and/or cerebellum. It is concluded that OKN testing has an important role in the assessment of the oculomotor system both as a visual response and as a motor behaviour