Torsional optokinetic nystagmus: response characteristics measured in the normal population and patients with ocular motor disorders

This thesis presents the first detailed study of the torsional optokinetic nystagmus (tOKN) response in the normal population and in patients with oculomotor disorders. The effects on the tOKN response of: (i) stimulus velocity, (ii) stimulus area, and (iii) aging, were investigated in the normal population. The tOKN response was also evaluated in patients with long standing oculomotor disorders, namely strabismus and infantile nystagmus. Torsional OKN responses were recorded using infrared video-oculography and were elicited with volunteers fixating the centre of a large-field rotating sinusoidal grating pattern. Torsional OKN responses were present in all normal young volunteers (n=20) to stimuli rotated in clockwise and anticlockwise directions, and a linear relationship was observed between log stimulus velocity and tOKN slow phase velocity. Torsional OKN also showed brisk responses to peripheral field stimulation in the same subjects. The first report of a significant increase with age in the proportion of absent tOKN responses is also described in normal subjects aged between 19-72 years (n=30). The tOKN response was investigated for the first time in strabismic patients (n=16), comparing horizontal and vertical OKN responses, and also in patients with infantile nystagmus (n=16). OKN responses from strabismus patients demonstrated consistent asymmetry in horizontal and vertical directions. However, a significantly higher incidence of absent tOKN responses in both intorsion and extorsion directions were observed in comparison to controls. Torsional OKN was present in 3 of 16 patients with infantile nystagmus. Torsional OKN is a well developed reflex in the normal population with the capacity to respond in proportion to stimulus velocity and area of stimulation. However, the tOKN response is dramatically affected by (i) the effects of aging, (ii) by the interruption of binocular visual development in patients with strabismus, and (iii) by the presence of infantile nystagmus.