Scanning laser ophthalmoscope fundus cyclometry in near-natural viewing conditions

Background: For a better understanding of motor and sensory adaptations in cyclodeviations, subjective and objective ocular torsion have to be measure d under the same conditions. The search coil technique and videooculography allow natural viewing but only assess relative cycloduction, the dynamics of torsion over a short period of time. Cycloposition, on the other hand, c an be measured by analysing the position of the foveola relative to the opt ic disc with fundus photographs but only in nonphysiological viewing. The a im of the study was to develop a technique that allows natural viewing cond itions during fundus cyclometry. Methods: The scanning laser beam of the SL O was deflected by 90 degrees with a semitransparent mirror in front of the patient's eyes. The patient was able to look through the semitransparent m irror with both eyes into the room, e.g. at Harms' tangent screen. The infr ared SLO images the central retina via the mirror through the undilated pup il. Digital image analysis quantifies the cycloposition of the eye. Control led head movements while fixating the centre of Harms' tangent screen allow measurements in reproducible gaze positions. Results: The semitransparent mirror reduces SLO image brightness, but image quality is sufficient for cy clometry after contrast enhancement. The laser light can be vaguely perceiv ed by the patient but does not interfere with natural viewing. Reproducibil ity of the measurement is within +/-1 degrees SD. Conclusion: Our modificat ion of SLO fundus cyclometry allows direct measurements of cycloposition in natural viewing conditions. This opens a new field for investigations of c yclodeviations and their sensory and motor adaptations.