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A Serra, LF Dell'Osso, JB Jacobs; Effects on Congenital Nystagmus (CN) of Combined Gaze-Angle and Vergence Variation: Therapeutic Implications . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2656.
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Purpose: To investigate the observation that once CN is damped by convergence in a binocular subject, it remains damped over a range of gaze angles. Methods: Ocular motility recordings were made using infrared reflection during fixation of targets at gaze angles varying between ±20° at different values of convergence ranging from far to 20D. The eXpanded Nystagmus Acuity Function (NAFX) was used to evaluate the CN waveform's relation to potential visual acuity at all fixation points. Results: During far fixation the subject exhibited a classic null (high NAFX value) with less damping (lower NAFX values) at gaze angles in both lateral directions from the null. During near fixation, the NAFX values were higher both at the null and at gaze angles to both sides of the null region; that is, the null region was broadened. The longest foveation domain (range of gaze angles in which the NAFX is within 10% of its peak value) at near was approximately twice as wide as at far. When plotted for a fixed gaze angle, the variation of NAFX with vergence exhibited hysteresis in the central 20° of gaze, being greater during divergence than convergence. Conclusions: Damping CN by means of induced convergence, either with base-out prisms or bimedial recession surgery, not only takes advantage of the vergence null (usually stronger than a coexisting gaze-angle null) but also provides a wider range of gaze angles with higher potential acuity. The null-broadening effects of vergence mimic those recently discovered for tenotomy. The hysteresis exhibited during vergence implies that the acuity achieved when fixating a target would be higher if one first fixated on a nearer point and then diverged to the target.
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