Sources of normal and anomalous motion in retinoscopy.
PURPOSE: Besides the classic "with," "against," and "neutral" absence of motion, retinoscopic reflexes can display anomalous "with" motion in myopia. A model is presented that explains the source of this anomalous motion, as well as quantifies the appearance of retinoscopic motion in myopia and hyperopia. METHODS: Various 2 x 2 matrices were created to describe schematic eyes for a +20 D trial lens, a Gullstrand #1 schematic eye, and an infant schematic eye. Rays from the retinoscope were traced paraxially through these matrices over a full transit of the retinoscope beam across the pupil. Retinal position of the edge of the reflex visible to the observer was plotted as a function of pupil sizes from 2 mm to 16 mm for -5.00 D and +2.00 D refractive errors for the +20 D trial lens. RESULTS: The edge of the retinoscopic reflex could be formed by one of two sources: the edge of the retinoscope beam itself, or the shadow cast by the beam against the edge of the pupil. Anomalous "with" motion arose in myopia when the edge of the reflex was formed by the edge of the beam. The edge of the beam was also visible in hyperopia but did not create anomalous motion. The retinoscope peephole was not involved in the formation of the edge of the reflex. The degree of anomalous motion increased with greater myopia and pupil size. Measured pupil sizes needed to completely eliminate anomalous motion agreed well with those predicted by the model, except at the largest pupil sizes. The limit for anomalous motion depended only on pupil size, refractive error, and working distance but not on whether the system matrix represented the trial lens, a Gullstrand #1 eye, or an infant eye. CONCLUSIONS: Seeing the edge of the beam at large pupil sizes during retinoscopy creates anomalous "with" motion in myopia but may make the reflex easier to see in hyperopia. Anomalous "with" motion in myopia can be managed by adjustment of pupil size, working distance, or net corrected refractive error. Aside from possible effects of aberrations, retinoscopic motion appears to be consistent across various paraxial optical systems for a given refractive error and pupil size.[1]References
- Sources of normal and anomalous motion in retinoscopy. Mutti, D.O. Optometry and vision science : official publication of the American Academy of Optometry. (2004) [Pubmed]
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