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Constance West, David Hunter; Displacement of Optical Centers in Over-the-Counter Readers: A Potential Cause of Diplopia. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1301.
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Induced prism in spectacle lenses, which may result from inadvertent displacement of optical centers, may worsen an existing heterophoria or even cause diplopia. Normal vertical fusional amplitudes are relatively small, making optical center displacement in the vertical meridian a particular concern. To evaluate the magnitude of this potential problem, we assessed the frequency and extent of clinically significant manufacturing variations in a random selection of over-the-counter reading glasses ("OTC readers.")
OTC readers of varying styles and powers were purchased from seven stores in three US states (West Coast, Midwest, and East Coast). The optical centers were marked using a focimeter. A prescription aligner was then used to measure interpupillary distance (PD) (distance between optical centers) and vertical displacement (relative distance of the optical centers from the lower frame edge). Monocular PD (distance of each optical center from its respective nasal frame edge) was also measured.
A total of 160 OTC readers were evaluated. The optical centers were vertically displaced by ≥2 mm in 24% (41 pairs) and by ≥3 mm in 11% (18 pairs), with a maximum displacement of 7 mm in one pair. Average PD was 64 mm (range, 58 to 74.5 mm), with PD <60 mm in 2% (3 pairs) and >70 mm in 3% (5 pairs). Monocular PD was asymmetric by ≥ 5 mm in 4% (6 pairs). One pair was found to have a 0.75 diopter power difference between lenses.
Some OTC readers have misaligned optical centers and other manufacturing defects that are of a magnitude sufficient to exacerbate a heterophoria and cause asthenopia or diplopia. Defects occur commonly enough that clinicians should routinely measure the optical centers and power of OTC readers when evaluating a symptomatic patient.
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