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Peter Ewen King-Smith, Thomas F. Mauger, Carolyn G. Begley, Patrice Tankam; Does the Peripheral Light Focusing Effect Explain the Strong Nasal Location Preference of Pterygia?. Invest. Ophthalmol. Vis. Sci. 2019;60(9):4701.
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© ARVO (1962-2015); The Authors (2016-present)
Pterygia are about 15 times more common nasally than temporally (Shiroma et al., 2009, Am J Ophthalmol 148, 766). Ultraviolet, UV, radiation is a major risk factor. Coroneo et al. (1991, Ophthalmic Surg 22, 705) proposed that the nasal preference is caused by the peripheral light focusing effect, PLF, in which UV at an oblique angle passes through temporal cornea and is concentrated on and damages nasal limbal stem cells. This study evaluates whether the PLF is sufficient to explain this nasal preference.
The cornea was modeled with custom software using the corneal surface and UV refractive index data from Kwok et al. (2004, Ophthal Physiol Opt 24, 119). Whereas they derived the peak intensity gain as a function of incident angle, (i.e., different positions were used for different incident angles) the current simulation derived intensity gain at a fixed position such as the nasal corneo-limbal junction, CLJ, in the horizontal plane. This provided a measure of the total PLF irradiation at this position, which was compared with total direct irradiation of nasal and temporal limbus at the corresponding positions.
Fig. 1A and B show intensity gain from the PLF as a function of incident angle, at the CLJ and on the limbus 1 mm outside the CLJ. For both positions, a measure of total irradiation was derived by integration of gain over incident angle. A corresponding measure of total direct irradiation of nasal and temporal limbus was similarly derived by integration over incident angle. For nasal irradiation it was assumed that the nose obstructs some 70o of irradiation reaching the surface of the nasal limbus, 30o from visual field obstruction by the nose plus 40o from slope of the limbal surface, Fig. 1C. Fig. 2 summarizes measures of overall irradiation of nasal and temporal limbus.
Fig. 2 indicates that the total UV irradiation of the nasal limbus is not greater than that of the temporal limbus. UV absorption and scattering in the cornea and limbal stroma should further decrease the relative contribution of the PLF. It thus seems that the PLF is not sufficient to explain the strong nasal location preference of pterygia. An alternative explanation worth considering (RH Elliot, 1920, “Tropical Ophthalmology”, p. 99) is that temporal to nasal flow of tears over the UV exposed cornea causes accumulation of toxins, etc., on the nasal limbus.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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