Eyeworld

Blinking temporarily restores the tear film, mixing the tear components and spreading the tear evenly across the ocular sur- face. However, between blinks the aqueous evaporates and the tear film degrades and becomes irregular. Research has demonstrated the effects of tear film irregularity on vision. According to Benito and colleagues, double-pass retinal im- aging demonstrated that increased light scatter in patients with dry eye worsens the image quality. 2 In addition, in retinal vessel contrast studies, Tutt et al. reported that tear film irregularities reduce retinal image quality by 20% to 40%. 1 Furthermore, topography stud- ies showed the tear breakup time during 15-second intervals between blinks reduced visual acuity by 6%. 3 Although image quality is highest immediately after a blink, image quality degradation and higher order aberrations occur more quickly between blinks in dry eyes compared with normal eyes. 4,5 In an era when patients spend more time viewing mobile phones, computer monitors, and other electronic screens—and blinking less—dry eye has become more prevalent, negatively affecting patients' quality of life. The tear film is the most critical refractive interface and must be maintained for optimum postoperative visual outcomes T he tear film is the eye's first and most important re- fractive interface; however, refractive cataract surgeons may take its role for grant- ed. Tear film irregularities can cause significant aberrations and visual distortions, affecting visual quality after refractive cataract surgery. Tear film structure The eye contains three refractive interfaces—the precorneal tear film, cornea, and lens. Approximately two-thirds (40 D) of the eye's optical power is derived from the cornea—including the tear film—and one-third from the lens (20 D), for a total power of about 60 D in the relaxed eye. When we examine the refractive index of each layer of the cornea, the greatest change in refractive index occurs between the air and tear film. 1 The tear film has three layers: the lipid layer, which prevents tear evaporation; the aqueous layer, the bulk of the tear film that contains the immune mediators; and the mucin layer, which helps the tear layer adhere to the ocular surface. The tear film thickness rang- es from 6 to 20 µm. If the film is smooth and uniform, it has minimal effect on the optical power of the cornea; however, the tear film thickness may vary depending on aqueous deficiency or aqueous evaporation. When the tear film becomes irregular, variations in the anterior radius and optical power may occur. Variable powers on the optical surface can induce significant higher order aberrations, and patients may have symptoms of diplopia, starbursts, glare, and shadowing. by Marjan Farid, MD The tear film: The neglected refractive interface Marjan Farid, MD Structure and composition of the tear film The neglected refractive interface: Impact of the tear film on refractive cataract surgery outcomes The greatest change in refractive index occurs between air and the precorneal tear film, making the tear film critical to the optical power of the eye.

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