Eyeworld

The ocular surface: The first refractive interface of the eye 6 by Marjan Farid, MD The contribution of the tear film to vision and ocular comfort Preoperative, intraoperative, and postoperative factors influence the tear film T he tear film plays a key role in visual outcomes and patient satisfaction after cataract and refrac- tive surgery. If tear film abnormalities remain undetected and untreated, they can alter preoperative measurements that can significantly affect postoper- ative refractive outcomes as well as cause postoperative discomfort and pain. Examining the tear film The eye has three refractive inter- faces: the pre-corneal tear film, cornea, and lens. The total optical power of the relaxed eye is 60 D; the corneal power (including the tear film) accounts for two-thirds and the lens power one-third. 1 The greatest change in the index of refraction occurs be- tween the air and tear film, giving the tear film the greatest optical power of any ocular surface (Fig- ure 1). 2 It consists of three layers: the lipid, aqueous, and mucin layers (Figure 2). The anterior radius of the curvature is 7.8 mm, and the thickness ranges from 6 to 20 µm. 3 Aqueous deficiency or evaporative dry eye produces tear film irregularity and can cause optical power changes through- out the cornea. Variable refractive powers on the ocular surface will cause significant higher-order aberrations. Blinking restores the tear film briefly, mixing tear compo- nents and spreading tears across the surface. Between blinks, however, aqueous evaporates and the tear film thins and becomes irregular. Dry eye worsens these effects, reducing visual acuity and increasing higher-order aberra- tions. 3,4 Using double-pass retinal imaging, Benito et al. demonstrat- ed that increased light scatter in patients with dry eye degrades image quality. 5 Retinal vessel con- trast studies by Tutt et al. showed that tear film irregularities can reduce retinal image quality by 20 to 40%. 2 In addition, topography studies by Németh et al. indicated that tear break-up during a 15-sec- ond pause between blinks reduced visual acuity by 6%. 6 Increased risk Preoperative diagnosis of OSD is important in patients having LASIK because dry eye is one of the most frequent adverse effects of the procedure. 7,8 Lee et al. reported that tear secretion decreased more after LASIK vs. PRK 6 months after surgery and stressed that dry eye treatment is essential after refractive surgery. 9 Additionally, patients having cataract surgery are already at high risk of dry eye because of their age, hormonal changes, and medications. In addition, their diets may be deficient in omega-3 fatty acids. Practice pearl: In the preoperative setting, always pause and evaluate the tear film prior to surgical planning. Ocular surface in- stability is the leading cause of the unhappy refractive cataract or laser patient. By taking that pause and treat- ing dry eye disease prior to surgery, you can significantly improve your postoperative outcomes and patient satis- faction. –Marjan Farid, MD " Patients having cataract surgery are already at high risk of dry eye because of their age, hormonal changes, and medications. " –Marjan Farid, MD The surgical process also contributes to dry eye and irrita- tion. The cornea and tear film can dry during preoperative dilation because the eyes are open and anesthetic drops decrease the blink rate. Mild surgical trauma to the cornea may result from the lid speculum or minor abrasion. Benzalkonium chloride (BAK) and proparacaine in the anes- thesia and topical drops affect endothelial cell integrity and tear function. 10,11 In addition, surgical inci- sions may disrupt corneal inner- vation and lengthen the interval between blinks. 12 In a prospective multicenter observational trial, Donnenfeld et al. reported that Cochet-Bonnet corneal sensitivity decreased most in regions adja- cent to limbal relaxing incisions (28 to 31%), which returned to near-normal levels at month three. 13 In a study of 48 eyes that had cataract surgery, goblet cell Conclusion OSD is very common in all patient populations, but it is extremely common in cataract patients. It is a common cause of patient dissatisfaction with refractive and cataract surgery. Therefore, it is critical to diagnose and manage OSD before surgery to obtain optimal outcomes. Proper diagnosis and treatment can improve surgeons' ability to choose the correct IOL type and power and improve postoperative results. References 1. Trattler WB, et al. Cataract and dry eye: Prospective Health Assessment of Cataract Patients' Ocular Surface (PHACO) Study. 2011 ASCRS•ASOA Symposium & Congress. 2. Cui X, et al. Assessment of corneal epi- thelial thickness in dry eye patients. Optom Vis Sci. 2014;91:1446–1454. 3. Epitropoulos AT, et al. Effect of tear osmolarity on repeatability of keratometry for cataract surgery planning. J Cataract Refract Surg. 2015;41:1672–1677. Dr. Holland is in practice at the Cincinnati Eye Institute. He can be contacted at eholland@holprovision. com. continued from page 4

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