Ocular Surface Characteristics of the Asian Eye

According to the market research company GFK Retail and Technology (1), the combined retail sales of contact lenses across the five key Asian markets of Hong Kong, Korea, Malaysia, Singapore and Taiwan totalled USD 592 million during 2009. The disposable contact lens category registered a nine percent growth in 2009, a year in which silicone hydrogels recorded the strongest growth rate, hitting a high of 24 percent in the second half of 2009.

It is well known that a practitioner has to factor in variations in anatomical ocular structure between people when choosing and fitting the most appropriate contact lens. A successful fitting with one particular lens design may be unsuitable for another person even though, for example, their central corneal curvature is similar. Practitioners need to pay attention to the anatomical and physiological differences between ethnicities and how these differences affect contact lens performance.

It has been reported that Asian eyes tend to have thicker and fuller upper eyelids, with an absent upper lid fold (2). There is more fatty tissue present in the upper eyelid (3) and the orbital fat content extends further inferiorly (upper lid) and superiorly (lower lid) in Asian compared to Caucasian eyelids (4).The lid position, tonus and aperture may cause difficulties with contact lens insertion and removal. Compared to Caucasians, Asians have a smaller and more oblique palpebral aperture (5,6). Asian eyelashes reveal a lower lift-up (angle between the root of the eyelash and the edge of the eyelid), a lower curl-up angle (angle between the root of an eyelash and its tip) and fewer lashes with a thicker transverse diameter (thicker and straighter) (7). In many Asian eyes, the upper lashes may rest against the cornea without causing discomfort, though this position may cause problems when trying to insert a contact lens.

Several studies have shown ethnic differences in ocular surface physiology. Asians tend to have a shorter average tear break-up time (10, 11) and lower tear volume (12, 13) compared to Caucasians The average Chinese eye has a smaller and steeper cornea, with less peripheral flattening than the average Caucasian eye (5). The horizontal visible iris diameter (HVID) varies significantly between Caucasians and Asians, with Asians showing smaller HVID in the 0.3mm range (8). Chenyak et al, investigated the differences between Asian and Caucasian eyes with respect to corneal geometry. They concluded that the corneas of many Asian subjects are more symmetrical in relation to the limbus, defined by the outer boundary of the iris. However, in Caucasian subjects, the corneal apex is located more nasally than the geometric center of the cornea (9). Displacement of the corneal apex will play a role in the location of the contact lens on the eye and can result in significant decentration.

Another important characteristic to consider is that corneal sensitivity is lower in Asians (5).This may be problematic in some cases, as Asian eyes are more susceptible to micro-trauma of the cornea during extended wear of contact lenses (14, 15). Moreover, a study by Albietz found that Asian eyes had greater ocular surface staining before and after LASIK (16). However, a further study detected no difference between Asian and Caucasian non-lens wearers with respect to corneal epithelial barrier function (17). When subjected to decreased oxygen levels (low Dk/t lenses, short-term eye closure, overnight wear), Asian eyes had a substantially greater increase in epithelial permeability (14) and increased endothelial bleb formation (18). Only very highly oxygen permeable lenses do not cause stress in Asian eyes (18).

A study by Long and McNally (19) found that only one of 87 Asian patients fitted with lotrafilcon A lenses was discontinued because of poor fitting, with 99% of eyes judged to have optimal or acceptable fitting. The 8.4 radius lens was chosen for 56% of eyes – 44% of eyes were fitted with the 8.6 mm radius. The 13.8 mm total diameter of lotrafilcon A lenses may be advantageous for the Asian eye that has a smaller HVID. Another study (20) conducted to investigate the clinical performance of lenses on Asian eyes found that only 24 of 41 eyes attained acceptable fit and satisfactory vision with an 8.7/14.5mm etafilcon A toric lenses. Approximately 40% of subjects were unable to obtain an acceptable fit with this one-fit lens, primarily because the lenses were too loose; this is most likely due to the steeper corneal shape of Chinese eyes. HVID and palpebral aperture size did not appear to influence the fit of this lens.

The future range of contact lens designs and materials must take into account the existence of ethnic differences in ocular anatomy and physiology that directly relate to the success of contact lens fitting. Better understanding of these ethnic differences in ocular characteristics may be helpful for practitioners' clinical decision making process, to achieve a more successful fitting outcome.

REFERENCES

1. http://www.gfkrt.com/news_events/market_news/single_sites/005718/index.en.html
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