LASIK and PRK, the most commonly performed refractive procedures, are great for a lot of people, but there are many newer procedures now that can help minimize the risk of complications. With regards to Asian patients in particular, there is fairly recent evidence that Asians are much more likely to develop dry eye problems than non-Asians after LASIK, even after factoring out the amount of myopia. It has also been shown that the corneal nerves do not seem to completely grow back to normal like we had once believed. Even if the nerves appear to grow back morphologically, their function is ultimately what must return to equilibrium.
What causes post LASIK dry eyes?
A sophisticated feed-back-loop links the corneal epithelium, stroma, nerves to the central nervous system, then back out through the facial nerve to the lacrimal gland and orbicularis fibers. When corneal nerves are damaged, the feed-back-loop is disrupted. As the sensation of dryness deminishes, the blink rates decrease and tear secretion is slowed.
LASIK induced neurotrophic epitheliopathy (LINE) initially described by Wilson, was found to correlate with blink rate. Because LASIK first requires the creation of a large corneal flap, a larger area of sub epithelial corneal nerves are damaged in LASIK than by PRK. This is believed to be part of the reason why PRK seems to have fewer problems with post operative dry eyes.
For patients especially at risk of developing dry eyes, nasal flap orientation was believed to help save a portion of the nerves (that enter the cornea from 3 and 9 o’clock) and reduce the incidence of dry eye symptoms. A recent prospective randomized clinical trial however found no significant difference in symptoms between the superior or nasal flapped patients, but did however find a correlation with the amount of preoperative myopia and ablative depth.
With this evidence, I believe that it is the deeper stromal corneal nerves that play a greater role in the development of dry eyes. Because LASIK begins ablation deeper within the stroma than PRK, this may also help explain why PRK patients suffer less from dry eyes. I will further discuss how each of the three different types of corneal nerves (cold sensors, nociceptors, and propriocepts) impact the blink rate on my Dry Eye page.
For people with higher degrees of myopia that require deeper ablations there are now more options that avoid corneal ablation. Phakic intra ocular lenses (P-IOLs) have been used for years in Europe but are starting to be used here in the US. Two types of lenses are currently FDA approved for use in myopic eyes. The Verisyse PIOL by Ophtec BV was approved 9/2004 for implantation infront of the iris. The Visian ICL (Implantable Collamer Lens) by STAAR surgical was approved 12/2005 for implantation behind the iris.
For highly myopic Asian patients who are particularly at risk of dry eyes, I believe this will become the best choice refractive procedure.