Effects of laser refractive surgery upon tear osmolarity

As you may be aware post-refractive surgery dry eye is the most common postoperative complaint by patients and can be extremely debilitating but it has also been linked with a higher risk of post-LASIK regression (laser in situ keratomileusis).²

Osmolarity is a marker for dry eye, which is central to the current definition by the dry eye workshop (DEWS):

As the aqueous portion of tear is reduced, the relative levels of dissolved materials increase, leading to hyperosmolarity. This can induce apoptosis and lead to further inflammation and result in permanent damage to the ocular surface and degradation of visual quality.⁴ Tear osmolarity has become increasing recognized as a diagnostic and prognostic 'bed side' test.

It should be stressed that tear osmolarity is a highly volatile parameter, which rises acutely if the patient is staring for even a small period, as with visual acuity testing, also recent topical drops (within 2 hours) will also alter osmolarity readings. Thus, the sequencing of the examination is important, where possible tear osmolarity should be the first examination performed.⁴

Post-LASIK dry eye

Post-LASIK dry eye affects up to half of patients at 1 week, 40% at 1 month, and 20–40% at 6 months. While typically transient, a significant cohort of patients experience severe symptoms. Surface refractive laser, including LASEK (laser assisted subepithelial keratectomy) and PRK (photorefractive keratectomy), is associated with transient postoperative dry eye symptoms. Patients with extreme dry eye such with Sjogren's syndrome should not be considered for corneal procedures.⁴

Post-LASIK dry eye describes a spectrum of diseases encompassing transient or persistent postoperative neurotrophic disease, tear instability, true aqueous tear deficiency, corneal and conjunctival epitheliopathy, and neuropathic pain states. Neural changes in the cornea and neuropathic causes of ocular surface discomfort may play a separate or synergistic role in the development of symptoms in some patients.

The post-LASIK neurotrophic effect and damage to goblet cells, are related to the corneal flap sectioning and suction effect, respectively. These effects may not be seen in surface refractive laser patients, so theoretically LASIK would have a more profound effect on the precorneal tear film and induce more dry eye disease. This neurtophic cornea is sometimes referred to as LASIK-induced neurotrophic epitheliopathy (LINE).⁴

Several factors are associated with the development of post-LASIK dry eye. Higher refractive correction, deeper ablation depth, narrow flap hinge and female gender have been repeatedly reported to be associated with an increased incidence of post-LASIK dry eye. Flap thickness has both been reported to be directly related to dry eye post LASIK and be unrelated.⁴ The dry eye effects of LASIK seem to have been comprehensively reported, however there was a paucity of data comparing LASIK and LASEK dry eye rates and especially in relation to tear osmolarity changes.