Dry Eye Dx and Tx
The Hormone-Dry Eye Connection, Part 2
BY WILLIAM TOWNSEND, OD, FAAO
My last column looked at the influence of sex hormones and thyroid hormones on dry eye. Hormones have access to every cell in the body and represent a major form of communication between diverse organs and tissues (Rocha et al, 2013). As health providers, we can appreciate that other hormones also trigger many dry eye conditions. This article will focus on insulin.
Is Insulin Linked to Dry Eye?
Insulin is released into circulation and affects multiple tissues and organs. It performs numerous functions including facilitation of glucose transport into certain cells, conversion of glucose into glycogen, and promotion of protein synthesis and growth.
Diabetes is associated with insulin deficiency and insulin resistance. It is also an established comorbidity for dry eye (Wang et al, 2012); but, because insulin is present throughout the body, it is challenging to establish a causal relationship between insulin and dry eye (Rocha et al, 2013).
Several animal studies may provide some insight into this relationship. In an animal model, Alves et al (2008) used streptozotocin to induce diabetes in rats. Within four to five weeks, they noted reduced tear film secretion and progressive histological damage to lacrimal glands. This study fails to demonstrate any direct relationship between insulin and dry eye, but it supports results of human studies suggesting that adequate insulin levels are necessary for maintaining lacrimal gland function, corneal epithelial cell proliferation, and ocular surface maintenance (Rocha et al, 2013).
Wang et al (2014) used a rabbit model to evaluate the impact of insulin-like growth factor-1 (IGF-1) versus placebo after LASIK. IGFs are proteins with high sequence similarity to insulin that bind to insulin receptors (http://en.wikipedia.org/wiki/Insulin-like_growth_factor). They found that compared to controls, study animals showed increased numbers of epithelial microvilli and regenerated nerve fibers, and they noted improvement in specific dry eye markers such as Schirmer’s test and tear breakup time (Wang et al, 2014). They concluded that “IGF-1 accelerates the early repair of the corneal epithelial ultrastructure and nerve regeneration after LASIK.”
So, can topical insulin be used to treat diabetic dry eye? Zagon et al (2009) evaluated ocular changes in rats with streptozotocin-induced diabetes. After five weeks, they noted onset of reduced tears (Schirmer’s test with anesthesia) and reduced corneal sensation in all subjects. They treated group 1 with topical naltrexone and artificial tears; group 2 received insulin drops and artificial tears. The naltrexone group showed improvement in both corneal sensitivity and tear production; in contrast, the insulin group showed improved corneal sensitivity, but no improvement in tear production. They concluded that naltrexone has potential for treating dry eye in diabetics. One consideration in the outcome of this study is how the results might have differed had they performed non-anesthetized Schirmer’s testing.
Hormones are incredibly powerful molecules, some of which have an established impact on the development and course of dry eye. We have touched on only a small number of the hormones that impact ocular surface disease. These molecules may be part of the next big advance in treating and managing dry eye. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #221.
Dr. Townsend practices in Canyon, Texas, and is an adjunct professor at the University of Houston College of Optometry. He is president of the Ocular Surface Society of Optometry and conducts research in ocular surface disease, lens care solutions, and medications. He is also an advisor to Alcon, B+L, CooperVision, Tearlab Corporation, and Vistakon. Contact him at firstname.lastname@example.org.