Dry Eye Dx and Tx
Implications of the Lacrimal Functional Unit
BY WILLIAM TOWNSEND, OD, FAAO
The April 2014 issue of The Ocular Surface featured an article titled, “Rethinking dry eye disease: a perspective on clinical implications.” (Bron et al, 2014). The authors, who are some of the world’s most respected clinicians and researchers in the field of dry eye disease, stated that their goal was to “provide an update on dry eye for the practicing clinician.”
The article states that “One concept retained from the DEWS report and emphasized here is that of the lacrimal functional unit (LFU) maintains ocular surface homeostasis by regulating tear flow, thus conserving the tear film and corneal transparency.”
Stern et al (1998) described the LFU as consisting of the cornea, conjunctiva, and lacrimal and meibomian glands connected by a neural network and the lacrimal drainage system. The neural response is initiated by activation of the afferent sensory nerves in the cornea and conjunctiva that stimulate efferent parasympathetic and sympathetic nerves that innervate the accessory and main lacrimal glands (Stern et al, 1998)
The Dry Eye WorkShop (DEWS) Report (2007) stressed the importance of the LFU in maintaining and preserving a healthy ocular surface (Stern et al, 2004). Dysfunction of one or more components of the LFU may negatively impact vision and ocular comfort (Dartt, 2009). The LFU model is valid, but fails to address the influence of hormones and other factors that contribute to maintaining the ocular surface (Stern et al, 1998).
One of the most challenging aspects of dry eye practice is assessing for underlying causative conditions, primarily because the etiology is often multifactorial (Bron et al, 2014). The concept of the “functional unit” can be helpful in evaluating dry eye patients. Systematic evaluation of the various components of the LFU provides a diagnostic pathway to aid in determining the most likely cause(s) underlying a patient’s dry eye. It is accepted practice to evaluate lids, tear layer, meibomian glands, and the nasolacrimal system, but easy to ignore the neurologic component of the LFU.
A good example is a diabetic patient who has dry eye. Manaviat et al (2008) reported a 54.3% incidence of dry eye in type 2 diabetics and a strong association between dry eye syndrome and duration of diabetes. Subjective findings in diabetics who have dry eye may be misleading, because subjects with a longer duration of diabetes and higher tear osmolarity report fewer dry eye symptoms (Fuest et al, 2014). This may be explained by progressive loss of corneal sensitivity in diabetics over time (Achtsidis et al, 2014).
The LFU compels clinicians examining dry eye patients to evaluate multiple factors, including reduced corneal sensation. We routinely perform the cotton wisp test, which is used to evaluate for neuropathy (Glaser, 1999) on diabetic patients. A surprising percentage display some degree of neuropathy. I will consider other implications of the LFU concept in my next column. CLS
For references, please visit www.clspectrum.com/references and click on document #239.
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 a consultant or advisor to Alcon, Allergan, NovaBay, TearScience, TearLab, and Science Based Health. Contact him at firstname.lastname@example.org.