Dry Eye Dx and Tx
One Eye Dry? Managing Unilateral Dry Eye, Part 1
By William Townsend, OD, FAAO
You might assume that when we note dryness in one eye, the fellow eye suffers the same fate. While this is often the case, we do encounter individuals who have monocular or asymmetric ocular dryness. This phenomenon is not confined to humans—Jianping et al (1998) reported monocular dryness in dogs. Monocular dryness might result from numerous conditions, some of which are pathological and have potential for sequelae. So when we address a case of unilateral “dry” eye, it is essential that we obtain thorough ocular, systemic, and social case histories.
Lacrimal Gland Innervation
To initiate the diagnostic workup for unilateral dry eye, we must understand innervation of the lacrimal glands. The parasympathetic nerve supply originates from the lacrimal nucleus of the facial nerve. These fibers branch off from the greater petrosal nerve and end in the pterygopalatine ganglion, where they synapse. Postganglionic fibers join the fibers of the maxillary nerve, branch off with the zygomatic nerve, and then branch off again to the lacrimal branch of the ophthalmic division of the facial nerve before reaching the lacrimal glands. This long and tortuous route is susceptible to trauma, tumors, and inflammatory disease. Any process that interrupts innervation may reduce lacrimation (Kosins et al, 2007).
In a rabbit study, Toshida et al (2007) reported that preganglionic denervation of the greater superficial petrosal nerve resulted in rose bengal staining of the conjunctiva, corneal fluorescein staining, increased blink rate, decreased tear film breakup time, decreased goblet cell density, and a 26-percent reduction in tear flow. This validates the principle that any condition disrupting parasympathetic innervation of the lacrimal glands will produce dryness. That interruption may occur at some distance from the glands.
Effects of Bell's Palsy
Bell's palsy is an acute peripheral-nerve palsy affecting the facial nerve (V), which supplies the muscles of facial expression. The obvious effects of Bell's palsy are easily observed and include unilateral weakness or complete paralysis of all the facial muscles. Paralysis of the orbicularis results in incomplete closure of the lids with exposure keratitis.
But less obvious is the fact that effects of this condition also impair tear secretion. Because the facial nerve also supplies parasympathetic fibers to the lacrimal and salivary glands, Bell's palsy can produce unilateral dryness. This may, in fact, be obscured by excessive tearing resulting from lid laxity and subsequent loss of apposition of the puncta to the globe (Tiemstra and Khatkhate, 2007).
More to Come
Asymmetric dry eye is a challenging and complex condition caused by neurologic and anatomic pathology. We will continue to explore this subject in my next column. CLS
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #198.
|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, Tear-lab Corporation, and Vistakon. Contact him at email@example.com.|