THE OCULAR SURFACE / OCTOBER 2003, VOL. 1, NO. 4 180 Clinical Science GARY N. FOULKS, MD, SECTION EDITOR ABSTRACT The human efferent tear ducts are part of the lac- rimal system. Because little knowledge exists concerning the physiology of the nasolacrimal system, and hence its patho- physiology, the nasolacrimal system has received almost no consideration as a possible factor in dry eye. The human naso- lacrimal ducts consist of the upper and the lower lacrimal canali- culus, the lacrimal sac, and the nasolacrimal duct. As a drain- ing and secretory system, the efferent tear ducts play a role in tear transport and nonspecific immune defense. Moreover, com- ponents of tear fluid are absorbed in the nasolacrimal passage and are transported into a surrounding vascular system. This system is similar to a cavernous body that is subject to auto- nomic control and regulates tear outflow. Tear duct-associated lymphoid tissue (TALT) is present in the efferent tear ducts, displaying the cytomorphological and immunophenotypic fea- tures of mucosa-associated lymphoid tissue (MALT). Under normal conditions, tear fluid components are constantly ab- sorbed into the blood vessels of the surrounding cavernous body. These vessels are connected to the blood vessels of the outer eye and could act as a feedback signal for tear fluid pro- duction, which ceases if these tear components are not ab- sorbed. In this way, dry eye could be initiated. Defective stimu- lation of TALT could result in abnormal immune deviation at the ocular surface, leading to an autoimmunological response that causes dry eye pathology. KEYWORDS dacryocystitis, dacryostenosis, efferent tear ducts, immune privilege, keratoconjunctivitis sicca, MALT, mucin, nasolacrimal ducts, tear drainage, tear fluid absorption I. INTRODUCTION ccording to a recent classification, dry eye (also termed sicca syndrome or keratoconjunctivitis sicca) is defined as a disorder of the tear film due to tear deficiency or excessive evaporation that causes dam- age to the interpalpebral ocular surface and is associated with symptoms of discomfort. 1 This common definition includes all forms of the sicca syndrome, both idiopathic forms and those associated with systemic disease (like Sjogrens syndrome). Keratoconjunctivitis sicca has demon- strably increased in Germany, as it has in many industrial nations, to become one of the most common ocular diseases. Although great advances have been made over the past 25 years in the knowledge of the pathogenesis, diagnosis, and treatment of dry eye and tear film disorders, many questions remain unanswered. One such question regards the pos- sible role of the nasolacrimal ducts in dry eye. II. ANATOMY OF THE EFFERENT TEAR DUCTS Tear fluid is drained by the nasolacrimal ducts (Figure 1A) into the inferior meatus of the nose. The lacrimal pas- sages consist of a bony passage and a membranous lacrimal passage. 2 The bony passage is formed anteriorly by the fron- tal process of the maxilla and posteriorly by the lacrimal bone. The membranous lacrimal passages include the lacrimal canaliculi, the lacrimal sac, and the nasolacrimal duct. The upper and lower canaliculi are lined by pseudo- stratified and stratified columnar epithelium and are sur- rounded by a dense ring of connective tissue, as well as by muscle fibers of the lacrimal portion of the orbicularis oculi muscle (Horner’s muscle). The lacrimal sac and the nasolac- rimal duct are lined by a double-layered epithelium com- posed of a superficial columnar layer and a deep, flattened layer of basal cells (Figure 1B). 2,3 Both layers sometimes ap- pear as a pseudostratified epithelium. Radnot and Bölcs 4 and Radnot 5 described kinociliae (motile ciliae) as a common find- ing of some epithelial cells in the nasolacrimal ducts. How- ever, most epithelial cells are lined by microvilli (Figure 1C). 6 Accepted for publication August 2003 From the 1 Institute of Anatomy and 3 Department of Ophthalmology, Chris- tian Albrecht Universität of Kiel, Kiel, Germany, 2 Department of Ophthal- mology, University Hospital Hamburg-Eppendorf, Hamburg, Germany Support by DFG Grant PA 738/1-4, Sicca Forschungsförderung Single copy reprint requests to: PD Dr. med. Friedrich P. Paulsen, Institute of Anatomy, Christian Albrecht University of Kiel, Olshausenstr. 40, 24098 Kiel, Germany, Tel.: +49 431 880 2597, Fax.: +49 431 880 1557, email: fpaulsen@anat.uni-kiel.de The authors have no financial interest in any concept or product mentioned in this article. Abbreviations are printed in boldface where they first appear with their definitions. ©2003 Ethis Communications, Inc. All rights reserved. Drainage of Tears: Impact on the Ocular Surface and Lacrimal System FRIEDRICH P. PAULSEN, MD, PHD, 1 ULRICH SCHAUDIG, MD, 2 ANDREAS B. THALE, MD 3 A