Hyperosmolarity: Effects on the Ocular Surface

Hyperosmolarity: Effects on the Ocular Surface

Hyperosmolarity I: Effects on the Ocular Surface

Gary N. Foulks, MD, FACS

 As our understanding of dry eye disease has grown more nuanced, we have come to realize that patients’ tears must be adequate both in quantity and quality. To maintain a healthy ocular surface, patients need enough tears to coat the eye, but tears must also have the proper chemical composition in order to nourish and protect ocular surface cells.
   
Hyperosmolarity

As a measure of tear film chemistry, osmolarity can be useful for evaluating the quality of patients’ tears. In general terms, osmolarity describes the quantity of solutes in a solution; in tears, it specifically refers to the concentration of small proteins and electrolytes, including sodium, potassium, and chloride. Although measuring osmolarity does not reveal the exact chemical composition of tears, it quantifies how concentrated they are, and research has shown that knowledge of tear film osmolarity can be clinically valuable for assessing dry eye disease.

While in theory tear film osmolarity could be either too low or too high, it is only the latter condition—termed hyperosmolarity—that is of clinical importance. In fact, because hyperosmolarity can result from either a decrease in tear secretion or an increase in tear evaporation—the two pathways that produce ocular dryness—hyperosmolarity is believed to be a feature common to all cases of dry eye disease.

Effects on the Ocular Surface

Hyperosmolarity is believed to play a central role in the disease process (Figure 1). Among its many effects, hyperosmolarity causes ocular surface discomfort and inflammation, both of which are nearly universal elements in dry eye disease.

In addition, hyperosmolarity causes ocular surface cell damage, which can be visualized by ocular surface staining. This damage occurs because ocular surface cell membranes are permeable; when they are exposed to hyperosmotic tears, water flows out of the cells in an attempt to balance the osmolarity of the intracellular fluid with the osmolarity of the surrounding tears. When this happens, ocular surface cells can become dehydrated, which damages cell membranes and changes the way proteins protect the ocular surface.

Addressing Hyperosmolarity

Because hyperosmolarity plays a central role in dry eye disease, I always consider whether a potential dry eye therapy will help to address this aspect of the patient’s condition. For this reason, I tend not to use punctal occlusion alone because it increases tear volume but does not necessarily improve tear film chemistry. In fact, patients with punctal plugs may have an adequate volume of tears, but in some cases still show signs and symptoms of dry eye disease because the osmolarity of the tears remains damagingly high.

I prefer treatments that address both tear volume and osmolarity, including artificial tears that are hypotonic or contain compatible solute mole­cules that address hyperosmolarity directly. Such treatments help to improve tear film chemistry, resulting in improvement in ocular surface health.

Gary N. Foulks, MD, FACS, is the Arthur and Virginia Keeney  professor of ophthalmology, University of Louisville, Louisville, KY, and is editor-in-chief of The Ocular Surface.