Variations in the osmolarity of topical eye drops may influence clinical performance, and this trait warrants greater attention, reports a study published in the May 2015 Cornea. Documenting a range of hypo- and hyperosmolarity levels in 87 prescription and OTC drops, the researchers call the effort “a first step in understanding the influence of osmolarity of ophthalmic formulations on the ocular surface.”1 Hyperosmolar eye drops could have therapeutic efficacy in treating corneal edema, they note.

Researchers in Germany evaluated the impact of hypo­osmotic and hyperosmotic conditions on ex vivo corneal thickness and integrity (specifically, glucose and lactate levels) in a rabbit model of induced corneal edema by filling the anterior chamber with a hypoosmolar solution. After 48 hours, two hyperosmolar solutions—Omnisorb (preserved with BAK) and Ocusaline (preservative-free)—were topically applied every 15 minutes over the course of one hour. 

Both “significantly reduced corneal swelling” on OCT, the authors found. Omnisorb reduced corneal thickness by 279μm vs. 258μm for Ocusaline. The authors say that this suggests eye drops containing preservatives may be more beneficial than preservative-free solutions in some cases, at least initially. There was no significant difference in treatment effect at 72 hours.

In the second part of the study, researchers categorized the osmolarity of 87 commercially available eye drops using freezing point osmometry. Forty-three showed an osmolarity value below a physiological tear osmolarity value of 289mOsm/L, while the other 44 demonstrated a higher osmolarity value. Overall, however, the majority of the products fell within a limited range around isoosmolarity, suggesting further experiments with application on a corneal model are necessary. 

Regardless, “it is important to keep in mind that the osmolarity of eye drops is not an isolated factor; instead, osmolarity is relevant concerning various product ingredients,” the researchers say. “Another factor is the viscosity of eye drops that prolongs their retention time on the ocular surface. This might alter the osmotic effects of eye drops as well.”  

1. Dutescu RM, Panfil C, Schrage N. Osmolarity of prevalent eye drops, side effects, and therapeutic approaches. Cornea. 2015 May;34(5):560-566.