contact lens care
New Rewetting Drops Improve Patient Comfort
BY MICHAEL A. WARD, MMSC, FAAO

Contact lens rewetting drops that contain sodium hyaluronate are poised to become major players in our armamentarium to combat lens-related dryness and contact lens drop out.

Figure 1a. HA viscoelastic properties during blinking: HA chains stretch in near parallel, linear arrangements.
Figure 1b. Fully relaxed, HA chains form an entangled meshwork between blinks.

Sodium hyaluronate is the sodium salt of hyaluronic acid (HA), which is a naturally occurring disaccaride biopolymer. HA is a structural polysaccharide composed of repeating units of N-acetylglucosamine and glucuronic acid. It's available in a wide range of molecular sizes and weights, and it has considerable viscoelastic properties and a high capacity to imbibe water (HA can bind up to 1000 times its weight in water). Sodium hyaluronate solutions strongly adhere to the mucin layer of the tear film, creating a persistent water-retaining layer that resists evaporation.

HA has unique viscoelastic properties: It thins during blinking and thickens between blinks. During blinks, shear stresses cause the HA molecules to align with one another, allowing a smooth spreading over the lens and ocular surface (Figure 1a). Between blinks HA thickens, forming a disarrayed meshwork that resists drainage and thereby increases tear retention and stability of the tear film (Figure 1b).

Origins and Use

Sodium hyaluronate is found in human synovial fluid of knee joints, the vitreous humor of the eye, cartilage, blood vessels, extracellular matrix and skin. HA naturally binds to receptors in the corneal endothelium.

Surgeons routinely use HA viscoelastic gels during intraocular surgeries to protect the corneal endothelium and other ocular structures. Physicians use injectable HA to treat joint conditions. It's also available as an oral nutritional supplement.

HA is the key water-retaining substance in human skin. Loss of HA and other polysaccharides results in skin sensitivity. Manufacturers use HA in lip balms and facial creams to form an air permeable, lubricating layer that keeps skin moist and soft.

HA in Rewetting Drops

Two lens rewetting drops currently contain HA: Aquify (CIBA Vision) and blink Contacts (Advanced Medical Optics) (Table 1). Both use dissipating preservative technologies to prevent toxicities.

HA contact lens rewetting technology promises to enhance lens wearing comfort by treating/preventing lens dryness. This welcome technology may curtail contact lens drop outs.

Mr. Ward is an instructor in ophthalmology at Emory University School of Medicine and Director, Emory Contact Lens Service.