Optometry - Journal of the American Optometric Association
Literature reviewA review of hyaluronan and its ophthalmic applications
Section snippets
Production of hyaluronan
In general, hyaluronic acid refers to the acid form and hyaluronate refers to the salt form. These 2 terms for hyaluronan, however, are often used interchangeably, irrespective of which form is present in a solution. When in the salt form, it is very often sodium hyaluronate.7 Although it can be extracted from bovine vitreous, rooster combs, or umbilical cords, this method is expensive, quantities are limited, and the resulting product can contain protein impurities. HA can also be produced by
Basic physicochemical properties of HA
HA is a high molecular weight, linear, unbranched polymer based on β1-4d-glucuronic acid (G1cA) and β1-3 N-acetyl-d-glucosamine (G1cNAc) in a repeating disaccharide unit.1, 2 HA is a member of a group of similar polysaccharides, which are also known as connective tissue polysaccharides, mucopolysaccharides, or glycosaminogylcans.1, 3, 7 These polysaccharides are known to influence the distribution of water in connective tissues, including the corneal stroma.8 HA is larger than the other
Basic biological functions
HA is found in the human body in the extracellular matrix of connective tissue in the skin, umbilical cord, and synovial fluid.1, 2, 3 In the eye, HA can be found in the vitreous, lacrimal gland, corneal epithelium, and conjunctiva.1, 2, 3, 15, 16 It has also been found in human tear fluid.17, 18, 19 HA is thought to possess anti-inflammatory properties and to play a role in cell functions, such as proliferation, differentiation, and migration, which play a role in inflammatory conditions, such
HA in the ocular anatomy and anterior eye
Frescura et al.18 collected tear samples via capillary tubes and found that the concentration of HA in human tears ranged from 12 to 1,900 ng/mL. They further showed that HA is secreted by conjunctival epithelial cells, which they suggest may be the source of HA in the preocular fluid. Fukuda et al.19 performed a similar study using capillary tubes to collect nonstimulated tears from normal patients and reflex tear samples from patients with corneal disease. They found HA was present in human
Use of HA to treat anterior eye problems
Many studies have been conducted to evaluate the safety and efficacy of HA in the anterior segment (see Table 1).17, 25, 26, 27, 28, 29 One study conducted to evaluate the effects of HA in Japanese white rabbits found no significant changes in general appearance, behavior, body weight, mortality, or food consumption when given drops of 0.125%, 0.25%, or 0.50% sodium hyaluronate or placebo 3 times daily over a 26-week period. Observations of the conjunctiva, cornea, iris, lens, and corneal
Application in contact lenses and contact lens care
Compared with the number of studies evaluating the use of HA as a treatment for dry eye, the literature on its use with contact lenses is relatively sparse. A few studies have reported on the use of HA with contact lenses in various types of applications.4, 14, 31, 32, 33, 34 The effect of sodium hyaluronate on peripheral staining in rigid contact lens wearers was studied in 1995 by Itoi et al.31 They used unpreserved sodium hyaluronate in a double-blind, multicentered study to evaluate 43
Summary
HA is a vital component of ocular physiology. It helps provide the required viscosity of the vitreous humor and hydrates the corneal epithelium. The status of HA as one of the eye’s natural lubricants has been utilized to develop ophthalmic applications, such as a replacement for the natural vitreous; for protection of the corneal endothelium from mechanical trauma during surgery; and to mimic natural tears in dry eye treatment. The possible applications of HA in ophthalmic products has
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Conflict of interest: This manuscript is supported by Bausch & Lomb, Inc.