Contact Lens Care & Compliance
Saline Is Not So Simple
BY MICHAEL A. WARD, MMSC, FAAO
Saline is a mixture of salt and water. Specifically, it is a salt (an ionic compound, usually a solid) disassociated in water, yielding a neutral charge. Saline solutions can be isotonic, hypertonic, or hypotonic. Normal saline (NS) solutions are typically made of sodium (metal cation) and chloride (anion) in a 0.9% w/v NaCl solution in water. With a pressure of approximately 310 mOsm, they are isosmotic with body fluids and the ocular surface.
Background of Saline Use
NS is the perfect solution for rinsing contact lenses. Historically, saline solutions have been used to rinse cleaners and disinfectants from lens surfaces or for lens rinsing and storage in conjunction with heat disinfection. Although heat disinfection has not been promoted for more than 20 years, commercial saline products may still be labeled for lens storage.
Some saline solutions list that they are indicated "for use following chemical disinfection and prior to lens insertion." This statement is a carryover from the 1980s before multipurpose disinfecting solutions (MPDSs) were available. Current-generation MPDSs are a much safer option for hydrogel lens storage.
Saline solutions for contact lens use may be preserved or nonpreserved, aerosol, multi-dose, or unit-dose. Multi-dose, nonpreserved saline products (e.g., Unisol 4 [Alcon] and PuriLens [The LifeStyle Company]) are borate-buffered. This may lead to corneal irritation, and the multi-use containers are prone to contamination, so we do not recommend their use with scleral lenses or for routine contact lens storage.
Aerosol salines remain sterile until sprayed from the pressurized can. Aerosol salines are a good choice for rinsing, but are not a good choice for scleral lens use because the nitrogen gas propellant may form bubbles under the lens as the gas escapes solution. I prefer nonpreserved, unitdose, sterile saline (e.g., Addipak [Teleflex Medical]) for scleral lens use. However, it is not labeled for ophthalmic use and is therefore considered off-label.
Multi-use saline solutions are commonly preserved with sorbic acid or a biguanide as their primary antimicrobial agents. Sorbic acid at 0.1% is a relatively weak, static inhibitor of microbial growth. The polyaminopropyl biguanide found in some saline solutions is a stronger antimicrobial, typically in low concentration at 0.00003%. The chelant ethylenediaminetetraacetic acid (EDTA) may be added to further enhance antimicrobial activity. The preservatives in salines are there only to preserve the solution; they are not strong enough to disinfect contact lenses.
Multi-Use Saline Concerns
Preserved salines are associated with a greater risk of chemical sensitivities, and nonpreserved salines have a greater risk of microbial contamination. Studies have reported on contamination of salines during normal use (Sweeney et al, 1992; Sweeney et al, 1999). They reported that unpreserved (multi-dose) saline bottles become heavily contaminated with Gram-negative bacteria within one week of patient use. Commercial multi-use preserved salines also became quickly contaminated during normal use. Preserved saline solutions are more likely to harbor Gram-positive bacteria, which may reflect contamination from patients' skin or ocular microbiota.
Choosing the correct saline is dependent on its intended use. Preserved salines in multi-use bottles are good for rinsing cleaners or disinfectants from lens surfaces. Unit-dose, nonpreserved saline is the best choice for scleral lens use. Saline does not disinfect contact lenses and should not be used for lens storage. CLS
For references, please visit www. clspectrum.com/references.asp and click on document #217.
Mr. Ward is an instructor in ophthalmology at Emory University School of Medicine and Director, Emory Contact Lens Service. You can reach him at email@example.com.