Contact lenses are medical devices requiring meticulous care, and the solution serves a precise, multi-faceted role in their maintenance. The fluid is engineered to clean the lens surface, disinfect against harmful microorganisms, and maintain hydration so the lens remains flexible and comfortable. When lenses are left soaking in the same solution for an extended period, these protective properties degrade, transforming a sterile environment into a potential hazard. This lapse in care compromises both the physical integrity of the lens and the health of the eye.
How the Solution Deteriorates Over Time
Contact lens solutions contain active chemical disinfectants, such as polyquaternium or polyaminopropyl biguanide, designed to neutralize microbes within a specified contact time. When the solution remains stagnant, the concentration of these agents drops. Disinfectants can lose their chemical potency over a prolonged period, or they can be absorbed into the lens material, which depletes the concentration in the surrounding fluid.
Even in a seemingly tightly sealed case, evaporation of the aqueous components of the solution can occur over time. Since the salts and other solutes do not evaporate, this loss of water increases the concentration of these remaining substances, leading to hypertonicity. A hypertonic solution is chemically unbalanced and can draw water out of the lens. When placed on the eye, this concentrated fluid can cause significant irritation and discomfort to the ocular surface. The diminished cleaning and hydrating properties mean that the lens is no longer adequately protected or prepared for safe wear.
The Danger of Microbial Proliferation
The most significant risk of prolonged storage is microbial growth, turning the lens case into a biofilm-rich breeding ground. Once disinfectants lose effectiveness, the stagnant solution allows bacteria, fungi, and, most concerningly, the parasitic amoeba Acanthamoeba to multiply. Pseudomonas aeruginosa is a common bacterium known for adhering to lens surfaces and causing severe eye infections.
The water-borne parasite Acanthamoeba is particularly dangerous because its cyst form is highly resistant to many multipurpose disinfecting solutions, often only being effectively killed by hydrogen peroxide systems. If the disinfectant is weakened, the parasite can survive and multiply on the lens. Wearing a contaminated lens can lead to serious conditions like microbial keratitis or Acanthamoeba keratitis. These infections involve inflammation and ulceration of the cornea, causing extreme pain, blurred vision, and potentially resulting in permanent vision loss or the need for a corneal transplant.
Physical Effects on the Contact Lens Material
Beyond the microbial hazard, a contact lens left in old solution is subject to physical damage that alters its performance and comfort. If the solution evaporates significantly, the lens can dry out, causing the soft polymer material to lose water and become brittle. This dehydration can lead to warping, permanently changing its curvature.
A warped lens will not fit the eye correctly, causing poor vision correction and mechanical irritation to the cornea. Furthermore, the solution stops actively removing tear components that deposit on the lens surface during wear, such as proteins, lipids, and calcium. These deposits build up, leading to a cloudy appearance and reduced surface wettability, which decreases comfort. This debris also reduces the oxygen transmissibility of the lens (Dk/t value). A damaged lens with a compromised Dk/t value can starve the cornea of oxygen, leading to complications like corneal swelling or neovascularization.
Safe Storage Protocols and Remediation
The standard safe storage period in a multipurpose solution is up to 30 days, though experts recommend changing the fluid weekly to maintain sterility. Hydrogen peroxide solutions often have a shorter maximum storage time, sometimes seven days, before a fresh disinfection cycle is required. Always consult the specific manufacturer instructions, as guidelines vary between brands.
For lenses left in stagnant solution for a short period past the recommended limit, a simple remediation strategy can be employed. The old solution must be discarded, the lenses gently rubbed and rinsed with fresh solution, and then soaked in new disinfectant for a minimum four-to-six-hour disinfection cycle before wear. If the lenses have been neglected for weeks or months, or appear dry, warped, or cloudy, the risk of irreparable damage or deep contamination is too high. In these cases, the safest course of action is to discard the lenses and replace them with a new pair.