Why was thimerosal removed from contact lens solutions?

Thimerosal, a mercury-based preservative, caused a high rate of allergic and toxic reactions in contact lens wearers, including conjunctival inflammation, corneal infiltrates, and contact dermatitis. Due to these widespread adverse effects, it has been largely eliminated from modern lens care products.

How can you tell if a patient has a solution sensitivity?

Look for persistent redness, discomfort, and corneal staining that correlates with solution use rather than lens wear time alone. The symptoms typically improve when the patient discontinues the solution. Toxic reactions tend to be dose-dependent and present quickly, while allergic reactions may develop after months of use and involve itching and papillary changes.

What is the best alternative for a patient with preservative sensitivity?

Hydrogen peroxide disinfection systems are the primary alternative. After the neutralization process is complete, the solution contacting the eye is preservative-free saline, eliminating preservative exposure entirely. This makes hydrogen peroxide systems ideal for patients who react to preserved multipurpose solutions.

Contact Lens Solution Incompatibilities: Preservatives, Reactions, and Material Interactions

Understanding Solution Incompatibilities

Contact lens solutions contain a complex mixture of surfactants, preservatives, buffering agents, and tonicity adjusters. When these chemical components interact poorly with a patient's ocular tissues, their lens material, or each other, solution incompatibilities occur. Recognizing and managing these reactions is essential for contact lens practitioners because they can mimic infection symptoms, cause patient dropout, and sometimes lead to serious corneal complications.

Preservative Sensitivities

Preservatives are necessary to prevent microbial contamination of lens care solutions, but they are also the most common source of adverse reactions.

Thimerosal

Thimerosal is a mercury-based preservative that was widely used in contact lens solutions before the 1990s. It caused significant allergic and toxic reactions in a large percentage of contact lens wearers, including:

Superior tarsal conjunctival inflammation
Contact dermatitis of the eyelids
Corneal infiltrates
Chronic conjunctival hyperemia

Due to these issues, thimerosal has been largely eliminated from modern contact lens solutions. However, it remains a classic example of preservative incompatibility that appears frequently on certification exams.

Polyhexamethylene Biguanide (PHMB)

PHMB (also called polyhexanide) is used at very low concentrations in many multipurpose solutions. While generally well-tolerated, some patients develop a solution sensitivity characterized by:

Diffuse corneal staining (often in a specific pattern)
Conjunctival redness
Lens discomfort that worsens throughout the day

Polyquaternium-1 (Polyquad)

Polyquad is a large-molecule preservative designed to be less likely to penetrate the corneal epithelium. It generally causes fewer sensitivity reactions than PHMB, though isolated cases do occur.

When a patient presents with persistent redness, discomfort, and corneal staining that correlates with solution use, suspect a preservative sensitivity. The first step is often switching to a preservative-free system such as hydrogen peroxide.

Solution-Material Interactions

The lens material itself plays a role in solution compatibility. Contact lens polymers can absorb and later release solution components, creating delayed reactions:

Silicone hydrogel lenses may absorb certain preservatives differently than conventional hydrogel materials due to their biphasic (silicone and hydrogel) structure
High-water-content lenses absorb more solution into their matrix, potentially concentrating preservatives against the corneal surface
Ionic lenses may bind with certain solution components through electrostatic interactions

This is why lens manufacturers sometimes recommend specific care systems for their products. A solution that works well with one lens brand may cause problems with another.

Mixing Solutions

Using multiple solution brands simultaneously or switching without proper transition creates additional risks:

Chemical interactions: Different preservative systems may react when mixed, producing byproducts that irritate the eye
pH mismatches: Solutions are buffered to specific pH ranges. Mixing can shift the pH enough to cause stinging or epithelial disruption
Residual contamination: Old solution remaining in the case when new solution is added dilutes the disinfecting concentration

Topping off the lens case with fresh solution instead of emptying, rinsing, and refilling. This practice dilutes the disinfectant concentration and can create a chemical environment different from what the manufacturer intended.

Recognizing Solution Reactions

Solution incompatibilities can present as toxic or allergic reactions, and distinguishing between them guides management:

Toxic Reactions

Dose-dependent (more exposure equals worse reaction)
Occur relatively quickly after lens insertion
Diffuse corneal staining, often in a characteristic pattern
Usually resolve when the offending solution is discontinued

Allergic Reactions

Not dose-dependent (even small amounts trigger a response)
May develop after weeks or months of uneventful use
Involve itching, papillary reaction on the tarsal conjunctiva
Require complete elimination of the allergen

Management Strategies

When solution incompatibility is suspected:

Switch to hydrogen peroxide: Preservative-free after neutralization, hydrogen peroxide systems eliminate preservative exposure entirely
Change solution brand: If hydrogen peroxide is not practical, switch to a solution with a different preservative system
Review lens material: Consider whether a different lens material might interact better with the patient's current or new solution
Educate on proper use: Reinforce the importance of not mixing solutions, replacing cases regularly, and rubbing lenses even with "no-rub" solutions

When switching a patient's solution, have them replace their lens case at the same time. Residual old solution in the case can prolong the incompatibility reaction even after the bottle is changed.

Key Takeaways

Thimerosal is a historic preservative that caused widespread allergic reactions and is now rarely used
PHMB and Polyquad are modern preservatives with different sensitivity profiles
Lens material affects how preservatives are absorbed and released
Mixing solutions or topping off cases creates chemical incompatibilities
Toxic reactions are dose-dependent; allergic reactions are not
Hydrogen peroxide systems are the primary alternative for preservative-sensitive patients