The Staining Grid study measured corneal staining area at baseline, and 2 and 4 hours after lens insertion. These observation points were chosen since the staining in question is likely due to preservative uptake into the lens during overnight soaking and subsequent release from the lens to the cornea upon lens insertion. Staining from this cause is most likely to be observed early in the lens wearing period.

Still unanswered?
A pressing question remained unanswered. How much of this toxicity-induced staining remains later in the day?

What we did?
To investigate this we undertook a randomized, double-masked study in which 26 subjects wore Renu MultiPlus and Purevision, a lens/solution combination previously observed to induce significant corneal staining. The staining area was measured every 2 hours for 12 hours after lens insertion. Purevision soaked in saline was also tested as a control and the same measurements were recorded for an additional 25 subjects.

What we found?
The figure below shows Staining Area (%) versus Time After Lens Insertion for both lens/solution combination tested. The staining area of Renu MultiPlus/Purevision peaked at 2 hours after lens insertion. The staining then consistently decreased as the solution preservative was depleted from the lens and corneal healing began. Approximately 7 hours after lens insertion the staining area crossed 10% (Green Zone limit). The control combination (saline/PureVision) induced minimal staining area (<2%) throughout the 12 hour testing period. Staining area of Renu MultiPlus/Purevision was statistically greater (p<0.05) than the control at each observation time except at baseline (before lenses were inserted) and at 12 hours after lens insertion.

Whats this mean?
This study emphasizes a couple of important facts concerning solution-induced staining.

  1) To effective observe this staining the patient needs to be examined within the first 4 hours after lens insertion. Observations later than this will likely miss solution-induced staining.
  2) If one assumes that clinically significant staining area begins around 10% then the Renu MultiPlus/Purevision combination results in this staining for approximately 50% of the patient's lens wearing day.