Corneal Physiology and Contact Lens Complications
Oxygen, edema, and the hypoxic cascade
Corneal oxygen demand drives most complications. The Holden–Mertz criteria set the minimum Dk/t to avoid corneal swelling at roughly 24 for daily wear and 34 for overnight wear (×10⁻⁹ units); below these thresholds aerobic metabolism falters, lactate accumulates, and stromal edema follows. Chronic hypoxia produces epithelial microcysts, striae, endothelial polymegethism, limbal hyperemia, and neovascularization - the last is the marker of sustained hypoxic insult and, once vessels invade more than about 1.5 mm, is not reversible.
Mechanical and fit-related problems include 3 and 9 o'clock staining (edge lift and wettability at the horizontal meridian), superior epithelial arcuate lesions from lid interaction with a tight soft lens, mucin balls with silicone hydrogels, and tight-lens syndrome with limbal blanching.
Inflammatory and infectious events must be separated. CLARE is acute, bilateral, and resolves quickly after lens removal. CLPU is a focal infiltrate with overlying epithelial defect, usually sterile. Microbial keratitis - classically Pseudomonas in extended-wear soft lens users - presents with a dense focal infiltrate, overlying defect, pain, and anterior chamber reaction; any infiltrate within the visual axis, a defect larger than 1 mm, or worsening at 24 hours warrants urgent referral.
Rule of thumb: red eye plus contact lens means remove the lens, treat, and never patch. Sterile infiltrates are usually under 1 mm, peripheral, and quiet; ulcers are larger, central, and symptomatic.