A 28-year-old construction worker sustains a severe alkali burn (calcium hydroxide) to the left eye. After immediate and prolonged irrigation with normal saline, the cornea shows significant opacification and the limbus is blanched. To chelate the residual alkali and prevent further penetration, which agent should be used as an adjunctive topical treatment?

## Chelation Therapy in Alkali Chemical Burns ### Why Alkali Burns Are More Severe **Key Point:** Alkali (especially calcium hydroxide and ammonia) causes saponification of lipids in cell membranes, leading to deeper penetration and more extensive tissue damage compared to acid burns. **High-Yield:** Alkali burns require **two-phase management**: (1) immediate irrigation to remove alkali, and (2) chelation therapy to bind residual cations (Ca²⁺, Mg²⁺) that continue to cause damage. ### Citric Acid as Chelating Agent **Mechanism:** 1. Citric acid is a tricarboxylic acid with three carboxyl groups 2. These groups form stable chelation complexes with divalent cations (Ca²⁺, Mg²⁺, Al³⁺) 3. Chelation prevents these cations from participating in further saponification and tissue damage 4. The citrate-cation complex is water-soluble and can be irrigated away ### Dosing and Administration - **Concentration:** 1% citric acid solution (or EDTA 0.5% as alternative) - **Application method:** Topical instillation or soaked pledgets applied to the ocular surface - **Duration:** 10–15 minutes of continuous application or repeated instillations - **Timing:** Should be started **immediately after initial saline irrigation** if alkali burn is suspected ### Comparison of Chelating Agents in Alkali Burns | Agent | Mechanism | Efficacy | Clinical Use | |-------|-----------|----------|---------------| | **Citric acid 1%** | Chelates Ca²⁺, Mg²⁺, Al³⁺ | High | First-line chelating agent | | **EDTA 0.5%** | Chelates divalent cations | High | Alternative to citric acid | | **Sodium bicarbonate 5%** | Neutralizes residual acid (not alkali) | Low for alkali | Used in acid burns, not alkali | | **Hydrogen peroxide** | Oxidizing agent, no chelation | None | Contraindicated; may worsen damage | | **Povidone-iodine** | Antimicrobial only | None | Adjunctive for infection prevention only | **Clinical Pearl:** The **limbal blanching** (whitening) observed in this case indicates severe alkali penetration with vascular thrombosis; this is a sign of full-thickness chemical burn and requires aggressive chelation therapy to prevent total corneal opacification and blindness. ### Post-Chelation Management **Key Point:** After chelation therapy, ascorbic acid (vitamin C) is initiated to prevent scarring, as described in the previous question. **Mnemonic:** **CACE** — Chelation, Ascorbic acid, Cycloplegics, Epithelialization support ### Why Citric Acid Is Superior to Alternatives - **vs. Sodium bicarbonate:** Bicarbonate is used for acid neutralization, not alkali chelation; it does not bind divalent cations effectively - **vs. Hydrogen peroxide:** Oxidizing agents can exacerbate tissue damage in chemical burns - **vs. Povidone-iodine:** Iodine is an antimicrobial, not a chelating agent; it does not prevent alkali penetration **Warning:** Delaying chelation therapy in alkali burns leads to irreversible corneal opacification and permanent vision loss. Citric acid must be applied within the first few hours of injury.