A 32-year-old factory worker presents with ocular chemical injury. On examination, the cornea shows a hazy appearance with loss of epithelium, but the limbal blood vessels remain intact and blanching is not observed. Which feature best distinguishes an alkali burn from an acid burn in this clinical context?

Explanation

## Distinguishing Alkali from Acid Burns ### Mechanism of Injury **Acid burns** cause coagulation necrosis of proteins, creating a protective eschar that limits deeper penetration. The cornea becomes opaque but the injury is often self-limiting. **Alkali burns** cause liquefactive necrosis, saponifying lipids in cell membranes. This allows continued penetration into deeper stromal and scleral tissues, causing progressive damage even after the initial insult. ### Key Discriminating Feature: Limbal Ischemia **Key Point:** The **limbal blood vessel damage and vascular blanching** is the hallmark of alkali injury. Alkali penetrates the cornea and sclera, damaging the delicate limbal vasculature. This leads to: - Vascular blanching (white appearance of limbus) - Progressive ischemia of the cornea and conjunctiva - Delayed epithelialization and scarring - Higher risk of corneal perforation and symblepharon formation In contrast, acid burns typically spare the limbal vessels because the eschar limits penetration. ### Comparison Table | Feature | Acid Burn | Alkali Burn | |---------|-----------|-------------| | **Corneal opacity** | Present (early) | Present (early) | | **Limbal ischemia** | Absent | **Present (key finding)** | | **Vascular blanching** | No | **Yes (pathognomonic)** | | **Depth of penetration** | Superficial | Deep (stromal + scleral) | | **White precipitate** | Yes (acid salt) | No | | **Prognosis** | Better | Worse | **High-Yield:** Limbal vascular damage is the single best clinical discriminator between alkali and acid burns. Its presence indicates alkali injury and predicts a more severe course requiring aggressive management (frequent irrigation, topical steroids, and close follow-up for scarring). ### Clinical Pearl The **Roper Hall classification** for chemical injuries uses limbal ischemia as a key prognostic criterion. Grade III and IV injuries (which carry poor prognosis) are characterized by >50% limbal ischemia, almost always seen in alkali burns. [cite:Parson's Diseases of the Eye 22e Ch 8]