Describe chemical and thermal injuries of the eye with their management.

Chemical Injury — Mechanism and Grading

• Chemical injury IS the only true ophthalmic emergency IN which treatment precedes examination. Irrigation begins immediately, before taking a history, before measuring the acuity and before any examination — because the damage continues for as long as the chemical remains in contact
• Alkali injury is far worse than acid, and the reason is chemical: an alkali saponifies the lipids of cell membranes and penetrates rapidly and deeply into the cornea, the anterior chamber and even the ciliary body.
• An acid, by contrast, coagulates and precipitates the epithelial proteins, and that coagulum forms a barrier limiting further penetration — so the injury is more superficial and self-limiting. The exceptions are hydrofluoric acid, which behaves like an alkali and penetrates deeply, and concentrated sulphuric acid, which also burns thermally

Emergency and Definitive Treatment

• 1.
• Irrigate immediately, at the point of injury and before anything else, with whatever clean fluid is to hand — tap water is entirely acceptable and delay is not.
• In hospital use normal saline or Ringer lactate through a giving set, at least 1 to 2 litres over 30 minutes → 2.
• Instil topical anaesthetic first, and use a lid speculum, since blepharospasm otherwise makes adequate irrigation impossible → 3.
• Double-evert the lids and sweep the fornices with a moistened cotton bud to remove particulate matter.
• This step is critical with lime and cement, which lodge as retained particles and continue to release alkali for hours.
• Irrigation without removing particles fails → 4.
• Check the pH with litmus paper in the fornix, and continue irrigating until it is neutral (7.0 to 7.4), then recheck after 20 to 30 minutes, since the pH may drift back up as chemical leaches from the tissues → 5.
• Only now take the history, measure the acuity and examine — assessing limbal ischaemia in clock hours, corneal clarity, the anterior chamber and the intraocular pressure → Do not attempt chemical neutralisation with an acid or alkali; the exothermic reaction causes a thermal burn on top of the chemical one
• Medical treatment has four aims:
• Promote epithelial healing — intensive preservative-free lubricants; ascorbate (vitamin C), topical and oral.
• Control inflammation — topical steroid intensively for the first 7 to 10 days.
• Prevent melting — citrate, which inhibits neutrophil activity, and oral doxycycline, which inhibits matrix metalloproteinases
• Prevent infection and complications — topical antibiotic; a cycloplegic for pain and to prevent synechiae; and pressure-lowering agents, since the pressure rises acutely from shrinkage of the collagen and later from angle damage
• Prevent symblepharon — daily sweeping of the fornices with a glass rod or a lubricated probe, and a symblepharon ring or conformer. This is simple, costs almost nothing, and prevents adhesions that are extremely difficult to correct later
• Surgical measures — early amniotic membrane transplantation.
• Thermal burns — the reflex blink and Bell phenomenon protect the globe, so lid burns are commoner than corneal burns; the threat is cicatricial ectropion and exposure, and early lid care and later grafting are the mainstays