Bacterial keratitis in the critically ill and comatose patient

The ocular surface is constantly exposed to a host of environmental pathogens, but infection is the exception rather than the rule. The natural defence mechanisms of the ocular surface are quite adequate to prevent infection. The lids offer mechanical and physical protection. A constant flow of the preocular tear film into the lacrimal sac, aided by the blink reflex, helps in flushing out contaminating microorganisms. Constituents of tears such as polymorphonuclear cells, immunoglobulins (IgA and IgG), complement components, lysozyme, lactoferrin, betalysins, orosomucoid, and caeruloplasmin prevent bacterial adherence and also kill the organisms. Constant evaporation of tears maintains the conjunctival sac at a temperature unfavourable for bacterial proliferation. Conjunctiva-associated lymphoid tissue (CALT) enables the ocular surface to mount an effective cell-mediated immune response. It is therefore uncommon for bacterial keratitis to occur in the absence of predisposing risk factors.