Ocular defence mechanisms

The eye is normally well protected against infections (Fig. 3.18.1). Physical factors such as the blinking action of the eyelid, the tear film and the conjunctiva help to prevent attachment of microorganisms. The outer eye contains antimicrobial compounds produced by the tear film (Fig.3.18.2) and phagocytic by conjunctival epithelial cells and polymorphonuclear leukocytes.

Fig. 3.18.1 Endophthalmitis caused by exogenous or endogenous sources.

Fig. 3.18.2 Antimicrobial components of tears.

A variety of different pathogens affect different anatomical structures, resulting in disease of varying severity, ranging from relative common and less severe (e.g. stye) to permanent loss of visual acuity or even blindness (retinochoroiditis). There are two main routes of infection. The most common is direct inoculation of pathogens into the eye (exogenous infections) through trauma (i.e. surgery, foreign body, contact lenses). The second route is much rarer and introduces pathogens into the posterior part of the eye (retina/choroids) via the bloodstream (endogenous infection) (Fig.3.18.1).

Infections of the external eye structures

Infections of the ocular soft tissue around the eye can present as a painful, red swelling (orbital cellulitis). The infection is rare and may derive from the parasinuses or following haematogenous spread of organisms. Streptococcus pneumoniae, Haemophilus influenzae and Staphylococcus aureus are often the cause and may be recovered from blood cultures; eye swabs are rarely useful. In many cases, empirical antibiotic treatment is initiated prior to any positive culture. If the infection is left untreated, it can progress into serious conditions such as subperiosteal abscess or life-threatening cavernous sinus thrombosis.

The eyelid is a common site of infection, affecting the lid margins (blepharitis) or its sebaceous glands (meibomian glands), causing a painful swelling (stye). Bacteria such as staphylococci are a common cause of this infection.

Conjunctivitis is a relatively common infection caused by bacteria (e.g. Chlamydia sp., S. pneumoniae, H. influenzae) or viruses (e.g. adenovirus, enterovirus). Symptoms are intense hyperaemia of the conjunctival vessels (‘red eye’), excessive discharge and a ‘gritty’ sensation in the eye. The discharge caused by bacterial conjunctivitis is thick and purulent, resulting in a ‘sticky eye’, whereas viral conjunctivitis presents with a watery discharge, often in both eyes. In some generalized viral infections such as measles, conjunctivitis is common. Some viral infections (particularly those caused by adenovirus) spread easily from person to person or through poorly disinfected ophthalmological equipment or shared eye protectors (shipyard eye).

Neonatal conjunctivitis caused by Neisseria gonorrhoeae (ophthalmia neonatorum) and Chlamydia trachomatis serotypes D–K (inclusion conjunctivitis) are serious conditions acquired from the female genital tract during birth. It can progress to keratitis, perforation and blindness. Trachoma is a severe C. trachomatis serotype A–C infection commonly found in the tropics in all age groups. Patients present with chronic inflammation of the conjunctival epithelium, which can lead to intense scarring and blindness. Early treatment with tetracycline in adults or erythromycin in children and pregnancy is essential.