External Eye and Lacrimal Infections

Typical organisms depend on the site of infection:

Infections of the lacrimal apparatus

Dacryoadenitis is often viral in aetiology, although gram-positive cocci (usually staphylococci) are not uncommon.

Dacryocystitis is usually a result of secondary infection, most commonly gram-positive cocci [6] and anaerobes, of an obstructed system caused by:

• Tumour
• Trauma
• Congenital malformation
• As a result of local infection
• Mucus plugging in dry eyes

Infections of the eyelid and associated glands

When infective, blepharitis and hordeolum are almost exclusively caused by staphylococci [7].
Blepharitis is often a non-infective local dermatitis, and can also be caused by viruses (e.g. herpes simplex), mites or lice.

Preseptal cellulitis

The majority of preseptal cellulitis cases are due to bacteria entering after local trauma (e.g. insect bites) of the skin. As such, the most common infecting organisms are skin commensals such as Staphylococcus aureus and Streptococcus pyogenes [8].
Less commonly, preseptal cellulitis may be caused by spread from local infections, including the sinuses. Organisms in these cases are similar to those for orbital cellulitis.

Orbital cellulitis

The most common cause of orbital cellulitis (more than 90% of cases) is direct spread from the ethmoid sinuses.
Other causes are:

• Direct spread from other structures around the orbit
• Local trauma permitting bacterial entry
• Haematogenous spread from distant site of infection

Consequently, the most common causes of infection are respiratory pathogens such as streptococci and Haemophilus influenzae type B (the latter less commonly since the introduction of routine HiB immunisation) [8].

Learning bite

A wide range of gram-positive and negative organisms can cause orbital cellulitis. Consideration of the likely underlying cause and route of spread can help to guide antibiotic choice.