The external structures of the eye—eyelids, conjunctiva, sclera, and cornea—are depicted in Figure 72-1. The eyeball comprises three layers. From the outside in, these tissues are the sclera, choroid, and retina. The sclera is a tough, white, fibrous tissue (i.e., “white” of the eye). The anterior (toward the front) portion of the sclera is the cornea, which is transparent and has no blood vessels. A mucous membrane, called the conjunctiva, lines each eyelid and extends onto the surface of the eye itself.

Figure 72-1 Key anatomic structures of the eye. (Modified from Thibodeau GA, Patton KT: *Anatomy and physiology*, ed 2, St Louis, 1993, Mosby.)

Only a small portion of the eye is exposed to the environment; about five sixths of the eyeball is enclosed within bony orbits shaped like four-sided pyramids. The large interior space of the eyeball is divided into two sections: the anterior and posterior cavities (see Figure 72-1). The anterior cavity is filled with a clear and watery substance called aqueous humor; the posterior cavity is filled with a soft, gelatin-like substance called vitreous humor.

Infections can occur in the eye’s lacrimal (pertaining to tears) system. The major components of the lacrimal apparatus include the lacrimal gland, lacrimal canaliculi (short channel), and lacrimal sac.

Resident Microbial Flora

Rather sparse indigenous flora is present in the conjunctiva sac. Staphylococcus epidermidis and Lactobacillus spp. are the most frequently encountered organisms; Propionibacterium acnes may also be present. Staphylococcus aureus is found in less than 30% of individuals, and Haemophilus influenzae colonizes 0.4% to 25%. Moraxella catarrhalis, various Enterobacteriaceae, and various streptococci (Streptococcus pyogenes, Streptococcus pneumoniae, other alpha-hemolytic and gamma-hemolytic forms) are found in a very small percentage of individuals.

Diseases

The eye and its associated structures are uniquely predisposed to infection by various microorganisms. The major infections of the eye are listed in Table 72-1 along with a brief description of the disease.

TABLE 72-1

Infections of the Eye

Infection	Description	Bacteria	Viruses	Fungi	Parasites
Blepharitis	Inflammation of the margins (edges) of the eyelids; (eyelids, eye lashes or associated pilosebaceous glands or meibomiam glawds) symptoms include irritation, redness, burning sensation, and occasional itching. Condition is typically bilateral	Staphylococcus aureus	Herpes simplex virus	Staphylococcues epidermidrs Malassezia furfar	Phthirus pulis
Conjunctivitis	Inflammation of the conjunctiva; symptoms vary according to the etiologic agent, but most patients have swelling of the conjunctiva, inflammatory exudates, and burning and itching	Streptococcus pneumoniae, Haemophilus influenzae, S. aureus, Haemophilus spp., Chlamydia trachomatis, Neisseria gonorrhoeae, Streptococcus pyogenes, Moraxella spp., Corynebacterium spp.	Adenoviruses, herpes simplex (HSV), varicella zoster. Epstein-Barr virus (EBV) influeza vius, pararryxovirus, rubella, HIV enterovirus, coxscukie A
Keratitis	Inflammation of the cornea; although there are no specific clinical signs to confirm infection, most patients complain of pain and usually some decrease in vision, with or without discharge from the eye	S. aureus S. pneumoniae, Pseudomonas, aeruginosa Moraxella lacunata, Bacillus spp.	HSV, adenoviruses, varicella zoster	Fusarium solani, Aspergillus spp., Candida spp., Acremonium, Curvularia	Acanthamoeba spp.
Keratoconjunctivitis	Infection involving both the conjunctiva and cornea; ophthalmia neonatorum is an acute conjunctivitis or keratoconjunctivitis of the newborn caused by either N. gonorrhoeae or C. trachomatis	Refer to agents for keratitis/conjunctivitis	Refer to agents for keratitis/conjunctivitis	Refer to agents for keratitis	Toxoplasma gondii, Toxocara
Chorioretinitis and uveitis	Inflammation of the retina and underlying choroid or the uvea; infection can result in loss of vision	Mycobacterium tuberculosis Treponema pallidum, Borrelia burgdorferi	Cytomegalovirus, HSV	Candida spp.	Toxoplasma gondii, Toxocara Treponema pallidum Brucella spp.
Endophthalmitis	Infection of the aqueous or vitreous humor. This infection is usually caused by bacteria or fungi, is rare, develops suddenly and progresses rapidly, often leading to blindness. Pain, especially while moving the eye, and decreased vision, are prominent features.	S. aureus, S. epidermidis, S. pneumoniae, other streptococcal spp., P. aeruginosa, other gram-negative organisms, Nocardia spp.	HSV Varicella zoster	Candida spp., Aspergillus spp., Volutella spp., Acremonium spp.	Toxocara, Onchocerca volvulus
Lacrimal infections, canaliculitis	A rare, chronic inflammation of the lacrimal canals in which the eyelid swells and there is a thick, mucopurulent discharge	Actinomyces, Propionibacterium propionicum
Dacryocystis	Inflammation of the lacrimal sac that is accompanied by pain, swelling, and tenderness of the soft tissue in the medial canthal region	S. pneumoniae, S. aureus, S. pyogenes, Haemophilus influenzae		C. albicans, Aspergillus spp.
Dacryoadenitis	Acute infection of the lacrimal gland; these infections are rare and can be accompanied by pain, redness, and swelling of the upper eyelid, conjunctiva discharge	S. pneumoniae, S. aureus, S. pyogenes

Pathogenesis

The eye has a number of defense mechanisms. The eyelashes prevent entry of foreign material into the eye. The lids blink 15 to 20 times per minute, during which time secretions of the lacrimal glands and goblet cells wash away bacteria and foreign matter. Lysozyme and immunoglobulin A (IgA) are secreted locally and serve as part of the eye’s natural defense mechanisms. Also, the eyes themselves are enclosed within the bony orbits. The delicate intraocular structures are enveloped in a tough collagenous coat (sclera and cornea). If these barriers are broken by a penetrating injury or ulceration, infection may occur. Infection can also reach the eye via the bloodstream from another site of infection. Finally, because three of the four walls of the orbit are contiguous with the paranasal (facial) sinuses, sinus infections may extend directly to the periocular orbital structures.

Epidemiology and Etiology of Disease

Blepharitis

Blepharitis is a bump that appears on the eyelid that is red, swollen, and resembles a pimple. Most bumps on the eyelid are caused by an inflamed oil gland on the edge of the eyelid commonly referred to as a stye. Bacteria, viruses, and, occasionally, lice can cause blepharitis, an infection of the eyelid surrounding the eye. Although occasionally isolated from surfaces surrounding the healthy eye, Staphylococcus aureus and S. epidermidis are the most common infectious agents associated with blepharitis in developed countries. Symptoms include burning, itching, the sensation of the presence of a foreign body, and crusting of the eyelids.

Viruses can also cause a vesicular (blister-like) eruption of the eyelids. Herpes simplex virus (HSV) produces vesicles on the eyelids that typically crust and heal with scarring within 2 weeks. Unfortunately, once this vesicular stage has resolved, the lesions can be confused with bacterial blepharitis.

Finally, the pubic louse Phthirus pubis has a predilection for eyelash hair. The presence of this organism produces irritation, itch, and swelling of the lid margins (edges).

Conjunctivitis

Bacterial conjunctivitis, commonly referred to as “pink eye,” is the most common type of ocular infection and may be caused by allergies or bacterial or viral infection. The principal causes of acute conjunctivitis in the normal host are listed in Table 72-1. Age-related factors are key in the identification of the etiologic agent. In neonates, neisserial and chlamydial infections are frequent and are acquired during passage through an infected vaginal canal. With the common practice of instilling antibiotic drops into the eyes of newborns in the United States, the incidence of gonococcal and chlamydial conjunctivitis has dropped dramatically. However, Chlamydia trachomatis remains responsible for one of the most important types of conjunctivitis, trachoma, one of the leading causes of blindness in the world, primarily in underdeveloped countries.

In children the most common causes of bacterial conjunctivitis includes Haemophilus influenzae, S. pneumoniae, and perhaps S. aureus. S. pneumoniae and H. aegyptius have been isolated from conjunctivitis epidemics. Inflammation of the conjunctiva is characterized by redness, itching, and discharge, and the condition is highly contagious; it can be transferred from one eye to the other by rubbing the infected eye and can easily be transferred to other individuals.

Numerous other bacteria may also cause conjunctivitis. For example, diphtheritic conjunctivitis may occur in conjunction with diphtheria elsewhere in the body. Moraxella lacunata produces a localized conjunctivitis with little discharge from the eye. Distinctive clinical pictures may also occur with conjunctivitis caused by Mycobacterium tuberculosis, Francisella tularensis, Treponema pallidum, and Yersinia enterocolitica.

Fungi may be responsible for this type of infection as well, often in association with a foreign body that has been introduced into the eye or an underlying host immunologic problem. However, these infections are infrequently encountered.

In adults, the etiology of conjunctivitis is usually viral, with adenovirus being the most common viral cause; 20% of such infections in children resulted from adenoviruses in one large U.S. study and 14% of infections in adult patients in another study. Adenoviruses types 4, 3, and 7A are common. Most viral conjunctivitis is self-limited but is highly contagious, with the potential to cause major outbreaks. Worldwide, enterovirus 70 and Coxsackie virus A24 are responsible for outbreaks and epidemics of acute hemorrhagic conjunctivitis.