Infectious keratitis

Infectious keratitis is characterized by corneal inflammation and defects caused by replicating bacteria, fungi, or protozoa. These infections can progress rapidly with devastating consequences, including corneal scarring and loss of vision. Thus, it is imperative to identify this condition promptly and begin an aggressive course of therapy to limit tissue damage. This chapter summarizes the current understanding of various clinical and pathophysiological aspects of infectious keratitis.

Clinical background

Key symptoms and signs

Clinical features of infectious keratitis include redness, tearing, edema, discharges, decreased vision, pain, and photophobia. The hallmark of keratitis is the appearance of diffuse or localized infiltrates within the corneal epithelium, stroma, and often the anterior chamber. Severe cases are denoted by necrotic ulceration of the epithelium and stroma.

Some clinical signs may be indicative of a particular infectious organism ( Table 7.1 ). Bacterial keratitis is often identified by the absence of epithelium and suppurative stromal infiltrates. Gram-negative bacterial infections are associated with hazy corneal rings and soup ulcerations, whereas Gram-positive infections tend to produce well-defined grayish-white infiltrates and localized ulcerations ( Figures 7.1 and 7.2 ). Fungal keratitis generally exhibits a slow progression, satellite lesions, and elevated infiltrates with undefined, feathery edges ( Figure 7.3 ). Some parasitic infections, like Acanthamoeba , are frequently misdiagnosed as fungal or viral because of the pseudodendritic appearance. In many cases, patients infected with parasites report disproportionate pain, which is characteristic of radial keratoneuritis ( Figure 7.4 ).

Table 7.1

Clinical features of infectious keratitis

Data from Matsumoto and Szliter et al.

Common pathogens Distinguishing clinical features Therapeutic strategies

  • Staphylococcus CNS (coagulase negative staphylococcus)

  • Staphylococcus aureus

  • Streptococcus pneumoniae

Localized ulcers: round or oval
Stromal infiltrates: discrete grayish-white Fluoroquinolones
Stromal haze: minimal, distinct borders Fluoroquinolones with cephalosporin

  • Pseudomonas aeruginosa

  • Neisseria gonorrhoeae

Undefined ulcers: soupy
Stromal infiltrates: dense and suppurative Aminoglycosides with cephalosporin
Stromal haze: intense immune rings

  • Candida albicans

  • Cryptococcus spp .

Ulcers, epithelial defects Polyenes
Stromal infiltrates: discrete grayish-white
Slow progression Imidazoles

  • Fusarium

  • Aspergillus

Slough: dry, elevated Fluorinated pyrimidines
Stromal infiltrates: feathery margins
Satellite lesions Imidazoles with fluorinated pyrimidine

  • Acanthamoeba spp .

  • Microsporidia spp .

Severe pain, radial keratoneuritis Cationic antiseptics
Stromal infiltrates: pseudodendritic Aromatic diamidines
Stromal haze: intense immune rings Azoles

Figure 7.1

Contact lens-associated bacterial keratitis caused by Staphylococcus aureus . Note discrete infiltrates and minimal corneal haze.

(Reprinted with permission of Macmillan Publishers from Whiting MAN, Raynor MK, Morgan PB et al. Continuous wear silicone hydrogel contact lenses and microbial keratitis. Eye 2004;18:935–937, copyright ©.)

Figure 7.2

Pseudomonas aeruginosa keratitis in a silicone hydrogel contact lens wearer. Note the undefined soup ulceration.

(Reprinted with permission of Macmillan Publishers from Whiting MAN, Raynor MK, Morgan PB et al. Continuous wear silicone hydrogel contact lenses and microbial keratitis. Eye 2004;18:935–937, copyright ©.)

Figure 7.3

Candida albicans keratitis in a patient with severe conjunctivitis.

(Reproduced with permission from O’Day D. Fungal keratitis. In: Albert DM, Miller JW, Azar DT, et al (eds) Principles and Practice of Opthalmology, 3rd edn. Amsterdam: Elsevier, 2008.)

Figure 7.4

Advanced keratitis caused by Acanthamoeba . Note classic ring infiltrate.

(Reproduced with permission from Parmar DN, Awwad ST, Petroll WM, et al. Tandem scanning confocal microscopy in the diagnosis of suspected acanthamoeba keratitis. Ophthalmology 2006;113:538–547.)

Epidemiology and risk factors

Incidence rates, risk factors, and causative agents of keratitis vary geographically and socioeconomically. Incidence in the USA is estimated to be 11 in 100 000, whereas rates in South-East Asia are near 800 in 100 000. The principal risk factors include trauma, contact or orthokeratology lens wear, ocular surface disease, ocular surgery, and systemic disease. In Europe, Japan, and USA, contact lens wear constitutes the major risk factor for infectious keratitis. Ocular trauma is the main predisposing factor in developing countries.

Among contact lens-related infections, Staphylococcus spp., Streptococcus spp., and Pseudomonas aeruginosa are the leading causes in temperate climates. In subtropical climates, like northern India, fungal keratitis has been strongly linked to contact lens wear, representing 20–30% of total isolates. Although rare in temperate climates, there has been a recent increase in fungal and parasitic keratitis associated with contact lens wear involving Fusarium and Acanthamoeba . These appear to be associated with specific contact lens care solutions and storage hygiene.

Infections due to ocular trauma are often attributed to fungal and mixed infections (fungi and bacteria). Candida and other yeasts are commonly reported in temperate climates and filamentous fungi, i.e., Aspergillus and Fusarium, in warmer climates.

Diagnostic workup

Preliminary diagnoses are based on clinical signs, symptoms, and patient history. Noninvasive techniques, such as slit-lamp microscopy, confocal microscopy, and histological examination of impression cytology, are often used. If bacterial keratitis is suspected, empirically based therapies are started immediately without definitive information about the organism. It is always advisable to confirm the presence and identity of an infectious agent. This can be accomplished by examining corneal scrapings using standard diagnostic staining, culturing, immunochemistry, and polymerase chain reaction techniques ( Table 7.2 ). Biopsies may be necessary if the disease is contained within the stroma. If the infectious agent is culturable, susceptibility profiles should be determined for optimizing treatment strategies.

Table 7.2

Diagnostic stains and standard culture media

Data from Matsumoto and Szliter et al.

Type of stain Organisms visualized/cultured Comments
Gram stain Bacteria, fungi, Acanthamoeba Peptidoglycan, teichoic acids – violet
Giemsa stain Bacteria, fungi, Acanthamoeba Acidophilic/basophilic – contrast
Acridine orange Bacteria, fungi, Acanthamoeba DNA – fluorescent orange
Calcoflur white Fungi, Acanthamoeba Cellulose/chitin – fluorescent blue
Gomori methenamine silver Fungi, Acanthamoeba Uric/urate particles – dark blue
Periodic acid–Schiff Fungi, Acanthamoeba Cell wall – pink
Hematoxylin and eosin Acanthamoeba Intracellular structures – contrast
Standard agar culture media
Blood agar * Bacteria, fungi, Acanthamoeba General purpose, including fastidious agents
Chocolate agar Bacteria, fungi General purpose, including fastidious agents
Brain–heart infusion agar Bacteria, fungi General purpose
Sabouraud dextrose agar Fungi
Escherichia coli overlay on non-nutrient agar Acanthamoeba
Standard liquid culture media
Brain–heart infusion broth Bacteria, fungi
Thioglycollate broth Bacteria Good for small inocula
Glucose neopeptone broth Fungi

* Ideal for culturing bacteria such as Staphylococcus, Streptococcus , and Pseudomonas .

Fungi can be recovered from standard bacterial media in the presence of antibiotics.

Only gold members can continue reading. Log In or Register to continue

Stay updated, free articles. Join our Telegram channel

Aug 26, 2019 | Posted by in OPHTHALMOLOGY | Comments Off on Infectious keratitis

Full access? Get Clinical Tree

Get Clinical Tree app for offline access