Living Library

Infectious Keratitis

CLINICAL DESCRIPTION:
Infectious keratitis refers to a family of conditions associated with microbial invasion of the corneal epithelium and stroma. The appearance can vary widely depending on the infecting organism and the stage of the disease at the time of presentation. The microorganisms most commonly isolated in cases of contact lens associated microbial keratitis are Pseudomonas and Serratia species. Typically there is significant necrosis of tissue and associated inflammation. Corneal ulceration, loss of epithelium with underlying stromal infiltrate, characterizes the classic presentation. Associated anterior chamber inflammation is common. The possibility of permanent visual loss from corneal scarring or perforation makes microbial keratitis the most devastating complication of contact-lens wear.
SYMPTOMS:
Microbial keratitis is associated with several rapidly developing symptoms. Initial awareness and irritation often progress in a matter of hours into pain and photophobia. Lacrimation is often profuse and there may also be purulent discharge. The eye is intensely red and the surrounding tissue is typically inflamed and swollen. Pain is usually sufficient to prompt the patient to seek professional care.
INCIDENCE: 
The incidence of infectious keratitis is quite low, however, contact lens wear, and particularly the overnight wear of contact lenses, is the main predisposing factor in its occurrence. The risk of corneal infection among contact-lens wearers is about 80 times that among healthy non-wearers. The generally accepted incidence among people who wear hydrogel contact lenses on an overnight basis is 20 in 10,000. By comparison, the incidence in people wearing soft contact lenses on a daily wear basis is approximately 4 in 10,000, and when rigid contact lenses are worn on a daily wear basis approximately 1 in 10,000. Although the introduction of disposable contact lenses did not decrease the risk of microbial keratitis it is hoped that silcone containing hydrogel lenses will make overnight wear of lenses safer.
ETIOLOGY:
It is generally accepted that a break in the epithelium is a prerequisite for establishment of microbial keratitis. That is to say, the intact epithelium presents a formidable defense against such invasion. In addition, an organism capable of invading and colonizing the tissue must be present. There are numerous risk factors that have been associated with infectious keratitis, most of which can be directly related to increased risk of epithelial break (e.g. trauma to the cornea) and/or increased likelihood of presence of infectious microorganisms (e.g. contaminated solutions, immune compromise).
Given the frequency with which epithelial breaks occur, for any of a multitude of reasons, and the ubiquitous nature of potentially pathological organisms, it is difficult to establish why a particular cornea becomes infected when so many other corneas, similarly at risk, do not.
MANAGEMENT:
There are currently two controversies regarding the management of infectious keratitis. The first has to do with the role of laboratory testing. The second is over the choice of first line treatment.
Traditionally, laboratory work up including cytology, cultures and sensitivities had been considered essential and standard of care. More recently, the relevance and feasibility of such work up have been questioned, particularly in light of the success of empirical treatment. Current thinking includes the strategy of conducting laboratory work up if initial empirical therapy fails to show improvement in 24 to 48 hours. On the other hand, there are still many authorities that recommend routine culturing of any suspect lesion.
Therapy is always initiated prior to receiving results of any laboratory testing, which may take up to 48 hours. Authorities differ however, in their recommendation for first line treatment. Several studies have indicated that commercially available fluoroquinalones are therapeutically equivalent to the traditional fortified cephazolin and aminoglycoside combination. Nevertheless, many corneal specialists continue to recommend the fortified combination for first line treatment of suspected microbial keratitis. Many others recommend initiating treatment with the fluoroquinolones and only changing if improvement is not evident or laboratory testing suggests a different course. Yet a third option is to combine a fortified cephazolin regimen with fluoroquinalone treatment.
In many cases (especially very central, severe or advanced cases), immediate referral to a corneal specialist is prudent and recommended, particularly if risk and suspicion levels are high. Initial antimicrobial treatment, although broad spectrum, is typically aimed at combating the more aggressive gram negative organisms, although in recent years the incidence of such infections appears to be on the decline, with a corresponding increase in the number of gram positive infections. Other treatment may become necessary in the event that the organism is identified as amoebic or fungal. Diligent care is essential and in extreme cases hospitalization is necessary to ensure proper treatment is carried out. Although the most effective treatments are administered topically, the frequency and complexity of treatment may be beyond that which can be trusted to home care.
Approximately ninety percent of infectious keratitis cases resolve without loss of best corrected vision, however, delay in treatment is associated with increasing risk of loss of vision.
KEY REFERENCES:
Lingel, NJ and L. Casser (2001). Diseases of the Cornea. Clinical Ocular Pharmacology. JD Bartlett and S.
OTHER REFERENCES:
Brennan NA, Coles ML. Extended wear in perspective. Optom Vis Sci 1997;74(8): 609-23.
Cheng KH, Leung SL, Hoekman HW, Beekhuis WH, Mulder PG, Geerards AJ, Kijlstra A. Incidence of contact-lens asssociated microbial keratitis and its related morbidity. Lancet 1999 Jul 17;354(9174):181-5.
Forster RK. .The Management of Infectious Keratitis as We Approach the 21st Century. CLAO J 1998;24(3): 175-180.
Dart JK. Predisposing factors in microbial keratitis: the significance of contact lens wear. Br J Ophthalmol 1988;72(12): 926-30.
Liesegang TJ. Contact lens-related microbrial keratitis: Part 1: Epidemiology. Cornea 1997;16(2):125-31.
Poggio EC, Glynn RJ, Schein OD, Seddon JM, Shannon MJ, Scardino VA, Kenyon KR. The incidence of ulcerative keratitis among users of daily-wear and extended-wear soft contact lenses. N Engl J Med 1989;321(12): 779-83.
Schein OD, Buehler PO, Stamler JF, Verdier DD, Katz J. The impact of overnight wear on the risk of contact lens-associated ulcerative keratitis. Arch Ophthalmol 1994;112(2): 186-90.

Cornea and Contact Lens Living Library

Infectious Keratitis

Edited by:
   Peter Bergenske, O.D. F.A.A.O., Pacific Uneversity College of Optometry
Barbara Robinson, O.D., F.A.A.O., Waterloo University College of Optometry