Q: I recently saw a patient who had a central corneal ulcer with a potentially poor prognosis for visual recovery. A nearby anterior segment specialist could not see the patient until much later that day, so I chose to treat the patient before obtaining cultures. What impact does prior treatment have on yielding growth on culture plates?

A: Recent studies indicate that prior antibiotic treatment of a corneal ulcer only slightly decreases the rate of positive cultures. However, it causes a significant increase in the number of days that the cultures must be incubated to yield an isolate.1,2

Thus, prior antibiotic treatment may delay the identification of the specific pathogen(s), possibly hampering healing time and concomitant microbiologically guided therapy, says St. Louis ophthalmologist Jay S. Pepose.

Staphylococcus aureus, as seen via blood agar medium.

So, did this practitioner do the right thing? Yes, say these doctors.

Time is of the essence because this is a sight-threatening condition, says optometrist Sherry J. Bass of New York City. So, even if you are unable to culture this patient or refer him to an anterior segment specialist right away, treat the patient empirically because chances are you are not dealing with an esoteric organism.

Dr. Pepose agrees. Certainly there is an urgency to initiate treat-ment in someone who has an ulcer in the central cornea because it involves the visual axis, he says.

Dr. Bass says that most of the corneal ulcers she sees are the garden-variety Staphylococcus aureus. These are effectively treated with the fourth-generation fluoroquinolones Vigamox (moxifloxacin, Alcon) or Zymar (gatifloxacin, Allergan). Studies have found that the success rate for empirical therapy for corneal ulcers is greater than 90%, says Dr. Pepose.3 However, the lesion should still be cultured in case it is clinically resistant or there is a poor response to the chosen therapy.

Dr. Bass agrees. After you place the patient on an antibiotic, have him cultured anyway as soon as possible (in this case, later in the day) to make sure he does not require a different form of medication, she says.

Optometrist Robert Ryan of Rochester, N.Y., says that optometrists should be prepared to manage patients who have suspected corneal infections. To do so, you should be able to culture a central corneal ulcer in a timely manner.

Because recent studies show that pretreated ulcers take a significantly longer amount of time to yield positive results when compared with non-pretreated ulcers, take a culture, and determine the sensitivity of the organism right away to ensure that your chosen form of medication is going to resolve the lesion in a timely manner, Dr. Ryan says.

You do not need to have sophisticated media, such as blood agar; your local collection lab may cour-ier the media to your office and await the sample.

The base of the lesion and margin should be scraped to maximize your yield. Then, place the scraping in transport or growth media, and give it to the collection facility to be plated, Dr. Ryan says.

There is some question, however, as to whether the positive culture yield with transport media rivals the same growth yield as plating done directly onto culture media in a collection facility.

Also, some practitioners argue that broad-spectrum fluoroquinolones make culturing unnecessary in most cases, especially without immediate involvement of the visual axis. Unfortunately, this case does make culturing necessary. Its very likely that the clinical response to these drugs is going to be favorable, says Dr. Ryan. But this is not always the case.

1. Marangon FB, Miller D, Alfonso EC. Impact of prior therapy on the recovery and frequency of corneal pathogens. Cornea 2004 Mar;23(2):158-64.
2. McDonnell PJ, Nobe J, Gauderman WJ, et al. Community care of corneal ulcers. Am J Ophthalmol 1992 Nov 15;114(5):531-8.
3. Kowal VO, Mead MD. Community Acquired corneal ulcers: the impact of cultures on management. Invest Ophthalmol Vis. Sci 1992;33:1210.

Vol. No: 142:9Issue: 9/15/2005