Q: There seems to be conflicting reports on how early ring infiltrates appear in Acanthamoeba keratitis (AK). Are they only possible weeks or months into the infection, or can they appear within days to weeks? Also, I know theyre not pathognomonic for AK, so when Im working up my differential diagnosis, what else should I consider?
A: Ring infiltrates dont often present initially in a case of Acanthamoeba keratitis, and if they are present early, the patients immune system is most likely suppressed, says Review co-chief clinical editor Christine W. Sindt, O.D., director of Contact Lens Service and associate professor of clinical ophthalmology at the University of Iowa.
Ring infiltrates are the presenting sign only in about 4% of AK cases, she says. Fifty-five percent present as punctate keratitis.
Generally, she says, rings are usually only present in the later stages of the disease, unless the patient is on a steroid regimensteroids accelerate the proliferation of trophozoites and the progression of disease.
Ring infiltrates may appear weeks to months after initial infection, says Dr. Sindt. Or, they may never appear at all, especially if the disease is caught early and treated before the amoebae cross Bowmans membrane.
These images show the progression of AK in the same patient over two months. Note the pronounced ring in the later image (right).
Photos courtesy: Christine W. Sindt, O.D.
Typically, the ring-shaped corneal ulceration, known as a Wessely ring, is attributable to the bodys reaction to an antigen, says Dr. James V. Aquavella, M.D., a professor at the University of Rochester Eye Institute.
Also, the presence of a contact lens on the cornea will affect the appearance of the lesion, he says. If you have a person who presents with AK not wearing a lens, the lesion will be more delayed than if the patient has continued to wear the lens.
But, Dr. Aquavella warns, dont jump to conclusions because you see a ring infiltrate. We dont have any idea what the immunology is when we look at the patient under the slit lamp. Lesions that are small infiltrates in ring form can also be caused by bacteria that penetrate from the surface into the corneal epithelium, he says.
Likewise, strong immunological involvement in cases of fungal or viral ulceration may also cause the formation of a ring, says Dr. Aquavella. Or, this can occur because of endotoxin being liberated from the bodies of the bacteria even after they have begun to be killede.g., in such cases as Pseudomonas or Staphylococcus, or other bacterial infection, you have a limbal lesion with a clear ring, and in the center, the deposition of the antigen antibody complexes.
Even herpes simplex keratitis may be associated with ring infiltrates, he says. Likewise, abuse of topical anesthetics can also cause ring infiltrates. But, many of these conditions present with rings as different as they are. For example, in a case of Staphylococcus marginal keratitis, the infiltrates form close to the limbus and areas around the corneaeach small infiltrate is shaped differently, but together, they form a ring, says Dr. Aquavella.
Examine the patients other symptoms in order to diagnose the condition. If it progresses slowly over days or weeks and smolders, thats more Acanthamoeba or fungal than bacterial, says Dr. Aquavella. But, if its acute, early and disrupts the epithelium, then consider a bacterial cause, such as Pseudomonas.
