Prof. Namrata Sharma, Prof. Cornea & Refractive Surgery Dr Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi.
Microbial Keratitis is an ocular emergency and carries a potential for blinding complications. As per the global estimates, around 1.5-2 million cases of corneal blindness can be attributed to microbial keratitis, per year. The incidence and etiological factors vary widely, based on the geographical location and the overall economic status of the nation. Preservation of structural anatomy and visual functions requires timely diagnosis and initiation of appropriate anti-microbial therapy. Over the past 2 decades, its management has undergone a paradigm shift from topical and systemic drug therapies to targeted drug delivery and the newer systems such as drug-eluting contact lens, micro-needle patch, and ocular inserts, in an attempt to improve and reduce toxicity of drugs.
Bacterial keratitis cases are usually managed either with monotherapy using fluoroquinolones or with combination therapy, depending on the size and the location of the ulcer. If the bacterial keratitis is less than 3 mm in size and not involving the visual axis, monotherapy is used. However, if the ulcer is greater than 3 mm in size and involves the visual axis, combination therapy is used. This generally comprises of cephalosporin or a glycopeptide and an aminoglycoside. The indiscriminate use of antibiotics has led to the emergence of multi drug resistance to these commonly prescribed antibiotics. Managing infections caused by organisms which are resistant to even the newer group of carbapenem antibiotics such as imipenem and meropenem, is an important cause for concern.
Fungal keratitis is another form of keratitis, mostly localised to the tropical countries and in nations with a higher prevalence of agricultural activities. The management is challenging in view of poor corneal penetration of the most commonly prescribed antifungal medications. Systemic antifungal drugs help in a limited spectrum of cases . To improve this, the targeted drug therapy has been used including intrastromal and intracameral treatment in conjunction with topical therapy which has been evaluated in lot of studies, most of them reporting no additional benefit of combined therapy over topical therapy alone. Newer formulations and drug delivery systems in the form of liposomes and nanoparticles are underway and hold promise for a better treatment of fungal infections.
Use of molecular methods for identification of fungal isolates has helped in the identification of several uncommon aseptate organisms which were once considered to be fungi with no response to the routine antifungal therapy. One such uncommon pathogen belongs to Pythium genus of Oomycota division. These cases have been seen to respond to the oxazolidinone class of antibacterials including Linezolid and the macrolide group of antibiotics. Pythium keratitis is the most devastating form of keratitis and is amenable to surgical management which should be done sooner than later.
The acanthamoeba keratitis is another sight threatening infection of the cornea, usually associated with contact lens wear. Establishing a correct and diagnosis remains a real challenge, majority patients present with a long history of pre-treatment with antivirals and corticosteroids. A combination therapy, comprising of biguanides and diamines, is helpful in controlling this infection. The disease usually runs a long-standing course, with slow response to medications and carries a risk of medication induced toxicity. Treatment is required for prolonged periods of time.
Microsporidial keratitis, which presents either in the form of punctate keratitis usually in immunocompromised patients, or as stromal disease mostly affecting immunocompetent patients, is another emerging corneal infection. Epithelial disease is self-limiting and is usually managed by a combination of prophylactic broad spectrum antibiotics and lubricants. Stromal disease, on the other hand, usually does not respond to medical therapy, almost always requiring keratoplasty. Establishing a correct diagnosis of the stromal variant is yet another challenge as most patients are wrongly treated on lines of viral keratitis first. Establishing an appropriate and timely diagnosis requires a high index of suspicion.
In the recent times, many efforts have been made both towards the diagnosis and treatment of microbial keratitis but obviously the bugs are smarter than any of the treating modalities and the war with the microbes continues.