La infectious keratitispolymicrobial keratitis, in particular, is a pathological condition that poses significant risks to vision and is therefore considered an ophthalmological emergency requiring immediate treatment. In fact, the incidence of endophthalmitis secondary to infectious keratitis ranges from 0.29% to 6% and the incidence of evisceration or enucleation surgery is still 1.8%.
Globally, an estimated 1.5 to 2 million people develop bacterial keratitis each year. The most common cause of bacterial keratitis is gram-positive bacteria, but it can also be attributed to gram-negative bacteria, fungi and viruses. Polymicrobial keratitis occurs when the corneal infection is caused by more than one pathogen and, in this case, the incidence ranges from 1.9% to 15.8% of infectious keratitis cases.
Risk factors and bacterial profile
Several studies have identified certain risk factors for the development of polymicrobial keratitis, the most frequent of which are:
- use of contact lenses;
- previous corneal transplant;
- advanced age;
- use of steroids;
- immunodepression (often associated with the most severe infections).
From a recent studyIt was found that the most frequent causative agent of infectious keratitis is Staphylococcus epidermidis while, among gram-negative bacteria, Pseudomonas aeruginosa. It has also been observed that polymicrobial keratitis caused only by gram-positive bacteria has a less aggressive course and a better response to treatment.
Polymicrobial keratitis may be characterised by the presence of 'ring' infiltrates around the infectious focus, which is rarer in fungal keratitis, while it is reported in Acanthamoeba, gram-negative and virus infection. Ring-shaped infiltrates result from an intense immune response mediated by interleukins, such as IL-6, which may be due to the increased immunogenic load following the presence of two or more infectious agents in the cornea.
Antimicrobial treatment and resistance
First-line treatment of polymicrobial keratitis includes the administration of ophthalmic antibiotics for topical use. Depending on the severity of the disease, the associated risk factors and the microbiological agents suspected of causing it, one or two antimicrobial agents are generally used. However, as seen, the use of topical steroid drugs is a significant risk factor for the development of this disease.
Although it has been shown that bacterial aetiology is present in 50-65% of all infectious keratitis and that fungal keratitis may account for up to half of all cases in tropical regions, corneal cultures are positive in only about 60% of cases. Therefore, the treatment of polymicrobial keratitis is very complex, as the diagnosis is sometimes not made accurately. However, inadequate treatment can lead to endophthalmitis, with loss of vision or even evisceration and enucleation; therefore, choosing antimicrobial therapy accurately is extremely important.
In the case of polymicrobial keratitis, different pathogens may have a different sensitivity to antibiotics and, therefore, the course of the disease may be more aggressive than in monomicrobial keratitis.
Analysis of the antimicrobial susceptibility pattern of bacterial isolates in polymicrobial keratitis revealed that both gram-positive and gram-negative organisms are more sensitive to netilmicin (92.9%). Netilmicin belongs to the aminoglycoside family and is a water-soluble, modified (N-ethyl) semi-synthetic derivative of isomycin, produced by the fermentation of Micromonospora inyoesis. In a recent study, the susceptibility of organisms isolated in polymicrobial keratitis to vancomycin, ceftazidime and azithromycin was found to be lower than that to netilmycin and fourth-generation fluoroquinolones. Polymicrobial keratitis thus represents, a condition with a high incidence, especially in patients with a history of contact lens wear, with coexisting autoimmune disease and in the presence of ring infiltrate. Of the available antibiotic treatments, netilmicin maintains high efficacy in both monomicrobial and polymicrobial keratitis. Netilmicin should, therefore, be considered among the first choices for the management of microbial keratitis, right from the first symptoms.