Although the cataract surgery is considered a safe procedure, endophthalmitis remains a rare but potentially serious complication. The bacteria that cause postoperative endophthalmitis may originate from the normal bacterial flora of the conjunctiva and eyelid. At present, however, there is no global consensus on endophthalmitis prophylaxis. Preparation of the ocular surface with topical povidone iodine has become a standard surgical procedure, both in Europe and the United States. Nevertheless, povidone iodine does not provide complete sterilisation of the ocular surface and, therefore, some additional prophylactic strategies are generally used to reduce the risk of endophthalmitis.
A recent survey by theEuropean Observatory of Cataract Surgery revealed that topical antibiotics are used in 42% of cataract surgeries before patients arrive in the operating theatre and in 87% of patients in the post-operative course. Similar results were reported in an online survey conducted by theAmerican Society of Cataract and Refractive Surgery.
In connection with these prophylactic strategies, however, it is also crucial to consider the problem of antibiotic resistance. Indeed, as bacterial flora and susceptibility to antibiotics vary geographically and over time, periodic local surveillance is necessary to anticipate possible antibacterial resistance phenomena.
Since, as mentioned, a patient's ocular flora can be the source of bacteria responsible for most cases of intraocular infections, reducing the bacterial load on the ocular surface or eradicating microorganisms that can reach the eye is the best strategy to reduce the risk of post-operative endophthalmitis.
Characterisation of ocular flora in patients undergoing cataract surgery
One studioconducted in Italy, aimed to characterise the ocular flora in a group of patients undergoing cataract surgery and determine their susceptibility profile to different ophthalmic antibiotics.
The study included 120 patients (49 men and 71 women) and cultures were positive in 72.5% of the cases. Most patients (47.1%) were positive for both conjunctiva and eyelid margin; 16.1% were positive for conjunctiva only and 36.8% for eyelid margin only. Thus, eyelid involvement was observed in 83.9% of the patients. These results are consistent with those reported in previous studies, which showed a range of culture positivity from 67% to 85%, with key involvement of the eyelid margin.
Furthermore, as observed in previous studies, most of the isolates identified in the study (70%) were Staphylococcus epidermidis e Staphylococcus aureus, microorganisms implicated in approximately 80% of postoperative endophthalmitis cases. The S. epidermitis methicillin-resistant (MRSE) represented 47% of the isolates of S epidermidis and MRSA (S. aureus methicillin-resistant) 30% to 64% of the total isolates of S. aureus.
Antibiotics and cataracts: the effectiveness of netilmycin
The isolated bacteria were then classified as susceptible, intermediate or resistant to a wide range of antibiotics.
Excluding antibiotics not available for topical use, the isolates were highly susceptible to netilmycin alone, while they showed high levels of resistance to azithromycin, ofloxacin and tobramycin. Approximately 30% of the isolates were resistant to at least 3 classes of antibiotics (e.g. fluoroquinolones, chloramphenicol and macrolides) and were therefore classified as multi-resistant.
MRSA and MRSE are highly susceptible to netilmicin. Chloramphenicol and tobramycin also showed good efficacy, while most methicillin-resistant strains were not susceptible to fluoroquinolones. This specific antimicrobial activity is clinically relevant because MRSA and MRSE are less likely to be eradicated with topical prophylaxis regimes and could easily induce infections.
In conclusion, among the antibacterials available for topical use, netilmicin was the only drug with an overall susceptibility rate higher than 90%. In fact, a 2-day topical administration with netilmicin in culture-positive patients was able to eradicate the micro-organisms in 94% of cases.
The results of the study indicate, therefore, that netilmicin could be considered a first-choice antibiotic in the preoperative setting due to its broad spectrum of activity that also covers multi-resistant strains, including MRSA and MRSE.
Netilmycin is also effective against aminoglycoside-resistant strains due to its resistance to most enzymes capable of inactivating them.
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