4 Discussion
Postoperative infection is one of the most common complications5,7 after cochlear implantation and conservative treatments are suggested to be primary and first-line regimens. For post-CI infection, a routine procedure may sequentially involve antibiotic treatments, pus puncture, revision surgery, and re-implantation8, which does not take heed of the specific clinical postoperative manifestations of CI and may cause repeated SFI. Therefore, a more accurate and efficient staging system is needed.
In the current study, the integrity of local skin is a significant index that greatly influences the diagnosis and efficacy of treatments for post-CI infection. Before the skin ruptures, oral or intravenous antibiotic therapy was prescribed. Puncture and etiology-guided antibiotics were administered after abscess was formed. Once the integrity of the skin flap was breached, the majority of patients were cured by re-implantation. These findings suggest that for all SFI cases, the integrity of skin flap can serve as a facilitating index for stratifying the clinical manifestations (Figure 3):
Stage I: Complete skin with redness and swelling or subcutaneous pus formation that can be properly addressed by conservative treatment.
Stage II: Ruptured pus formation or exposed implant that can be remedied by re-implantation.
Of note, the revision surgery was attempted to prevent the cochlear equipment from explant, which proved to be ineffective at any stage in the current study. Thus, re-implantation plays an important role in the prognosis of patients at stage II.
This stratification system only targets the post-CI SFI, whose bacterial culture was a positive result. The skin injury, from antenna pressure stratified by the National Pressure Ulcer Advisory Panel5 and foreign body response including knot and device, is not involved in the current stratification system. The similar symptoms of infection, antenna pressure, and foreign body response may pose some confusion in choosing a correct system, which needs a more comprehensive study.
It is worth mentioning that in all cases of post-CI infection, no obvious malfunctions were reported in cochlear implants, which suggests that post-CI infection only affects the bone groove and skin flap where the receiver/stimulator is located while the cochlear electrodes are safe. But some studies have reported that the function of the device was affected after SFI9,10, which is related to persistent infection and the hypersensitivity caused by the infection9. Therefore, such complications should also be considered.
The rate of postoperative infection in the current study is similar to that of several previous studies, within the range of 1.6%~8.2%11. The incidence of complications has remained stable for many decades5,12, no matter how experienced the surgeon is with cochlear implantation. Staphylococcus and pseudomonas are recognized as the most easily-colonized bacteria on the surface of implants in most studies5,8. But for all cases of infection in our center, conservative treatment, even guided by culture results, cannot eradicate SFI thoroughly, which may relapse in weeks or months.
We found a jelly-like substance on the flap-covered side, which concentrated in the center and scattered around, and observed a bacterial biofilm under the scanning electron microscope. We suspect that antibiotic-resistant bacterial infection may be one of the recurrent reasons, while the formation of biofilm has a greater impact on the recurrent SFI.
Biofilm is easily found on the surface of the device, and most distributed in the center of the device9. This may be due to the absence of a silicone package over the magnetic pole in the center of the equipment, which indicates that the design and material improvements are needed for the receiver-stimulators so as to reduce the probability of infection after CI. Tea tree oil (the essential oil of Melaleuca) was found to remove methicillin-resistant Staphylococcus aureus from the surface of the implants13-15. Bioactive glass particles can inhibit the mature bacterial biofilm on the surface of the cochlear equipment16. But the availability and safety of these materials await further exploration.
Bacterial biofilm is a complex ecosystem. Multi-microbial aggregates composed of a variety of bacteria are embedded in the exopolysaccharides (EPS), mainly composed of polysaccharides, protein-nucleic acids, and lipids17. Antibiotics only inhibit the planktonic bacteria released by the biofilm, but fail to eliminate the biofilm18. Bacteria are generally located in the middle layer of the membrane and are wrapped with the EPS19, which makes it difficult for antibiotics to penetrate every layer of the biofilm and clean bacteria up. On the other hand, the implanted cochlear may provide acquired conditions for the biofilm to resist the environmental changes and the human immune system20, which usually terminates when the antenna is removed from the body by procedure18. This complexity may explain the recurrent bacterial infection and the irresponsive antibiotic treatment.
In the revision surgery of SFI, the surface of the receiver-stimulator was covered with the jelly-like biofilm, which was similar to the substance found by Jiri et al21. On the electron microscopic images at 5000 times, a morphology of typical dense network can be observed in the bacterial biofilms, with pipe-shaped channels, which is in line with the morphology reported by Gi Jung Im et al9.
How to eradicate biofilm is a challenging handicap. We managed to remove the granulation around the implant and soaked the receiver-stimulator of the implant with 1.5% hydrogen peroxide and the aqueous solution of betadine for 30 minutes as reported by Jiri Skrivan et al.21, and covered with part of temporalis to protect the equipment, but the results were negative. Mohnish Grover et al. chose to re-grind a new bone groove which was located away from the original bone groove and infected area to place the receiver3. This is a method that can be used as a reference, but it will damage the skull of patients. The original place with a weak skull is vulnerable to intracranial trauma when hit by force.
Aseptic enhancement during the initial surgery can reduce the patients’ pain and preserve the expensive devices via inhibiting the formation of biofilm. The CI surgery can only commence after the complete curation of inflammation in the ears5. Preoperative skin preparation in the operating room significantly reduces the postoperative infection rate22,23, including waxing the patient’s hair a few days before CI, lavaging the patients’ external auditory canal with 70% alcohol daily with 0.5% chlorhexidine, and wiping the skin of the surgery area5.
The position of CI incision also impacts the infectious rate. Kabelka et al. found that the postoperative SFI rate of the incision behind the ear was about 15 times lower than that of the incision in the ear5. Gawecki et al. found that when the short C-type incision behind the ear was compared with the long incision, the infection rate dropped from 2.43% (11/452) to 1.28% (8/624)5. In our cohorts, all cases were operated with short C-type incision behind the ear and the overall infectious rate was lower than in many institutions11.
Ceftriaxone sodium injection is generally used as a preventive medication for 3 days after the initial surgery. Garcia Valdecasas et al. reported that ceramic-coated cochlear implants such as MED-EL implants and Cochlear Nucleus and Advanced Bionics implants employ titanium silicon for the artificial coating, using ceftriaxone sodium before surgery and clarithromycin after surgery, in which the postoperative infection rate is significantly lower than that of ceftriaxone sodium alone5. This can be a different inspiration. Lisa Kirchhoff et al.24 tried to utilize bioactive glass (BAG) of type S53P4 as a promising tool for the reduction of biofilm formation.