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.