Discussion

In the context of cochlear implantation, the presence of a canal wall down mastoid cavity represents a challenge for peri- and post-operative management, especially when there is a large meatoplasty. Many surgical techniques and different approaches have been developed to obtain good results with low complication rates, but no technique is currently considered the ‘gold standard’.

Many authors advocate subtotal petrosectomy with overclosure of the external auditory canal, combined with Eustachian tube plugging and fat obliteration, for managing radical cavities prior to cochlear implantation. This technique was introduced in the late 1950s, and Ugo Fisch introduced the term ‘subtotal petrosectomy’ in 1965.^{Reference Prasad, Roustan, Piras, Caruso, Lauda and Sanna14} In the early years, subtotal petrosectomy was considered hazardous in light of increased incidences of recurrent disease, and there was a general hesitancy amongst surgeons in adopting this procedure. However, over the past couple of decades, there has been a growing interest in subtotal petrosectomy, and the indications for the procedure have also increased. In fact, with the development of MRI and diffusion-weighted MRI in particular, and refinements in the surgical techniques and better microscopes, a significant reduction in residuals and recurrences has been observed.^{Reference Prasad, Roustan, Piras, Caruso, Lauda and Sanna14} The rationale for using subtotal petrosectomy in these cases is to obtain an isolated cavity with a low risk of infections, and to diminish the risk of extrusions.^{Reference Free, Falcioni, Di Trapani, Giannuzzi, Russo and Sanna5,Reference Polo, Del Mar Medina, Aristegui, Lassaletta, Gutierrez and Aranguez15,Reference Karatzanis, Chimona, Prokopakis, Kyrmizakis and Velegrakis16}

Polo et al.,^{Reference Polo, Del Mar Medina, Aristegui, Lassaletta, Gutierrez and Aranguez15} in an interesting series with 110 cases, described their vast experience with subtotal petrosectomy. A very low rate of array extrusion (4.55 per cent) is reported, but only 33 patients had previously undergone canal wall down mastoidectomy and the percentage of array extrusion in this subgroup of patients is not specified. Previously, Vincenti et al.^{Reference Vincenti, Pasanisi, Bacciu, Bacciu and Zini17} reported the long-term results of 12 patients after subtotal petrosectomy and one- or two-stage cochlear implantation after open cavity surgery. They reported one patient with residual cholesteatoma and one with wound breakdown at the external meatus. Good results were also reported by Szymanski et al.;^{Reference Szymanski, Ataide and Linder18} these authors did not observe any complications in seven patients where a subtotal petrosectomy was performed after previous open cavity surgery. In the largest series of subtotal petrosectomy cases reported in the literature, published in 2017, Prasad et al.^{Reference Prasad, Roustan, Piras, Caruso, Lauda and Sanna14} analysed the results of 460 patients. In 91 cases, subtotal petrosectomy and cochlear implantation was performed, with a very low rate of electrode extrusion of 1.1 per cent. A previous surgical procedure was the indication for cochlear implantation with subtotal petrosectomy in only 48.3 per cent of cases. The type of surgery performed and the exact number of patients with a previous large meatoplasty is not specified.

Those who advocate maintaining a canal wall down mastoid cavity following cochlear implantation refer to the simplicity of the procedure, the improved post-operative visualisation and the ability to clean the cavity. Furthermore, maintaining the canal wall down cavity decreases the risks related to external auditory meatus overclosure, such as post-operative infections, dehiscence of large pre-operative meatoplasties, development of residual cholesteatomas, fat necrosis, mucoceles, cavity infections, meningitis and complications related to the fat donor site.^{Reference Free, Falcioni, Di Trapani, Giannuzzi, Russo and Sanna5,Reference Casserly, Friedland and Atlas9,Reference Postelmans, Stokroos, Linmans and Kremer12,Reference Lyutenski, Schwab, Lenarz, Salcher and Majdani19} In particular, the development of residual or recurrent cholesteatomas causing a masked cavity infection with meningitis is a dreaded complication. Thus, early detection of a residual or recurrent cholesteatoma can be considered a priority. For this, maintaining the otoscopic view into the external auditory canal after pseudo mastoid obliteration may be beneficial during the required long-term follow up of patients with cholesteatoma. It is especially important in patients with a cochlear implant, where MRI follow up is limited.

However, maintaining an open cavity is not without risk, and both electrode exposure and extrusion have been reported, including electrode extraction with mastoid cavity cleaning, which is needed in these patients once or twice a year.^{Reference Issing, Schonermark, Winkelmann, Kempf and Ernst2} In order to reduce the risk of extrusion, many surgical techniques emphasise the meticulous elevation of the cavity's fibroepithelial lining, minimising the risk of leaving residual squamous tissue, while providing adequate soft tissue coverage of the electrode. Several different techniques have been described for electrode coverage, using combinations of bone pâté, fibrin glue, bone cement, perichondrium, bone matrix, cartilage, abdominal fat and local musculoperiosteal flaps.^{Reference Himi, Harabuchi, Shintani, Yamaguchi, Yoshioka and Kataura20–Reference Craig and Lavy22} The principal limitation is that most published data relative to these techniques are based on small case series, and thus a definitive conclusion about outcomes cannot be obtained.

This retrospective study shows that partial mastoid obliteration with the cartilage palisade technique may be a safe option for cochlear implantation when a large meatoplasty exists and closure of the external auditory meatus is not easy to perform. The decreased blood supply surrounding the open cavity, and the tension placed on the double layer closure of a large meatoplasty, increase the risk of complications related to external auditory canal blind sac closure.

This study's findings are in line with reports on cochlear implantation regarding subsequent and simultaneous reconstruction of the posterior bony canal wall and obliteration of the mastoid cavity.^{Reference Tamura, Shinkawa, Ishida and Sakai23,Reference Balk, Schwarz, Wolber, Anagiotos and Gostian24}

We recognise that there are several limitations to our study. In a similar way to all retrospective reviews, ours is limited by the availability and accuracy of medical records, and we recognise that we are reporting on a small cohort of patients. Despite this, we consider that pseudo-obliteration of the canal wall down mastoid cavity with a cartilage multi-layered palisade reconstruction covering the electrode can be advocated as a feasible alternative technique in selected patients who present with a large meatoplasty and a stable mastoid cavity.