Discussion

Cochlear implantation can be performed by a number of methods, of which cortical mastoidectomy with a posterior tympanotomy is the most common. Over the years, refinements in the surgical techniques have made the posterior tympanotomy approach highly successful, with minimal complications.^{Reference Rajan, Teh, Prepageran, Kamalden and Tang5} However, the posterior tympanotomy approach poses a risk, although rare, of injury to the facial nerve, particularly in malformed ears. This can occur as a result of bony overhangs or an anomalous course of the facial nerve. In addition, because of anatomical variations, visualisation of the round window membrane may be obscured.^{Reference Roland, Wright and Isaacson6} The direct line of sight obtained with the microscope through the posterior tympanotomy may not provide adequate surgical exposure, and sometimes it may be difficult to identify the round window and surrounding landmarks. This occurs in up to 11 per cent of adult cases and 22 per cent of paediatric cases, and conventional bony cochleostomy may then be required.^{Reference Leong, Jiang, Agger and Fitzgerald-O'Connor2} In order to avoid these complications, complementary techniques are considered.

Over the last two decades, endoscopic middle-ear surgery has gained considerable popularity, and endoscopes are now being used during cochlear implant surgical procedures. Using endoscopes in ear surgery can improve optics and magnification, providing clearer, brighter and better quality images. The endoscope also enables a wider field of view, and is able to access hidden areas that are difficult to view using a microscope.^{Reference Roland, Wright and Isaacson6} The round window niche is one such hidden area that the endoscope can accurately identify. These advantages help to reduce complications and enable atraumatic electrode insertion in the scala tympani for hearing preservation.^{Reference Rajan, Teh, Prepageran, Kamalden and Tang5}

Patients with middle- or inner-ear anomalies may particularly benefit from endoscopic-assisted cochlear implantation. Marchioni et al.^{Reference Marchioni, Soloperto, Guarnaccia, Genovese, Alicandri-Ciufelli and Presutti3} have described endoscopic-assisted cochlear implantation in patients with middle- and inner-ear anomalies. In patients with advanced otosclerosis obliterating the round window, identification of the fustis, an anatomical landmark to the round window, allows accurate siting of cochleostomy. Additionally, the wider visual field provided by endoscope reduces the need for a more extensive approach.

This study describes endoscopic-assisted cochlear implantation via a conventional posterior tympanotomy approach. In cases with limited visibility of the round window, endoscopic examination through the facial recess can be performed, to obtain a panoramic and magnified view of the round window and surrounding structures. This facilitates a membranous cochleostomy for electrode insertion. Our study demonstrated a significant improvement in visualisation of the round window membrane when using the endoscope, in all patients. This allowed electrode insertion via a round window or extended round window approach in nearly all of the patients. In two patients with Mondini dysplasia, conventional bony cochleostomy had to be performed because of unfavourable anatomy.

In the literature, several studies describe transcanal endoscopic-assisted cochlear implantation.^{Reference Marchioni, Soloperto, Guarnaccia, Genovese, Alicandri-Ciufelli and Presutti3,Reference Dia, Nogueira, O'Grady and Redleaf7,Reference Migirov, Shapira and Wolf8} Its advantages include avoidance of drilling the mastoid cavity, better round window visualisation and no risk of facial nerve injury. However, in a recent study, Tarabichi et al.^{Reference Tarabichi, Nazhat, Kassouma and Najmi9} demonstrated that the posterior tympanotomy approach offered the most favourable trajectory for electrode placement. In their radiological evaluation of the position of the round window in relation to posterior tympanotomy, these authors highlighted that the transcanal approach without mastoidectomy did not favour the angle to the axis of the basal turn of cochlea, and the electrode was likely to be deflected off the wall of the cochlea following the trajectory of basal turn. In addition, the transcanal approach is prone to iatrogenic annulus and tympanic membrane injuries, especially in children, as well as electrode extrusion.^{Reference Dia, Nogueira, O'Grady and Redleaf7,Reference Marchioni, Grammatica, Alicandri-Ciufelli, Genovese and Presutti10}

This study describes the auxiliary role of the endoscope in conventional transmastoid microscopic cochlear implantation. The main benefit of endoscope-assisted cochlear implantation is improved visibility of the round window region. This may reduce the risk of complications and is of particular benefit in patients with congenital malformations. However, before embarking on endoscopic-assisted cochlear implantation, surgeons should be trained and well versed in both endoscopic and microscopic ear surgery.