Cochlear implant electrode insertion: in defence of cochleostomy and factors against the round window membrane...

The round window membrane (RWM) is an increasingly popular route for electrode insertion in cochlear implantation especially for hearing preservation. Limitations to this route include anatomical, physiological, and surgical aspects. The established soft-tissue cochleostomy route for electrode insertion is thought to place the basilar membrane and spiral ligament at risk. However, the mammalian model response to soft-tissue cochleostomy has not yet been quantified.

Firstly, an on-line literature search was conducted to gather evidence of the anatomical and physiological functions of the RWM and adjacent structures. Secondly, experimental guinea pigs underwent left soft-surgery cochlestomy. Four weeks post-operatively they were euthanased and the cochlea's harvested for histology. Surgical damage to the cochlea and auditory neurons was assessed.

The literature review with regard to the RWM anatomy revealed evidence for difficulty in approach/visualization, possible absence, and impedance of electrode insertion by the hook region. It also has a number of higher functions including immune defence and absorption/secretion of molecules. Experimental cochlea's 4 weeks post-soft-tissue cochleostomy showed only mild and localized inflammatory response adjacent to the scala tympani cochleostomy site. There was no spiral neuronal ganglion loss.

The RWM route may be compromised or absent. Electrode insertion via the RWM could interfere with its higher functions. Mammalian soft-tissue cochleostomy has been shown to elicit a limited tissue response and does not reduce the number of cochlear spiral ganglion neurones. It should therefore remain within the hearing implant surgeon's armamentarium.