Evaluation of round window accessibility to cochlear implant insertion.

Emphasis on hearing preservation has led to recognition of the round window membrane (RWM) as a portal for the cochlear implant electrode array. The St Thomas' Hospital (STH) classification was devised to evaluate the accessibility of RWM electrode insertion. The objectives of this study were: (1) to prospectively evaluate the STH classification in selecting the appropriate cochlear insertion route in "RWM-intentioned" cases, and (2) to ascertain if RWM accessibility differs from adults to children. This was a prospective cohort study of consecutive patients (adult and paediatric) undergoing cochlear implantation at a specialist auditory implant centre. Visibility of the RWM was graded according to the STH classification after an "optimal" posterior tympanotomy had been performed and any overhang of the bony round window niche removed without breaching the RWM. Most adult (89 %) and paediatric (78 %) cases had more than 50 % of the RWM exposed (Types I and IIa). Cases having less than 50 % of RWM exposed (Types IIb) or none exposed (Type III) were twice as common in children (p = 0.004). 96 % of Type I and 63 % of Type IIa cases underwent RWM insertion through a membranous cochleostomy. 71 % of Type IIb necessitated an extended membranous cochleostomy. All Type III cases required a conventional bony cochleostomy. When more than 50 % of RWM was visible (Types I and IIa), 88 % underwent a membranous cochleostomy. For "RWM-intentioned" cases, the STH classification can be a sensible method of relating RWM visibility/accessibility to the optimal route for insertion. This study also demonstrates that RWM insertion is more challenging in children.