Choosing an IC electrode
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- Category: Cochlear Implants and NHS
- Last Updated on Monday, 30 September 2013 09:45
- Written by Nadia Giarbini MD
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Each child candidate to cochlear implantation should receive the best possible electrode array for his/her unique cochlear anatomy. Complete cochlear coverage and optimal frequency-to electrode allocation should be guaranteed to ensure the best possible outcomes in speech perception and sound quality (Lee J et al., 2010).
Cochleae may differ significantly in size and shape from one another, as can individual cochlear duct lengths. Figure 1 depicts the variations in human cochlear duct length as from Hardy M, 1838 and Miller JD, 2007.
Figure 1. Variations in cochlear duct length
Surgical techniques and electrodes technology need to ensure that the delicate neural structures in the cochlea are left undamaged. Hearing preservation, in fact, is essential for two main reasons.
First, helping to ensure that the neural structures in the cochlea are left undamaged is critical in enabling recipients to benefit from future therapies and technologies. It is likely that any future interventions, be they device, biological, or pharmaceutical in nature, will be more successful in a cochlea where the neural structures have been preserved.
Secondly, many children with severe-to-profound hearing loss may still be able to hear some low frequency sounds. It is important to preserve this residual hearing during cochlear implant surgery as it enables the individual to use the natural hearing in combination with a cochlear implant (electro-acoustical stimulation). Several studies demonstrate that this provides the user with the best possible listening experience (Helbing S et al., 2010; Skarsynski H et al., 2011).
It has been established that the scala tympani is the ideal location for an electrode array (Adunka O & Buchman C, 2007). Once in the scala tympani, the electrode array should not perforate the basilar membrane and thereby destroy the delicate neural structures of the cochlear duct.
The choice of the appropriate electrode array can be done with the aid of radiological imaging. For this, the diameter of the basilar turn is calculated on the computed tomography (CT) scan (Escudè B et al., 2006). Its value, labelled “A”, is measured as the largest distance from the round window to the opposite lateral wall, crossing the modiolus (Figure 2).
Figure 2. Two-dimensional reconstruction from HRCT data of the basal turn of the cochlea using a 1.0-mm layer, minimum intensity projection. Distance A is measured using the scanner system.
The “A” value may then be used for pre-operative assessment of the appropriate electrode variant, by using the graphs reported in Figure 3.
Figure 3. Selection of electrode arrays for round window or cochleostomy approaches.
The “A” value is only indicated for patients with normal cochlear anatomy. To use the appropriate graph depending on whether a Round Window or Cochleostomy approach is recommended.
The final positioning of the cochlear electrode array can be determined post-operatively by means of X-ray images as described by Xu J et al. [2000].
References
Adunka O. & Buchman C. Scala Tympani Cochleostomy 1: Results of a Survey. The Laryngoscope, 2007, 117: 2187-2194.
Hardy M. The Length of the Organ of Corti in Man. American Journal of Anatomy, 62(2), 1938, p. 179-311.
Helbig S; Baumann U; Hey C; Helbig M. Hearing Preservation After Complete Cochlear Coverage in Cochlear Implantation with the Free-Fitting FLEXSOFT Electrode Carrier. Otol Neurotol. 2011 Aug;32(6):973-9.
Lee J; Nadol JB; Eddington DK. Depth of Electrode Insertion and Postoperative Performance in Humans with Cochlear Implants: A Histopathologic Study. Audiol Neurootol. 2010 July; 15(5): 323–331.
Miller JD. Sex differences in the length of the organ of Corti in humans. J Acoust Soc Am. 2007 Apr;121(4):EL151-5.
Skarzynski H; Lorens A; Zgoda M; Piotrowska A; Skarzynski PH; Szkielkowska A. Atraumatic round window deep insertion of cochlear electrodes. Acta Otolaryngol. 2011 Jul;131(7):740-9.
Xu J, Xu SA, Cohen LT, Clark GM. Cochlear view: postoperative radiography for cochlear implantation. Am J Otol. 2000 Jan;21(1):49-56.