The Impact of Maturational Changes in Cochlear Position on Cochlear Implantation

 

Background

Three dimensional CT imaging provides good data regarding the positional changes in normal temporal bone development. Attention has been focused on the position of the basal turn in relation to other structures of the temporal bone. The following studies demonstrate that such changes occur rapidly between birth and 4 years of age and then may occur slowly until adulthood with a possible bump during puberty.

McRackan et al (2012) found age-related variation between adults and children in the orientation of the facial recess to the round window membrane (RWM) (1). Children had greater angulation than adults averaging 6.2°(p=0.01) between adults and children. Thus surgeons can expect a narrower view of the RWM in children and may find slightly more difficulty in electrode insertion. The authors also showed that maturation of the EAC over time simplifies the approach to the facial recess and RWM. The angle between the facial recess and EAC is more acute (tighter) in young children. This supports the findings of Lloyd et al (2010) who showed that the axis of the inferior segment of the basal turn becomes more parallel to the axis of the trans-mastoid approach angle over time (2).

New Knowledge

Jackson et al (2014) studied CTs of 713 children to show a significant angular change between the basal turn axis and the trans-mastoid/facial recess approach during the first 4 years of life (3). They found that during normal development, the mastoid—scalar angle becomes more obtuse (nearly straight), reducing insertion complexity. Another factor that can complicate implantation of young children is incomplete mastoid pneumatization. Jackson et al also note that the small mastoid cavities of young children also add complexity.

Hui-Ying et al (2015) reported similar findings and concluded that a more anterior cochleostomy would be useful in many children. More significantly, the authors suggest that pre-op CT should be used to determine the axis of the inferior segment of the basal turn in relationship to the EAC (4).

Take Home

There are many differences in optimum surgical technique for young children and adults, especially in babies between ages 6 and 12 months. Differences may include delicate soft tissues, skull often less than 2 mm thick, active marrow in diploic spaces that bleeds freely and may cause volume problems, constricted mastoid cavity, more acute angle between the EAC and facial recess and, as noted in these papers, a more acute angle of the axis of S. tympani re: the facial nerve.

All of these are manageable, but attention to insertion angle in young children should receive more attention by surgeons. RWM cochleostomies may require a more inferior electrode insertion trajectory in children. Whether bony cochleostomy is preferable in young children due to the greater scalar angulation has not been demonstrated. Similarly, whether routine pre-operative CT should be favored over MR or used in addition to MR has not been addressed adequately.

References

1. McRackan TR, Reda FA, Rivas A, et al. Comparison of cochlear implant relevant anatomy in children versus adults. Otol Neurotol 2012; 33:328–334.

2. Lloyd SK, Kasbekar AV, Kenway B, et al. Developmental changes in cochlear orientation—implications for cochlear implantation. Otol Neurotol 2010; 31:902–907.

3. Jackson NM, Givens VB, Carpenter CC, Allen LM, Morrell BB, et al. Cochlear trajectory in pediatric patients. Laryngoscope 2014. Doi: 10.11102/ lary.24984

4. Hui-Ying L, Ke-Guang C, Lin Y et al. The age-related positional and orientational changes of the human cochlea. Acta Oto-laryngologica 2015. 135:205-210.