Cochlear Rotation and Its Relevance

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cochlear implants international , Vol. 11 Suppl. 2, September, 2010, 5763

2010 W. S. Maney & Son, Ltd DOI 10.1179/146701010X12726366067815

Cochlear Rotation and its Relevance toCochlear ImplantationSimon KW LloydDepartment of Otolaryngology, Manchester Royal Infirmary, UK

Anand V Kasbekar and Bruno KenwayDepartment of Otolaryngology, Cambridge University Hospitals, UK

Toby PrevostDepartment of Applied Medical Statistics, Cambridge University, UK

Maurice Hockman Johannesburg Cochlear Implant Group, South Africa

Timothy BealeDepartment of Radiology, Royal National Throat Nose and Ear Hospital,

UK John GrahamDepartment of Otolaryngology, Royal National Throat Nose and EarHospital, UK

Objectives: To investigate changes in cochlear orientation with age and dis-cuss the implications of any change with respect to cochlear implantation.Study Design: Cross-sectional study of computerized tomography scans ofthe temporal bones in patients with no congenital abnormalities.Patients: 159 patients were included in the study making a total of 318 ears.The age range was nine months to eighty five years.Intervention: Axial compututed tomography scans showing the basal turn ofthe cochlea were identified. The angle of the basal turn of the cochlea wasmeasured by drawing a line through the long axis of the basal turn andmeasuring its angle with a line drawn through the midsagittal plane. Thepatients were grouped according to age and a one way analysis of variance

was used to identify any statistically significant change in basal turnangulation. Inter- and intra-observer errors were calculated and presented as


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58 SIMON KW LLOYDet al.

repeatability co-efficients. The basal turn angles of 3 difficult cases ofcochlear implantation were related to the findings.Results: The mean basal turn angle was 54.6 (range 46.863.8 ; SD 3.5).There was a statistically significant reduction in the angulation of the basalturn with increasing age (F = 10.1; p= 0.002). The majority of the changeoccurs between the ages of eleven and fifteen. The inter-observer reliabilityco-efficient was 4.8. The intra-observer reliability co-efficient was 2.0. The 3difficult cases all had basal turn angles that were at the upper limit of thenormal range.Conclusions: There is a statistically significant reduction in basal turn angu-lation relative to the midsagittal plane with increasing age. However, careshould be taken in interpreting these results in light of the inherent error inthe measuring technique although the intra-observer repeatability coefficientwas only 2.0. The more obtuse angulation of the basal turn in children may

have implications for cochlear implantation.keywords anatomy, cochlea, cochlear implant, temporal bone

Introduction

It has become doctrine in Otolaryngology that the labyrinth reaches adult size andmorphology by 16 to 20 weeks gestation(1). However, there is some evidence thatsubtle changes may occur to the labyrinth into adulthood(2).

Following review of the computed tomography (CT) images of three paediatricpatients in whom cochlear implant was difficult, it appeared that the orientationof the basal turn of the cochlear was more obtuse relative to the midline than inother patients. The authors therefore decided to analyse a series of patients undergo-ing computed tomography imaging in order to investigate changes in cochlear orien-tation with age and discuss the implications of any change with respect to cochlearimplantation.

Methods

This study was a cross-sectional study of CT scans of the temporal bones in patientswith no congenital abnormalities. The most representative axial slice through thebasal turn of the cochlea was identified. A line was drawn through the midline andthrough the long axis of the basal turn. The midline was defined by a line drawnthrough the junction of the face of the sphenoid bone and the nasal septum anteri-orly and through the internal occipital protruberanceposteriorly. The angle betweenthese bisecting lines was then measured using radiological software (Figure 1).

One hundred and fifty nine patients were included in the study making a total of318 ears. The mean age was 33.7 years (range 9 months to 85 years). There were 73females and 86 males.

The patients were grouped according to age (as shown in Table 1).The age catego-ries were narrower for the younger groups as it was assumed that any observedchange in cochlear orientation was more likely to occur in the early years. All angles


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59COCHLEAR ROTATION AND ITS RELEVANCE TO COCHLEAR IMPLANTATION

figure 1 Axial high resolution CT scan of the temporal bones showing the basal turn ofthe cochlea in a six year old and the technique used to measure the angle of the basal turnrelative to the midsagittal plane. In this case the basal turn is obtusely angled with an angleof 63.21 degrees to the midline. Length refers to the length of the line representing themidline is not relevant to the measurement of basal turn angle.

were measured by a single observer (BK). However, in order to check intra-observererror, the measurements of thirty three randomly selected patients were repeated bythe same observer more than 1 week after the original measurements were taken.Similarly, inter-observer error was checked by comparing angles measured by the firstobserver in fifty nine randomly selected patients with those of a second observer. Forthe purposes of analysis, the right and left ear angles were averaged to produce asingle angle for each individual patient. A one way analysis of variance was used toidentify any statistically significant change in basal turn angulation. Inter- and intra-observer errors were calculated and presented as repeatability co-efficients according

to the Bland Altmann technique. The basal turn angles of the three difficult cases ofcochlear implantation were related to the findings.
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Results

All data were normally distributed. The mean basal turn angles for right and leftears respectively were 54.9 (Range 46.464.0; SD 3.7) and 54.2 (Range 45.564.3;SD 3.8). These were strongly correlated (Pearson correlation co-efficient = 0.768).Following averaging of the right and left basal turn angles for each patient, the meanbasal turn angle was 54.6 (Range 46.863.8; SD 3.5). This data is summarised inFigure 2. Table 1 summarises the basal turn angles according to age stratification.This data is shown in the form of a box-whisker chart in figure 3. There was a sta-tistically significant reduction in the angulation of the basal turn with increasing age(F= 10.1; p = 0.002). The majority of the change occured between the ages of 11 and15. The inter-observer reliability co-efficient was 4.8. The intra-observer reliabilityco-efficient was 2.0. The 3 difficult cases all had basal turn angles that were at theupper limit of the normal range (66, 63, 71).

Discussion

Current opinion in the literature investigating post-partum developmental changes inthe labyrinth is still divided. However, it would seem that some significant althoughsubtle changes in labyrinthine morphology may be possible even after completeossification of the otic capsule. Data regarding cochlear orientation in particular issparse but the results of Jeffery and Spoor suggest that there is a significant age-related reduction in the angle of the cochlea relative to the sagittal plane duringfoetal life although they found that postnatal changes in this angle were not statisti-

cally significant(2). The findings of this study suggest that there may be subtle change sin cochlear orientation post-partum. However, it is important to bear in mind that

TABLE 1

TABLE SHOWING THE MEAN BASAL TURN ANGULATION RELATIVE TO THE MIDSAGITTAL PLANEGROUPED INTO AGE CATEGORIES.

Age Category (years) n Mean (degrees) SD

01 6 55.5 4.025 14 57.0 3.9

610 10 55.3 5.0

1115 13 56.2 3.0

1620 15 54.8 3.8

2130 18 53.9 2.9

3140 24 54.5 2.7

4150 20 53.4 3.5

5160 16 53.2 3.6

6170 11 54.5 3.77185 12 53.6 2.5

Total 159 54.6 3.5


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figure 2 Histogram illustrating the distribution of cochlear angles relative to the sagittal

plane.

the measurement error inherent in the technique used in this study, although small,means that the results should be interpreted carefully.

It is possible that the observed changes in orientation of the basal turn of thecochlea do not reflect remodeling of the otic capsule itself but instead result fromchanges in the morphology of the cranium during post-partum development. It iswidely accepted that remodeling of the otic capsule is inhibited once the labyrinth isfully developed and that this inhibition is required for maintenance of normal hear-

ing. In contrast, the cranial vault and skull base undergo considerable change duringpost-partum development.

The growth of the cranium is not linear and differs between the sexes. There is aperiod of rapid growth between the ages of 1 and 4 years and another around theages of 11 and 12 years following which the spheno-occipital synchondrosis fuses(3).These periods of rapid growth occur earlier in females. The temporal changes docu-mented in this paper appear to mirror those of the cranium as a whole. However,it should be noted that the basicraniummay have a more linear growth patterncompared with the rest of the cranium.

It is clear from the results of this study, that there is considerable variation in the

normal range of cochlea orientation. This is consistent with the findings of otherauthors(4). It should also be noted that although congenitally abnormal cochleas were


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excluded from the study, during the screening process it was clear that the majorityof congenitally abnormal cochleas tended to have a greater basal turn angle relativeto the sagittal plane (unpublished data). These findings may have implications forcochlear implantation, particularly in the paediatric population. In the 3 cases ofdifficult cochlear implantation presented, each had a particularly obtuse basal turnangle that was at the upper extreme of the normal distribution curve. This may haveresulted in the difficulties experienced during implantation.

Kennedy has demonstrated using histological and endoscopic examination of thecochlea after electrode insertion that there is often significant damage to the spiralligament in the basal turn following insertion and that passage of the electrodebeyond the point at which resistance is felt can result in much more significantcochlear damage including damage to the basilar membrane(5). Although the clinicalsignificance of such damage remains unclear, it is likely that a smooth linear approachinto the basal turn is preferential to a more oblique approach. In cases with anobliquely angled basal turn it may be advantageous to consider an approach via themiddle ear rather than through a posterior tympanotomy. Other factors may alsoinfluence the ease of access to the basal turn. These include the size and orientation

of the facial recess, the position of the facial nerve and the chorda tympani togetherwith the orientation of the posterior canal wall of the external auditory canal.

figure 3 Box-whisker chart showing mean and quartiles for basal turn angles of each agecategory. Circles represent outlying cochlear angles.


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The position of the cochleostomy is critical if damage to the inner ear structures isto be avoided. This is particularly true now that it has become clear that it is possibleto preserve residual hearing in patients undergoing cochlear implantation. It shouldbe noted, however, that in the initial cases described here, no specific attempt wasmade to preserve hearing as the cases pre-dated the introduction of soft surgical tech-niques in our department. The ideal position of the cochleostomy in order to preventinner ear damage is controversial and recommendations in the literature vary fromdifferent positions in the otic capsule to round window insertion. A recent paper byLi et al.(6) has suggested that the bone around the round window niche should beremoved and that a cochleostomy immediately anterior and inferior to the roundwindow membrane should be performed as the critical structures of the inner ear areat their greatest distance from the cochleostomy site in this position and access to thescala tympani is optimal. Similarly, in the cochlear implant patients included in thecurrent study, it was advantageous to remove bone around the round window inorder to identify the scala tympani.

Conclusions

There is considerable variation in the normal range of cochlear orientation betweenindividuals and there appears to be a significant reduction in the angle of the basalturn of the cochlea relative to the midsagittal plane with increasing age. This findingchallenges the traditional view that the morphology of the labyrinth does not changefollowing birth. However, care should be taken in interpreting these results bearingin mind the measurement error inherent in the study design.

Obtuse angulation of the basal turn relative to the sagittal plane may make

cochlear implantation more difficult. Children are more likely to have an obtuselyangled basal turn that may predispose children to difficult implantation. We recom-mend measuring the angle of the basal turn of the cochlea relative to the mid-sagittalplane in all preoperative scans, to identify cases in which the basal turn of the cochleamay be in an unusual location.

References

Bast, T.H. 1942. Development of the otic capsule. VI. Histological changes and variations in the growing bonycapsule of the vestibule and cochlea. Ann Otol Rhinol Laryngol , 51: 34357.

Jeffery, N. & Spoor, F. 2004 Feb. Prenatal growth and development of the modern human labyrinth. J Anat ,204(2): 7192.

Farkas, L.G., Posnick, J.C. & Hreczko, T.M. 1992 Jul. Anthropometric growth study of the head. Cleft PalateCraniofac J , 29(4): 3038.

Erixon, E., Hogstorp, H., Wadin, K. & Rask-Andersen, H. 2009 Jan. Variational anatomy of the human cochlea:implications for cochlear implantation. Otol Neurotol , 30(1): 1422.

Kennedy, D.W. 1987 Jan. Multichannel intracochlear electrodes: mechanism of insertion trauma. Laryngoscope ,97(1): 429.

Li, P.M., Wang, H., Northrop, C., Merchant, S.N. & Nadol, J.B., Jr. 2007 Aug. Anatomy of the round windowand hook region of the cochlea with implications for cochlear implantation and other endocochlear surgicalprocedures. Otol Neurotol , 28(5): 6418.

Email: [email protected]
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Cochlear Rotation and Its Relevance

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