Taste change following cochlear implantation

Taste change following cochlear implantation

SIMON LLOYD, LEAH MEERTON, RICCARDO DI CUFFA, JEREMY LAVY, JOHN GRAHAM, Cochlear Implant Programme, Royal National Throat Nose and Ear Hospital, London, UK

ABSTRACT Taste change is a well documented but under emphasised complication of middle ear surgery and is usually related to damage to the chorda tympani nerve (CTN). However, the taste outcomes following cochlear implantation have not been previously recorded. One hundred and forty one patients who had received cochlear implants between January 1997 and April 2006 were questioned using a postal survey regarding changes in the sense of taste following cochlear implantation. Sixty seven per cent of questionnaires were returned. Forty three patients (45%) experienced changes in taste following their surgery. Eighteen patients (19%) said that their symptoms had not resolved by the end of the follow up period (mean 51 months). CTN division produced taste change in 86%. Fifty per cent of patients who had CTN preservation developed taste change, presumably related to occult trauma to the nerve. The difference in the number of patients describing taste change in the two groups was statistically signifi cant (p < 0.05). Resolution of symptoms took a mean of 20 weeks. For those patients in whom taste change was noted, resolution of symptoms was more likely if the CTN was divided rather than preserved although the difference was not statistically signifi cant (p < 0.5). This may refl ect neuronal injury during surgery which then fails to heal normally. These results are comparable to taste outcomes in patients undertaking middle ear surgery for non-infl ammatory disease such as stapedectomy. The symptoms can be extremely troublesome and may not resolve with time. Careful consideration of the CTN intra-operatively is recommended and in the event of injury, the taste outcome may be better if the nerve is divided rather than left in situ but damaged. These fi ndings have signifi cant implications for patients as bilateral implantation becomes more commonplace. Copyright © 2007 John Wiley & Sons, Ltd.

Keywords: tastes, cochlear implant, chorda tympani

Cochlear Implants InternationalCochlear Implants Int. 8(4), 203–210, 2007Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/cii.342

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Cochlear Implants Int. 8(4), 203–210, 2007Copyright © 2007 John Wiley & Sons, Ltd DOI: 10.1002/cii

Lloyd et al.204

Introduction

The chorda tympani nerve (CTN) carries pre-ganglionic secretomotor fi bres to the submandibular ganglion which then pass to the submandibular and sublingual glands. It also carries taste fi bres from the fungiform and fi liform papilliae in the anterior two thirds of the tongue. These fi bres pass in the lingual nerve, through the infratemporal fossa and enter the tympanic cavity as the CTN through the petrotympanic fi ssure. The nerve passes posteriorly across the tympanic cavity in the upper part of the tympanic membrane, between its inner and middle layers, passing medial to the neck of the malleus. It then turns inferiorly to join the verti-cal portion of the facial nerve within the mastoid bone.

During surgery, the nerve can be damaged at any point in its passage across the middle ear or within the mastoid bone. This often results in a change in the sensa-tion of taste (dysgeusia) (Bull, 1965; Moon and Pullen, 1963; Gopalan et al., 2005; Saito et al., 2001; Nin et al., 2006). This has been well documented in stapes surgery (Bull, 1965; Moon and Pullen, 1963) and, to a lesser extent, in other types of middle ear surgery (Gopalan et al., 2005; Saito et al., 2001; Nin et al., 2006). The characteristics and duration of this taste change are variable and depend on the nature of the injury to the nerve. The patient may complain of a metallic, bitter or salty taste in the mouth (Bull, 1965). Alternatively, they may describe a numbness of the tongue. These symptoms may improve or even resolve with time but they can have a considerable impact on the quality of life of patients which is often under-appreciated by the surgeon.

To date, there has been no documentation of taste outcomes following cochlear implantation. During the approach to implantation it is necessary to perform a posterior tympanotomy. This triangular space allows access to the middle ear so that a cochleostomy can be performed. The boundaries of the posterior tympanot-omy are the facial nerve posteriorly, the CTN anteriorly and the fossa incudis superiorly. While in a proportion of patients, the CTN may not be exposed during the drilling of a posterior tympanotomy, the nerve is at risk and occasionally it is necessary to sacrifi ce the nerve in order to obtain adequate access. Thus, as with other forms of middle ear surgery, taste change is a possible complication of coch-lear implantation and is the subject of this study.

Methods

This study is a retrospective survey conducted using a postal questionnaire.English speaking patients over the age of 18 who had undergone cochlear

implantation between January 1997 and April 2006 were identifi ed. Each patient was sent a questionnaire asking about their sense of taste pre-operatively; whether there had been any taste change following the operation and if so what the char-acteristics of the taste change were; how long any taste change had lasted; whether they still had symptomatic taste change. They were then asked to score the degree of taste abnormality from 1 to 7 (where 1 was normal taste and 7 very abnormal


Taste change following cochlear implantation 205

taste) pre-operatively, 2 weeks post-operatively and currently. A notes review was then performed to investigate whether the CTN had been seen intra-operatively; whether the nerve had been divided; the type of implant inserted. Statistical analysis was performed using the chi square test for categorical data and the Student t test for quantitative data. Statistical signifi cance was defi ned as p < 0.05.

Results

One hundred and forty one questionnaires were posted. Ninety six were returned giving a return rate of 67%. The mean age at implantation was 57 with a range of 23 to 87. The male to female ratio was 11:13. Forty fi ve patients had an implant on the right and 51 had an implant on the left. Sixty nine patients had a Nucleus device, 22 had an Advanced Bionics device and fi ve had a Med-El device. The mean follow up period was 51 months with a range of 2 to 99 months.

Ninety fi ve patients had normal taste pre-operatively. Forty three patients (45%) had abnormal taste post-operatively. Eighteen of these patients continued to complain of symptoms throughout the follow up period representing a long term taste change in 19% of patients. The characteristics of the taste change are shown in Table 1. The majority of patients complained of a metallic taste in their mouth but the symptoms described amongst the whole study population were quite diverse. The mean time to recovery was 20 weeks with a range of 1 to 52 weeks.

Table 2 shows the taste scores at each of the time points recorded for the whole population. The mean taste score post-operatively was 2.89 which dropped to 1.73 at the end of the follow up period refl ecting an improvement in taste sensation in most patients with time. These were both signifi cantly different from the pre-opera-tive scores, at the p = 0.005 level. If the data for those patients whose taste was affected are isolated the mean taste score for this sub-population was 5.11 with a mode of 7. This suggests that when taste change occurs the symptoms are quite

Table 1: Characteristics of taste change

Taste No. of patients

Metallic 12No taste 6Numbness 3Dry 4Salty 3Tingly 2Bitter 1Burnt 1Other 7


Lloyd et al.206

severe. At the end of the follow up period the mean score for this group was 2.54 with a mode of 1 refl ecting resolution of symptoms in the majority of patients.

Of the 96 patients included in the study, 72 sets of notes were identifi ed. Review of the operating records showed that 24 patients had CTN preservation docu-mented. Fourteen had their CTN divided. In 34 patients CTN status was not recorded. This either refl ects the fact that the CTN was not seen or that its status was simply not documented. Table 3 relates the taste symptoms to the intra- operative CTN status. Among those patients who had no taste change, there was a high rate of CTN preservation, as one would expect. Interestingly, two patients had their CTN divided but did not notice any taste change. Some of those that had temporary taste change had had their CTN divided but a signifi cant number had documented CTN preservation. Similarly, in those with permanent taste change, there were a number with CTN division but a higher proportion with CTN preservation.

If these data are analysed in a different way (as shown in Table 4) CTN section-ing resulted in symptoms in 86% of patients. Sixty two per cent of these had reso-lution of their symptoms. CTN preservation produced symptoms in 50% of patients of whom only 42% had resolution of their symptoms. The difference between the chorda preserved group and the chorda sacrifi ced group in terms of the numbers of

Table 2: Mean taste scores: 1 = normal; 7 = very abnormal

Whole population

Pre-op Post-op Current

1.10 2.89 1.73

Patients with taste change

Post-op Current

Mean Mode Mean Mode

5.11 7 2.54 1

Table 3: Relationship between taste change and chorda status

No change (n = 30) Temporary (n = 25) Permanent (n = 17)

Chorda status Chorda status Chorda status

P Di NM P Di NM P Di NM12 2 16 5 8 12 7 4 6

P = chorda preserved; Di = chorda divided; NM = no mention of chorda status



patients with symptoms was statistically different (p < 0.05). For those that had taste change, the difference in the numbers of patients whose taste change resolved in the two groups was not statistically signifi cant (p < 0.5). The time to resolution of symptoms in the CTN preserved and sacrifi ced groups were 7 weeks and 23 weeks, respectively. Whilst this is a considerable difference it was not possible to analyse these data statistically because of the low number of patients in each group.

When the CTN was not mentioned in the operating record, there were still a large number who described symptoms. This presumably represents undocumented division or damage to the CTN or that the CTN was thought to be intact and therefore not mentioned but was in actual fact damaged without the knowledge of the surgeon.

Discussion

As one would expect, absence of symptoms in this study group seems to refl ect preservation of the CTN in most cases although there were two exceptions in which the CTN was divided but the patients’ taste was preserved. Documented CTN preservation does not however, necessarily mean that taste sensation is pre-served. In this study 50% of patients thought to have CTN preservation had symptoms. This either refl ects inaccurate documentation of CTN status or unno-ticed damage to the nerve. It is possible that such damage could occur without exposure of the nerve as a result of transmitted thermal injury from the diamond burr. Conversely, CTN sectioning appeared to result in taste change in 86% of cases.

It is interesting to examine the number of patients whose symptoms resolve in the CTN preserved and divided groups. It is clear that although division of the nerve results in a large number of patients with symptoms, a relatively high propor-tion of these patients (67%) have resolution of their symptoms. This contrasts with those patients in the CTN preservation group of whom only 42% had resolution of their symptoms. This suggests that incomplete injury to the nerve is more likely than complete division of the nerve to result in permanent taste change. This phenomenon has been documented elsewhere in the literature (Gopalan et al., 2005; Saito et al., 2001; Saito et al., 2002).

There is no literature available on taste outcomes following cochlear implant surgery. The closest comparison in terms of surgical procedure and risk to the CTN

Table 4: Relationship between chorda status and taste symptoms

TN sectioned (n = 14) CTN preserved (n = 24) CTN not mentioned (n = 34)

12 (86%)67% resolved



comes from data on taste change following stapedectomy (Bull, 1965; Moon and Pullen, 1963). The CTN is always within the surgical fi eld during this proce-dure and, apart from otosclerosis, these ears are usually free of disease, either previ-ously or currently. In such series, the proportion of patients experiencing taste change post-operatively is around 45% (Moon and Pullen, 1963) which compares closely with the results of this study. Similarly, resolution of symptoms occurs in around 75% by 6 months. In Bull’s series (1965), CTN division resulted in symp-toms in around 80% of cases, which again is remarkably similar to the results here.

The taste outcomes in otological series including surgery for chronic middle ear disease are slightly different. Gopalan et al. (2005) found that 14% of patients had taste disturbance post-operatively but 92% of these had recovered by 1 year. They noted that stretching of the nerve resulted in a higher likelihood of symptoms (28%) than nerve sectioning (13%). It appears from their data that patients are much less likely to develop changes in taste following surgery on ears which have been chronically infl amed. The proportion with symptoms in this group was 10% compared to 57% in patients having surgery for non-infl ammatory disease. Saito et al. (2001) published similar results although their data suggested that sectioning of the nerve results in a higher proportion of symptomatic patients than touching the nerve. They did not have a category for stretching or otherwise traumatising the nerve and it is unclear how they defi ne touching. The relatively low incidence of taste disturbance in patients undergoing surgery for chronic middle ear disease may result from a gradual deterioration in CTN function as a result of the chronic infl ammation. There may be a gradual compensation for this with the resultant absence of taste defi ciency.

It therefore appears that the results of this study are comparable to the taste outcomes on studies looking at non-infl ammatory disease of the middle ear. The importance of looking after the CTN is highlighted by these results. In particular, the surgeon needs to be aware that the CTN can be damaged even if it is not exposed within the mastoid bone. This may refl ect thermal injury, and irrigation during drilling of the posterior tympanotomy should minimise the risk of damage in this situation. It also appears that, in the event that the CTN is damaged, it may be better in terms of long term taste outcomes to completely divide the nerve rather than leave a damaged nerve in situ.

There are a number of mechanisms by which taste function may regenerate following injury to the chorda. Saito et al. (2002) described regeneration of the CTN in 42% of patients following its division during middle ear surgery for chole-steatoma. However, the number of nerve fi bres within the regenerated CTN was considerably reduced compared to healthy nerves. Saito et al. also described recov-ery of function in intact nerves, presumably resulting from neuropraxia. Kveton and Bartoshuk (1994) put forward the proposal that the CTN serves to inhibit the glossopharyngeal nerve whilst it is intact. Once the CTN is divided this inhibition is lost and the glossopharyngeal nerve takes over taste sensation in the anterior tongue with a resultant recovery in the sense of taste. A further proposed


208 Lloyd et al.



mechanism is that of contralateral neuronal ingrowth, whereby nerves adjacent to the dysgeustic area grow into the affected area to replace the fi bres that have been lost. Finally, the overall sense of taste may not result from neuronal input from specifi c areas of the tongue but from the mouth as a whole. Thus loss of a small part of the overall surface area available for taste, as in CTN injury, may not have a long term effect on the ‘whole mouth taste sensation’.

This study provides some insight into the taste outcomes following cochlear implantation. However, the data were retrospectively collected and the return rate, although acceptable for a postal study, could have been higher. Since the patients were responding to the questionnaire after some time had elapsed follow-ing their original surgery, the responses may well have been infl uenced by the accuracy of the patients’ own recollections. A prospective study is currently under-way in which careful documentation of each patient’s post-operative taste sensa-tion is performed using a similar questionnaire to the one used for this study. There are no validated taste questionnaires directly applicable to otological surgery, although there is a validated instrument on smell and taste used in patients with head and neck cancer (Goldberg et al., 2005). It was felt that this was not appro-priate in an otological setting. CTN status is also being recorded more rigorously during implantation. Electogustatometry and objective assessment of taste could also be performed but these often have little relevance to clinical outcome in terms of symptoms.

There is increasing evidence that bilateral implantation provides better hearing outcomes than single side implantation and as a result the number of bilateral implantees will inevitably increase over the next few years. CTN preservation will become an extremely important issue in these patients as bilateral nerve damage may leave these patients with a signifi cantly reduced quality of life, returning one sense modality at the cost of another. Increased awareness of the CTN in patients undergoing implantation should reduce the risk of its damage and avoid unneces-sary suffering on the part of the patient.

References

Bull TR (1965) Taste and chorda tympani. Journal of Laryngology and Otology 79: 479–493.Goldberg AN, Shea JA, Deems DA, Doty RL (2005) A chemosensory questionnaire for patients

treated for cancer of the head and neck. Laryngoscope 115: 2077–2086.Gopalan P, Kumar M, Gupta D, Phillipps JJ (2005) A study of chorda tympani nerve injury and

related symptoms following middle ear surgery. Journal of Laryngology and Otology 119:189–192.

Kveton JF, Bartoshuk LM (1994) The effect of unilateral chorda tympani damage on taste. Laryngo-scope 104: 25–29.

Moon CN, Pullen EW (1963) Effects of chorda tympani section during middle ear surgery. Laryngo-scope 73:392–405.

Nin T, Sakagami M, Sone-Okunaka M, Muto T, Mishiro Y, Fukazawa K (2006) Taste function after section of chorda tympani nerve in middle ear surgery. Auris Nasus Larynx 33: 13–17.

Saito T, Manabe Y, Shibamori Y, Yamagishi T, Igawa H, Tokuriki M et al. (2001) Long term follow up results of electrogustometry and subjective taste disorder after middle ear surgery. Laryngoscope 111: 2064–2070.


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Saito T, Shibamori Y, Manabe Y, Yamagishi T, Igawa H, Ohtsubo T et al. (2002) Incidence of regeneration of the chorda tympani nerve after middle ear surgery. Ann Otol Rhinol Laryngol 111: 357–363.

Address correspondence to: Mr S. Lloyd, Flat 2, 7 Eton Avenue, London NW3 3EL, UK. Email: [email protected]

Taste change following cochlear implantation

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