Morphological and functional study of regenerated chorda tympani nerves in humans

Taste Disturbance After Endoscopic Tympanoplasty with Tragal Cartilage Graft and Improvement Strategies

Objectives: To explore the risk factors of early postoperative taste disturbance (EPTD) after type I endoscopic tympanoplasty and operative modification strategies to improve taste disturbance. Methods: This was a controlled study. One hundred and twenty-four patients who underwent type I endoscopic tympanoplasty with tragal cartilage graft were separated evenly into control and modified groups. The full-thickness tragus cartilage graft was placed close to the bony annulus to ensure drum integrity in the control group, and in the modified group, the cartilage graft was not in contact with the posterior-superior bony annulus, and the inferior-posterior of the scutum. Univariate and multivariate models were used to analyze the possible factors affecting EPTD and the prognosis of taste recovery. Results: The incidence of EPTD was significantly lower in the modification group (24.19%) than in the control group (56.45%) (OR: 4.24, 95% CI: 1.93-9.33, P < .001). Surgical manipulation of the chorda tympani nerve resulted in a higher incidence of EPTD (OR: 2.43; 95% CI: 1.06-5.57, P = .035). The size of the graft did not affect taste disturbance. No difference in the taste recovery rate was observed between the control and test groups (Z = -1.57, P = .116) after 3 months. The recovery rate of patients with manipulated chorda tympani nerves was still lower than that of patients without at 3 months (Z = -2.74, P = .006). Conclusion: Modified surgery and reduced manipulation of the chorda tympani nerve effectively reduce EPTD. Manipulated chorda tympani nerves may have a persistent effect on taste recovery.

Impact of Nutritional Intervention on Taste Perception-A Scoping Review

The aim of the present scoping review was to evaluate the impact of experimental meal loads or observational diet changes/habits on taste tests in both healthy subjects and patients. A systematic search performed in PubMed, Scopus, and Institute for Scientific Information (ISI) Web of Science electronic databases retrieved, respectively 2981, 6258, and 7555 articles from January 2000 to December 2020. A total of 17 articles were included for full-text review. Literature results were stratified according to the observational/interventional approach, the involvement of healthy subjects or patients, the taste test, and the meal/dietary changes. The present scoping review reinforced the notions postulating that certain taste tests (for example focusing on fatty acid, salt, or sugar) might be specifically influenced by the nutritional intervention and that other ones might be susceptible to a wide span of changes beyond the extent of tastant included in the specific food changes. This could also depend on the inhomogeneity of literature trend: The short duration of the intervention or the random type of meal load, unsuitability of the taste test chosen, and the presence of underlying disorders. Future studies for a better comprehension of taste tests reliability in relation to specific food changes are thus to be fostered.

Hedgehog Signaling Regulates Taste Organs and Oral Sensation: Distinctive Roles in the Epithelium, Stroma, and Innervation

The Hedgehog (Hh) pathway has regulatory roles in maintaining and restoring lingual taste organs, the papillae and taste buds, and taste sensation. Taste buds and taste nerve responses are eliminated if Hh signaling is genetically suppressed or pharmacologically inhibited, but regeneration can occur if signaling is reactivated within the lingual epithelium. Whereas Hh pathway disruption alters taste sensation, tactile and cold responses remain intact, indicating that Hh signaling is modality-specific in regulation of tongue sensation. However, although Hh regulation is essential in taste, the basic biology of pathway controls is not fully understood. With recent demonstrations that sonic hedgehog (Shh) is within both taste buds and the innervating ganglion neurons/nerve fibers, it is compelling to consider Hh signaling throughout the tongue and taste organ cell and tissue compartments. Distinctive signaling centers and niches are reviewed in taste papilla epithelium, taste buds, basal lamina, fibroblasts and lamellipodia, lingual nerves, and sensory ganglia. Several new roles for the innervation in lingual Hh signaling are proposed. Hh signaling within the lingual epithelium and an intact innervation each is necessary, but only together are sufficient to sustain and restore taste buds. Importantly, patients who use Hh pathway inhibiting drugs confront an altered chemosensory world with loss of taste buds and taste responses, intact lingual touch and cold sensation, and taste recovery after drug discontinuation.

Regenerative peripheral neuropathic pain: novel pathological pain, new therapeutic dimension

After peripheral nerve damage, injured or stressed primary sensory neurons (PSNs) transmitting pathological pain (pathopain) sensitize central nervous system (CNS) neural circuits and determine behavioral phenotypes of peripheral neuropathic pain (PNP). Therefore, phenotypic profiling of pathopain-transmitting PSNs is vital for probing and discovering PNP conditions. Following peripheral nerve injuries (PNIs), PNP might be potentially transmitted by distinct classes of damaged or stressed PSNs, such as axotomized PSNs without regeneration (axotomy-non-regenerative neurons), axotomized PSNs with accurate regeneration (axotomy-regenerative neurons), and spared intact PSNs adjacent to axotomized neurons (axotomy-spared neurons). Both axotomy-non-regenerative neurons and axotomy-spared neurons have been definitely shown to participate in specific PNP transmission. However, whether axotomy-regenerative neurons could transmit PNP with unique features has remained unclear. Recent studies in rodent models of axonotmesis have clearly demonstrated that axotomy-regenerative neurons alone transmit persistent pathological pain with unique behavioral phenotypes. In this review, we exclusively review this novel category of PNP, reasonably term it ‘regenerative peripheral neuropathic pain’, and finally discuss its potential clinical significance as a new therapeutic dimension for PNIs beyond nerve regeneration.

Oral sensory nerve damage: Causes and consequences

Oral sensations (i.e., taste, oral somatosensation, retronasal olfaction) are integrated into a composite sense of flavor, which guides dietary choices with long-term health impact. The nerves carrying this input are vulnerable to peripheral damage from multiple sources (e.g., otitis media, tonsillectomy, head injury), and this regional damage can boost sensations elsewhere in the mouth because of central interactions among nerve targets. Mutual inhibition governs this compensatory process, but individual differences lead to variation in whole-mouth outcomes: some individuals are unaffected, others experience severe loss, and some encounter sensory increases that may (if experienced early in life) elevate sweet-fat palatability and body mass. Phantom taste, touch, or pain sensations (e.g., burning mouth syndrome) may also occur, particularly in those expressing the most taste buds. To identify and treat these conditions effectively, emerging clinical tests measure regional vs. whole-mouth sensation, stimulated vs. phantom cues, and oral anatomy. Scaling methods allowing valid group comparisons have strongly aided these efforts. Overall, advances in measuring oral sensory function in health and disease show promise for understanding the varied clinical consequences of nerve damage.

Long-term Follow-up Results of Regeneration Process of Fungiform Taste Buds After Severing the Chorda Tympani Nerve During Middle Ear Surgery

OBJECTIVE

To elucidate the regeneration process of fungiform taste buds after severing the chorda tympani nerve (CTN) by confocal laser scanning microscopy in vivo.

METHODS

In 7 consecutive patients whose CTN was severed during tympanoplasty, an average of 10 fungiform papillae in the midlateral region of the tongue were periodically observed, and the number of taste buds was counted until 12 to 24 months after surgery. Gustatory function was assessed by EGM.

RESULTS

EGM thresholds showed no response within 1 month after surgery in any patient. All taste buds had disappeared until 13 to 71 days after surgery. Regenerated taste buds were first detected 5 to 8 months after surgery in 5 of the 7 patients. EGM thresholds recovered to their preoperative values in 2 patients. In these 2 patients, the number of regenerated taste buds gradually increased in combination with a recovered taste function. However, a time lag existed between taste bud regeneration and taste function recovery. EGM thresholds did not recover in the other 3 patients with regenerated taste buds, suggesting that these taste buds were immature without gustatory function.

CONCLUSION

The long-term regeneration process of fungiform taste buds could be clarified using confocal laser scanning microscopy.

The chorda tympani degenerates during chronic otitis media: an electron microscopy study

CONCLUSION

Chorda tympani nerve specimens from ears with chronic inflammatory middle ear disease exhibit structural signs of degeneration. These correlate well with taste disturbance. Simultaneously, they exhibit signs of regeneration, which may explain the ability for taste recovery.

OBJECTIVES

The chorda tympani, the major taste nerve, runs uncovered through the middle ear cavity. This situation exposes it to various forms of middle ear pathology. A difference has been noticed regarding taste symptoms pre- and postoperatively between inflammatory and non-inflammatory diseases. The present study aimed to investigate ultrastructural changes of chorda tympani in different forms of inflammatory middle ear disease, such as chronic suppurative otitis media and cholesteatoma, as compared with normal.

METHODS

Five chorda tympani specimens were collected from healthy middle ears of patients subjected to surgery for acoustic neuroma, to be used as normal controls, and five from middle ears with chronic otitis media or cholesteatoma, where the nerve could not be saved during the operation. Light and electron microscopy were performed.

RESULTS

For all five nerves from diseased ears, microscopy showed a higher percentage of axon and myelin sheath degeneration than in the normal controls. Furthermore, three of the five also exhibited sprouting.

Surgery of the ear and the lateral skull base: pitfalls and complications

Surgery of the ear and the lateral skull base is a fascinating, yet challenging field in otorhinolaryngology. A thorough knowledge of the associated complications and pitfalls is indispensable for the surgeon, not only to provide the best possible care to his patients, but also to further improve his surgical skills. Following a summary about general aspects in pre-, intra-and postoperative care of patients with disorders of the ear/lateral skull base, this article covers the most common pitfalls and complications in stapes surgery, cochlear implantation and surgery of vestibular schwannomas and jugulotympanal paragangliomas. Based on these exemplary procedures, basic "dos and don'ts" of skull base surgery are explained, which the reader can easily transfer to other disorders. Special emphasis is laid on functional aspects, such as hearing, balance and facial nerve function. Furthermore, the topics of infection, bleeding, skull base defects, quality of life and indication for revision surgery are discussed. An open communication about complications and pitfalls in ear/lateral skull base surgery among surgeons is a prerequisite for the further advancement of this fascinating field in ENT surgery. This article is meant to be a contribution to this process.

Impact of chorda tympani nerve injury on cell survival, axon maintenance, and morphology of the chorda tympani nerve terminal field in the nucleus of the solitary tract

Chorda tympani nerve transection (CTX) has been useful to study the relationship between nerve and taste buds in fungiform papillae. This work demonstrated that the morphological integrity of taste buds depends on their innervation. Considerable research focused on the effects of CTX on peripheral gustatory structures, but much less research has focused on the central effects. Here, we explored how CTX affects ganglion cell survival, maintenance of injured peripheral axons, and the chorda tympani nerve terminal field organization in the nucleus of the solitary tract (NTS). After CTX in adult rats, the chorda tympani nerve was labeled with biotinylated dextran amine at 3, 7, 14, 30, and 60 days post-CTX to allow visualization of the terminal field associated with peripheral processes. There was a significant and persistent reduction of the labeled chorda tympani nerve terminal field volume and density in the NTS following CTX. Compared with controls, the volume of the labeled terminal field was not altered at 3 or 7 days post-CTX; however, it was significantly reduced by 44% and by 63% at 30 and 60 days post-CTX, respectively. Changes in the density of labeled terminal field in the NTS paralleled the terminal field volume results. The dramatic decrease in labeled terminal field size post-CTX cannot be explained by a loss of geniculate ganglion neurons or degeneration of central axons. Instead, the function and/or maintenance of the peripheral axonal process appear to be affected. These new results have implications for long-term functional and behavioral alterations.

Taste function after stapes surgery

OBJECTIVE

A manipulation of chorda tympani nerve (CTN) is frequently necessary during the surgical therapy of stapedial ankylosis. The aim of this study was to re-assess the taste function before and after stapes surgery in patients with unilateral stapes ankylosis.

METHODS

Eighteen patients (14 female and 4 male) with unilateral stapedial ankylosis were included. Taste and olfactory function were measured preoperatively, 3 days and 3 months after surgery by questionnaire, chemical taste test, electrogustometry, and Sniffin'Sticks. The patients who reported deterioration in taste and revealed pathological taste test results were re-investigated 8-12 months after surgery. Postoperatively 11 patients were treated intravenously for 3 days for inner ear protection with 1g of cortisone. The gustatory and olfactory results were compared with age and sex specific normative data.

RESULTS

A significant transient decrease of gustatory measurement values was found on the ipsilateral two thirds of the anterior tongue. 12 out of 17 patients whose CTN was slightly manipulated during stapes surgery reported tongue sensations, such as numbness or gustatory blindness of the ipsilateral tongue side. The measures of chemical taste test significantly decrease in lateralized taste test on the ipsilateral side and in whole mouth testing. There was no significant increase of the EGM measures at the ipslateral tongue. In 11 of 12 patients with symptoms, complaints recovered within one year after surgery completely. The factor "cortisone" did not have a significant effect on the taste test results after surgery.

CONCLUSION

In conclusion, even after minor CTN manipulation the rate of postoperative taste disorders or tongue symptoms after stapes surgery is high. The symptoms appear to be transient. Therefore CTN should be preserved in stapes surgery after mild trauma of the CTN. Transient taste alteration should be mentioned prior to stapes surgery.

Light and Electron Microscopic Observation of Regenerated Fungiform Taste Buds in Patients with Recovered Taste Function after Severing Chorda Tympani Nerve

Objectives:

The aim of this study was to evaluate the mean number of regenerated fungiform taste buds per papilla and perform light and electron microscopic observation of taste buds in patients with recovered taste function after severing the chorda tympani nerve during middle ear surgery.

Methods:

We performed a biopsy on the fungiform papillae (FP) in the midlateral region of the dorsal surface of the tongue from 5 control volunteers (33 total FP) and from 7 and 5 patients with and without taste recovery (34 and 29 FP, respectively) 3 years 6 months to 18 years after surgery. The specimens were observed by light and transmission electron microscopy. The taste function was evaluated by electrogustometry.

Results:

The mean number of taste buds in the FP of patients with completely recovered taste function was significantly smaller (1.9 ± 1.4 per papilla; p < 0.01) than that of the control subjects (3.8 ± 2.2 per papilla). By transmission electron microscopy, 4 distinct types of cell (type I, II, III, and basal cells) were identified in the regenerated taste buds. Nerve fibers and nerve terminals were also found in the taste buds.

Conclusions:

It was clarified that taste buds containing taste cells and nerve endings do regenerate in the FP of patients with recovered taste function.

Potassium channels as a potential therapeutic target for trigeminal neuropathic and inflammatory pain

Previous studies in several different trigeminal nerve injury/inflammation models indicated that the hyperexcitability of primary afferent neurons contributes to the pain pathway underlying mechanical allodynia. Although multiple types of voltage-gated ion channels are associated with neuronal hyperexcitability, voltage-gated K⁺ channels (Kv) are one of the important physiological regulators of membrane potentials in excitable tissues, including nociceptive sensory neurons. Since the opening of K⁺ channels leads to hyperpolarization of cell membrane and a consequent decrease in cell excitability, several Kv channels have been proposed as potential target candidates for pain therapy. In this review, we focus on common changes measured in the Kv channels of several different trigeminal neuropathic/inflammatory pain animal models, particularly the relationship between changes in Kv channels and the excitability of trigeminal ganglion (TRG) neurons. We also discuss the potential of Kv channel openers as therapeutic agents for trigeminal neuropathic/inflammatory pain, such as mechanical allodynia.

Cutting the chorda tympani: not just a matter of taste

Introduction:

Chorda tympani injury as a complication of middle-ear surgery has been extensively studied with regard to its effects upon taste. However, the chorda tympani also carries parasympathetic fibres to the salivary glands of the oral cavity. To date, little has been reported about the effect of chorda tympani section upon salivary function.

Setting:

Tertiary care centre.

Material and methods:

We report a case series of three patients with bilateral chorda tympani lesions. Chorda tympani function was assessed using ‘taste strips’ and unstimulated sialometry. A careful history of oral symptoms was taken.

Results:

All patients showed transient or permanent bilateral ageusia of the anterior two-thirds of the tongue, and a decreased resting salivary flow rate. In addition, all patients suffered from transient or persistent, distressing xerostomia.

Conclusion:

Taste disorders may occur after middle-ear surgery but they are mostly transient, even when the chorda tympani nerves are sectioned bilaterally. In contrast, bilateral chorda tympani lesions may lead to severe, persistent and distressing xerostomia. Based on this neglected aspect of chorda tympani function, we emphasise the importance of preserving the chorda tympani whenever possible.

Alteration of primary afferent activity following inferior alveolar nerve transection in rats

Background

In order to evaluate the neural mechanisms underlying the abnormal facial pain that may develop following regeneration of the injured inferior alveolar nerve (IAN), the properties of the IAN innervated in the mental region were analyzed.

Results

Fluorogold (FG) injection into the mental region 14 days after IAN transection showed massive labeling of trigeminal ganglion (TG). The escape threshold to mechanical stimulation of the mental skin was significantly lower (i.e. mechanical allodynia) at 11-14 days after IAN transection than before surgery. The background activity, mechanically evoked responses and afterdischarges of IAN Aδ-fibers were significantly higher in IAN-transected rats than naive. The small/medium diameter TG neurons showed an increase in both tetrodotoxin (TTX)-resistant (TTX-R) and -sensitive (TTX-S) sodium currents (I_Na) and decrease in total potassium current, transient current (I_A) and sustained current (I_K) in IAN-transected rats. The amplitude, overshoot amplitude and number of action potentials evoked by the depolarizing pulses after 1 μM TTX administration in TG neurons were significantly higher, whereas the threshold current to elicit spikes was smaller in IAN-transected rats than naive. Resting membrane potential was significantly smaller in IAN-transected rats than that of naive.

Conclusions

These data suggest that the increase in both TTX-S I_Na and TTX-R I_Na and the decrease in I_A and I_k in small/medium TG neurons in IAN-transected rats are involved in the activation of spike generation, resulting in hyperexcitability of Aδ-IAN fibers innervating the mental region after IAN transection.

Chorda tympani nerve function after middle ear surgery

OBJECTIVE

To assess chorda tympani nerve function before and after different types of middle ear surgery. The hypothesis was that in cases with cholesteatoma, iatrogenic injury to the nerve results in less postoperative taste disturbance than those undergoing myringoplasty or stapedectomy.

STUDY DESIGN

Prospective questionnaire study.

SETTING

ENT Department of a district general hospital in England.

PATIENTS

Adult patients undergoing middle ear surgery in which there would be elevation of the anulus.

MAIN OUTCOME MEASURE

Patients' change in taste sensation after surgery with follow-up to demonstrate subsequent recovery.

RESULTS

A significantly higher incidence of taste disturbance follows myringoplasty and stapedectomy than for procedures in which the initial diagnosis was cholesteatoma, regardless of the extent of injury sustained by the nerve. The most common complaint after chorda tympani nerve injury is of a metallic taste sensation.

CONCLUSION

Despite the small patient cohort, the results support the hypothesis that iatrogenic chorda tympani nerve injury in surgery for cholesteatoma results in less postoperative taste disturbance than that for otosclerosis. This may be because the disease process of cholesteatoma renders the nerve hypofunctional. The situation after myringoplasty is less clear. A larger study may help validate these findings.

Gustatory function in chronic inflammatory middle ear diseases

HYPOTHESIS

Changes of gustatory function after ear surgery have been studied extensively. However, little is known on the influence of repeated/chronic inflammation within the middle ear on taste.

STUDY DESIGN

Prospective study.

MATERIALS AND METHODS

Forty-six patients suffering from either cholesteatoma (n = 25) or chronic otitis media mesotympanalis (n = 21) received quantitative gustatory tests. None of these patients had been operated on before these investigations.

RESULTS

Side by side comparison showed a significantly lower taste function on the anterior two thirds of the tongue ipsilateral to the site of inflammation, regardless of the diagnosis. Further analyses exhibited a trend toward greater impairment in relation to the severity of the inflammatory process.

CONCLUSION

These data are proof that taste function changes in relation to chronic middle ear diseases. It further shows that many of these alterations go unnoticed by the patients.

Evaluation of Taste Sensation Following Tongue Reconstruction by Microvascular Forearm Free Flap

PURPOSE

The objective of this study was to examine the restoration process of taste perception in patients after reconstruction of the tongue using microvascular forearm flaps.

PATIENTS AND METHODS

Twelve patients, aged 28 to 70 years with a mean of 59.6 years consisting of 9 men and 3 women, were enrolled in this study. Examination results based on the filter paper disk and whole mouth methods were evaluated on a 6-point rating scale.

RESULTS

In none of the patients was it possible to detect any perception of the test tastes in the regions under the control of the chorda tympani and glossopharyngeal nerves on the reconstructed side, indicating a "nondetectable" grade 6 result. The evaluation results for the normal tongue region under the control of the chorda tympani in the patients after tongue reconstruction showed a mean grade of 4.7 for the sweet, 4.9 for the salty, 4.6 for the sour, and 4.4 for the bitter taste. Mean grade determined by the whole mouth method was 3.6 for sweet, 3.3 for salty, 3.0 for sour, and 3.6 for bitter.

CONCLUSION

No taste perception was detected in the regions of the tongue reconstructed with forearm flaps. These results led us to assume that compensatory taste perception must occur in other regions of the oral cavity because none of our postsurgery patients showed any serious taste disorder.

To divide or manipulate the chorda tympani in stapedotomy

The objective of this study is to determine the nature, duration and severity of chorda tympani symptoms in post-stapedotomy patients when the nerve has been known to have either sustained surgical manipulation or division. The study design was retrospective and blind, based at a tertiary Otology Referral Center (Gloucestershire Royal Hospital). All successive patients undergoing stapedotomy operated upon by the senior author (JMR) between November 1991 and October 1998 were included. The main outcome measures were postoperative graded dysgeusia (questionnaire) and post operative electrogustometry score, comparing the chorda tympani cut group and the chorda tympani preserved group. Sixty stapedotomies were performed in the study period. After a minimum interval of 8 months, questionnaire data were obtained in 55/60 (92%), and electrogustometry studies were carried out in 48/60 (80%). Symptoms of dysgeusia were experienced in the chorda tympani nerve cut (CC) group in 95% and in the chorda preserved (CP) group in 52%. For those with symptoms, duration was 6.7 months (+/-4.9 SD) and 3.4 months (+/-3.7 SD) for the CC and CP groups, respectively. From the electrogustometry data, after 8 months, the risk of total loss of response is significantly higher in CC patients (94%) than in CP patients (25%) (P = 0.0001). Also at 8 months, 54% of CP patients had normal ipsilateral electrogustometry responses compared with 6% in the CC group. In conclusion, cutting the chorda tympani results in significantly greater symptoms than when subjected to manipulation only, and these symptoms are likely to be more long lasting. Therefore, it is our recommendation that where possible the chorda be preserved. The risk of severe chorda symptoms when the chorda is cut is small (<5%). Preoperative consent to include dysgeusia is advised.

Taste function in elderly patients with unilateral middle ear disease

OBJECTIVE

It is generally said that taste function deteriorates naturally with age. Taste function after middle ear surgery in elderly patients has not been clarified.

MATERIAL AND METHODS

The taste function of the chorda tympani nerve on the normal and diseased sides was examined before and after middle ear surgery using electrogustometry in 79 patients aged>60 years, and the findings were compared with those in 228 young and middle-aged patients.

RESULTS

The threshold of electrogustometry on the normal side increased significantly with increasing age (p<0.0001). The rate of thresholds that were off the scale was highest in the>70 years age group. The preoperative threshold on the diseased side increased significantly with increasing age in patients with chronic otitis media (p=0.0029) and cholesteatoma (p<0.0001). In patients with chronic otitis media, the postoperative threshold of the>60 years age group tended to be higher than that of the<60 years age group.

CONCLUSIONS

These findings suggest that the taste function of the chorda tympani nerve deteriorated on the diseased side as much as on the normal side in elderly patients. Therefore, in most cases, we do not have to pay as much attention to the chorda tymapani nerve when performing surgery in elderly patients compared to young and middle-aged patients.

Electrogustometry: strengths, weaknesses, and clinical evidence of stimulus boundaries

Electrogustometry is well established as a clinical tool for the estimation of taste detection thresholds. Nevertheless, the user is sometimes unaware of the impact of superficially minor procedural and psychophysical factors upon the reliability and comparability of threshold estimates. The inherent strengths and limitations of the procedure are outlined, and aspects of the control and specification of the stimulus that moderate threshold measures are discussed. In addition, threshold estimates from two individuals with severe unilateral taste loss are used to illustrate the level at which anodal dc current may elicit common, rather than taste, sensation. Where chorda tympani section is complete and historical (older than 7-14 days), very high stimulus levels, conservatively over 5 micro A/mm2 (100 micro A linear current with a 5-mm diameter electrode), are required to activate trigeminal responses.

Rate of recovery of taste function after preservation of chorda tympani nerve in middle ear surgery with special reference to type of disease

To study the recovery of function of the chorda tympani nerve, we examined by electrogustometry 79 patients (83 ears) with both preoperative normal function of the nerve and operative preservation of the nerve, every 2 days during hospitalization and at 6 months after surgery. For symptoms such as tongue numbness and taste disturbance, patients with noninflammmatory (NI) diseases (13/20 or 65.0%) showed a significantly higher rate of symptoms than did patients with chronic otitis media (COM; 13/35 or 37.1%) at 2 weeks after surgery (p = .032). The patients with NI diseases (5/20 or 25.0%) tended to show a higher rate of symptoms than did the COM patients (2/35 or 5.7%) or cholesteatoma patients (2/28 or 7.1%) at 6 months after surgery. The rate of recovery of the EGM threshold to normal at 2 weeks after surgery was significantly lower in NI disease patients (6/20 or 30.0%) than in COM patients (23/ 35 or 62.9%) or cholesteatoma patients (19/28 or 67.9%; p = .015 and .008, respectively). Thus, the patients with NI diseases had postoperative symptoms and elevation of EGM threshold more frequently than did the patients with inflammatory diseases.

Incidence of regeneration of the chorda tympani nerve after middle ear surgery

We retrospectively reviewed 52 patients who underwent middle ear surgery during which the chorda tympani nerves were severed and who then underwent secondary surgery 1 to 5 years later. In 22 patients (42.3%), regenerated chorda tympani nerves (entire length of the tympanic segment) were detected in the submucosal layer of the reconstructed eardrum during the secondary surgery. Before the secondary surgery, 16 patients (30.8%) showed threshold recovery on electrogustometry. When 5 regenerated nerves were observed by transmission electron microscopy, myelinated nerve fibers were detected in a small fascicle or connective tissue, but the number of myelinated axons was significantly decreased (7.4% to 84.6%; p = .01) compared with that in normal subjects (1,911 +/- 324; n = 4). There was a significant difference in the incidence of regeneration between the group with end-to-end anastomosis (5/5 or 100%) and that with nerve gap defects (17/47 or 36.2%; p <.05); this finding suggests that repair of the sectioned nerve produces a better incidence of regeneration than leaving the nerve unrepaired.

Expression of activating transcription factor 3 and growth-associated protein 43 in the rat geniculate ganglion neurons after chorda tympani injury

The purpose of this study was to evaluate the degree of damage in the geniculate ganglion and its target organ as a result of chorda tympani (CT) injury. We performed unilateral transection of the rat CT and examined expression of the activating transcription factor 3 (ATF3), a neuronal injury marker, and the growth-associated protein 43 (GAP-43), a regeneration-associated molecule. The mean proportion of ATF3-immunoreactive (ir) neurons in the geniculate ganglion was approximately 32% at 3 days after CT injury, but these neurons were never detected in the naive ganglion. Using in situ hybridization, the mean percentage of GAP-43 mRNA-labeled neurons (signal : noise ratio > or = 10) was observed to have increased significantly to approximately 60% for 1-7 days after CT injury, while that in the naive ganglion was < 15%. The results of morphological studies using scanning electron microscopy and immunohistochemistry indicated that atrophic change and reduction of protein gene-product 9.5-ir fibers in the denervated papillae, mainly in the intragemmal region, were observed after CT injury. Increase in GAP-43 mRNA, suggesting CT axonal regeneration, may have a role in recovery from taste disorders. However, this regenerative process may be involved in abnormal activity in the axotomized neurons or the adjacent intact neurons and so one must not disregard the existence of injured geniculate ganglions when considering the treatment of diseases that cause CT injury.

Long-term follow-up results of electrogustometry and subjective taste disorder after middle ear surgery

OBJECTIVES

The present study compares the long-term follow-up results of electrogustometry with patient reports of taste dysfunction after middle ear surgery.

STUDY DESIGN

Retrospective review of 371 patients who underwent middle ear surgery.

METHODS

Patients were divided into the following groups depending on the degree of manipulation or surgical damage to the chorda tympani nerves: the no-touch group (group 1 [n = 109]); the touch group (group 2 [n = 149]); and the severed nerve group (group 3 [n = 113]). Electrogustometry was periodically performed over the course of several years.

RESULTS

The incidences of postoperative subjective taste disorder in groups 1, 2, and 3 were 2.8%, 25.5%, and 38.9%, respectively. Although the subjective taste disorder usually recovered within 1 to 2 years after surgery in all groups, it persisted for more than 2 years in 2.7% of the touch group and 5.3% of the severed nerve group. Concerning postoperative electrogustometric results, in the no-touch group, 8.3% of patients showed threshold elevation on electrogustometry, but the elevated thresholds completely recovered in all cases. In the touch group, 45% of patients exhibited elevated electrogustometric thresholds on their first postoperative test, including 32.9% who subsequently had complete electrogustometric recovery, 10.1% who subsequently had incomplete recovery, and 2% who failed to recover during the follow-up period. In the severed nerve group, none of the patients was responsive to the electrical stimulus on the first postoperative test, including 8.8% of patients who subsequently exhibited complete electrogustometric recovery, 32.7% who later had incomplete electrogustometric recovery, and 58.4% who never recovered any electrogustometric responsiveness. Nerve repair in the severed nerve group produced better recovery, as measured electrically.

CONCLUSIONS

The incidence of postoperative subjective taste disorder was low, although inconsistent with the high incidence of threshold elevation on electrogustometry, especially in the severed nerve group. Preservation or repair of the chorda tympani nerve is recommended in order to maintain or recover gustatory function.