Postoperative Tinnitus after Vestibular Schwannoma Surgery: A Neglected Entity
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.280639
Source of Support: None, Conflict of Interest: None
Keywords: Cochlear nerve resection, management algorithm, mechanism, postoperative tinnitus, Tinnitus Handicap Inventory scoring
Evolution in the management of vestibular schwannomas (VS) surgery using operating microscope; intraoperative facial, trigeminal, and lower cranial nerve and brainstem auditory evoked response (BAER) monitoring; and stereotactic radiosurgery has significantly improved the outcome of facial nerve and hearing preservation. Abundant literature exists regarding the surgical outcome of facial nerve and hearing preservation following surgery for VS. Despite being one of the most disturbing symptoms for patients in the postoperative period, literature analyzing the status of tinnitus following VS surgery is considerably sparse.,,
Tinnitus (Latin – “ringing”) is the subjective perception of continuous or intermittent sound in the ear without any external acoustic stimuli. It is the second most common presenting symptom (65–73%) in patients of VS, and in 10% of the cases, it is the principle symptom., The potential mechanisms of preoperative tinnitus with VS are ephatic coupling of cochlear nerve fibers by compression, cochlear dysfunction by ischemia or by biochemical degradation, efferent system dysfunction following compression of the nerve fibers in the inferior vestibular nerve, and cortical reorganization following hearing loss., The evolution of tinnitus is unpredictable and is unrelated to the hearing and cochlear nerve preservation surgery., It may persist, improve, or worsen following surgery. Despite its significant impact on the quality of life, tinnitus persisting after VS surgery has been rarely analyzed. The subjective nature of this entity makes its quantification and assessment a cumbersome procedure. Consequently, it is an often-neglected entity both during the preoperative counseling for VS surgery and as well as during the postoperative evaluation. Consequently, postoperative persistent tinnitus (PPT) becomes troublesome for both the surgeons as well as the patients as it is an unexpected sequel of surgery that has significant postoperative physical and psychological ramifications. The objective of our study was to analyze the factors affecting postoperative tinnitus, highlight its mechanisms, and assess the impact of cochlear nerve resection on its persistence.
This prospective study enrolled 69 consecutive patients with an untreated unilateral sporadic VS operated via a microscopic retrosigmoid–suboccipital approach from July 2014 to June 2015. In all patients, the cochlear nerve was not separately identifiable from the tumor capsule and was resected during the surgery. Intraoperative facial nerve monitoring was used during all surgeries. Two patients with bilateral VS were excluded from the study. Therefore, we analyzed a total of 67 patients to assess the impact of complete removal of the VS as well as cochlear nerve resection on postoperative tinnitus. The data collected in each patient included the demographic profile, size and consistency of the tumor, pre and postoperative hearing evaluation with pure tone audiometry (PTA), and facial nerve function (based on the House–Brackmann scale). The Tinnitus Handicap Inventory (THI) score was used to assess pre and postoperative symptoms of tinnitus. Postoperative hearing evaluation was also performed using a PTA. Patients with PPT were further analyzed using brain stem auditory evoked response (BAER) performed on the contralateral side with normal hearing. All patients underwent a postoperative contrast-enhanced computed tomographic (CT) scan 48–72 hours following the definitive surgery for VS.
Hearing levels in both ears were assessed based on the PTA obtained and compared with the PTA obtained for a normal population. Hearing loss was classified as normal hearing (30 dB loss); mild hearing loss (31–50 dB loss); moderate hearing loss (51–70 dB loss); severe hearing loss (71–90 dB loss); profound hearing loss (>90 dB loss) [WHO classification of haring loss (1980)]. We analyzed the results of PTA thresholds measured at clinically useful speech frequencies of 0.5, 1, 2, and 4 kHz.
The tumor size was calculated based on preoperative MRI using Jackler grading system. The size of the tumor was classified as G1: intracanalicular tumors; small sized (G2): <10 mm; medium sized (G3): 11–25 mm; large sized (G4): 25–40 mm; and giant (G5): >40 mm).
Intraoperative findings including the vascularity and consistency of the tumor, its adherence to neurovascular structures, presence of cystic component within it, and division of the cochlear division of the eighth cranial nerve in proximity to the tumor (in case it was separately discernible from the tumor capsule) were recorded.
Postoperative contrast-enhanced CT scan was performed to exclude the presence of residual tumor in all patients at the time of discharge. VS patients with persistent tinnitus underwent postoperative BAER on the normal side contralateral to the tumor. To evaluate the results of BAER, we used the Hannover classification (H1: preserved complex of waves I, III, and V with normal or prolonged latencies of waves I–III ≤2.66 ms; H2: preserved complex of waves I, III, and V, but pathologically prolonged latencies of waves I–III >2.66 ms; H3: present waves I and V, but missing wave III; H4: only one present wave, usually wave I; H5: no response).
Tinnitus Handicap Inventory scoring and its clinical significance
The intensity of tinnitus was quantified subjectively by self-reporting questionnaires. The THI is widely used in a clinical context to assess tinnitus-related self-reported handicaps and to report treatment outcomes., Its test–retest reliability has been reported to be high, and a high convergent validity with other measures of tinnitus distress has been reported. A total of 25 questions related to tinnitus were asked and the resultant score obtained was recorded. Based on this score, the intensity of tinnitus was graded as: Grade 1 – slight (THI 0–16); Grade 2 – mild (THI 18–36); Grade 3 – moderate (THI 38–56); Grade 4 – severe (THI 58–76); Grade 5 – catastrophic (THI 78–100).
The clinical spectrum of the patients was recorded as frequency of patients and percentage. The factors affecting preoperative tinnitus were analyzed using Pearson's Chi-square test or Fischer exact test, in which a P value of less than or equal to (≤) 0.05 was considered significant. The effect of individual factors in determining the PPT after cochlear nerve resection were analyzed with the help of Wilcoxon signed ranked test (a P value of ≤0.05 was considered significant).
Clinical spectrum of patients and its influence on preoperative tinnitus
The clinical characteristics of patients having a unilateral VS are summarized in [Table 1]; 28/67 (41%) patients presented with tinnitus. There was no significant difference in the age distribution in groups with or without tinnitus (mean age 41 ± 11 years and 40 ± 14 years, respectively). There was no sex preponderance in the two groups (P = 0.29). Twenty-eight patients (100%) with tinnitus and 32 (78%) patients without tinnitus were found to have no useful hearing. Patients with a mild sensory neural hearing loss (SNHL) (P = 0.03) and severe SNHL (P = 0.00) had a statistically significant incidence of preoperative tinnitus. There was no statistical correlation between the tumor size and its consistency with the incidence of preoperative tinnitus [Table 2].
Postoperative tinnitus score after vestibular schwannoma excision
Complete tumor removal was achieved in all 67 patients which was confirmed by an early postoperative contrast-enhanced CT scan. Each patient underwent a PTA and tinnitus evaluation after 1 week of surgery and during the follow-up visit after 3 months to determine any changes in the THI score [Table 3]. Of the patients with preoperative tinnitus [n = 28 (41%)], 24 (85%) patients had a significantly improved postoperative THI score. In 15 of the 24 patients, the tinnitus subsided completely. In 3 (4%) patients, the THI scores remained unaltered and 1 (1.4%) patient reported worsening of THI scores. Of the 39 patients without a preoperative tinnitus, 4 (10%) developed a postoperative tinnitus. The THI score repeated at a 3-month follow-up revealed no further changes. Thus, in 17 (25%) postoperative patients (9 with the improved but persisting, 3 with unaltered, 1 with worsened, and 4 with fresh tinnitus), the tinnitus was persisting up to a follow-up of 3 months.
Factor influencing postoperative tinnitus status
On comparisons of preoperative and postoperative THI score on individual basis, and after analyzing impact of various factors on postoperative tinnitus, we found that the preoperative hearing status had a direct impact on the incidence of postoperative tinnitus. In patients with mild-to-moderate SNHL the incidence of postoperative tinnitus was less; however, in patients with severe SNHL (P = 0.00), postoperative tinnitus incident was significantly increased [Figure 1] and [Table 3]. Profound SNHL was negatively correlated with the incidence of postoperative tinnitus (P = 0.04, odd ratio = 0.077, 95% CI, 0.009–0.637). Moreover, large (P = 0.002) and giant sized tumor (P = 0.04) had significantly improved postoperative THI scores [Figure 2] and [Table 3]. Patients with solid (P = 0.02) and mixed consistency (P = 0.001) had significant improvement in postoperative THI grade; however, no improvement was observed in those with cystic consistency (P = 1.00). The latter observation lacked statistical significance due to a small sample size (n = 2); a large study with more representation of patients with cystic VS is needed for a more precise analysis of the influence on postoperative tinnitus. An improvement in postoperative tinnitus was seen in all age groups young age (P = 0.018), middle age (P = 0.010), elderly age (P = 0.063), however, this association lacked statistical significance in the elderly age group (50 years or more) [Table 3].
Result of BAER study in patients with postoperative persistent tinnitus (n = 13)
For evaluating patients with postoperative persistent tinnitus, 13 patients underwent BAER study [Table 4]. On the operated side, majority of the patients (11 patients) showed an H5 grade (no response), whereas 2 patients had an H4 grade response. On the contralateral side, all patients showed an H1 grade response (preserved complex of waves I, III, and V with normal or prolonged latencies of wave I–III ≤2.66 ms).
Assessment of tinnitus is mandatory during the management of VS as there are high chances (nearly 46%) of persistent, and often vexatious, postoperative tinnitus. Preoperative tinnitus, linked to the degree of SNHL (higher incidence in severe SNHL compared to mild-to-moderate/profound SNHL), is dependent on an intact cochlear nerve functioning. However, PPT is dependent on other mechanisms (brainstem/ipsilateral cochlear nuclei compression and cortical reorganization) as it persists despite cochlear nerve resection.
Prevalence and types of tinnitus associated with vestibular schwannoma
In our study, the incidence of preoperative tinnitus was seen in 28/67 (41%) patients. In 17 patients (including those with persistent tinnitus as well as those with freshly developed tinnitus in the postoperative period), the tinnitus was persisting at a follow-up of 3 months following a definitive surgery for VS. The outcome of tinnitus after VS surgery quoted in the literature varies with the type of surgical approach adopted. Kameda et al. reported an incidence of 34% of postoperative tinnitus after a retrosigmoid approach. Among these patients, tinnitus disappeared in 25.2% patients, improved in 33%, remained unchanged in 31.6%, and worsened in 9.9% patients. Martin et al. reported a 35% incidence of postoperative tinnitus with disappearance of tinnitus seen in 66% of the patients with the same retrosigmoid approach. Postoperative tinnitus may be spontaneous, gaze-evoked, or noise-induced tinnitus. Spontaneous tinnitus after surgery remains similar in character, intensity, and pattern of sound as the preoperative tinnitus. It may worsen, remain unchanged, or may disappear completely. Prevalence of gaze-evoked type of tinnitus was reported in 20–36% patients., The prevalence of persistent tinnitus in patients with VS is, therefore, quite high. The majority of surgeons are often dismissive of the patient's complaints and of the disability emanating as a result of this persisting tinnitus, and consider them innocuous in nature simply because they do not have the knowledge and means to address the problem.
Etiopathology and mechanism
The mechanism of preoperative tinnitus in the presence of VS is poorly understood. It may originate at any location along the auditory pathway from the cochlea to the auditory cortex. The prevalent theory suggests that a VS compresses the eight nerve and leads to the subsequent afferent degeneration of cochlear hair cell that discharge repetitively and stimulate auditory nerve fibers in a continuous manner. Spontaneous activity in individual auditory nerve fibers, hyperactivity of the auditory nuclei in the brainstem, or a reduction in the usual suppressive activity of the central auditory cortex on peripheral auditory nerve activity may also be responsible for tinnitus., This proposed hypothesis is supported by the fact that complete excision of a VS with a simultaneous vestibulocochlear nerve section ablates both afferent and efferent pathways and often suppresses the tinnitus. In our study, in 85% patients with no useful hearing and a large/giant-sized tumor, the tinnitus improved after surgery. In 53% of the patients, the cochlear nerve resection along with the complete excision of the VS completely ameliorated the tinnitus.
In our study, preoperative tinnitus was linked to the degree of SNHL (with a higher incidence in patient with a severe preoperative SNHL compared to mild-to-moderate as well as profound SNHL). Thus, the presence of preoperative tinnitus was dependent on an intact cochlear nerve functioning and on a particular decibel range of hearing loss.
The effect of cochlear nerve resection on postoperative tinnitus after VS surgery has rarely been analyzed. We utilized the retrosigmoid approach with cochlear nerve resection. Among 28 of the 67 patients with a VS who had a preoperative tinnitus, in 15 (53%) patients the tinnitus completely subsided; in 13 (46%) it persisted (tinnitus improved, worsened or unchanged). In our study, in 24 (85%) patients, tinnitus either improved or was completely cured following the retrosigmoid approach and complete tumor excision along with cochlear nerve resection. Other studies in consonance with this data have shown a 45–55% improvement in the severity of tinnitus after cochlear nerve resection., Kameda et al., however, concluded that there was no difference in the incidence of tinnitus based on whether or not hearing was preserved or the cochlear nerve was resected during surgery. Martin et al. concluded that preservation of cochlear nerve but loss of preoperative hearing was identified as the main factor for both the persistence of tinnitus persistence and new-onset tinnitus.
An interesting observation made in this study was the persistence or development of tinnitus following successful excision of the VS and nonpreservation of the vestibulocochlear nerve in 17 of our postoperative patients. Because an abnormal focus emanating from the vestibulocochlear nerve can no longer explain this persisting postoperative tinnitus (since the nerve has already been resected along with the tumor), other mechanisms may play a role. Elevated protein levels in the cochlear perilymph following VS surgery was observed but cannot account for the persistent tinnitus as the peripheral auditory system is no longer in communication with the central nervous system following vestibulocochlear nerve resection., Cochlear nuclear dysfunction caused by brainstem compression by a large/giant-sized VS or cortical reorganization due to the preoperative severe SNHL with tinnitus have also been proposed as the possible etiopathogenetic mechanisms. The auditory pathway from the level of the cochlear nerve nuclei in the brainstem has a bilateral representation. However, a BAER evaluation in the normal ear revealed Hannover grade I (normal) wave recording, effectively ruling out brainstem dysfunction. Cortical reorganization following vestibulocochlear nerve resection may be responsible for PPT despite cochlear nerve resection. However, this unexplored arena could not be evaluated in the present study due to the limited knowledge and resources prevalent as of now.
Impact of various surgical approaches on postoperative tinnitus
Using stereotactic radiosurgery, the translabrynthine corridor or the middle fossa approach for VS management did not alter the tinnitus at a group level, even though some individual patients may experience less tinnitus after treatment., However, some studies have shown a worsening in the intensity of tinnitus after stereotactic radiosurgery due to the cochlear nerve toxicity induced by radiation. The retrosigmoid surgery, on the other hand, seems to confer a significant advantage in the management of tinnitus, significantly decreasing its severity, except when the approach was adopted for hearing-preservation surgery., Our study also revealed that the retrosigmoid approach had a significant impact on improving the postoperative tinnitus.
Management of postoperative tinnitus
Majority of postoperative tinnitus are of mild variety and not distressing to the patient. However. in 2–6% of the cases, it may be severe enough to significantly affect the quality of life., Identification of patients with severe and distressing tinnitus by THI grading in the preoperative setting may allow therapy to commence while the patient awaits surgery. Prognosis regarding the outcome of tinnitus amelioration must be incorporated in the preoperative counseling of patients undergoing VS surgery. In patients in whom tinnitus persists, various forms of physical as well as psychogenic therapies facilitate the habituation of the patient to his/her tinnitus in the postoperative period. Various therapies that are indicated in patients having a tinnitus associated with a VS include the tinnitus retraining therapy (TRT), the masking therapy, the sound devices, and various pharmacological agents like tricyclic antidepressants, gamma aminobutyric acid (GABA) analogues such as benzodiazepines and herbal medicines like Ginkgo biloba [Figure 3].,,
Limitation of study
Two major limiting factors were recall bias and selection bias. During evaluating the THI score, some patients were not able to complete the questionnaire due to their concomitant depression. This indirectly affected the statistical analysis. A preoperative functional MRI and the comparison of its baseline images with a subsequent scan repeated in patients with a persistent postoperative tinnitus (as well as its comparison with a normal control group) would help to better localize the foci of tinnitus before and after surgical intervention and give a better idea regarding the ensuing cortical reorganization in patients with a large/giant VS with no functional hearing and a persistent tinnitus. Further, there are no methods currently available to clinically evaluate the medium and long latency responses following auditory stimulation. BAER allows precise evaluation of the auditory pathway only up to the level of lateral lemniscus-inferior colliculus. These are obvious confounding factors affecting the evaluation of these patients.
The biggest challenge in evaluating tinnitus is its subjectivity. THI grading used for our series was also obviously entirely subjective, and hence, the responses given by the patients were bound to be influenced by subjective patient parameters.
A significant number of patients harboring a large/giant VS have preoperative tinnitus that may persist in the postoperative period. The preoperative hearing status has a significant impact on the persistence of postoperative tinnitus with patients with a severe SNHL having a significant incidence of postoperative persistent tinnitus compared to those having mild-to-moderate or profound SNHL loss. Complete resection of the VS as well as cochlear nerve resection using the retrosigmoid suboccipital approach often reduces/completely abolishes the tinnitus in the postoperative period. PPT may, however, persist in the postoperative period despite cochlear nerve resection signifying that other central mechanisms such as cochlear nuclear dysfunction or cerebral reorganization may be the culpable factors. Further functional imaging studies using the functional magnetic resonance and the positron emission tomographic imaging would perhaps further elucidate the underlying central mechanisms for the persistence of tinnitus despite cochlear nerve sectioning during surgery for a large/giant VS in patients with no useful hearing.
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Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4]