Keywords
sudden hearing loss - low tone - prognosis
Introduction
Low-tone sudden sensorineural hearing loss (SSHL) is an already known clinical entity,
presenting with low-frequency hearing loss and preservation of high-tone hearing,
without vertigo.[1] Abe[2] described SSHL as an independent disease after studying 39 patients in 1982. Since
then, low-tone SSHL was often discussed in Japan and Korea, but less documented in
the European or North American studies. The incidence of low-tone SSHL is estimated
to be ∼ 40 to 60/100,000 based on Japanese regional surveys.[3]
On the other hand, high-tone SSHL has not been extensively studied so far, which is
characterized by elevation of the thresholds in the high-frequency range and preservation
of low-, middle-frequency hearing. In this study, we retrospectively reviewed the
records of patients with the diagnosis of low-tone SSHL and compared their hearing
thresholds outcome, hearing recovery and long-term symptoms with those of patients
affected by high-tone SSHL. Predictive factors affecting the hearing outcome in a
follow-up of ∼ 3 years after treatment were also examined.
Methods
The study sample consisted of 47 patients divided in 2 groups, the low-tone SSHL group
(27 patients, 10 males, 17 females), and the high-tone SSHL group (20 patients, 8
males, 12 females). The patient characteristics of the two groups are summarized in
[Table 1]. Retrocochlear disease, otologic surgery, acoustic trauma (or barotrauma), acute
or chronic otitis media and Ménière disease were excluded from this study.
Table 1
Patient characteristics with low- and high-tone sudden sensorineural hearing loss
|
Gender
(males/females)
|
Age
(range in years, mean ± SD)
|
Side
(right/left)
|
Delay of treatment (range in days, mean ± SD)
|
Follow-up (in years, mean ± SD)
|
|
Low-tone (n = 27)
|
10/17
|
19–72, 44.1 ± 13.1
|
13/14
|
0–14; 2.7 ± 3.3
|
3.3 ± 2.9
|
|
High-tone (n = 20)
|
8/12
|
16–64, 41.4 ± 13.5
|
11/9
|
0–60; 8.6 ± 14.2
|
2.8 ± 1.6
|
|
Post-treatment
|
p = 0.537
|
p = 0.496
|
p = 0.433
|
p = 0.089
|
p = 0.947
|
Abbreviation: SD, standard deviation.
Low-tone type SSHL was defined as the hearing loss for which the average from 3 low
frequencies (125, 250, and 500 Hz) was ≥ 30 dB, and the average from 3 high frequencies
(2,000, 4,000 and 8,000 Hz) was ≤ 20 dB.[4] High-tone SSHL was always associated with a sensation of loss of auditory acuity,
tinnitus or ear fullness and was characterized by at least a 15-dB difference in hearing
level at high frequencies (4,000 and/or 8,000 Hz) in comparison with that of the healthy
side; moreover, hearing thresholds at other frequencies were within the normal limits
on the affected side.
All the patients were treated in our clinic with intravenous steroids (dexamethasone,
8mg, 3 times daily and tapered every 3 days over 10 days). After discharge, the patients
of the low-tone SSHL group were followed for a mean of 3.3 years (standard deviation
[SD]: 2.9 years), and those of the high-tone SSHL group for a mean of 2.8 years (SD:1.6
years).
The age, gender, affected side, delay of treatment and follow-up period were also
statistically studied in relation to the hearing recovery in the two groups. To better
study the effect of the time gap between the onset of sudden hearing loss and treatment,
each group was divided into 2 subgroups, the early and the late; the early subgroup
started the treatment within 2 days of SSHL onset, and the late subgroup, in which
the treatment was delayed for more than 2 days of SSHL onset.
At the last follow-up, the final outcome in each group was recorded as follows: complete
recovery: final hearing level within 0 to 20 dB, or the average gain was 10 dB or
less between 2 ears. Partial recovery: the average gain was improved 10 dB or more
when compared with the initial audiogram. Unchanged: the final average gain was within
10 dB. Moreover, each band of frequencies was statistically compared before and after
the treatment, for the low-tone SSHL group (125, 250, 500 Hz) and for the high-tone
SSHL group (4,000 and 8,000 Hz, respectively). The patients were also asked to report
on persistent tinnitus, possible recurrences of sudden hearing loss or episodes compatible
with Ménière disease.
Statistics
The variables in this study included gender, age, affected side, follow-up period,
hearing thresholds at each frequency pre and posttreatment, time gap between the onset
of sudden hearing loss and the start of treatment.
Absolute and relative frequencies for all demographic and clinical variables were
obtained. All variables were checked for normality via the Kolmogorov-Smirnov test.
Non-parametric tests were used where applicable. Collinearity among scores and other
variables in the study was assessed via a correlation matrix, using Pearson r or Spearman
ρ correlation coefficient. The Student t-test or the Mann-Whitney U-test was used for independent sample comparisons. Paired
t-test or paired Wilcoxon signed rank test was used for paired comparisons. Pearson
Chi square was used for categorical comparisons. The Type I error probability associated
with all tests in this study was set to 0.05. The statistical analyses were performed
using the IBM SPSS Statistics for Windows, version 23.0 package (IBM Corp., Armonk,
NY, USA).
Results
The pattern of hearing levels (based on pure tone average at each frequency) of pre
and posttreatment is illustrated in [Fig. 1] and [2] for the low-tone and high-tone SSHL groups, respectively. A very statistically significant
improvement in hearing was found for all low frequencies (125, 250, 500 Hz) in the
low-tone SSHL group ([Table 2]). In the high-tone SSHL group, a significant hearing recovery was revealed at 4,000 Hz;
however, there was no improvement in hearing at 8,000 Hz ([Table 3]).
Table 2
Low-tone sudden sensorineural hearing loss group; pure tone audiogram average threshold
by frequency (dB HL) pre and posttreatment
|
Hz
|
Pre
|
Post
|
p Value
|
|
125
|
49.44 ± 6.699
|
20.56 ± 10.860
|
< 0.001
|
|
250
|
48.15 ± 6.954
|
19.26 ± 10.442
|
< 0.001
|
|
500
|
41.48 ± 10.725
|
15.93 ± 8.775
|
< 0.001
|
Table 3
High-tone sudden sensorineural hearing loss group; pure tone audiogram average threshold
by frequency (dB HL) pre and posttreatment
|
Hz
|
Pre
|
Post
|
p-value
|
|
4,000
|
40.00 ± 24.815
|
28.25 ± 20.981
|
0.016
|
|
8,000
|
64.00 ± 17.137
|
54.75 ± 21.611
|
0.081
|
Fig. 1 Audiometric configuration of low-tone sudden sensorineural hearing loss based on
average hearing thresholds, pre- and posttreatment, at each frequency (SD: standard
deviation).
Fig. 2 Audiometric configuration of high-tone sudden sensorineural hearing loss based on
average hearing thresholds, pre- and posttreatment, at each frequency (SD: standard
deviation).
According to [Table 4], at the final follow-up, the low-tone SSHL group showed better hearing outcome compared
with the high-tone SSHL group. Complete hearing recovery was observed in 77.7% of
patients of the low-tone SSHL group compared with 15% of cases of the high-tone SSHL
group.
Table 4
Hearing recovery at the last follow-up in low- and high-tone sudden sensorineural
hearing loss
|
Complete
|
Partial
|
Unchanged
|
|
Low-tone (n = 27)
|
21 (77.7%)
|
3 (11.1%)
|
3 (11.1%)
|
|
High-tone (n = 20)
|
3 (15%)
|
6 (30%)
|
11 (55%)
|
Note: The low-tone sudden sensorineural hearing loss group showed better hearing outcome
compared with the high-tone sudden sensorineural hearing loss group (Pearson Chi-squared
test, p < 0.001)
In our study, gender, age, affected side and follow-up period had no statistically
significant impact on hearing outcome ([Table 1]); the earlier onset of treatment would possibly affect the hearing recovery in the
low-tone group (statistical trend), but the same would not happen in the high-tone
SSHL group. No statistically significant difference (p = 0.088) was found between the early and the late subgroup in both groups related
to the time elapsed (less/more than 2 days) from the onset of hearing loss to the
start of treatment.
At the last follow-up (∼ 3 years) in the low-tone SSHL group, 3 (11%) patients were
still complaining of tinnitus, and another 6 (22%) patients had recurrences of low-tone
SSHL in the diseased ear, half of them in the first year of the first attack; Ménière
disease occurred in the diseased ear of another 2 (7%) patients at 1.5 and 10 years
after SSHL onset, respectively. Overall, no complete relief from symptoms was reported
in 40% (11 patients) of cases in the low-tone SSHL group.
Compared with the low-tone SSHL group, a higher proportion (15/20 cases, 75%) of patients
in the high-tone SSHL group reported persistent symptoms. From these, 13 (65%) patients
still had tinnitus in the diseased ear and another 2 patients suffered from 2 (10%)
recurrences; 1 patient experienced a 2nd episode of SSHL in the diseased ear 1 month
later, and the other patient 3 years later. In this group, no patient experienced
episodes compatible with Ménière disease.
Discussion
Most authors have already reported that the prognosis of low-tone SSHL (or upward-sloping
audiogram) is better than that of high-tone SSHL (or downward-sloping audiogram),[1]
[5] although this is not in agreement with other studies.[6]
[7] In our study, high-tone SSHL was audiometrically defined as thresholds elevation
restricted to high-frequencies, in contrast to low-tone SSHL. According to the results,
patients with hearing loss in the high-frequency band clearly had poorer recovery
rates than those with hearing loss in low-frequency band (77.7% versus 15%). The symptoms
in high-tone SSHL were quite similar to those in low-tone SSHL, characterized by a
sensation of loss of auditory acuity, tinnitus, autophony or fullness in the affected
ear; moreover, female preponderance and peak incidence during the fourth decade of
life[8] seem to be common in these two clinical entities ([Table 1]).
Our treatment for low-tone SSHL group was based on intravenous steroid with complete
recovery of hearing in 77.7% of patients, which is comparable to the findings of other
relatively recent studies.[1]
[9] Jung et al[1] found an audiometric improvement rate of 76% with oral steroids alone (follow-up:
8 weeks), which was better than 50% achieved after intratympanic steroid injections
alone and 76.9% with combination of the two methods. In a study with almost 2 years
of follow-up, Roh et al[9] demonstrated complete hearing improvement in 75% of patients after treatment mainly
with oral steroids, which was very close to the results obtained in our last follow-up.
However, Morita et al[10] showed lower recovery rates with oral steroid alone (63%) but with 2 months follow-up.
It has also been advocated that when diuretics were added to steroids the recovery
rates are significantly improved (78.2% to 83.9%).[4]
[10]
[11]
In high-tone SSHL, the hearing recovery was statistically significant after treatment
at all frequencies, except at 8,000 Hz ([Table 3]). We have no explanation why the treatment for high-tone SSHL is ineffective at
8,000 Hz. It is possible that this region of cochlea at the end portion of the basal
turn is most vulnerable to damage, reflecting the gradual downward-sloping hearing
loss in presbycusis or ototoxicity with degradation of hearing thresholds at higher
frequencies. It has been reported that the levels of glutathione, an antioxidant,
tend to be lower at the most basal cochlear turn hair cells and inversely increasing
toward the apex;[12] glutathione peroxidase is an enzyme that alter reactive oxygen species to less damaging
forms.[12]
The pathophysiologic mechanism of low-tone SSHL is still unclear. According to studies,
the low-tone SSHL may be a variant of Ménière disease, or the beginning period of
Ménière disease.[1]
[13] Yamasoba et al,[14] using glycerol test and electrocochleogram (elevated SP/AP ratio), suggested that
low-tone SSHL might be caused by endolymphatic hydrops. Fuse et al[15] reported that the etiology of low-tone SSHL involves an autoimmune response of the
endolymphatic sac that induces endolymphatic hydrops. On the other side, Choi et al[11] supported that low-tone SSHL might be a different disease entity from Ménière disease,
since only 1 among 18 patients who had been checked with electrocochleogram during
acute low-tone SSHL showed elevated SP/AP ratio. Moreover, Wu and Young[16] demonstrated that low-tone SSHL should be differentiated from Ménière disease on
the basis of vestibular evoked myogenic potentials (VEMPs), since most patients with
low-tone SSHL revealed normal VEMPs; in contrast, 50% of Ménière disease patients
with low-tone hearing loss showed abnormal VEMPs. In our study, low-tone SSHL progressed
to Ménière disease in only 2 (2/27, 7%) patients at 1.5 and 10 years after the SSHL
onset. Similarly, Yamasoba et al[14] reported that 5 (11%) out of 45 patients followed up for more than 3 years developed
Ménière disease. Compared with low-tone SSHL cases, our patients with high-tone SSHL
did not develop Ménière disease, although initially there were a few complaints of
lightheadedness and instability. It seems that low-tone SSHL is not the same disease
as Ménière disease because a limited proportion of low-tone SSHL cases progressed
to Ménière disease.
However, recurrences occurred more frequently in the low-tone SSHL (22%) than in the
high-tone SSHL group (10%). It has been postulated[17] that patients with low-tone SSHL suffering of recurrent episodes of hearing loss
tended to have endolymphatic hydrops (higher summation potential/action potential
[SP/AP] ratio of electrocochleography), whereas those without recurrent episodes do
not. According to previous reports,[1]
[3]
[9]
[14] the incidence of recurrences after low-tone SSHL onset has ranged from 9 to 45%,
the majority of them were documented within one year of the first attack.[3]
[9] Indeed, Oishi et al[5] supported that if the low-tone SSHL patients display hearing fluctuations within
one year after the initial attack, about half of them exhibited high- and pan-frequency
hearing loss within 10 years of onset; nevertheless, with regard to progression of
hearing loss at high frequencies we should consider the effect of aging.
In both groups, no factors such as gender, age, affected side, follow-up period, hearing
thresholds at each frequency pre and posttreatment have been found to be significant
in prognosis. The earlier onset of treatment would possibly affect the hearing recovery
in the low-tone SSHL group (statistical trend). In addition, a prompt treatment with
steroids within 2 days from the onset of low- or high-tone SSHL was not apparently
related to the final hearing outcome. However, it has been advocated that age,[11] co-occurrence of tinnitus,[1] degree of pretreatment hearing loss on the affected side[1] and time interval between onset and start of treatment[11] are positive prognostic factors of hearing recovery in low-tone SSHL.
Conclusion
High-tone SSHL, which is characterized by hearing loss at high frequencies, shows
poor recovery rates and residual symptoms, such as tinnitus; low-tone SSHL has a more
favorable hearing outcome but is more often associated with recurrences and less commonly
progressed to Ménière disease. Further studies are needed, as well as more extensive
research, to better understand the etiology of both low- and high-tone SSHL.
