Mechanisms of tinnitus
+ Author Affiliations
Abstract
The generation of tinnitus is a topic
of much scientific enquiry. This chapter reviews possible mechanisms of
tinnitus, whilst
noting that the heterogeneity observed within the
human population with distressing tinnitus means that there may be many
different mechanisms by which tinnitus can occur.
Indeed, multiple mechanisms may be at work within one individual. The
role
of the cochlea in tinnitus is considered, and in
particular the concept of discordant damage between inner and outer hair
cells is described. Biochemical models of tinnitus
pertaining to the cochlea and the central auditory pathway are
considered.
Potential mechanisms for tinnitus within the
auditory brain are reviewed, including important work on synchronised
spontaneous
activity in the cochlear nerve. Whilst the number
of possible mechanisms of tinnitus within the auditory system is
considerable,
the identification of the physiological substrates
underlying tinnitus is a crucial element in the design of novel and
effective
therapies.
Hypotheses regarding mechanisms of tinnitus generation abound. Given the heterogeneity observed in the tinnitus population1,
it may be considered that no single theory, model or hypothesis will
explain the presence of tinnitus in all those affected.
Thus, the mechanisms described in this chapter are not
mutually exclusive, and multiple mechanisms may be present in an
individual
with tinnitus. The focus of this review is upon
physiological mechanisms of tinnitus generation rather than the
psychological
impact that tinnitus may have, or therapies and
treatments.
The word tinnitus derives from the Latin tinnire meaning ‘to ring’, and in English is defined as ‘a ringing in the ears’2. In an attempt at a scientific definition, McFadden3 considered that: ‘tinnitus is the conscious expression of a sound that originates in an involuntary manner in the head of
its owner, or may appear to him to do so’. This definition has been widely adopted.
Tinnitus is a common experience in adults and children. Adult data from the MRC Institute of Hearing Research4
indicate that, in the UK, 10% of adults have experienced prolonged
spontaneous tinnitus, and that in 5% of adults tinnitus
is reported to be moderately or severely annoying. In
1% of the adult population, tinnitus has a severe effect on quality
of life. The incidence data from the MRC study
indicate that 7% of the UK adult population have consulted their doctor
about
tinnitus, and 2.5% have attended a hospital with
regard to tinnitus. Up to one-third of children experience occasional
tinnitus,
and in approximately 10% tinnitus has been bothersome5.
A complex relationship between epidemiological factors and tinnitus has been identified4.
The prevalence of tinnitus increases with age and with hearing
impairment. Women are more likely to report tinnitus than
men, and occupational noise and lower socio-economic
class are also associated with increased tinnitus. These factors are
not independent of each other, and further work is
needed in this area.
A large number of descriptors of tinnitus
have been reported, the most common being hissing, sizzling and
buzzing, these reflecting
the clinical finding that tinnitus is usually high
pitched. An individual may localise tinnitus to one ear or other, to
both,
within the head or occasionally external to the head.
In a clinical context, many individuals may hear more than one tinnitus
sound.
Tinnitus is an element of the symptom
profile of several significant otological pathologies (such as
otosclerosis, vestibular
schwannoma and Menière's disease) that necessitate
medical or surgical treatment. Whilst such conditions are rare within
both
the general and tinnitus-complaint populations, there
is a consensus that an informed clinical opinion should be sought by
an individual with troublesome tinnitus (especially
when unilateral) in order to exclude such pathologies. This review does
not consider pathology-specific mechanisms other than
the cochlear dysfunction implicated in sensorineural hearing loss.
Cochlear models
Any model which considered the cochlea
in isolation from the rest of the auditory pathway in relation to
tinnitus would not
now be considered adequate, but there are
situations where cochlear dysfunction has been implicated in tinnitus
generation.
Spontaneous oto-acoustic emissions
The concept that a normal healthy cochlea may produce low intensity tonal or narrow-band sound in the absence of any acoustic
stimulation (spontaneous oto-acoustic emissions, SOAEs) was introduced by Gold in 19486 as an element of a model of active processes within the cochlea. The identification of such activity7 (see Kemp this volume) was greeted with enthusiasm by the scientific community concerned with tinnitus as ‘our hope was that
they corresponded to their owner's tinnitus and thus, at long last, we could measure tinnitus objectively’8.
This hope was not well founded, as it became clear that whilst 38–60% of normal-hearing adults have measurable SOAEs, the
majority of such individuals are not aware of this activity9. Penner and Burns10
noted that when SOAEs do occur in the ear of a tinnitus patient, they
rarely correspond to the judged frequency of the tinnitus.
These authors considered that if a SOAE could be
suppressed by a suitable low-level external tone without affecting the
tinnitus
perception, and, conversely, if tinnitus could be
masked in an individual without affecting the SOAE, then the inference
of
physiological independence could be made. This
suppression/masking paradigm has been used to determine the incidence of
tinnitus
complaint caused by SOAEs. Penner11 found that 4.1% of a series of tinnitus patients (n = 96) had tinnitus originating as an SOAE. Baskill and Coles12 found an incidence of 2%, and Coles (cited in Penner13) of 4.5%.
One additional piece of evidence that SOAEs are not largely responsible for tinnitus generation is as follows. SOAEs are largely
abolished by aspirin (salicylate)14, but tinnitus perception is not generally improved by salicylate, there being only one report of such an experience15, this in a case where SOAE and tinnitus were demonstrably linked. Penner13 notes that the treatment of SOAE-generated tinnitus with salicylate is done at the risk of ototoxic hearing loss and the
possible generation of new tinnitus perceptions.
Discordant damage of IHC and OHC
Jastreboff16
noted that intense noise and ototoxic agents initially damage the basal
turn of the cochlea, and outer hair cells (OHCs),
and only later affect inner hair cells (IHCs) if
continued or repeated, IHCs being more resistant to such damage17.
The inference was made that, within a partially affected organ of
Corti, there will be an area with both OHCs and IHCs affected,
an area with OHCs are affected but IHCs are intact,
and an area with both intact. In the second of these three categories,
the coupling between the tectorial membrane and the
basilar membrane would be affected, to the extent that the tectorial
membrane
might directly impinge upon the cilia of the IHCs,
thus causing them to depolarise. Clinical support for such modification
of auditory afferent activity leading to tinnitus
perception has been cited, in that some patients with tinnitus and
high-frequency
hearing loss match their tinnitus frequency to the
point at which the loss begins18,,19. The role that increased neural activity in the auditory periphery may have in tinnitus generation is considered in detail
below. Jastreboff20
went on to consider not only the afferent activity generated by the
IHCs, but also the possibility that afferent activity
of the IHCs might be interpreted in the light of
attempted (but failed) reduction of cochlear gain via OHCs, giving rise
to
increased perceived intensity. It was further
suggested that this model might apply to both permanent and temporary
discordant
damage, the example of temporary damage being
temporary tinnitus associated with temporary threshold shift following
noise
exposure. Chery-Croze et al21
noted that, in an area where IHC damage was present, any efferent
inhibition of the OHCs in that area will be reduced due
to the reduced afferent input. That efferent
innervation may be shared with neighbouring OHCs partnering undamaged
IHCs, due
to the diffuse nature of efferent innervation (one
fibre for 20–30 OHCs), and so the undamaged area neighbouring the
damaged
IHCs may also have reduced efferent inhibition,
giving rise to a highly active area of the basilar membrane, resulting
in
tonal tinnitus.
LePage22
suggested an alternative mechanism by which an area of the basilar
membrane with damaged OHCs but intact IHCs might contribute
to tinnitus generation. The role of the normal OHCs
in fixing the operating point of IHCs was considered, that is an
ability
of OHCs to control the sensitivity of IHCs by
setting the operating point on the IHCs' transfer characteristic to a
value
which the brain normally interprets as no sound.
This point would not actually correspond to zero sound input, but a
sound
level regarded as background. A loss of motility in
OHCs might reduce the ability to set the operating point of the IHCs
appropriately,
thus causing a ‘virtual’ sound input, so that this
normally inaudible activity might be perceived as tinnitus. If this were
to occur over a short length of the basilar
membrane, the perception would be interpreted according to the tonotopic
frequency
normally transduced at that point, and hence would
be tonal. LePage notes that if there were functional OHCs adjacent to
the
dysfunctional OHCs, then no loss of audiometric
sensitivity might be evident. Zenner and Ernst23 suggested that tinnitus generated by such a mechanism should be classified as ‘DC motor tinnitus’.
A further role for OHC in tinnitus has been suggested by Patuzzi24,
who noted that OHC dysfunction may cause excessive release of
neurotransmitter from IHCs following an increase in the endocochlear
potential. This phenomenon might then lead to a
‘rate tinnitus’, so called because of the excessive rate of glutamate
release
from IHCs. Patuzzi predicted that the tinnitus
percept would have a ‘hiss’ quality.
Biochemical models
A biochemical model of peripheral tinnitus has recently been proposed25
based partly on the clinical observation that adult humans with
distressing tinnitus have experiences of agitation, stress
and anxiety, and partly on cochlear neurochemistry.
Endogenous dynorphins (associated with stress) are postulated to
potentiate
the excitatory function of glutamate within the
cochlea, mimicking the action of sodium salicylate in increasing
spontaneous
neural activity.
The biochemistry of the central auditory system has also been considered in the tinnitus literature. A role for serotonin
(5-HT) in persistent tinnitus was postulated by Simpson and Davies26,
based on the consideration that disrupted or modified 5-HT function
might cause a reduction in auditory filtering abilities
and in tinnitus habituation (see later). The
identification of a role of 5-HT in persistent distressing tinnitus is
important
as it may facilitate the development of effective
pharmacological intervention. The need for investigation of the effect
of
selective serotonin re-uptake inhibitors upon
tinnitus is urgent27.
Non-cochlear mechanisms of tinnitus generation
Considerable attention has been paid to the possible involvement of cochlear mechanisms in tinnitus generation, but in recent
years the interest of the scientific community has shifted towards retro-cochlear and central mechanisms16,,28–31. In many cases, the models and hypotheses proposed do not preclude a role for the cochlea, but have as their primary concern
neural mechanisms of tinnitus generation and persistence.
Jastreboff neurophysiological model
In a review of tinnitus from a neurophysiological perspective, Jastreboff16
considered a role for ‘signal recognition and classification circuits’
in persistent tinnitus, that function as neural networks
becoming tuned to the tinnitus signal, even when
that signal is transitory, fluctuating or intermittent. It was suggested
that cochlear processes might be involved in the
generation of weak tinnitus-related activity, but since the majority of
individuals
with normal hearing perceive tinnitus-like sound in
quiet surroundings32,
it was not necessary for a lesion of the auditory system to be present
for tinnitus to be heard. The Jastreboff ‘neurophysiological
model’, which involves the auditory perceptual,
emotional and reactive systems involved in tinnitus, was published in
199633 and in slightly more detailed form (Fig. 1) in 199934.
In many individuals after a short period of awareness of
tinnitus-related activity, a process of habituation occurs, such
that the activity is no longer consciously
perceived. However, in cases where there is some ‘negative emotional
re-inforcement’,
described as fear, anxiety or tension, limbic
system and autonomic activation cause the activity to be enhanced and
perception
persists. The distinction between the perception
of, and the behavioural and emotional reaction to, tinnitus was
explicit,
as was the potential for a feed-back loop between
these processes. A treatment protocol arising from this perspective, and
based upon facilitating habituation to both the
tinnitus signal and to the reaction to that perception, has been
entitled
Tinnitus Retraining Therapy33.
The Jastreboff model has been widely accepted as a synthesis that has
utility for patients, clinicians and researchers alike.
Whilst direct empirical evidence to support this
model has not been forthcoming, the concepts involved are congruent with
a modern understanding of the auditory system. A
potential criticism is that the model does not represent the full
complexity
and dynamism of the human auditory system, but if
the primary aim was a model of tinnitus that was easily understood by
patients
then this may have been intentional.
Increased neural activity
Evans et al35
noted that then contemporary theories of tinnitus generation made the
assumption, either implicit or explicit, that it was
associated with spontaneous overactivity of the
cochlear nerve. This was at odds with the literature which indicated
that
experimentally induced chronic cochlear pathology
resulted in a decrease in such spontaneous activity. Such a decrease had
been reported by Kiang et al36 on the basis of a study involving kanamycin-deafened cats. Evans et al35, however, reported that doses of salicylate in the cat equivalent to blood-concentration doses known to induce tinnitus in
humans (in excess of 300–400 mg/l) had the effect of increasing spontaneous activity. Tyler37 noted the different methodology of these studies, and that the recording from single units in the cochlear nerve might miss
hyperactivity occurring elsewhere. Eggermont30
also considered the discrepancy between these findings, and concluded
that increased spontaneous activity in the human cochlear
nerve was unlikely to be involved in tinnitus
generation (assuming that animal data can be applied to humans) since
tinnitus-inducing
events in humans are as likely to reduce
spontaneous activity as increase it.
Increased neural activity at levels
above the cochlear nerve may be implicated in tinnitus generation.
Increases in spontaneous
activity in the dorsal cochlear nucleus (DCN) in
the golden hamster after intense sound exposure have been reported38–40. Salvi et al41
subjected chinchillas to intense sound exposure (2 kHz tone, 105 dB
SPL, 30 min) and reported increases of spontaneous activity
in the inferior colliculus and the dorsal cochlear
nucleus; in addition, they noted tonotopic re-organisation in these
structures.
Increased activity in the inferior colliculus has
also been reported after salicylate administration in the rat42 and the guinea pig43. Chen et al44
studied the effect of intense sound exposure (125 dB SPL, 10 kHz tone, 4
h) on spontaneous activity in the DCN of the rat.
They found an increase in bursting spontaneous
activity and a decrease in regular (simple spiking) spontaneous
activity. The
authors suggested that such activity might
represent increased auditory efferent activity.
Increased cortical activity in the gerbil following salicylate administration has been demonstrated using 2-deoxyglucose methods45 and c-fos immunochemistry46, one study43 using impulse noise as well as salicylate to induce cochlear dysfunction. Wallhausser-Franke and Langner47
also noted evidence of increased activity in the amygdalae of these
animals, and considered this a response to induced tinnitus,
though they noted that the changes may have been
produced by the stress of the animals. Langner and Wallhauser-Franke48 proposed a model for tinnitus generation based on these findings. The lack of increased activity in the ventral cochlear
nucleus (VCN)49
after salicylate administration was claimed as evidence that the
reported effects of salicylate are not due to increased
afferent activity in the cochlear nerve. The
altered activity reported in the DCN was suggested to result either from
increased
efferent activity from the cortex or inferior
colliculus (IC), or from a lack of inhibition from other cochlear
nucleus units.
The amplification of spontaneous activity within
this feedback loop, influenced also by processes of attention (involving
the reticular formation) and the limbic system
(specifically the amygdala) was thought to be the cause of tinnitus
perception.
A mechanism of disinhibition in the IC and DCN has been proposed30,,50. In the DCN type II/III, interneurones act in an inhibitory manner upon spontaneously active type IV neurones30.
If these inhibitory interneurones have reduced afferent input due to
peripheral auditory dysfunction, there may be a loss
of inhibition of the spontaneous activity of the
type IV neurones, thus resulting in abnormally high spontaneous activity
which might be audible. Eggermont30 furthered this proposal, suggesting, after Moller51, additional disinhibition in the IC.
Synchronisation of spontaneous neural activity
Eggermont30 has reviewed the evidence for a theory that tinnitus may result from the imposition of a temporal pattern upon stochastic
cochlear nerve activity. Hudspeth and Corey52 reported that, in the saccular hair cells of the bull frog, an increase in the concentration of extracellular calcium could
lead to increased firing. Eggermont29
proposed that if such a calcium-induced increase were present in
dysfunctional human cochleae, then it might lead to enhanced
neurotransmitter release from IHC, and thence to
increased activity in auditory nerve fibres, some of the spikes
occurring
in bursts. This pattern of activity (burst-firing)
may mimic that seen in response to sound stimulation. Burst-firing can
occur in the cat auditory nerve after exposure to
kanamycin36, and in the rat inferior colliculus after salicylate administration42. Increased burst-firing in the rat DCN following intense sound exposure has also been reported44. Kaltenbach31 argued that a link between such bursting activity and tinnitus perception is problematic, in the light of the finding of
Ochi and Eggermont53,,54
that, in the cat, no increase in cortical bursting activity is
demonstrated after administration of salicylate or quinine.
It is possible, however, that bursting activity in
the auditory periphery could be re-coded as a rate change in more
central
nuclei.
Eggermont50 proposed that the synchronised activity of a small number of fibres in the auditory periphery may give rise to a sensation
of sound, and thus of tinnitus. Moller55
drew an analogy with hemifacial spasm and trigeminal neuralgia
patients, noting that the surgical decompression of vessels
impinging upon the Vth cranial nerve relieved
trigeminal neuralgia, and upon the VIIth cranial nerve relieved
hemifacial spasm.
Moller noted that these cranial nerves were
sensitive to such compression at the root entry zone, where they were
covered
by myelin. He hypothesised that compression of the
nerve caused cross-talk between nerve fibres, the breakdown of the
myelin
insulation of the nerve fibres establishing
ephaptic coupling between them. This concept was applied to the
cochlear-vestibular
nerve, which is covered by central myelin for the
majority of its length, and hence is vulnerable to compression from
blood
vessels or tumours impinging upon the nerve, such
as vestibular schwannoma. Such compression and consequent ephaptic
coupling
might lead to tinnitus perception, if
synchronisation of the stochastic firing in the human cochlear nerve is
perceived as
sound. Eggermont29
modelled the effect of ephaptic interaction between fibres of the
auditory nerve, and proposed that the effect of the interaction
was to increase the number of interspike intervals
around 10 ms. He concluded that the ephaptic interaction model had a
‘potential
real-life parallel in the demyelinating effects of
tumours of the eighth nerve’ (e.g. vestibular schwannoma).
Several terms have been used for such measures of synchronised activity – ensemble spontaneous neural activity (ESNA)56, average spectrum of electrophysiological cochlear activity (ASECA)57, ensemble spontaneous activity (ESA)58, and spectrum of background neural noise (SNN)59. Evidence for synchronised spontaneous neural activity associated with tinnitus is emergent. Martin et al60
recorded spontaneous auditory nerve activity from 10 cats pre- and
post-salicylate administration using an incoherent spectral
averaging technique, which allows the
identification of continuous signals that have consistent frequency
characteristics.
It was noted that the results needed to be
interpreted with caution because of the physiological stress salicylate
places
upon the animal. Marked changes in the spectral
analysis of auditory nerve activity pre- and post-salicylate
administration
were reported, with a new peak of activity centred
at or near 200 Hz being identified in all post-administration
recordings.
A higher-frequency, broader peak was also
identified. Two animals in whom saline was administered did not
demonstrate the
new peaks of activity. Cazals et al61
reported the effects of long-term salicylate administration in the
guinea pig. Changes (specifically an immediate decrease
followed by a progressive increase) in spontaneous
activity recorded from the round window predated changes in hearing
sensitivity,
which the authors felt was an indication of
high-frequency salicylate-induced tinnitus as this would be expected in
humans
under such conditions. Martin56
described spectral average recordings from the cochlear nerve of 14
human adult patients undergoing cerebellopontine angle
surgery. In 12 patients with tinnitus, a prominent
peak in the spectral average near 200 Hz was reported (see Fig. 2). Further animal studies report that ESNA is influenced by contralateral acoustic stimulation62.
Fig 2.
Comparison of
power spectra of spontaneous auditory nerve activity recorded from a
patient with pre-operative tinnitus (upper)
and a patient with no pre- or postoperative
tinnitus (lower). It should be noted that the sharp spikes in both
traces are
the result of 60 Hz noise and its harmonics.
From Martin56, with permission.
The origin of peaks within the
spectrum of spontaneous neural activity recorded from guinea pigs
undergoing salicylate administration
was explored by McMahon and Patuzzi63. They questioned the assumption of Cazals and Huang57 that the peaks are indicative of synchronous activity. Two spectral peaks were identified. A peak at 170 Hz was thought to
be consistent with the 200 Hz peak previously reported by Martin56 in humans (see Fig. 2).
A spectral peak at 900 Hz was thought to arise from resonance of the
primary afferent nerve membrane, with a potential
contribution from neurones with similar membrane
properties in the ventral cochlear nucleus. Evidence favouring the
existence
of these two peaks has also been reported by
Searchfield et al64. McMahon and Patuzzi suggested that the peaks of spontaneous activity recorded at 200 Hz and 900 Hz may in future be used
to determine the location of physiological generators of tinnitus.
Medial efferent system involvement
Eggermont50
suggested that the efferent system might influence the perceived
intensity of tinnitus, and associated annoyance, based on
the observation that stressful situations may
exacerbate tinnitus, and that techniques such as biofeedback may reduce
tinnitus.
In addition, the connection of the auditory
efferent system with the reticular formation within the brain stem had
been linked
with the persistence of tinnitus as an alerting
stimulus by Hazell and Jastreboff19. Jastreboff and Hazell61 additionally suggested a role for the efferent system in modulating a cochlear mechanism of tinnitus generation.
Veuillet et al66
investigated the possibility that dysfunction of the medial efferent
system was involved in tinnitus perception by measuring
the suppressive effect of contralateral noise upon
transient evoked oto-acoustic emissions (TEOAE) in subjects with
tinnitus
localised to one ear only. The hypothesis that
efferent dysfunction in the tinnitus ear would result in a smaller
suppressive
effect of noise upon TEOAE amplitude than in the
non-tinnitus ear was only marginally supported. Large intersubject
variability
in the suppressive effect was noted.
Lind67
also measured the suppressive effect of contralateral broad-band noise
on TEOAE in 20 patients with unilateral tinnitus and
symmetrical hearing, finding no significant
difference between the suppression effect in tinnitus and non-tinnitus
ears.
An alternative mechanism of efferent system involvement in tinnitus perception has been suggested by Robertson et al68,
following experimental evidence that, in the guinea pig, olivocochlear
inputs to the cochlear nucleus can be excitatory,
thus directly affecting ascending activity in the
auditory pathway, separately from influence upon the cochlea. Efferent
dysfunction
might, therefore, be implicated in tinnitus
perception generated at a brain stem level. However, a review69
of tinnitus experience following vestibular nerve section in humans,
which involves ablation of the medial efferent pathway
in the inferior vestibular nerve, indicated that
total medial efferent dysfunction was not associated with troublesome or
exacerbated tinnitus.
Somatic modulation
The modulation of tinnitus by somatosensory input was considered by Levine70.
Patients were first interviewed about their experiences of somatic
modulation of their tinnitus, such as changes in pitch
or intensity associated with face stroking or head
movements. They were then asked to perform manoeuvres of a few seconds'
duration to test for somatic effects, including
teeth clenching, pressure on the occiput, forehead, vertex and temples,
head
turning and shoulder abductions. In the interview,
16 of 70 patients reported that they could somatically modulate their
tinnitus
(23%). On testing, however, 48 patients (68%)
reported modulation of their tinnitus with at least one of the
manoeuvres. The
pattern of modulation reported was highly variable
involving changes in intensity (both increase and decrease) and pitch.
In all cases, these changes were transient. The
results led Levine to conclude that ‘somatic modulation appears to be a
fundamental
attribute of tinnitus’, and to propose interactions
between auditory perception and somatosensory input at the dorsal
cochlear
nucleus. Higher centres where such interaction also
occurs (such as the SOC and IC) were not considered as somatically
modifiable
tinnitus is largely localised to one or other ear,
and it was thought that binaural interactions in the SOC and higher
centres
would not have given rise to tinnitus heard to just
one side. Levine71,,72 also noted that cranial nerves V, VII, IX and X converge in the medullary somatosensory nuclei (MSN; Fig. 3) and that anatomical links between the MSN and the DCN had previously been described73. The ability of some mammals to incorporate information about pinna position in sound localisation is indicative of such
a pathway74. Levine hypothesised that decreases in inhibitory MSN input to the DCN (specifically inhibition) might result in disinhibition
of DCN activity leading to increased activity and the perception of tinnitus. Levine71
noted potential criticisms of this model. The DCN may not be the site
of somatic and auditory interaction involved in tinnitus.
The argument that the lateralisation of the
tinnitus perception being evidence for the role of DCN somatic
modulation of tinnitus
is strong, but a role of the extralemniscal pathway
in interactions between somatic and auditory pathways, as proposed by
Moller et al75,
is also worthy of consideration and would allow unilateral tinnitus
perception. Another potential criticism is that, whilst
the anatomical links between the MSN and DCN have
been identified in the cat, the situation is humans is less clear, and
in
particular no pathway from the cunate/spiral tract
of cranial nerve V to the DCN has been identified.
Fig 3.
Schematic diagram of suggested interaction between somatic and otic pathways (following Levine72 with permission).
Analogies with pain
Analogy with chronic pain
Analogies between pain and tinnitus have been made many times in the literature (see House & Brackmann76 and Evans77
for early examples). It has been noted that: (i) pain, like tinnitus,
can arise from a great variety of lesions; (ii) there
is no one specific mechanism for pain perception;
(iii) pain is a subjective phenomenon that is difficult to quantify; and
(iv) treatment of pain symptoms is difficult and
often ineffective79,,80. More specifically, Moller1,,80,81
considered the analogy between tinnitus and chronic pain in terms of
peripheral generation and of central persistence once
the acute injury has resolved. Whilst chronic pain
is often a consequence of peripheral injury, that injury may not in
itself
account for the sustained nature of chronic pain.
Moller81 considered that the involvement of the CNS in such sustained perception was indicated by the relevant literature (see Basbaum
& Jessell82
for a comprehensive review). Such involvement implies plasticity within
the CNS. Similarly, while tinnitus is often associated
with peripheral auditory dysfunction, that
dysfunction may not account for the sustained and distressing tinnitus
perception.
Emotional and environmental influences upon pain
perception have been noted82.
The consequent large variation between individual experience of pain,
makes the development of effective therapy very difficult.
Cortical re-organisation, tinnitus, and analogies with phantom pain
The possible analogy between tinnitus and phantom limb pain was first drawn by Goodhill in 195083. The concept that cortical re-organisation similar to that involved in phantom limb pain84 might occur in auditory cortical areas following change in the auditory periphery was first reviewed in detail by Meikle85 and more recently by Salvi et al86.
The precise tonotopicity that has been demonstrated in the central
auditory pathways means that de-afferentation of a specific
portion of the cochlea will, in the short-term,
lead to reduced activity in the cortical area with corresponding
characteristic
frequency (CF). If similar measurements are made
some months later, that area is again responsive to sound, but many
neurones
now have CFs adjacent to that of the lesioned
region86. This phenomenon has been demonstrated in animals87,,88
(one study in particular reported that even a modest noise induced
hearing loss resulted in significant cortical re-organisation89) and in humans90,,91. One consequence of this re-organisation is that a disproportionately large number of neurones will be sensitive to frequencies
at the upper and lower borders of the hearing loss. Salvi et al86 proposed that spontaneous activity in these areas might be perceived as tinnitus. Meikle85
suggested that the mechanism of such re-organisation might be the
disinhibition of previously weak synaptic connections,
and that the area of re-organisation might be
limited to 1–2 mm, leading her to suggest that cortical re-organisation
effects
larger than this might represent re-organisation at
a lower level in the auditory pathway where the tonotopic maps are
smaller
(the inferior colliculus for example).
Re-organisation of the tonotopic map in the IC of the chinchilla
following a high-frequency
cochlear lesion has been demonstrated92.
Evidence for re-organisation of the auditory cortex being a mechanism of tinnitus generation in humans was reported by Mulnickel
et al93 and Dietrich et al91.
Whilst these studies involve small numbers of subjects, there are early
indications that the identification of tinnitus
mechanisms involving re-organisation and plasticity
within the central auditory system may facilitate the development of
novel
pharmacological therapies for tinnitus60,,94.
The future
This review indicates that there are
many potential mechanisms for tinnitus, and so the population of people
with troublesome
tinnitus will be heterogeneous in aetiology and
experience, as is observed in clinical practice. It is envisaged that
the
objective of tinnitus mechanism research in coming
years will be to determine the validity and relevance of the hypotheses
regarding tinnitus generation to the clinical
population, and to use that evidence to design effective clinical
treatments.
Key points for clinical practice
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There are multiple potential mechanisms of tinnitus, and this accounts for the heterogeneity evident in the clinical population.
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The development of new and effective treatments will be greatly facilitated by identification of mechanisms in humans.
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The analogy between tinnitus and phantom limb pain, and the possibility of a role for 5-HT dysfunction in tinnitus, indicate the possibility of effective clinical intervention in tinnitus where these mechanisms are evident.
Acknowledgments
Discussions with Ian Winter and his
detailed critical review of the manuscript were extremely helpful in
writing this chapter.
Brian Moore was a kind and diligent editor. Thanks
also are due to David Moffat, Ross Coles and Jonathan Hazell for
nurturing
my interest in tinnitus.
Footnotes
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Correspondence to: Mr David M Baguley, Audiology Department (Box 94), Addenbrooke's Hospital, Hills Road, Cambridge CB2 2QQ, UK
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