FROM:
Chiropractic & Osteopathy 2006 (Jan 19); 14 (1): 2 ~ FULL TEXT
Joseph O Di Duro
BACKGROUND: The first chiropractic adjustment given in 1895 was reported to have cured deafness. This study examined the effects of a single, initial chiropractic visit on the central nervous system by documenting clinical changes of audiometry in patients after chiropractic care.
CASE PRESENTATION: Fifteen patients are presented (9 male, 6 female) with a mean age of 54.3 (range 34-71). A Welch Allyn AudioScope 3 was used to screen frequencies of 1000, 2000, 4000 and 500 Hz respectively at three standard decibel levels 20 decibels (dB), 25dB and 40dB, respectively, before and immediately after the first chiropractic intervention. Several criteria were used to determine hearing impairment. Ventry & Weinstein criteria of missing one or more tones in either ear at 40dB and Speech-frequency criteria of missing one or more tones in either ear at 25dB. All patients were classified as hearing impaired though greater on the right. At 40dB using the Ventry & Weinstein criteria, 6 had hearing restored, 7 improved and 2 had no change. At 25dB using the Speech-frequency criteria, none were restored, 11 improved, 4 had no change and 3 missed a tone.
CONCLUSIONS: A percentage of patients presenting to the chiropractor have a mild to moderate hearing loss, most notably in the right ear. The clinical progress documented in this report suggests that manipulation delivered to the neuromusculoskeletal system may create central plastic changes in the auditory system.
From the Full-Text Article:
Discussion
The current observational study cannot prove a cause and
effect relationship. The limitations to this current study
are the small sample size and that there was no blinding
of the investigator though patients were blinded to the
fact that hearing would be tested post-chiropractic care.
Furthermore, no true control group or randomization of
testing sequence was employed and potential alternative
explanations as to the natural history of hearing loss may
explain our results, for example some learning effect of the
test.
Possible mechanisms
The auditory system is inherently plastic, permitting us to
learn to identify new voices, speak new languages and sing
new songs. The rapid changes observed in our sample
group were characteristic of those occurring in central
adaptive mechanisms [16]. These central plastic changes
are most likely the result of relatively simple alterations in
the balance of excitatory and/or inhibitory inputs produced
by manipulative care when examining central auditory
processing.
Cortical mechanisms
Each primary sensory cortex, in this case the auditory and
somatosensory, project to nearby higher order areas of
sensory cortex, called unimodal association areas, that
integrate afferent information for a single sensory modality
[17]. The unimodal association areas in turn project to
multimodal sensory association areas that integrate information
about more than one sensory modality. Animal
experiments indicate that dynamic cortical reorganization
of the representation or tonotopic map of the cochlea, the
primary organ for hearing, occurs when the cochlea is
lesioned [16]. Specifically, cortical regions deprived of
normal peripheral input show expanded representation of
lesion-edge frequencies. Reorganization of cortical and
behavioural activity associated with sensory deprivation
has also been demonstrated in humans [16]. Therefore, it
is possible that a long standing decrease in activation of
the auditory cortex and primary association areas, which
may occur in insidious hearing loss, could produce a central
auditory processing disorder (CAPD) [18] and that, in
turn, could serve to explain the areas of hearing loss and
rapid restoration seen in our patient group.
The concept of central plasticity (i.e. the central nervous
systems ability to adapt to environmental influences) presumes
that changes in one sensory modality may create a
convergence upon other areas of the cortex that integrate
that information into a polysensory event. Some authors
have pointed to the site of this neuronal plasticity as characteristic
of the non-primary auditory thalamus and cortex
[18]. Cortical integrity relating to task-conditioned
speech sounds is reflected in lateralized supratemporal
cortical responses possibly in concordance with the left
hemispheric dominance in language [19]. A certain level
of left/right dissociation in the processing of tones within
the speech sound range may be reflected in the significantly
greater unilateral hearing loss which we recorded in
the right ear. If this is the case, then the changes induced
by chiropractic evoked somatosensory potentials via
physical adjustments create changes in both hemispheres
as indicated by our data. We noted that despite generalized
and predominantly right-sided deficit detected in the
audiograms of each patient, the total number of tones recognised
post chiropractic care surprisingly became evenly
distributed and symmetrical (Table 2). This may a global
change in neural activation rather than a change in one
specific modality.
Thalamic mechanisms
Recent electrophysiological evidence has changed the traditional
view that language and memory being primarily
in the cortex to focus on the role of subcortical structures
[20]. Loss of language function in a patient after a focal
infarct of the left ventral lateral thalamic nucleus extending
to the anterior part of the pulvinar [21] exemplified
the way the left thalamus brings online the cortical networks
involved in language processing. This form of
"selectively engaging" positioned the thalamus as integral
in activating post-synaptic areas [22]. This concept places
the thalamus as an alerting system activating a mosaic of
specific discrete cortical areas appropriate to a particular
task and maintaining other cortical areas in a state of relative
disengagement (inhibition). Asymmetric hemispheric
responses to speech sounds are well documented,
however thalamic as well as cortical specialisation to language
has also been demonstrated, the left being more
involved [20]. New evidence derived from a battery of
studies on patients undergoing stereotatic thalamic operations
for the treatment of chronic pain, dyskinesias, (Parkinsonism)
dystonia and tremor demonstrated that when
the ventral lateral thalamus, long considered the "motor"
area of the thalamus, was stimulated on the left, performance
on tests involving simple speech sound was
enhanced. However, when lesions were administered to
the left thalamus, dichotic listening performance was
impaired [23]. The results suggest that the thalamus is
involved in generating a "specific alerting response" that
acts as a gating mechanism which controls the input and
retrieval of specific items [23]. Specifically, activation of
the reticular nucleus of the thalamus changes an "arousal
threshold", thereby affecting language processing and
learning. As an integrating group of neurons that connect
to every level of brain tissue, it appears that the left thalamus
plays a central role in manifesting arousal control
and contributing to excitation or inhibition of the auditory
system.
In a study of 500 participants, Carrick [24] examined the
central effects of cervical spinal manipulation on the
changes in dimensions of the visual field's blind spot. His
results suggest that cervical manipulation has a strong significant
ability to change and increase contralateral thalamic
and cortical activity. Carrick postulated that changes
in amplitude of muscle stretch receptors and joint mechanoreceptors
from manipulation change the amplitude of
somatosensory receptor potentials, which in turn, influence
the frequency of firing of cerebello-thalamocortical
loops responsible for maintaining a central integrated
state of the cortex [24].
Brainstem mechanism
The changes in a persons' ability to hear tones at speech
threshold would fall under the classification of central
adaptive changes or plasticity. There is no doubt that central
plastic changes occur in the brainstem, specifically at
the level of the vestibular nerve. Central plastic changes
and recovery in vestibular nuclei adapt so rapidly that
complete unilateral labyrinthectomy (complete damage
to one labyrinth) should create extreme vertigo and
imbalance. However, patients can become asymptomatic
in less than two weeks [25]. Spontaneous regeneration
and recovery of hearing function of central auditory pathways
after transection of the ventral cochlear tract in the
pons have been noted in young rats [26]. Plastic changes
in the auditory system have been noted to take place
much faster in central systems than in peripheral system
following a reversible cochlear damage (the primary
receptor for hearing) [27]. In an animal model, employing
similar frequencies and decibels to those in our study,
an acid was administered at the inner hair cells (the location
of the auditory nerve synapse) in the cochlea. This
excitotoxic damage is reversible and in time hearing was
restored. The investigators discovered that the inferior colliculus
evoked potential (IC-EVP) was restored much
more rapidly than the compound action potential (CAP
overall) measured for the auditory nerve. This restoration
was so fast that the IC-EVP was restored to nearly 80% of
baseline at between one to five days, while the CAP over-
all remained below baseline even at 30 days. Furthermore,
the CAP amplitudes remained depressed while the IC-EVP
amplitudes tended to overshoot their baseline values by
some 20% [27]. In other words, when the threshold for
hearing was compared, no difference could be discerned
between the response threshold from peripheral and central
measurements, though the synaptic areas did not contribute
equally to these the adaptive or plastic changes.
This research offers a new perspective on central plasticity
and it is important to note that these rapid changes were
measured at the level of the inferior colliculus (IC) does
not mean that the IC is the site of plastic change. It may be
the case that functional and possibly structural changes
have occurred at lower levels of the brainstem and are
merely being reflected "upstream" in the response of neurons
in the IC.
Another possible site for confluence of somatic and acoustic
input is the vestibulo-cochlear system within the brain
stem. Unilateral hearing loss is frequently noted in persons
with vertigo [28-30]. In fact, between 8% to 44% of
vertigo cases are associated with a chronic ipsilateral sensorineural
hearing loss [28]. The vestibular nuclei integrate
signals from the vestibular organs and visual system
with that of the somatic system. Therefore, it is possible
that changes in the vestibulo-cochlear system of the brainstem
brought about through afferent information of
somatic structures affected by chiropractic adjustments
may influence the integrity acoustic processing and hearing.
Conclusion
A percentage of patients seeking chiropractic care have a
mild to moderate hearing loss, identified by audiometry.
In accordance with other reports, the clinical progress documented
here suggests chiropractic care may benefit hearing
loss and that chiropractic adjustments to various areas
of the spinal column and locomotor system may have an
effect on central auditory processing, though alternative
explanations can not be disregarded. There is a difference
in the unilateral aspect of the hearing deficit noted in the
right ear of patients in this current study as reported in
others. The observations documented in this case series
provide limited support to previous works indicating that,
when hearing is tested immediately after a single chiropractic
adjusting visit, hearing may be improved in both
ears. Further research in this area is required, in the form
of a well designed randomised controlled trial.
Competing interests
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