The role of inflammation in the pathogenesis of tinnitus

Tinnitus is the perception of sound without an external source. Chronic
tinnitus is prevalent among 5 to 15% of the general population and negatively
affects quality of life in 20% of cases (Eggermont and Roberts, 2004). No
curative treatment is available, making it a substantial medical and
socioeconomic problem.
The exact pathophysiology of tinnitus remains unknown. It is thought that
tinnitus is generated in the brain as a result of damage to the auditory tract
somewhere between the cochlea and brain. Cochlear damage because of noise
exposure is the major cause of such deafferentation in the onset of tinnitus
(Agrawal et al., 2009, 2008). In the past decade several studies demonstrated
an inflammatory response under various damaging conditions causing tinnitus,
including noise exposure. In animal models of tinnitus, the expression of the
proinflammatory cytokines Tumor Necrosis Factor-α (TNF-α) and interleukine-1β
(IL-1β) was increased throughout the whole auditory tract (Mennink et al.,
2022). In humans, results on complete blood count measures (mean platelet
volume, platelet distribution width, platelet count and
neutrophil-to-lymphocyte ratio) and cytokine levels were conflicting (Mennink
et al., 2022). Szczepek et al. (2014) found a correlation between serum levels
TNF-α and tinnitus loudness. Only two studies evaluated cytokine concentrations
in tinnitus patients. Weber et al. (2002) found an increase of IL-6 in tinnitus
patients, and Haider et al. (2020) found a decrease in IL-10. However, neither
study included tinnitus patients based on the degree of hearing loss, or
accounted for hearing loss in the analysis, despite that inflammation has also
been implicated in hearing loss (Frye et al., 2019; Fuentes-Santamaría et al.,
2017). On the other hand, studies about inflammation in hearing loss did not
(always) exclude tinnitus. Thus, it remains unclear whether the change in
cytokine concentrations in Weber et al. (2002) and Haider et al. (2020) are
specific to tinnitus, and whether potential effects remained unsignificant
because the presence of hearing loss is a confounder. Therefore, the aim of
this study is to evaluate the presence of inflammation in blood samples of
tinnitus patients with normal hearing.

Primary Objective: To evaluate cytokine levels in blood samples of human
subjects with tinnitus and normal hearing, compared to healthy controls.

Secondary Objective(s): To evaluate complete blood count measures (neutrophil
count, platelet count etc.) in blood samples of human subjects with tinnitus
and normal hearing, compared to healthy controls, and to assess the relation
between tinnitus characteristics and inflammatory serum marker concentrations.

This research protocol describes a cross-sectional study that will take place
at the University Medical Center in Groningen at the department of ENT &
Audiology. In this study, we intent to examine whether inflammation is present
in tinnitus patients without hearing loss. Inclusion is planned from
February/March 2022 to September 2022, or until the target number of inclusions
is acquired. Patients who have been referred to the tinnitus consultation at
the outpatient clinic will be asked to participate in this study. If a patient
is eligible for the study, his/her primary caregiver will inform the patient
orally and/or in writing. Normally, patients attending the tinnitus
consultation complete a tinnitus questionnaire and undergo audiometric testing
for standard care. For this study, we access this data and additionally,
patients will have two samples of blood drawn. In these samples, cytokine
concentrations and complete blood count measures will be determined. Moreover,
the ENT department has a database with tinnitus patients that attended the
tinnitus consultation previously. Eligible patients will be contacted by the
audiologist (primary caregiver). If they give consent for being contacted for
this study, they will be contacted by the researcher about this study and will
be informed orally and/or in writing. Patients that attended the tinnitus
consultation <= 5 years ago and still suffer of tinnitus are eligible, assuming
they fulfill the inclusion criteria. These patients are asked to visit the UMCG
once to fill in a short version of the tinnitus questionnaire and to have two
tubes of blood drawn by venipuncture. Additionally, we ask permission to review
their audiogram if it is younger than 1 year, and audiometric testing is done
if their audiogram is older than 1 year.
For controls, healthy volunteers will be recruited with flyers. When someone
contacts the researcher that he/she is willing to participate, the researcher
will make sure that he/she does not fulfill an exclusion criterium. If this is
the case, a visit at the UMCG is planned. First, audiometric testing is
performed to make sure the participant fulfills the inclusion criterium
regarding hearing thresholds. If eligible, the participant will also complete a
questionnaire and have two tubes of blood drawn by venipuncture.

There are no immediate benefits for patients to participate in this study.
Complications of venipuncture are minor (bruising or hematoma, diaphoresis,
hypotension, syncope, cellulitis and phlebitis). Except for bruising and
hematoma, the other complications are uncommon. When they do arise however,
they can be treated (Galena, 1992). Therefore, possible risks are minimal.