Long-term prognosis of residual tinnitus after SSNHL
Over time, the degrees of tinnitus in patients with different initial
tinnitus levels were all alleviated to a certain extent. The degree of
tinnitus relief and healing time are only related to the initial degree
of tinnitus and the median recovery time for all the patients was 23
months.
Previous studies on residual tinnitus after SSNHL have shown a tendency
for gradual improvement (13,
16). In 2013, Michiba T found that
tinnitus appeared to improve 6 months after treatment but did not
remarkably change between 6 and 24 months
(13). In this article the median healing
time showed that 2 to 3 years may be required for tinnitus to recover.
Such improvements may be associated with the plasticity of the central
auditory pathway. The tinnitus habituation theory
(5) proposes that the brain selects which
stimuli to ignore, and only the information that is considered valuable
or dangerous will be perceived by the brain. According to the theory,
most people consider the tinnitus of low informational value and does
not require a reaction. However, tinnitus-related distress occurs when
this processes is malfunctioning, which is more likely at times of
increased stress, subsequently straining the cognitive resources causing
chronic tinnitus and creating a vicious cycle with the emotional system
(5). Recent studies have confirmed that
once hearing loss starts, the most active compensation occurs in the
thalamus, but not all of these signals can be transmitted to the
auditory cortex. Meaningless and nonhazardous sounds are adapted to and
ignored by the frontal-striatal circuits in the midbrain when patients
do not pay attention to tinnitus over time. The frontal-striatal circuit
region mainly consists of the ventromedial prefrontal cortex, the
nucleus accumbens, and other important areas. These circuits can be
collectively called the tinnitus clearance system as a gatekeeper of
tinnitus. This explanation accounts for the clinical observation that
most tinnitus cases resolve naturally (5,
22, 23) .
However, if a problem arises with this ‘gatekeeper’ , the ability to
clear tinnitus signals will be impaired. As a result, tinnitus may
persist for a prolonged period. Factors affecting the function of this
gatekeeper may include but are not limited to sleep, migraine, estrogen
decline(22), trauma, depression, and
sleep-disordered breathing(23,
24). However, some patients cannot
self-heal and continue to experience slight intermittent tinnitus. Such
patients should be given adequate education and counseling to relay that
tinnitus is a “scar” on the auditory system, which may rationalize
tinnitus, thus eliminating patients’ irrational thoughts and fear of
tinnitus.