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.