Chronic, subjective tinnitus can be defined as the conscious perception of a sound in the absence of an external auditory stimulus over an extended period of time (Esmaili and Renton, 2018). An estimated 11-30% of the global population suffers from tinnitus (De Ridder et al., 2021). Tinnitus is a heterogeneous disorder in that many types of auditory sensations are categorized as tinnitus percepts (Baguley et al., 2013). These include persistent tones, chords (a set of frequencies consisting of multiple notes), roaring, buzzing, clicking, or hissing (NIDCD, 2017). The percept can be unilateral or bilateral, inconstant, or constant. Some individuals can modulate their tinnitus percept with head, neck, or jaw movement (Levine et al., 2007; Pinchoff et al., 1998; Simmons et al., 2008). Although tinnitus percepts can be essentially any sound, they differ from auditory hallucinations in that there is no associated meaning to them (Baguley et al., 2013). Since percepts in tinnitus are heterogeneous, perceivable only by the affected person, and must be reported by the patient, tinnitus is difficult to classify. To complicate matters, there is no universal agreement on the clinical definition of tinnitus (De Ridder et al., 2021).

Most patients with tinnitus also have hearing loss (Tan et al., 2013), although it is possible to have hearing loss without tinnitus or tinnitus without measurable hearing loss. Why tinnitus is present in some, but not all, patients affected by hearing loss is not clear. One potential explanation is that types of hearing loss can vary (e.g. affected frequency range, degree of loss) and have different causes (single traumatic noise exposure, age-related hearing loss, synaptopathy, etc.). Different types of noise exposure may cause different patterns of hearing loss, and certain patterns of hearing loss may or may not result in tinnitus (Tan et al., 2013). Another explanation is that individual differences in cochlear function, such as the ability to detect frequency differences (frequency selectivity), may play a role in which individuals develop tinnitus (Tan et al., 2013). Prevalence of tinnitus increases with increasing age, which is hypothesized to be due to the natural decrease in the number of functioning nerve fibers in the inner ear that occurs with age (Moller, 2007). However, exactly why certain individuals will develop tinnitus while others do not is unknown.

Tinnitus is often co-morbid with hyperacusis and hearing loss (Nondahl et al., 2011). Hyperacusis is hypersensitivity to sounds or the perception of ordinary sounds as louder than normal and with uncomfortable intensity (Schecklmann et al., 2014). Hyperacusis is found in 40% of tinnitus patients (Baguley, 2003) and up to 86% of hyperacusis patients also report tinnitus (Anari et al., 1999).

There are likely genetic differences that underly the development of tinnitus. Heritability of tinnitus is reported between 31-43% (Bogo et al., 2017; Cederroth et al., 2019; Maas et al., 2017). A GWAS of the UK biobank found a single-nucleotide polymorphism heritability of 6% with 6 significant tinnitus loci (Clifford et al., 2020). A systemic review of the genetic contribution to noise-induced tinnitus found multiple replicated genes and genetic variants between genetic tinnitus studies, including ANK2, AKAP9, and TSC2 (Perez-Carpena et al., 2024).

The perception of any sound is usually not distressing but can have detrimental effects on well-being. Tinnitus sufferers experience higher than average rates of daily frustration, annoyance, anxiety, long-term depression, insomnia, stress, and emotional exhaustion compared to the general population (Hebert et al., 2012; Langguth, 2011; McKenna et al., 1991). For tinnitus sufferers, quality of life can be predicted by “tinnitus-fear” or catastrophizing (Cima et al., 2011; De Ridder et al., 2021) and individuals with presumedly similar tinnitus loudness and awareness often have dissimilar tinnitus-associated burdens (Schlee et al., 2017). Therefore, there is an argument to distinguish between tinnitus and the associated suffering (as tinnitus and tinnitus disorder respectively) (De Ridder et al., 2021).

Subjective tinnitus is thought to be caused by dysfunction in the central auditory system (Tunkel et al., 2014). Such changes should be detectable as changes in central nervous system (CNS) function. However, there is currently no objective electrophysiological or imaging method to distinguish individuals with tinnitus from those without tinnitus. Not only does this make it very difficult to diagnose tinnitus, but it also impedes research and the development of treatments for this condition. This review provides an overview of tinnitus assessment, the potential causes of tinnitus, and the status of an electrophysiological test for tinnitus.