1Pathophysiology Overview — A Peripheral Trigger, A Central Disease

FThe two-stage idea

For most of the twentieth century tinnitus was hunted in the ear, on the assumption that a phantom sound must have a sound-generating source. That search largely failed, and a more powerful framework replaced it: tinnitus is a two-stage disorder [2004]. In the first stage a peripheral trigger — usually a loss of cochlear input — disturbs the balance of activity entering the auditory brain. In the second stage the brain itself responds, and that response is what we hear.

The clinically important consequence is that the ear lights the fuse but the brain holds the flame. Cutting the auditory nerve after tinnitus is established rarely abolishes it, and many patients have tinnitus with an audiogram that looks normal . The generator has moved centrally.

TMaladaptive neuroplasticity

The central response is not random; it is the auditory system trying to defend its own activity level. When afferent drive falls, neurons up-regulate their excitability to restore a homeostatic set-point — a process of central gain [2014]. The same plasticity that lets a healthy brain adapt to a quiet room becomes maladaptive here: gain is turned up on a missing signal, and internal neural noise is amplified until it crosses into perception [2011].

This reframes tinnitus as a disorder of plasticity rather than of the cochlea alone — a maladaptive form of the brain’s normal capacity to change [2016]. It also explains why a single peripheral insult produces different outcomes in different people: the percept depends on how the brain reacts, not only on how much input was lost.

TWhy loudness is not the audiogram

A recurring clinical paradox is that tinnitus loudness correlates poorly with the degree of hearing loss. Patients with mild thresholds can have severe tinnitus, and patients with profound loss may have none. This dissociation is expected once tinnitus is understood as a central, gain-driven phenomenon: what matters is the change in drive and the brain’s reactive amplification, not the absolute threshold [2010].

Distress dissociates even further from the acoustics. Emotional and attentional networks decide how intrusive the percept becomes, so loudness and bother are separable dimensions [1990]. Treatment that never changes the sound’s measured loudness can still transform the experience.

FMap of the chapter

The chapter walks the signal from periphery to perception. We begin with the peripheral generators — hair-cell and synapse loss — then trace the central consequences: spontaneous hyperactivity, synchrony and bursting, the dorsal cochlear nucleus as an early generator, central gain, tonotopic reorganisation, and thalamocortical dysrhythmia [2014].

From there the story becomes a network. Auditory cortex, limbic and emotional circuits, and attention/salience systems are added, then integrated by predictive-coding and neuropathic-pain frameworks, and finally by the unifying models that connect mechanism to treatment [2016]. Reading the chapter in order is reading the signal’s journey.

CWhat this means at the bedside

The two-stage model carries direct clinical messages. Because the periphery is the trigger, restoring input — hearing aids or a cochlear implant — can reduce tinnitus in some patients by lowering central gain [2014]. Because the brain sustains the percept, therapies that retrain central activity (sound therapy, neuromodulation, counselling) have a rational target even when the ear cannot be repaired [2016].

And because loudness, generation and distress are separable, evaluation must measure more than thresholds. A normal audiogram does not exclude a real, mechanistically grounded tinnitus — it simply means the relevant lesion may be hidden, deeper, or already central .