Mechanisms of tinnitus generation

1
Mechanisms of tinnitus generation

• Carol A. Bauer
• Current Opinion in Otolaryngology Head and Neck
  Surgery 2004,12413417
• R1 ??

2
Introduction

• Tinnitus auditory sensation without external
  stimulus.
• 620, (13 interferes with daily life)
• Theories of tinnitus pathophysiology
• aberrant peripheral neural activity
• central neural sources
• central dysfunction peripheral source of
  abnormal input
• More than one physiologic mechanism

3
Peripheral sources of tinnitus

• Hair cell damage ? stereocilia decoupling from
  tectorial membrane ? noise? from molecular motion
  within the hair cells ? tinnitus
• Baseline deviation from random activity of
  auditory nerve in the absence of stimulation
• High-rate pulsatile electrical stimulation to the
  cochlea ? suppression of tinnitus (5/11)
• Loss of tonic random afferent input ? loss of
  inhibition within brainstem auditory structures

4
Central sources of tinnitus

• Complete eighth nerve section normal
• Peripheral injury ? central changes
• acute (acoustic trauma) / slowly progressive
  hearing loss.
• Inhibition? / excitation? dorsal cochlear
  nucleus, inferior colliculus
• Complex change in glutamatergic transmitter
  release in ipsilateral cochlear nucleus after
  noise exposure.
• Initially damaged hair cells, neural fibers
  degenerated ? acute? in glutamatergic release
• 2 weeks later glutamatergic release? and uptake
  ?
• 90 days later long-term?of residual
  glutamatergic synapses

5
Plasticity and tinnitus

• Neural plasticity long-term alterations in
  central neural function after peripheral sensory
  receptor damage
• Bidirectional information modulation within
  auditory pathway corticofugal / corticopetal
  projections between auditory cortex and brainstem
  nuclei
• Disturbing tinnitus fail to develop the normal
  habituation in response to a repetitive
  non-informative sound.
• Auditory enrichment or sound therapy long-term
  exposure to low-level (15 dB SPL) sound

6
Plasticity and tinnitus

• Tinnitus ? maladaptive cortical reorganization
  (ex phantom limb pain)
• Magnetoencephalography subjective tinnitus
  loudness / frequency ?? primary auditory cortex.
• Psychophysical training with frequency
  discrimination task ? plastic changes in
  cortical representation of a range of frequencies
• Exposure to continuous low-level background sound
  (auditory enhancement) ? shift in loudness
  judgments

7
Somatosensory / vascular factors

• Electrical excitation of median nerve ?
  somatosensory system ? modulate the
  characteristics and loudness of tinnitus.
• PET imaging orofacial maneuvers (jaw clenching)
  ? changes in blood flow in temporal lobe /
  hippocampus ? modulation of tinnitus loudness.
• Injury to head / neck ? brainstem somatosensory
  nuclei ? inappropriate excitation of auditory
  pathway (dorsal cochlear nucleus) ?
  craniocervical tinnitus

8
Somatosensory / vascular factors

• Cochlear implants isometric movements of head,
  neck, or jaw muscles ? 5080 change in tinnitus
  loudness
• Trigeminal ganglion ? excitatory and inhibitory
  projections ? synapse within ventral and dorsal
  cochlear nucleus
• Stimulate trigeminal ganglion ? 2-deoxyglucose
  uptake? in ipsilateral and contralateral lateral
  lemniscus and inferior colliculus
• Guinea pig capsaicin ? trigeminal control of
  cochlear blood flow
• Capsaicin agonist of type 1 vanilloid receptor
  (VR-1), nonselective cation channel in small- to
  medium- diameter primary afferents in the
  somatosensory system.

9
Somatosensory / vascular factors

• Immunoreactive fibers trigeminal origin.
• electrical stimulation of trigeminal ganglion ?
  plasma extravasation from cochlear vessels.
• inflammatory conditions exacerbate tinnitus.
• Spiral and vestibular ganglia of rats VR-1 and
  5-lipoxygenase.
• tinnitus generation by aspirin and nonsteroidal
  ototoxicity.

10
Conclusion

• Tinnitus appears to be significantly affected in
  complex ways by somatosensory, limbic, and motor
  influences.
• Effective treatments will certainly emerge from
  these new areas of research.