The possible mechanisms of tinnitus have been incompletely understood and research is underway to elucidate this question further. After a brief introduction to tinnitus in my previous blog, in this blog I elaborate various findings in history, examination, anatomy and physiology of hearing that help us understand the possible mechanisms of tinnitus.
Tinnitus is an abnormal perception sound in the absence of a sound.
Anatomy and neurophysiology of hearing
To understand the possible mechanisms of tinnitus generation one has to understand the neurophysiology of hearing. While a detailed explanation of it is out of the scope of this blog, I will attempt to give a simplified version for my readers.
Ear consists of three parts which have been named before:
- External ear – The pinna, canal till the outer surface of the eardrum. This is part of our auditory system that collects sound and directs it to the middle ear. One can relate this to the satellite dish that collects the signals for cable TV.
- Middle ear – It consists of an air filled chamber that starts from the inner aspect of the eardrum to the bony cover of the inner ear medially. This chamber is connected to the eustachian tube and mastoid air cells for ventilation. The eustachian tube connects to the nose and maintains atmospheric pressure in the middle ear. Other contents of the middle ear are three bones namely Malleus, Incus & Stapes, two muscles – tensor tympani and stapedius and a nerve called chorda tympani. Main function of the middle ear is to amplify the sound signal collected by the eardrum and conduct it to the inner ear via the oval window.
- Inner ear – The membranous cochlea, utricle and saccule and the three semicircular canals are housed in bony cavity and together make up the inner ear. Cochlea is the site of hearing where the mechanical stimulus (sound vibration) is converted to electrical impulses which are then carried by the auditory nerve to the brain.
Neural pathway from the cochlea to the auditory cortex is a very complex pathway. We can think of it like a long train journey which originates in the cochlea, with multiple stops(ganglions and nuclei) where the trains(neural tracts) have to be changed to reach the final destination that is the auditory cortex.
The three parts of the ear upto the auditory nerve are called the periphery of the auditory system, rest of it till the auditory cortex is called the central auditory system.
At any given time our periphery (the ear) is feeding an electrical stimulus to the central auditory system which is being identified as a sound, or in case of absence of sound stimulus perceived as silence.
Now that we are equipped with the basic idea of our auditory system, I will attempt to explain the mechanism of tinnitus.
Possible mechanisms of tinnitus
Hearing loss is identified in almost 60% of patients who present with tinnitus. That leads us to the question – is tinnitus the sound of silence?
At level of external & middle ear:
We can understand this question with examples of patients who present with conductive hearing loss due to various causes like impacted wax, otitis media or otosclerosis. On being evaluated these patients often complain of tinnitus which disappears on correction of the cause.
At level of inner ear (cochlea):
Coming more medially to the cochlea, in most patients that present with persistent continuous high pitched tinnitus, it has been associated with sensorineural hearing loss and can be correlated with the audiogram. Tinnitus is also a very common finding after acoustic trauma (i.e. exposure to a very loud sound). Acoustic trauma transiently shifts the auditory threshold higher and damages the outer and inner hair cells.
By associating the finding of tinnitus in short term acoustic trauma and in sloping sensorineural hearing loss, malfunction of outer and inner hair cells was identified as one of the points of origin of tinnitus.
However cochlea could not be the only origin point for tinnitus as tinnitus is also reported by patients who have had auditory nerve transection post head injury. Therefore, tinnitus must have other origin points that could be centrally located.
At levels beyond the ear (centrally originating tinnitus)
Centrally originating tinnitus can originate at any of the sites of synapses of neurons (earlier mentioned train stations i.e. ganglions and nuclei). Then, tinnitus is essentially a manifestation of misfiring neurons activating synapses involved in the auditory pathway.
In our nervous system there are neurons and neurotransmitters that play an inhibitory role, that is, they keep in check these misfiring which lead to development of tinnitus. When these inhibitory neurons get damaged or the levels of inhibitory neurotransmitters drops it manifests as tinnitus in the auditory system.
After discussing the various mechanisms of tinnitus we conclude that tinnitus is a manifestation of imbalance of excitatory and inhibitory nerve impulses in the auditory system which can happen at various levels.
For management and treatment an attempt at diagnosing the level of origin of tinnitus should be made, but, if no cause is identified, patient should be counselled about the benign course of tinnitus and symptomatic treatment may be advised. Treatment of tinnitus will be discussed in my next blog.