Working of the hearing (auditory) system :-

Our system of hearing comprises of 2 sections viz. a peripheral section which is our ear and a central section located in the brain which carries the sensation from the ears to the auditory area of the cerebral cortex. The auditory area of the cerebral cortex (called auditory cortex) is the area of the brain which is dedicated to and specialised in interpreting the sound which comes to our ears. The ear receives the sound in the form of sound energy which is a form of vibration. This vibrating energy enters the external part of the ear (called external auditory meatus) and vibrates the ear-drum (technically known as tympanic membrane). This vibration of the tympanic membrane is picked up by a chain of small bones called malleus, incus and stapes which not only conduct this vibration to a specialised organ called cochlea but also amplify the vibration several folds. This amplified vibration stimulates the cochlea. The cochlea is the transducer of the hearing system. The function of the cochlea is to convent the vibratory energy into electrical energy. Once this has been achieved, this electrical energy enters the nerve of hearing (called auditory nerve) and carries the sensation through different parts of the brain to the auditory cortex, where the sensation of sound is analysed and interpreted. For proper hearing each and every part of this system right from the external auditory meatus to the auditory cortex has to be normal. A disorder in any of them will cause deafness.

The ear has 3 sections viz.- the external auditory meatus, the middle ear and the inner ear. The external ear is the area from the pinna (technically called auricle) to the ear drum. The middle ear is from the ear drum to the cochlea, it consists of the 3 small bones called ossicles which are placed in a closed space (called tympanum) filled with air. The inner ear is the portion of the ear deeper to this and it houses the transducer ( called cochlea) and also the organ of balance (called vestibular labyrinth).

Disorders of the auditory system :-

Each section of the ear has diseases specific to it and specific tests (investigations) are there to identify disorders in each portion. The common causes of disorder in the external auditory meatus is collection of either wax or fungal debris or foreign body in it. To diagnose this no investigation is required and your doctor can see it directly and clean it with instruments. This deafness due to blockage of the external ear is usually very slight. The middle ear comprises of the ear drum, the ossicles and the air space within the cavity of the middle ear. The common diseases affecting this portion are e.g. perforation in the ear drum (called CSOM) a stiffness or damage to the chain of small bones in the ear and collection of fluid in the middle ear space (called middle ear effusion). A perforation can usually be diagnosed just by visualising the ear-drum; however the other middle ear disorders require special investigations for confirmation. Any deafness due to a disorders in the external auditory meatus or in the middle ear is called conductive deafness. because the primary function of these portion is the conduction of sound to the inner ear and disorders in these areas impede the conduction of sound to the cochlea. A perforation in the ear drum or a stiffness of the ossicular chain can be corrected surgically. Collection of fluid in the middle ear is usually treatable by medicines but may sometimes require surgical management. The diseases of the inner ear, i.e. the cochlea is difficult to treat. Disorders of the inner ear not only cause a deafness called sensorineural (perceptive) deafness but also may case a peculiar sensation of buzzing sounds in the ear called tinnitus. However tinnitus is not specific only of cochlear damage and sometimes disorders of the middle ear and / or the portion of the auditory nerve in the brain can also cause tinnitus. Deafness due to disorders of the inner ear are commonly refractory to medical and surgical methods and usually hearing aids are the only option. Deafness may also occur due to diseases of the nerve carrying the sensation from the cochlea to the brain . One common disorder is acoustic neuroma which is primarily a tumor of the nerve of nerve of balancing but damages the nerve of hearing due to its close proximity to it. This is a sinister disease and if left untreated can be life threatening. Deafness due to disorder of the nerve is called retrocochlear deafness and deafness due to disorders of the nerves which carry the sensation of hearing still higher up to auditory cortex are called central deafness.

The common investigations for hearing :-

Different audiological investigations help us to diagnose the nature of deafness and localise the site of disorder. The commonest investigation for deafness is Puretone Audiometry. It measurers hearing acuity (i.e. how perfectly the subject can hear) and tells us whether is deafness is conductive (disorder in ext. aud. meatus and / or middle ear) or sensorineural (disorder in the inner ear or in the nerve of hearing in the brain) or whether the deafness is mixed ( i.e. a disorder combining both the conductive apparatus as well as the inner ear / nerve of hearing). Tympanometry is also a common audiological investigation. It assess the structural integrity of the middle ear. It helps us to diagnose the nature of the disorder in the middle ear in cases of conductive or mixed deafness. It can tell us whether there is any stiffness of the ossicular chain or whether the ossicular chain is broken or whether there is collection of fluid in the middle or the ear drum had become immobile due to adhesions in the middle ear. If Audiometry test has diagnosed the deafness to be sensorineural in type, there are some specialised test like- SISI, ABLB, Tone Decay, Speech Audiometry and Acoustic Reflex tests which can tell us whether the disorder is in the cochlea or in the nerve of hearing. The BERA test is a test for sensorineural deafness and is usually done to objectively assess the site of lesion in retrocochlear type of sensorineural deafness (i.e. if a lesion is expected in the auditory nerve or in the neural pathways which carry the sensation through the brain). It also is used to objectively assess the the hearing acuity of children who cannot responded properly to the puretone audiometry test.