Hearing loss and deafness

The mechanism that allows hearing is very complex and any alteration in any of its stages can lead to decreased hearing ability. Sounds are picked up by the pinna, which transmits and amplifies them through the external auditory canal, causing the tympanic membrane to vibrate. This vibration is transmitted by the ossicles of the middle ear (hammer, anvil and stapes), which in turn transmit it through the oval window to the cochlea, the anterior labyrinth, where sound travels through a liquid and stimulates the cilia of the organ of Corti, the structure in which vibration is transformed into a nerve stimulus.

This nerve impulse is transmitted through the auditory fibers to the spiral ganglion, from which arises the auditory nerve that goes to the brain stem and from there the stimulus continues to the cerebral cortex, where the nerve stimulus is interpreted.

Any alteration in this transmission chain could produce a hearing deficit, be it acquired or congenital.

How is it produced?

Within hearing loss, a differentiation must be made between those that are due to an alteration in the physical mechanisms of sound transmission, the so-called conduction, and those derived from alterations in nerve transmission, which are called neurosensory. Some types of deafness can be mixed, with both conductive and sensorineural alterations.

• The transmission hearing loss They can be due to an obstruction of the external auditory canal caused by wax plugs, tumors or foreign bodies. Otitis media, tympanic perforation, Eustachian tube abnormalities, or pressure or trauma injuries can cause some degree of hearing loss.
• The otosclerosis It is an alteration that affects the bone of the bony labyrinth, where the cochlea is located. More frequent in women and bilateral in 80% of cases, there is an alteration of the bone tissue, which is replaced by disorganized bone. It mainly affects the stapes, the contact area of ​​the stapes and the oval window, with which the conduction of sound is altered.
• Another form of conductive hearing loss is tympanosclerosis, in which the subumucosa of the middle ear degenerates and calcifies, with which the structures become more rigid and the transmission of sound is hindered. It is usually due to recurrent infections.

Sensorineural hearing loss is hereditary in 50% of cases. 25% are acquired and up to 25% are of unknown cause. Those acquired can be due to transplacental infections, such as toxoplasmosis, measles, adenovirus, herpes, cytomegalovirus or bacterial infections, as well as by incompatibilities of the Rh group, radiation or ototoxic substances. In adulthood, they can be due to infections, trauma, malignant tumors, metabolic disorders, or ototoxic substances such as aminoglycosides, some chemotherapeutic agents, loop diuretics, or some antimalarials.

However, one of the main causes of acquired sensorineural hearing loss in adults is presbycusis, which affects 25% of people over 60 years of age. The hair cells of the organ of Corti, which capture sound vibrations and transform them into a nerve impulse, are injured, causing people to suffer from generally bilateral hearing loss, especially in noisy environments.

Acoustic trauma, either from isolated exposure to loud sound over 140 decibels or from chronic unprotected exposure to noise over 85 decibels, can also cause sensorineural hearing loss.

Symptoms

The main symptom of an auditory pathway alteration is hearing loss, which can be mild (loss of less than 40 decibels), moderate (between 40 and 70 decibels), severe (greater than 70 decibels) or absolute, which is called kphosis. .

It should be assessed if it is unilateral or bilateral, if the onset has been sudden or progressive and if it is associated with other symptoms, such as earache, otorrhea, tinnitus or balance disturbances. Patients with otosclerosis, unlike those with presbycusis, report hear better in noisy environments, which is known as Willis paracusis, and yet they hear worse when chewing, a phenomenon called Weber paracusis.

Diagnosis

The diagnosis will be based initially on a correct questioning of the patient, ruling out exposure to ototoxic substances, recent acoustic or physical trauma, ear infections or decompression situations (air travel, diving,…). It is usually normal unless there is an active infection, external auditory canal obstruction, or tympanic perforation. In children it is essential to make an early diagnosis, assessing whether there is a family history, infections during pregnancy, cranial malformations, low gait at birth, exposure to ototoxic agents or trauma. Audiomotor reflexes will be explored, that is, the mobility of the extremities when faced with a loud sound and their ability to locate the source of the sound.

Acumetry or examination with tuning forks to assess whether the airway or bone conduction is affected in the transmission of sound.

The audiometry It is a test that allows quantitative and qualitative assessment of the patient's auditory perception. With it, it is possible to discern whether the hearing loss is transmission, sensorineural due to cochlear alteration or sensorineural hearing loss due to alteration of the auditory pathways posterior to the cochlea. In its different forms, it can assess the minimum threshold of auditory perception, the existence or absence of distortion of the sound wave and the ability of the listener to discriminate verbal sounds.

The impedance it is an assessment of the resistance of the tympanic membrane and the ossicle chain to the passage of sound. If there are alterations in the structures of the middle ear, the impedance will be altered. There are two types of impedance tests, tympanometry, which makes it possible to measure the resistance of the structures of the middle ear to sound, and the study of the stapes reflex, which assesses the adaptability of the ossicle chain to a very high sound stimulus. .

Otoacoustic emissions are a record through the external auditory canal of the nerve activity produced by the cochlea. It is a good system to rule out sensorineural hearing disorders in neonates.

Brainstem auditory evoked potentials are an electrode recording of the transmission of the auditory nerve impulse from the auditory nerve to the trunk. It allows to assess neurosensory alterations of the posterior pathways to the cochlea.

Treatment

Treatment should be that of the cause whenever possible, especially in those reversible hearing loss secondary to obstructions by foreign bodies, wax plugs, otitis or tumors.

With regard to hearing loss due to otosclerosis, it is necessary to perform a surgical intervention in which the incus stapes is disarticulated, removed and replaced by a prosthesis. In tympanosclerosis, surgical treatment can be considered, but the results are not very promising. In pressure trauma cases, treatment should be conservative, with analgesics, non-steroidal anti-inflammatory drugs, or corticosteroids.

In the case of sensorineural hearing loss or those of transmission that cannot be solve surgically, the treatment must be early, enhancing the remaining hearing capacity that the patient may have. Hearing aids can be external hearing aids, with a microphone that converts sound into an electrical impulse, amplifies it and transforms it back into a sound wave, or hearing implants, which can be middle ear, for transmission hearing loss, cochlear, which require that the auditory nerve function normally, or brain stem, for patients with both cochlear and auditory nerve alterations.

Precautionary measures

It is essential to detect hearing loss in children early. Likewise, special precaution should be taken during pregnancy, to avoid the administration of substances that may be ototoxic, as well as to avoid exposure to high intensity sounds without adequate hearing protection.