The mechanism that allows hearing is very complex and any change in any of its stages may lead to a decrease in hearing. The sounds are picked up by the ear, which transmits and amplifies through the ear canal and the eardrum vibrate. This vibration is transmitted the middle ear bones (hammer, anvil and stirrup), which in turn transmit it through the oval window to the cochlea, the previous maze where sound propagates through a fluid and stimulates organ of Corticilia, the structure in which the vibration becomes a nervous stimulus. The nerve impulse is transmitted through the spiral ganglion auditory fibers, from which comes the auditory nerve that goes to the brain stem and hence the continuing stimulus to the cerebral cortex, which interprets the nerve stimulus.
Any alteration in this chain of transmission could produce a hearing impairment, whether acquired or congenital.
Within hearing loss should differentiate between those that are due an alteration of the physical mechanisms of sound transmission, driving calls, and those arising from changes in nerve transmission, called sensorineural. Some types of hearing loss can be mixed with both conductive and sensorineural disorders.
Transmission hearing loss may be due to an obstruction of the ear canal caused by earwax, tumors or foreign bodies. Otitis media, perforated eardrum, alterations of the Eustachian tube, or crush injuries or trauma can cause some degree of hearing loss.
Otosclerosis is a disorder that affects the bone of the bony labyrinth, where is located the cochlea. More common in women and bilateral in 80% of cases, there is an alteration of bone tissue, which is replaced by disorganized bone. Mainly affects the stirrup, the contact area of this n the oval window, thereby driving the sound is altered.
Another form of conductive hearing loss is the tympanosclerosis, in which the middle ear subumucosa degenerates and becomes calcified, with which the structures become more rigid and sound transmission is hampered. This is usually due to recurrent infections.
Are inherited sensorineural hearing loss in 50% of cases. 25% are acquired and up to 25% are of unknown cause. The acquired may be due to transplacental infection, such as toxoplasmosis, rubella, measles, adenovirus, herpes, cytomegalovirus or bacterial infections, such as Rh incompatibility group, radiation or ototoxic substances. In adulthood may be due to infection, trauma, malignancy, metabolic or ototoxic substances such as aminoglycosides, some chemotherapy, loop diuretics or some antimalarials.
Anyway, one of the leading causes of acquired sensorineural hearing loss in adults is presbycusis, which affects 25% of people over age 60. The hair cells of the organ of Corti, which capture sound vibrations and turn them into a nerve impulse, are injured, which people generally suffer bilateral hearing loss, especially in noisy environments.
Acoustic trauma, either by exposure to intense sound isolation of more than 140 decibels or chronic exposure to unprotected noise exceeding 85 decibels, can also cause a sensorineural hearing loss.
The main symptom to an auditory pathway impairment is hearing loss, which may be mild (less than 40 dB loss), moderate (40 to 70 decibels), severe (more than 70 dB) or absolute, is called cophosis .
You should assess whether unilateral or bilateral, whether the onset was sudden or progressive, if associated with other symptoms such as otalgia, otorrhea, tinnitus and balance disorders. The patients with otosclerosis, unlike those with presbycusis relate better hear in noisy environments, what is known as paracusia Willis, worse yet hear chewing phenomenon called Weber paracusia.
The diagnosis is based on a correct initial questioning of the patient, exposure to ototoxic substances ruling, acoustic trauma or recent physical, ear infections or decompression situations (air travel, diving, …).Otoscopy usually normal unless there is an active infection, obstruction of the ear canal or eardrum perforation. In children it is essential to make an early diagnosis, assessing if a family history, infections during pregnancy, cranial malformations, low birth step, exposure to ototoxic agents or trauma. It will scan them audiomotores reflexes, ie, the mobility of the limbs to a loud sound and ability to locate the source of the sound.
The tuning fork or exploration acumetría assess whether the airway or bone conduction are affected in sound transmission.
Audiometry is a test that can quantitatively and qualitatively assess the patient’s auditory perception. With it you can tell if the transmission is hearing loss, sensorineural by cochlear or sensorineural impairment by altering auditory pathways after the cochlea. In its various forms can evaluate the minimum threshold of auditory perception, the existence or absence of distortion of the sound wave and the discriminating power of word sounds by the listener.
The impedance is an assessment of the reluctance of the tympanic membrane and the ossicles to the passage of sound. In case of alterations of the structures of the middle ear impedance will be altered. Two types of tests impedance tympanometry, which measures the strength of the structures of the middle ear to the sound, and the stapes reflex study, which assesses the adaptability of the ossicles to a very high sound stimulus .
Otoacoustic emissions are a record through the ear canal of the nervous activad produced by the cochlea. It is a good system to rule sensorineural hearing impairment in neonates.
Evoked potentials are auditory brainstem using electrodes record the auditory nerve impulse transmission from the auditory nerve to the trunk. Neurosensory allows assessment of airway disorders after the cochlea.
Treatment should be to the cause whenever possible, especially in reversible hearing loss secondary to obstruction by foreign bodies, earwax, ear infections or tumors.
With respect to the hearing otosclerosis is required by surgical intervention in which the stirrup is disarticulated anvil is removed and replaced by a prosthesis. In tympanosclerosis can raise surgical treatment, but the results are not very encouraging. In cases of trauma by pressure treatment should be conservative with analgesics, NSAIDs or corticosteroids.
In the case of sensorineural hearing loss or those transmission can not afford surgery, treatment should be early, boosting the remains of hearing that the patient may have. Hearing aids can be external headset with a microphone that converts sound into an electrical impulse, which amplifies and converts it back into sound wave, or hearing implants, which may be of middle ear transmission for hearing loss, cochlear, which require that the auditory nerve function normally, or stem, for patients with both cochlear alterations as the auditory nerve.
It is essential to detect hearing loss in children early. Also, special caution during pregnancy, to prevent the administration of substances that may be ototoxic and avoid exposure to high intensity sounds without hearing protection.