Conductive hearing loss is a condition resulting from an impaired passage of acoustic waves through outer and middle ear. Common pathologies causing conductive hearing loss are impacted cerumen, otitis media and otosclerosis.
Older children and adults often state their inability to hear and describe whether they suffer from mild, moderate or severe hearing loss, whether only one or both ears are affected. In case of bilateral CHL, the severity of symptoms may differ between both ears. Hearing impairment may be the predominant or even the only symptom experienced, but in some cases, other complaints are reported. A feeling of fullness or pressure in the ear and otalgia are commonly noted. In detail, the following symptoms may be observed in patients presenting with impacted cerumen, otitis media and otosclerosis:
Neonates and infants don't typically provide such detailed anamnestic data. Here, parents may observe their child to be stubborn, not to interact with their surroundings. Pre-lingual CHL, i.e., hearing impairment that manifests before the acquisition of language and speech takes place, may cause the child to vocalize little and to show delays in cognitive development. Otalgia is often expressed by frequent touching of the ears, but may also lead to continuous indisposition and restlessness.
Besides anamnesis and physical examination, otoscopy and audiologic tests are usually carried out first. With regards to the latter, functional tests don't necessarily allow for a precise localization of the cause of hearing impairment. Some tests do require active participation of the patient and are therefore not suitable for neonates and infants. Auditory brainstem response and otoacoustic emissions can be measured in such patients, though . In fact, they are commonly applied in newborn hearing screenings. Pure-tone and speech audiometry are reserved for elder children and adults. This also applies to tuning fork tests like Weber test and Rinne test that may be indicated to distinguish CHL from SNHL.
Because in CHL patients, acoustic wave propagation through the outer or middle ear is disturbed, chances are good that an otoscopic examination of the external auditory canal and the tympanic membrane yield pathologic findings. While a tentative diagnosis of impacted cerumen may be confirmed this way, pneumatic otoscopy and tympanometry are often required in otitis media patients . Otosclerosis affects the ossicles in the middle ear and tympanometry and diagnostic imaging (computed tomography) have to be performed in order to detect a decrease in ossicle mobility and alterations in bone structure, respectively. Exploratory tympanotomy may also be considered. If these measures don't allow for a reliable diagnosis, additional techniques have to be applied. Serological tests are helpful to detect underlying infectious diseases. Neoplasms are usually diagnosed by means of magnetic resonance imaging.
If feasible, causal therapy should be provided.
To date, causal treatment is not available for patients suffering from otosclerosis. Here, hearing aids may be applied to improve sound perception. In severe cases, non-functional ossicles have to be replaced with the corresponding protheses. Stapedectomy is commonly performed. Similarly, surgical interventions are necessary to reconstruct parts of the auditory system if a child is born with developmental defects causing CHL.
Prognosis depends on the underlying disease. Reconstructive surgery is often necessary to treat patients suffering from developmental defects and congenital CHL. In case of otitis media or impacted cerumen, the outcome is generally very good because causal therapy is available. This does not apply to otosclerosis patients and many affected individual have to undergo surgery in order to regain their ability to hear. Few pathologies associated with acquired CHL have an unfavorable prognosis, e.g., malignant otitis externa.
CHL may be congenital or acquired, whereby the latter is much more common than the former.
Congenital CHL is the result of developmental defects like malformation of one or more ossicles and fixed stapes . Furthermore, microtia, aural atresia and more complex craniosynostosis may be the cause of CHL in neonates. Congenital rubella syndrome and hearing loss due to prenatal infection with measles virus, mumps virus or other pathogens rather induce congenital SNHL.
CHL may be acquired at any age and hearing impairment is often claimed by patients suffering from otitis externa, impacted cerumen, exostosis of the external auditory canal, otitis media (especially if an effusion is present) or tympanic membrane perforation. Also, traumatic lesions of the external or middle ear may hinder sound wave propagation. Temporal bone fractures may cause CHL, SNHL or mixed hearing loss . Otosclerosis is an acquired disease of as of yet unknown origin. Besides impacted cerumen, it is the most common cause of CHL in adults. Here, both progressive limitation of the mobility of the stapes as well as resorption and formation of the bony labyrinth in the inner ear hinder ossicle vibration and pressure wave propagation through perilymph and endolymph. Tumors like cholesteatoma, osteoma or paraganglioma may also lead to CHL. Of note, congenital cholesteatoma have repeatedly been described. Such tumors are rarely diagnosed in pediatric patients, but the presence of cholesteatoma in individuals without a medical history of otorhinolaryngologistic diseases led to the hypothesis of them having an embryologic origin .
Although incidence and prevalence of CHL are lower than of SNHL, the former is still a common condition. According to a study recently conducted in Australia, prevalence of CHL among neonates averages 0.3% . Interestingly, significant differences have been observed between indigenous and non-indigenous Australians, with the former being affected about twice as often as the latter (prevalence of CHL among infants 35% vs. 18%). These findings imply racial differences that may also exist with regards to ethnicities in other continents. It has to be noted that gender and racial predilection as well as age distribution of CHL may vary depending on the underlying disease, but, in general, women and men, patients of all races and ages may suffer from CHL.
Sound waves pass auricles, external auditory canal, tympanic membrane, ossicles and oval window, before reaching the inner ear and the organ of Corti. Here, mechanical stimuli are converted into electrical impulses which are conducted to higher brain centers. As per definition, CHL affects the passage of acoustic waves through outer and middle ear, whereas SNHL is diagnosed if cochlear signal conversion, conduction via the vestibulocochlear nerve or processing in the auditory cortex are disturbed. Accordingly, CHL generally results from the inability of acoustic waves to pass a mechanical obstacle blocking any of the aforementioned anatomical structures or impeding wave propagation. In contrast, SNHL may be provoked by mechanical or functional anomalies, e.g., by neuronal death after cerebral ischemia.
In detail, CHL may be caused by:
Depending on their pathogenesis, developmental defects may be prevented by complying with vaccination plans, or by avoiding possible sources of infection. It is generally recommendable to keep healthy during pregnancy and to refrain from consumption of alcohol and drugs.
With regards to acquired CHL, the following measures may be helpful to prevent hearing loss:
The auditory system consists of outer, middle and inner ear as well as the vestibulocochlear nerve (corresponding to the eighth cranial nerve) and processing brain structures. Mechanical or functional impairment of any of those organs may lead to hearing impairment, hearing loss and eventually deafness. Depending on the precise site of lesion, different forms of hearing loss are distinguished :
Clinical presentation, therapy and prognosis of CHL depend on the underlying disease. In most cases, CHL is acquired, develops progressively, but may be treated with medication or surgery. The outcome is often better than in SNHL patients, because CHL is rarely associated with irreversible nerve damage.
Hearing loss refers to a decreased ability to recognize sounds and speech, and this condition may result from a wide variety of diseases. Depending on the precise site of tissue damage, various types of hearing loss are distinguished: If an illness affects outer or middle ear, a patient may develop conductive hearing loss (CHL). This term is based on the fact that acoustic waves, which correspond to propagating alterations in air pressure, are conduced through these organs until they reach the cochlea in the inner ear. In contrast, patients whose cochlea is unable to convert pressure into electrical signals, or who present with problems transmitting those signals to the brain, are diagnosed with sensorineural hearing loss.
CHL may be congenital, i.e., present at birth, or, more commonly, acquired. Developmental defects associated with malformations of auricles, external auditory canal or ossicles are possible causes of congenital CHL, and they may be triggered by infectious diseases like rubella. The most frequent forms of acquired CHL are impacted cerumen, otitis media and otosclerosis. In case of the former, accumulated cerumen blocks the auditory canal and does impede sound waves to reach the tympanic membrane and the middle ear. In order for pressure waves to pass the middle ear, vibrations of the tympanic membrane have to be transmitted to the ossicles, which are located behind it. If a patient suffers from otitis media, the mobility of tympanic membrane and ossicles may be reduced, and consequently, sound perception may be impaired. Otosclerosis is typically diagnosed in adults and affects the ossicles in the middle ear.
CHL patients may experience symptoms besides hearing loss, e.g., a feeling of fullness or pressure in the ear and pain, but these largely depend on the underlying disease. Similarly, treatment and prognosis differ according to diagnosis. In general, drug therapy, reconstructive surgery as well as hearing aids may be considered possible therapeutic options.