Conductive Hearing Loss
Anatomy and Pathophysiology:
Conductive hearing loss (CHL) refers to impairment of auditory function due to abnormal transmission of sound in the outer or middle ear, resulting in reduction or elimination of sound stimulus reaching the cochlea. Briefly, vibratory sound waves are funneled into the external auditory canal (EAC) by the pinna. At the termination of the external ear, the sound waves cause the tympanic membrane (TM) to vibrate, moving the attached ossicular chain within the middle ear space. The stapes then contacts the oval window, transmitting the vibration into the fluid-filled inner ear. A number of pathologic processes can disrupt this chain of events by occluding the external ear (blocking sound waves from reaching the tympanic membrane) or impairing tympanic membrane or ossicular movement. The maximal possible amount of conductive hearing loss is approximately 60 dB, which represents the amplification in sound intensity due to air conduction in the external and middle ear.
In the external ear, acquired causes of CHL include cerumen (earwax) impaction, occlusion of the external auditory canal due to inflammation or edema (such as in cases of otitis externa), foreign body, exostosis (benign bony outgrowth within the canal), and tumor. Of these, cerumen impaction is by far the most common. Tumors of the external canal are rare, but may originate from the canal skin or glandular tissue.
Congenital causes of CHL in the external ear can include malformations of the auricle (microtia) and EAC. EAC atresia or stenosis may occur in isolation or in association with other otologic defects (such as microtia or ossicular malformation). Certain syndromic conditions characterized by craniofacial anomalies may also include malformation of the EAC, such as Treacher-Collins syndrome and Goldenhar syndrome.
Perforation or absence of the tympanic membrane also causes conductive hearing loss; the extent of loss depends on the size of perforation and status of the ossicular chain. Myringosclerosis is a condition involving scarring of the tympanic membrane. The accumulation of hyaline and calcium plaques within the tympanic membrane can decrease TM mobility and may rarely produce some degree of CHL, although usually mild. Myringosclerosis may be a complication of acute otitis media or pressure equalization tube placement. Other changes in TM compliance and mobility may result from middle ear conditions, such as poor aeration or, commonly, accumulation of serous effusion due to Eustachian tube dysfunction.
A wide range of middle ear conditions can produce CHL by impairing the action of the ossicular chain. The presence of a mass in the middle ear space can cause erosion or compression of the ossicles. Tumors of the middle ear are uncommon, but include hemangiomas, adenomas, and glomus tympanicum. Cholesteatoma frequently results in hearing loss.
Congenital ossicular abnormalities may occur as part of a syndrome, in conjunction with other otologic malformations, or in isolation. Numerous ossicular anomalies are possible, including abnormal fixation, hypoplasia, and aplasia. A large percentage of abnormalities affect multiple ossicles and may involve fusion or joint disarticulation. Hypoplasia or aplasia of the round and oval windows can also occur, often in association with ossicular malformations. The list of congenital syndromes that produce some form of middle ear abnormality is too extensive to discuss in its entirety in this text. Some of the more notable of these syndromes include Apert syndrome, the CHARGE syndrome, Goldenhar syndrome, Pierre-Robin syndrome, Treacher-Collins syndrome, and several trisomies. Teratogen effect (such as due to thalidomide) and congenital infections (such as congenital syphilis and rubella) can also produce middle ear anomalies.
Otosclerosis is a localized bony disorder characterized by increased osteoblastic and osteoclastic activity in the middle (and occasionally inner) ear. Early in the disease course, this results in the replacement of normal bone with vascular, spongy bone (otospongiosis). Eventually, the condition results in foci of dense, sclerotic bone formation. Most commonly, these foci involve the region adjacent to the stapes footplate, resulting in fixation of the stapes. The etiology of the disease is unknown, although in many cases it appears to be inherited in an autosomal dominant manner with variable penetrance. The development of otosclerotic foci has also been associated with infection with measles virus; however the implication of this correlation remains unclear.
Other acquired causes of ossicular disruption include traumatic and iatrogenic injury. Temporal bone fractures that disrupt the otic capsule can result in a number of injuries to the external ear, TM, and ossicles. Of these, incudostapedial joint dislocation is the most common ossicular injury, although fractures and other ossicular joint injuries can also occur.
Because many of the common etiologies of CHL are covered in other chapters (and other rare entities are beyond the scope of this text), the remainder of this chapter will address general principles regarding the approach to CHL, with a particular focus on otosclerosis.
Based on routine autopsy findings, the incidence of otosclerosis has been reported to be as high as 13% of the general population, although many of these cases are likely subclinical (asymptomatic). Symptomatic disease has been estimated to affect 0.5-2% of the Caucasian population in the United States. Otosclerosis is less common among African-American and Asian-Americans. Women are more likely to be affected than men, with a female/male ratio of 2:1. Certain hormonal factors may exacerbate the condition; symptoms may worsen with pregnancy or estrogen therapy. The average age at presentation is 20-45. The disorder is rare in childhood.
The onset of otosclerosis is typically insidious, such that hearing loss may be quite significant by the time medical attention is sought. Most patients begin to notice functional impairment when conductive hearing loss reaches the 25-30 dB range.The nature of the disease is slowly progressive, although exacerbation of symptoms may occur with pregnancy. Bilateral involvement is common, although the extent of disease is sometimes asymmetric.
Patients with conductive hearing loss due to chronic conditions may not notice gradual deterioration of auditory function until brought to attention by friends or family. In preverbal children, hearing loss may be more difficult to discern clinically; symptoms may include inattentiveness or failure to respond to speech or environmental sounds. In school age children, a decline in academic performance may be an indicator of auditory impairment. A 25-30 dB CHL is typically sufficiently severe to prevent speech comprehension.
In early otosclerosis, patients may report better hearing in noisy environments (paracusis), which is strongly suggestive of the diagnosis. Patients may also speak softly due to enhanced bone conduction of their own voices. Tinnitus is common, but associated vestibular symptoms, such as vertigo or dizziness, are rare.
Differential Diagnosis of Conductive Hearing Loss:
- External Ear
- Cerumen impaction
- Canal inflammation or edema (e.g., in otitis externa or due to trauma)
- Foreign body
- Canal atresia or stenosis
- Cutaneous origin (basal cell carcinoma, squamous cell carcinoma, melanoma)
- Glandular origin (adenoid cystic carcinoma, pleomorphic adenoma, ceruminous adenoma or adenocarcinoma)
- Tympanic Membrane
- Middle Ear
- Middle ear effusion
- Congenital anatomic abnormality
- Ossicular discontinuity or hemotympanum due to trauma
- Tumor (e.g., glomus tympanicum, adenoma, hemangioma)
- Involvement by systemic disease (e.g., osteogenesis imperfecta, Wegener’s granulomatosis)
In any patient presenting with a new complaint of hearing loss, the timing of symptom onset and progression should be elucidated, along with the presence of associated symptoms, such as tinnitus, vestibular dysfunction, or facial palsy. Any prior otologic diagnoses or procedures and family history of otologic disease should be noted.
A thorough otoscopic evaluation can identify many causes of CHL, such as cerumen impaction and other external auditory canal obstruction, as well as presence of tympanic membrane perforation and middle ear. The presence of a middle ear mass may also be visible on otoscopy or otomicroscopy. In otosclerosis, the otoscopic examination does not usually reveal any significant abnormality. The middle ear space should appear well pneumatized, and the tympanic membrane exhibits normal mobility on pneumatic otoscopy.
Tuning fork tests are useful for confirming subjective complaints of hearing loss and can determine the side of greater deficit. The technique for performing the Weber and Rinne tests is given in Examination of the Ear. In pure conductive hearing loss, the Weber test lateralizes to (sounds louder in) the worse-hearing ear, while the Rinne test shows bone conduction to be greater than air conduction (negative test). The combined results of performing the Rinne test using three tuning fork frequencies can estimate the extent of conductive hearing loss as shown in the below table..
|Rinne Test Result|
|Extent of CHL||256 Hz||512 Hz||1024 Hz|
|Mild (20-30 dB)||Negative (BC>AC)||Positive (AC>BC)||Positive (AC>BC)|
|Moderate (30-45 dB)||Negative (BC>AC)||Negative (BC>AC)||Positive (AC>BC)|
|Severe (45-60 dB)||Negative (BC>AC)||Negative (BC>AC)||Negative (BC>AC)|
Audiologic assessment, including pure tone audiometry, speech testing, and acoustic immittance testing, should be ordered to further characterize the hearing loss. These tests are discussed in further detail in Vocal Fold Lesions. In otosclerosis, audiometry typically reveals a low-frequency conductive hearing loss (although a mixed hearing loss is also possible with involvement of the cochlea). As the disease progresses, increasing hearing loss may be seen at higher frequencies. The Carhart notch describes a dip in the bone conduction threshold at around 2000 Hz; it is a characteristic audiometric sign of otosclerosis. Tympanometry in otosclerosis generally produces a tympanogram with normal peak pressure (type A tympanogram) but dampened amplitude, which worsens with increasing stapes fixation. Most patients with otosclerosis have absent acoustic reflexes at the time of presentation; however, early in the disease, a characteristic diphasic reflex pattern may be seen, which is considered pathognomonic.
Radiographic imaging is indicated in cases of conductive hearing loss where tumor or cholesteatoma is suspected, or if the facial nerve is involved. It is also useful for delineating anatomy in cases of congenital malformations or trauma. Imaging is not necessary for most cases of otosclerosis; exceptions include patients with associated vestibular dysfunction or sensorineural hearing loss, pediatric patients (due to the high likelihood of associated anatomic malformations), or patients with unusual presentation.
The treatment approach to CHL depends on the etiology of the hearing loss. Some causes of CHL are directly reversible or treatable, such as cerumen impaction or infection. Persistent otitis media with effusion can be treated with tympanostomy and pressure equalization tube placement (see Tympanostomy and Tube Placement). Some anatomic malformations may be also addressed via surgery, such as canalplasty for some cases of external canal atresia or placement of ossicular prostheses for ossicular chain anomalies. Ossicular prostheses may be partial (partial ossicular replacement prosthesis, PORP) or total (TORP); the former addresses incus and malleus defects in the presence of an intact stapes, while the latter replaces all three ossicles.
Mild cases of otosclerosis are often observed with annual audiograms, particularly if the condition is unilateral and the patient has no functional complaints. Pharmacotherapy for otosclerosis includes agents that prevent bone absorption or promote bone formation, such as fluoride and bisphosphonates. The effects of these agents appear to have significant individual variability; they may be most beneficial in the earlier stages of disease to slow progression. Most patients with symptomatic CHL resulting from otosclerosis can be treated surgically with stapedotomy or stapedectomy and placement of an ossicular replacement prosthesis.
In many cases where nonsurgical management is preferred (or when surgical intervention is contraindicated or not feasible), CHL can be ameliorated with the use of hearing aids. No firm guidelines exist regarding patient candidacy for hearing aid use, although the degree and type of hearing loss, as well as word recognition scores, all play a role in determining whether the patient will benefit from a hearing aid. Patients with moderate hearing loss or greater are more likely to pursue amplification, but some patients with mild or minimal hearing loss find that they benefit from hearing aid use as well. In general, patients with better word recognition scores tend to have better results from amplification, but low word recognition scores do not necessarily mean that hearing aids will be ineffective. Often, patient motivation and the presence of impaired communication ability are the deciding factors in successful amplification.
A full discussion of amplification technology is beyond the scope of this text, but, to summarize, the main styles of hearing aids include completely in the canal (CIC), custom in the canal (ITC), custom in the ear (ITE), behind the ear (BTE), and open-fit mini BTE. The choice of hearing aid style depends not only on the degree and configuration of hearing loss, but also on patient anatomic factors (such as pinna shape or meatus diameter) and functional considerations (such as patient’s manual dexterity and amount of cerumen production). Most modern hearing aids are digital devices that have the ability to process incoming sound, allowing for reduction of feedback and selective amplification of speech versus background noise. For most patients, binaural amplification (hearing aids in both ears) produces better results than a single hearing aid.
Despite the effectiveness of traditional hearing aids in many cases, patient willingness to obtain or use hearing aids is often low. Patients may cite a variety of reasons for not wearing a hearing aid, including technical complaints (such as feedback or sound quality), discomfort, or cosmetic stigma. This has led to an interest in development of implantable hearing devices that can deliver sound directly to the middle ear, with the goal of providing clearer sound quality with less feedback and no obstruction of the external canal. In general, these devices are implanted via a mastoidectomy and facial recess approach, with at least some portion of the device attached internally to the ossicles.
Finally, bone-conduction hearing aids are an option for patients with CHL or mixed hearing loss who cannot use traditional hearing aids. Examples include patients with chronic otorrhea or external canal atresia. Conventional bone-conduction hearing aids consist of a bone conductor that is held in place over the mastoid cortex with a headband (or eyeglasses); however, efficacy of this style of hearing aid is limited by sound attenuation due to skin and soft tissue overlying the mastoid. Patients may also experience discomfort due to pressure from the bone conductor. The osseointegrated bone-anchored hearing aid (BAHA) avoids these issues by implanting the bone conductor directly on the mastoid bone via a surgical procedure.
Complications, Prognosis, and Follow-Up:
Studies have demonstrated significant quality of life benefit for hearing impaired patients who use hearing aids, regardless of degree of hearing loss. However, only an estimated 20% of patients with hearing loss own hearing aids, and only a subset of those patients are likely to wear their hearing aids on a regular basis.
Complications of bone-anchored hearing aid placement are primarily related to soft tissue reaction or infection at the site of implantation. Rarely, the conductor may fail to osseointegrate. Studies of patient outcomes after BAHA have suggested that optimal success rates are seen in patients with a pure tone average of 45 dB or less, although a beneficial effect is typically still seen for pure tone averages of up to 60 dB.
Up to 90% of patients with otosclerosis report a significant improvement in hearing following surgical intervention. An estimated 2% of patients report worsening hearing loss, including a very small percentage of patients ( < 0.5%) who experience complete sensorineural hearing loss in the operative ear. In patients who derive no benefit from stapes surgery, or who have a recurrence of disease following surgery, revision surgery may be attempted. However, the success rate after revision surgery is lower than for initial surgery and the risk of hearing loss is increased.
- Conductive hearing loss (CHL) refers to impairment of auditory function due to abnormal transmission of sound in the outer or middle ear, resulting in reduction or elimination of sound stimulus reaching the cochlea.
- External ear causes of CHL include cerumen impaction, canal inflammation or edema, foreign body, exostosis, tumor, and auricular or canal malformation (either isolated or as part of a syndromic condition, such as Treacher-Collins).
- Middle ear causes of CHL include otitis media with effusion, tumor (such as glomus tympanicum), cholesteatoma, congenital ossicular abnormalities (either isolated or as part of a syndromic condition), traumatic injury, and otosclerosis.
- Otosclerosis is a localized bony disorder characterized by replacement of ossicular bone with dense, sclerotic bone, resulting in immobility of the ossicles; it can often be treated surgically with stapedotomy or stapedectomy and placement of an ossicular replacement prosthesis.
- Tuning fork tests and audiologic studies are useful in characterizing the nature and degree of hearing loss; in otosclerosis, audiometry typically shows a characteristic dip in the bone conduction threshold at around 2000 Hz (Carhart notch).
- Hearing aids have been shown to increase quality of life across multiple areas in hearing impaired patients, although use and compliance remains low; newer technology has led to improvements in hearing aid comfort, cosmesis, and performance, including the advent of implantable middle ear devices.