Conductive Hearing Loss

Etiology

Conductive hearing loss can be produced by any defect in the pinna, external auditory canal, tympanic membrane, and ossicles, up to the footplate of the stapes. Problems with the external ear and auditory canal may be due to congenital abnormalities such as aural atresia, which results in the incomplete formation of the external ear. This anomaly occurs due to the maldevelopment of the first and second branchial arches and the first branchial cleft.[4] More commonly, problems arise at the external canal by obstruction from debris, wax, or foreign bodies. Perforation of the tympanic membrane can also cause a conductive hearing loss.[5] Perforation can be attributed to trauma using cotton swabs to clean the ear, barotrauma from deep water diving, or as a sequela of otitis media.

Pathologies in the middle ear resulting in a conductive hearing loss include acute otitis media and otitis media with effusion (commonly referred to as glue ear). Otitis media with effusion is the most common cause of acquired hearing loss in children.[6] In most cases, it is a transient issue, which often recovers spontaneously, and no further management other than monitoring is required. However, in some children, hearing loss can have disastrous consequences on speech and language development. Middle ear effusions in an adult should raise suspicion of a nasopharyngeal tumor (particularly a unilateral effusion or a refractory effusion) and should be investigated as such.[7] Another serious cause of conductive hearing loss is cholesteatomas. These occur due to the presence of squamous epithelium in the middle ear or other aerated areas within the temporal bones. Although they are not tumors, they can be locally destructive and therefore require surgical removal.[8] Restoration of hearing is a secondary objective behind the formation of a safe, dry ear.

A defect in the ossicles of the middle ear will also give a conductive hearing loss. This could be due to head trauma resulting in ossicle discontinuity, or it could be due to a disease process such as otosclerosis. Otosclerosis is an osseous dyscrasia within the temporal bone.[9] Spongy bone from the otic capsule gets replaced by sclerotic bone. This process predominantly affects the anterior oval window, resulting in calcification of the stapes or the annular ligament. Otosclerosis is twice as common in women and presents in early adulthood with a gradual onset of conductive hearing loss due to the stapes' fixation.

Epidemiology

Conductive hearing loss is common in younger patients due to conditions such as otitis media with effusion. A study on primary school children found the prevalence of hearing loss to be 15%, with 88.9% of those being conductive.[10] Another study with preschool children in a South African community showed that 19% had hearing loss, with 65% of those being conductive.[11] They found that 9% of children had impacted cerumen, causing a hearing loss in 19% of them. In a Canadian study in school children from kindergarten to grade 6, hearing loss was found in 19%, with 93% of those being conductive. They found perforations of the tympanic membrane in 37% if unilateral loss and 46% in the bilateral loss.[12]

In low-middle income countries, hearing loss secondary to otitis media can be as high as 26%.[13] Otosclerosis prevalence in the white population is 0.04% to 1% but increases to 5% in Asians, and is associated with bilateral hearing loss in up to 80% of cases.[14] In the elderly population, hearing loss is mostly attributed to presbycusis, which is sensorineural.[15]

History and Physical

A thorough history can point towards the cause of the hearing loss. When obtaining the child's pertinent history, it is essential to ask about speech and language development and whether the appropriate milestones have been reached. Whether the child had preceding or recurrent upper respiratory tract infections must be ascertained, as this may suggest an otitis media with effusion. A conductive hearing loss can manifest as inappropriate behavior or inattention in the classroom.[16] 

Other essential things in the history of children and adults include the onset of hearing loss, whether there was any trauma, and the presence of associated symptoms such as vertigo, otorrhoea, otalgia, and facial weakness. It would be prudent to ask about nasal discharge and weight loss in the older patient, indicating a postnasal space tumor. A child with a history of otitis media during the first two-three years of life are at risk for a mild-to-moderate conductive hearing loss affecting phonological production. They can have difficulty perceiving strident or high-frequency consonants, such as sibilants. During speech they typically fail to produce these consonants, since they do not perceive them. This failure of sound production is called stridency deletion.

It is important to ask about family history and birth history to exclude any familial syndromes. Otosclerosis has an autosomal dominant with an incomplete penetrance mode of inheritance. It usually presents in the third decade of life and is twice as common in women as men. It has a gradual onset, and the hearing loss is bilateral in 80% of cases.[17] The patient with otosclerosis may comment that their hearing is better with background noise (paracusis Willisii). This phenomenon can occur as well in other causes of conductive hearing loss.

A full otolaryngology examination is mandatory for patients with hearing loss. Both ears must be examined with an otoscope or microscope. The examiner can see an obstruction of the ear canal with cerumen, debris, or a foreign body. Stenosis of the canal, which may be congenital or the consequence of repeated infections, can be seen. The tympanic membranes must be visualized to exclude acute infections, effusions, perforations, hemotympanum, or the presence of a cholesteatoma. Approximately 90% of patients with otosclerosis have normal tympanic membranes, while 10% have a pink tinge called Schwartz’s sign.[18] If an adult is noted to have an effusion, then a flexible nasoendoscopy should be performed in the clinic to view the postnasal space.[19]

Weber and Rinne tuning fork tests are useful screening tests, which can be carried out in the clinic to see whether hearing loss is conductive or sensorineural.[20] They are better suited for unilateral hearing loss and when the hearing loss is not of the mixed type. Cases of bilateral hearing loss or mixed hearing losses are better assessed using pure tone audiometry rather than tuning fork tests. To perform the Weber test, a 512Hz tuning fork is struck on the clinician's knee or elbow, and the vibrating fork is placed in the vertex of the patient’s head of the midline of the forehead. A normal result is a tuning fork being heard equally in both ears. In a conductive hearing loss, the tuning fork will be heard louder in the affected ear. In a sensorineural hearing loss, the tuning fork will be heard louder in the unaffected ear. This is used in conjunction with the Rinne test, where again a vibrating 512Hz tuning fork is placed, this time on the mastoid process until the patient can no longer hear it, and then 1cm away from the external acoustic meatus. A normal result is when air conduction is heard better than bone conduction (paradoxically called Rinne positive). If bone conduction is superior to air conduction (Rinne negative), a conductive hearing loss is present.

Evaluation

Pure tone audiometry is the mainstay of investigations for hearing loss. It can confirm the presence of hearing loss, quantify the severity, and determine the nature of the hearing loss.[21] Headphones deliver sounds in varying loudness over 250 to 8000 Hz. The patient notifies the audiologist when they hear the sounds 50% of the time. This is a measure of the air conduction threshold and is recorded in decibels (dB). The bone conduction threshold is determined by placing a transducer on the mastoid process. An air-bone gap exists when bone conduction is superior to air conduction.[22] This is significant when the gap is over 10dB and marks a conductive hearing loss. Usually, an air-bone gap over 40dB indicates pathology within the ossicular chain rather than solely a tympanic membrane pathology. A Carhart notch is where there is a depression in bone conduction of around 10 to 15dB at 2kHz.[23] This is indicative of stapes fixation, as seen in otosclerosis.

Pure tone audiometry is suitable for patients over four years old. For younger children, there are alternative tests such as play and visual reinforcement audiometry. These are all subjective hearing tests. Acoustic impedance audiometry, also called tympanometry, provides some objective evidence for the examination. Tympanometry generates a graph of the tympanic membrane's compliance and gives useful information about the middle ear pressures.[24]

The work-up for a patient with a suspected cholesteatoma should include a computed tomographic scan of the petrous temporal bones. Brain and cervical magnetic resonance imaging with gadolinium may be subsequently recommended to exclude tumor pathologies of the posterior fossa, temporal bone, and neck/oropharynx areas.

Treatment / Management

The treatment of conductive hearing loss is dependent on the underlying condition. Any foreign body should be removed under direct visualization, with the aid of a microscope if required.[25] Wax and debris which is obstructing the ear canal should be amenable to micro-suction. Perforations of the tympanic membrane often heal independently and only require a clinic follow-up evaluation in 6-8 weeks to ensure total healing.[26] If the perforation has not sealed, then a myringoplasty may be necessary; however, the hearing effects are somewhat more unpredictable.

Otitis media with effusion usually resolves by itself and can just be monitored every three months. However, if persistent bilateral otitis media with effusion occurs over three months, and the hearing loss in the better ear is over 25-30dB, grommet insertion may be valuable.[27] Grommets enable ventilation of the middle ear to the external auditory canal, rather than ventilating to the nasopharynx via the Eustachian tubes. A myringotomy is performed, and any middle ear fluid is suctioned out before a grommet is placed in the anteroinferior quadrant. Adenoidectomy can also be performed if there is associated hearing loss with the effusions.[28] The most common complications of grommets are infections and rarely tympanosclerosis. Grommets are favored to tympanostomy tubes (T-tubes) as the latter have higher rates of complications.[29](A1)

Cholesteatomas require complete surgical excision. The two main approaches to a mastoidectomy are canal wall down, which involves an endaural or postaural incision, or canal wall up, which involves a postauricular incision.[30] Trauma resulting in ossicle discontinuity may require an ossiculoplasty. Management options for otosclerosis include conservative measures such as watchful waiting, hearing aids, and fluoride supplementation. Surgery may be indicated when the air-bone gap is over 20 dB and involve total/partial stapedectomy or stapedotomy.[31] Conductive hearing loss that is not amenable to medical or surgical management can be treated with hearing aids. There are various types: air conduction hearing aids, bone conduction hearing aids, and bone-anchored hearing aid.

Differential Diagnosis

The differential diagnosis for conductive hearing loss is extensive. A thorough history and examination, together with pure tone audiometry, will point towards the underlying cause. It is essential to confirm that the hearing loss is conductive rather than sensorineural, guiding subsequent investigations and management.

• Defect in the pinna, external auditory canal, tympanic membrane, and ossicles
• Aural atresia
• External canal obstruction
• Tympanic membrane perforation
• Acute otitis media
• Otitis media with effusion
• Nasopharyngeal tumor
• Cholesteatoma
• Otosclerosis
• Ossicle discontinuity after head trauma

Prognosis

Prognosis is very much dependent on the cause of the conductive hearing loss. Simple conditions such as wax impaction and otitis media with effusion have excellent outcomes.[32][33] Even when hearing loss is not reversible or amenable to medical or surgical management, hearing aids have a high satisfaction rate amongst patients.[34][35]

Complications

The following complications may be observed:

• Conductive hearing loss in children can lead to disastrous speech and language delays and impact their education if not promptly diagnosed and treated.[36]
• Cholesteatomas can have significant invasion and destruction of local structures.
• Some conditions causing conductive hearing loss will produce a permanent hearing loss if untreated.