Acquired mixed hearing loss refers to combined conductive and sensorineural hearing losses which are not present at birth and/or develop after the first month of life (beyond the “neonatal” period).
In addressing causes of mixed hearing loss, it is useful to divide discussion into conductive and sensorineural components – not only for diagnosis, but particularly as one considers potential treatment modalities. Clearly, there can be many causes for mixed hearing loss (nearly any combination of conductive and/or sensorineural causes when combined can cause a mixed hearing loss).
Table 1 lists many of these causes, categorized by type of hearing loss as well as anatomic location. Not emphasized here are congenital disorders causing mixed hearing loss. See: Congenital mixed hearing loss. Incidences of these pathologies vary greatly from childhood through adolescence and into adulthood. While many etiologies are mentioned in here in brief, the focus of this summary is largely on single etiologies responsible for both conductive and sensorineural hearing loss. Some of these may have a genetic predisposition, or be present at birth (considered by some as “congenital”) but may not manifest until early or even late adulthood (i.e., otosclerosis, enlarged vestibular aqueduct (EVA), etc.). As a result, they are included in discussion here.
As insinuated above, the vast majority of hearing loss in children is conductive in nature from fluid in the middle ear space. The 2004 consensus clinical practice guideline published in the journal of Pediatrics indicates 90% of children will present to the physician’s office with an otitis media with effusion (OME) at some time before school age. The overall rate of OME approaches 80% per ear (90% of all children) between ages 6 months and 4 years old; 30–40% of these children will have recurrent or persistent OME (“Otitis media with Effusion Clinical Practice Guideline” 2004). Among causes of sensorineural hearing loss (SNHL) in children, most are congenital (occurring secondary to infection, environmental causes or inherited defects). Using this data, one may infer that the most common cause ofmixed hearing loss is likelymiddle ear effusion causing conductive hearing loss (CHL) in children with congenital sensorineural hearing.
The focus of this article is refined to causes of mixed hearing loss in young children outside of common middle ear dysfunction. Among single etiologies for mixed hearing loss in early childhood, congenital causes must be high on the differential of the evaluating otolaryngologist. These losses are often not discovered until more formal audiometric data is obtained (ages 2–3). Many of these syndromic and nonsyndromic causes are discussed in the article: Congenital mixed hearing loss.
Similar to the aforementioned combination of isolated conductive and sensorineural hearing losses in children, adolescent and adult onset mixed hearing loss may simply represent cerumen impaction (the most common cause for adult onset CHL) superimposed on presbycusis (the most common cause for sensorineural hearing loss - SNHL in adults) (Isaacson 2010).
Setting aside obvious external or middle ear sources for the conductive component of the mixed hearing loss (the diagnosis and management which are discussed separately), one must consider less common singular disease processes which could be implicated. In order for a single lesion to affect both air and bone conduction, it must impair transmission of sound to or through the middle ear space and additionally cause deficits in sensorineural processing of the fluid wave or electrical impulse carried away from the cochlea.
In the case of trauma, this can be accomplished by isolated but simultaneous injuries to discrete areas of the hearing organ. As an example, otic capsule violating fractures or compression of the eighth cranial nerve (both causing sensorineural hearing loss - SNHL) in addition to ossicular avulsion, discontinuity or hemotympanum (causing conductive hearing loss - CHL) can together form a mixed hearing loss.
A few middle or external ear pathologies can cause a mixed loss via direct invasion of the otic capsule and the middle ear space (i.e. invasive cholesteatoma or other neoplasm). Diffuse, systemic diseases, particularly those with the tendency to cause granulomas or global hypofunction of both mechanical and neural elements (vascular or endocrine disorders), have the potential to create a mixed loss. Endocrine disorders specifically are known to cause eustachian tube obstruction (edema, mass effect) in concert with cochlear degeneration (secondary to tectorial membrane or hair cell erosion, or nerve fiber degeneration) to induce a mixed hearing loss (Pellitteri et al. 2010).
Other mechanisms of impairment are based upon either bony defects or the process of bony remodeling. Most of these rely on the so-called “third-window effect”. A proposed pathophysiology for this phenomenon has been well described by Merchant and Rosowski (see Fig. 1 – “Air-Bone Gap in Thirdwindow” lesions). As the caption describes, these lesions in the bony capsule allow distension in response to a fluid wave on the scala vestibuli side of the cochlear partition. This allows for dissipation of the vibratory energy transmitted through the perilymph and out through this defect. This dissipation of energy leads to less deflection of the cochlear partition and less hair cell stimulation (decreasing air thresholds). This same less rigid area in the bony capsule of the cochlea provides increased translation of boneconducted sounds into the cochlea. Some authors indicate the increased pressure gradient between the scala vestibuli (now expandable) and scala tympani (relatively rigid volume) allows for increased bone conduction thresholds.
There are other etiologies for which a sensorineural hearing loss (SNHL) is commonly described, yet a conductive hearing loss (CHL) is sometimes observed (Meniere’s syndrome, intralabyrinthine schwannomas, and others) (Merchant and Rosowski 2008). For these, there remains no clear explanation for the mechanism of the conductive component of hearing loss.
With universal newborn hearing screening that is in place in most hospitals in the United States, infants are often identified with hearing loss shortly after birth (Katbamna et al. 2008). In children that are not identified with newborn screening, the symptoms of hearing loss can be variable and often subtle. Careful attention is required to developmental (speech and language) milestones. Lack of babbling, or failure to localize to sound as an infant or poor performance in school and/ or lagging in speech development in older children may raise concerns about a child’s hearing. Parents may comment on high volume while watching television or the need for repetition when speaking to their children. Because of the large proportion of infectious etiologies, symptoms of acute otitis media may accompany these and should be ruled out. Parents and primary care physicians bear the largest burden in identifying concern for hearing loss and appropriately referring patients for further otologic and audiometric evaluation (Katbamna et al. 2008). Further appropriate examination and workup for infants and young children relies on an understanding of the congenital etiologies for mixed conductive hearing loss.
In older children, adolescents, and adults, a complete history of present illness should include: onset and duration of hearing loss, progression, laterality, severity, and presence of associated symptoms such as otalgia, autophony and/or hyperacusis, vertigo, oscillopsia, tinnitus, aural fullness, or otorrhea. A history of head trauma can be helpful in directing workup. Review of systems should include other neurologic symptoms which could represent meningitic infection or stroke. Past medical and surgical history of otologic infections (recurrent acute otitis media, congenital infections) or surgery (tympanostomy tubes, cholesteatoma removal, middle ear exploration) as well as vascular disease, endocrine, or autoimmune disorders must be considered. Social history of tobacco use or IV drug use may shift the differential diagnoses (Katbamna and Flamme 2008). Probing for a family history of history loss or vertigo is important.
Physical exam should include an otoscopic exam (binocular microscopy if possible) for evaluation of external or middle ear anatomy (including Schwartze’s sign) with pneumatic otoscopy testing for both tympanic membrane mobility and Hennebert’s sign (vertigo in response to pressure changes). Tuning fork evaluation should be performed systematically. The Weber test assesses on the laterality of losses (localizing toward conductive lesions, away from sensorineural ones). The Rinne test assesses for bone versus air conduction (bone conduction greater than air on the side of conductive lesions). If the patient endorses autophony, placing a tuning fork on the medial or lateral malleolus to assess for supranormal bone conduction sensitivity may be revealing. A careful cranial nerve exam must include facial nerve assessment for impairment.
The key to confirming a diagnosis includes full audiometric analysis including: air and bone conduction pure tone audiometry to quantify the proportion of CHL versus SNHL (see Fig. 2 – Mixed Hearing Loss Audiogram). Speech discrimination scoring is useful as it is often decreased relative to pure tone average losses for retrocochlear lesions. Tympanometry may confirm perforation or suggest a tympanic membrane mobility abnormality. Acoustic reflex testing can help confirm a diagnosis of otosclerosis (see below). In infants, otoacoustic emissions (OAE’s) or auditory brainstem response (ABR) testing may be required (see Congenital Mixed Hearing Loss).
In patients with third-window lesions, bone conduction may actually be supranormal, which can be obscured by varying severities of sensorineural hearing loss but is, nonetheless, appropriate to request supranormal threshold testing if suspected (Merchant and Rosowski 2008).
Determining a definitive diagnosis for patients with mixed hearing loss often requires multiple forms of imaging. CT has been reported to be the modality of choice for imaging the temporal bone and middle ear space. Contrast-enhanced MRI has been used to evaluate internal auditory canals and membranous labyrinth in sensorineural hearing loss (St. Martin and Hirsch 2008).
Children pose a particular diagnostic challenge secondary to the difficulty in obtaining key symptomatology as well as the possibility of delayed diagnosis of congenital lesions in addition to the above discussed incidence of middle ear dysfunction contributing to the conductive component of hearing loss (Ahmad 2008). CT scans are expeditious (making them more tolerable to young patients) but require radiation exposure; a growing concerning in pediatric patients. Even noncontrast studies can reveal otosclerosis, osteogenesis imperfecta, or X-linked stapes gusher (see below) (Rodriquez et al. 2007). However, if the etiology for the SNHL cannot be adequately explained by CT findings, an MRI is warranted.
In adults, contrast-enhanced high resolution CT will provide additional detail in cases of trauma to the otic capsule or ossicular chain (Ahmad 2008). MRI is again indicated in either asymmetric sensorineural hearing loss (inter-aural difference of greater than 15 dB at 2 frequencies or 10 dB at 3 frequencies) or if the diagnosis is unclear from the CT images.
Some authors describe exploratory tympanotomy or determining Umbo velocity although neither technique is widely employed (Merchant and Rosowski 2008).
In patients who present with mixed hearing loss (particularly bilaterally) and vertigo, Vestibular Evoked Myogenic Potential (VEMP) testing may be of diagnostic benefit, and in cases requiring surgical intervention, it may provide information about severity, then guiding laterality for initial intervention.
When suspicious of infectious, endocrine, or inflammatory diseases, consider appropriately directed laboratory evaluations: CBC (acute infection with WBC elevation, peripheral eosinophilia in cases of Churg–Strauss syndrome), cANCA (Wegener’s granulomatosis), TSH and free T4 (hypothyroidism), ESR and/or CRP (elevated in some neurologic and autoimmune disorders).
The proposed mechanisms in this advanced form of otosclerosis are believed to be secondary to bony remodeling that extends from the footplate onto the promontory. As that remodeling continues, it can progress to involve the cochlear endosteum (Cureoglu et al. 2010). This may cause a SNHL in addition to the classic CHL of otosclerosis. Some authors describe cavitating otosclerosis near the middle and apical turns of the cochlea which they propose cause a “third-window” effect (Makarem 2008). It is unclear if the disease is genetically predisposed (proposed autosomal dominant inheritance with incomplete penetrance) or caused by infectious etiologies (several reports of an associative link with rubeola virus) (Davis 2010). Onset is generally in the third to fourth decade of life (Isaacson 2010). Audiograms typically show a persistent air-bone gap with closure at 2,000 Hz. This Carhart’s notch is thought to be from stapes fixation preventing the normal ossicular resonance.
Many criteria have been described to diagnose cochlear otosclerosis:
- conductive hearing loss and sensorineural hearing loss with good speech discrimination scores,
- vertigo (less common),
- absence of acoustic reflexes,
- binocular microscopic visualization of a reddish blush on the promontory in active lesions (Schwartze’s sign), and
- a family history of otosclerosis (Cureoglu et al. 2010).
Non-contrast-enhanced CT may show peri-cochlear hypodense “double ring” appearance secondary to demineralization of the otic capsule around the cochlea (see Fig. 3 – Otosclerosis on CT). More commonly described is a demineralized (dark) fissula ante fenestram (at the stapes footplate where the disease process is often most physiologically active) (St. Martin and Hirsch 2008). MRI may show nonspecific intermediate signal enhancement around the same areas (see Fig. 4 – Otosclerosis on MRI) (Cureoglu et al. 2010). The disease process is generally progressive with surgical therapy aimed at improving only CHL; although mild improvements in SNHL have been reported with surgical or medical intervention.
Enlarged Vestibular Aqueduct (EVA) or Large Vestibular Aqueduct Syndrome
The vestibular aqueduct provides a communication between the intracranial cavity and the bony vestibule. Like otosclerosis, when present a patient can have a mixed hearing loss without visible evidence of middle ear disease on exam. While congenital etiologies are described (see Congenital Mixed Hearing Loss and Section. “Perilymphatic Fistula” below), cases of acquired EVA often manifest as sudden onset hearing loss that often follows head trauma. Audiogram often shows low-frequency air-bone gap (similar to superior semicircular canal dehiscence-discussed below). Fine-cut temporal bone CT is diagnostic, with a pathologically enlarged vestibular aqueduct (greater than 1.5 mm in diameter) (St. Martin and Hirsch 2008) (see Fig. 5 – CT and MRI of EVA). The mainstay of treatment is counseling to avoid future head trauma.
Endolymphatic Hydrops (Meniere’s Disease)
Meniere's disease. The well-known triad of low-frequency SNHL with tinnitus and vertigo are the typically described minimal criteria for diagnosis of Meniere’s disease (Crane et al. 2010). Some suggest the cause of associated CHL lies in the “third-window” effect described above; others have proposed that the stapes’ vibratory distortion causes the conductive component of the loss. Despite the unclear pathophysiology, recent studies show more than 25% of Meniere’s patients will have a conductive loss (Yetis¸er and Kertmen 2007). Diagnosis relies on the characteristic duration of vertigo, confirmation of low-frequency SNHL in addition to a (likely low frequency) air-bone gap indicating a CHL. MRI may show some mild enhancement of the cochlea whereas CT imaging is likely to be normal. Treatment follows a ladder of escalating therapies from medical to hearing non-ablative, and then hearing ablative (see Meniere’s Disease - technical article).
Temporal Bone Trauma or Otic Capsule Trauma
While ▶temporal bone trauma can cause any type of hearing loss, classically longitudinal fractures (more recently described as otic capsule violating fractures) have been associated with sensorineural hearing loss. Because of the high incidence of conductive losses with any temporal bone trauma, these patients present with a mixed loss audiogram. CHL is most frequently caused by hemotympanum, perforation (EAC fractures avulsing the notch of rivinus), or ossicular discontinuity (most frequently incudostapedial joint) (St. Martin and Hirsch 2008). Hemotympanum may take weeks, even months to resolve; however, if follow-up audiograms show a persistent conductive hearing loss (greater than 30 dB), ossicular fracture or discontinuity must be considered. The sensorineural loss (nearly entirely associated with otic capsule violating fractures) may be from: disruption of blood supply to the cochlea or membranous labyrinth, injury to the cochlear nerve, bleeding into the cochlea, or obstruction of the endolymphatic sac causing SNHL in a similar mechanism to endolymphatic hydrops (see Sect. “Vestibular Dysfunction, Meniere’s Disease” above), or perilymphatic fistula (see Sect. “Perilymphatic Fistula below). High-resolution CT can confirm otic capsule violation as well as ossicular dislocation or fracture (see Fig. 6 – Temporal Bone Fracture), and although this remains an indication for surgical exploration (reconstruction of the ossicular chain with hydroxyapatite versus partial or total ossicular reconstruction prosthetics), discretion must be employed to determine if intervention beyond amplification is truly warranted. Caution is warranted even more in cases of bilateral hearing loss (Brodie 2010).
Perilymphatic Fistula (PLF)
A perilymphatic fistula, or “gusher,” is aptly named for the profuse flow of cerebrospinal fluid immediately on opening the vestibule (either iatrogenically or traumatically) (Crane et al. 2010). The resulting fluid leak (CSF) is thought to arise either from a widened cochlear aqueduct or a defect in the fundus of the IAC (habenula perforata) (St. Martin and Hirsch 2008). A rare occurrence, some even debate the existence of spontaneous lesions. It is most commonly associated with DFN-3 (Xlinked deafness with stapes gusher) and congenitally fixed stapes footplate. The overall iatrogenic incidence noted byHouse et al. is quoted as 0.03%or less ofmiddle ear surgeries, including syndromic and non-syndromic patients (House and Cunningham 2010). Following fistula creation, hearing loss is commonly fluctuating with mixed hearing loss on audiogram (largely sensorineural loss with low-frequency air-bone gap and preservation of the acoustic reflexes). It is important therefore to evaluate preoperative CT and MRI for widening of the IAC and vestibule, potentially with enlarged vestibular aqueduct as well in patients with suspected syndromic hearing loss or trauma-related losses. Radiographic evaluations may show increased signal density on T2-weighted images near the oval window on MRI or CT findings of air within the labyrinth (St. Martin and Hirsch 2008). Treatment for PLF is surgical exploration, plugging or sealing the dehiscent area with a tissue graft or fibrin-based glue, using a lumbar drain if necessary postoperatively (House and Cunningham 2010).
Superior Semicircular Canal Dehiscence (SSCD)
Loss of bone covering the superior semicircular canal causes diversion of perilymph from both the cochlear and vestibular systems with increased intra-abdominal or intracranial pressures ▶Superior Semicircular Canal dehiscence (SSCD). This causes vestibular manifestations (vertigo with straining) in addition to an airbone gap, often with an apparent SNHL. As mentioned above, especially with a mild premorbid SNHL, the additional air-bone gap may appear to indicate significant conductive and sensorineural loss. Even without premorbid SNHL, some bone thresholds can be supranormal (particularly low frequency, with resolution of the air-bone gap at high frequencies) (Martin 2009). History is suggestive of pressure-related vertigo, autophony or hyperacusis, and correlating hearing loss. Exam findings may be (classically) positive for Tullio’s and Hennebert’s sign, often with the ability to detect bone conduction using a 256 Hz tuning fork placed on the medial malleolus (Merchant and Rosowski 2008). High-resolution CT (1mm) images are reformatted both in the plane of the canal (Poschl plane) and perpendicular to the plane of the canal (Stenver plane) in order to reveal the bony defect (see Fig. 7 – Semicircular Canal Dehiscence) (St. Martin and Hirsch 2008). Treatment is surgical ablation of the canal with plugging (tissue graft and/or fibrin glue) or resurfacing.
In its acquired form, chronic retraction of the tympanic membrane (or otologic surgery) causes nests of trapped keratin producing cells in the middle ear space ▶Cholesteatoma.Although nonmalignant, the presence of even a small collection of squamous debris in the middle ear space can cause significant damage to the ossicular chain and surrounding structures (described pathophysiologies including proliferation of bacteria and associated osteolytic enzymes that erode bony structures). Typically, patients often present with otorrhea, tinnitus, otalgia, with occasional vertigo (indicating invasion of the lateral canal) and potentially facial nerve weakness. Otoscopic exam is often diagnostic. The typical losses caused by middle ear cholesteatoma are minimal and of a conductive nature. (However, bone conduction is often preserved by transmission of vibration through the mass.) Audiograms that confirm SNHL in addition to this CHL warrant CT imaging to determine the extent of the erosion. Often, scutal erosion with soft tissue density opacification of Prussak’s space can be seen on highresolution CT (see Fig. 8 – Cholesteatoma). Treatment is largely operative, but depends on extent of tumor and patient-specific considerations (some are followed with serial debridements in a clinic setting) (Isaacson 2010).
Smoking-Related Hearing Loss
Described particularly among adolescents, presumed etiologies for smoking-related mixed hearing loss surround a vascular insult or inflammatory process in the cochlea and middle ear structures that increases susceptibility to SNHL from noise exposures (Katbamna and Flamme 2008). Additionally, a significant portion of these patients have CHL from middle ear dysfunction (caused by irritation of the respiratory cilia or mucosa in the nasopharynx). History alone is the source for diagnosis in concert with audiometric analysis, although one may see a retracted TM as evidence of negative pressure in the middle ear space. Imaging (both CT and MRI) tends to be normal. Smoking cessation may help prevent further loss and may allow improvement in eustachian tube dysfunction; however, it is unlikely to improve incurred SNHL (Kozak and Grundfast 2009).
Osteogenesis Imperfecta (OI)
Commonly known as “brittle bone disease,” OI (▶Distraction Osteogenesis) can encompass one of many connective tissue disorders. The genetic inheritance patterns vary from autosomal dominant to recessive, based on the type of collagen deficiency and locus (COL1A/COL1A2). Type I (autosomal dominant) arises from a substitution in amino acid structure rendering the collagen triple helix structure less durable (Marini 2011). The result is loose, inefficient joint motion, rapid bone turnover, and sclerosis (causing stapes fixation). In addition, weak bone allows for microfractures, which are suspected to cause a distributed “third-window” effect and lead to SNHL. Fifty percent of diagnosed patients will have resultant hearing loss, the majority (54%) of which are mixed (Cheung et al. 2005). It can appear audiometrically identical to otosclerosis but exam findings elsewhere (blue sclera, hypermobility, fractures, mitral valve prolapse, easy bruising) make OI easily distinguishable. Imaging of the middle and inner ear is similar to otoslcerosis, with diffuse skeletal findings in addition. Surgical complications in patients with OI are significant, as all surrounding skin, soft tissue, and bony structures are more susceptible to damage. As a result, treatment is often medical (bisphosphonates) (Cheung et al. 2005).
Paget's Disease (Osteitis Deformans)
Likely secondary to mutations that upregulate TNF-beta RANK ligand pathway, IL-6 overproduction causes hyperstimulation of osteoclastic activity. The consequence is excessive remodeling of the axial skeleton, including ossicular chain and otic capsule (Cheung et al. 2005). Often not clinically present until the fourth decade of life or later, bony remodeling can cause histologically visible lytic lesions, sclerosis, or microfractures of the temporal bones. Eventually the otic capsule is entirely replaced and the membranous labyrinth is obliterated (although rarely in isolation from the axial skeleton). Audiograms show a mixed hearing loss, with the CHL at low frequencies and SNHL greatest at high frequencies (possibly from neural foramina encroachment) (Cheung et al. 2005). Skull radiography shows classic “cotton wool” appearance, with CT findings of stenotic EAC and IAC (St. Martin and Hirsch 2008). Treatment is medical and includes chemotherapeutics and calcitonin therapy (Cheung et al. 2005).
Autoimmune Disorders (Labyrinthitis Ossificans)
Underlying cause for labyrinthitis ossificans begins with meningitic infections. Both viral (CMV, rubeola, mumps) and bacterial (Streptococcus pneumoniae, Hemophilus influenza, and Neisseria meningitidis) causes have been suggested (Davis 2010). These infections travel through the cochlear aqueduct (which is often small and open in children) from the subarachnoid space and into the perilymphatic spaces (scala tympani). In bacterial infections, particularly S. pneumoniae, this response can lead to hemorrhage in to the intralabyrinthine spaces and quickly cause a fibrous response followed by ossification. Suppurative immune responses (to viral etiologies as an example) can similarly cause obliteration of the scala spaces (Cheung et al. 2005; Mark 2005). Early in the course of disease these changes create MRI enhancement, and the increased porosity of the blood/brain barrier then allows contrast enhancement which is visible on CT as well as T2-weighted MRI (see Fig. 9 – CT of labyrinthitis ossificans). Appropriate medical management of acute or chronic infections is paramount. However, once labyrinthitis ossificans is noted, hearing amplification versus cochlear implantation will be required, with careful attention to imaging to ensure patent scala for placement of the electrode (Davis 2010).
▶Granulomatous diseases of the temporal bone are overall uncommon. Specific underlying diagnoses may range from histiocytosis to ▶Wegener’s granulomatosis to Churg–Strauss syndrome. Exam will likely be significant for symptoms mimicking otitis externa (bloody otorrhea) or even mastoiditis (painful swelling of pre- and postauricular regions with facial nerve palsy or symptoms of peripheral vestibular lesions). Spontaneous perforation has been reported, and TM thickening may be observed on exam. Audiograms show mixed (but largely conductive) hearing losses. Symptoms are often responsive to topical (or systemic) steroid therapy as indicated for other manifestations of underlying disease processes (Bauer and Jenkins 2010).
In children with congenital hypothyroidism (cretinism), the prevalence of mixed (and progressive) hearing loss is significant. In adults with hypothyroidism, the loss tends to be both less severe and less prevalent. The conductive component of a mixed loss may come from malformation of the ossicular chain or eustachian tube dysfunction (presumably from edema). Most infants with SNHL resolve with oral supplementation of thyroid hormone; however, prolonged hypothyroidism causes atrophy of the tectorial membrane and hair cells at the basal turn which, if not noted early in infancy, will cause substantial and permanent impairment (Pellitteri et al. 2010).
Immunization against bacterial etiologies associated with meningitis is important in preventing CNS infections causing labyrinthitis.
Population studies suggest a higher incidence of otosclerosis in geographic areas of low or no fluoride in drinking water (Cureoglu et al. 2010). Avoidance of head trauma (i.e., contact sports) is recommended for children with diagnosis of EVA.
Hearing protection is the mainstay of prophylaxis against further sensorineural hearing loss.
Many with mixed hearing losses will meet indications for hearing amplification, which remains one of the first-line treatments and most effective mechanisms for improving quality of life in these patients. Options are largely behind the ear amplification and in canal amplification. The choice between these options relies on patient preference, anatomy, amplitude of loss, and other comorbid conditions (see Hearing aids).
Relatively few effective medical therapies are available that have shown statistically significant benefits for causes of mixed hearing loss. Exceptions apply to metabolic or endocrine disorders, otosclerosis, and diffuse third-window lesions (e.g., Paget’s disease, granulomatous disorders, and hypothyroidism).
Options for addressing the bony remodeling in otoslcerosis include both medical and surgical options. Sodium fluoride has been postulated to have inhibitory activity against proteolytic enzymes, thus reducing bony resorption. In one double-blinded randomized controlled study in Europe and several non-controlled studies, patients had a statistical decrease in recurrence and/or worsening of hearing loss with sodium fluoride therapy (60 mg/day initially then 20 mg for maintenance). Some physicians add calcium and vitamin D to this regimen (Cureoglu et al. 2010; Linthicum 2009). Higher frequency SNHL of less than 50 dB responded best. Recent literature proposes bisphosphonates may also be effective by inhibiting osteoclastic activity (Cureoglu et al. 2010).
With few notable exceptions, surgical therapy aims to improve conductive hearing loss, making the sensorineural losses more easily aidable or providing implantable modes of amplification.
When etiologies are discrete, the conductive loss can often be nearly resolved (e.g., ▶tympanostomy tube placement, ▶canalplasty, atresia reconstruction for EAC obstruction, ▶Ossicular chain reconstruction for traumatic interruptions).
Surgical options for otosclerosis (mentioned in brief above and more comprehensively elsewhere) include middle ear exploration with ▶complete stapedectomy, ▶partial stapedectomy, ormicrofenestration. Except in extenuating circumstances, surgeons generally choose the worse hearing ear for initial procedures. Bone conduction thresholds have been shown to improve in 75% of patients, with 85% of those patients having retained their gains years later (Yazdi 2009). Even in patients with apparently severe SNHL, some authors still advocate for middle ear exploration and stapedectomy (due to the limits of bone conduction testing to 70 dB, the patient may appear to have no response and a profound mixed loss, when they may have some function and appear to improve after surgery). Conversely, for progression to profound disease with decreasing speech discrimination scores despite being appropriately aided, cochlear implantation may be an option (House and Cunningham 2010).
Certainly in etiologies with debilitating side effects such as superior semicircular canal dehiscence or refractory Meniere’s disease, surgical intervention is dictated for control of symptoms. However, a frank discussion must be entertained between operating physicians and patients as not everyone will see a significant improvement in hearing and even among those that do, hearing amplification may still be required. For those with an otherwise asymptomatic mixed hearing loss, a reasonable approach is to pursue amplification until disease progression warrants further intervention. In superior semicircular canal dehiscence, surgical intervention commonly requires middle cranial fossa approach with either re-roofing or ablation of the dehiscent area. Hearing loss generally improves with correction of bony defect (either by ablation or re-roofing the defect) (Wilkinson 2008).
For temporal bone neoplasms and for cholesteatoma, surgical resection is often indicated; however, improvement of hearing thresholds is unlikely to be the primary objective or probable outcome of these procedures.
Implantable Hearing Devices
Hearing amplification (before or after surgical correction) that reaches profound thresholds, or provides poor return of speech discrimination warrants consideration of implantable devices (▶Implantable Hearing Devices). The cause in most cases is inability of conventional hearing aids to provide enough gain for incoming signals. Generally, the maximum level of output needed to effectively aid speech and sound is approximately 50 dB above the pure tone average (Slatterly 2005). When this limit is reached without effective amplification, (usually occurring at severe to profound thresholds), implantable devices may be indicated.
Bone Anchored Hearing Aid (BAHA)
Candidates for osseointegrated cochlea stimulators (like the BAHA) are those with CHL or mixed hearing losses that are not effectively treated using standard surgical techniques (poor surgical candidates), or those ineffectively rehabilitated with traditional hearing aids due to severity of loss or inability to tolerate conventional hearing aids (Lustig and Della Santina 2010). With the advantage of relatively superficial surgical intervention, BAHA has been proven effective for severe mixed hearing loss which are too significant for even newer digital hearing aids, particularly when the conductive component is greater than 30 dB (Tjellstrom 2010; Flynn 2009) (see ▶Bone-Anchored Hearing Aids (BAHAs)).
Cochlear Implantation has FDA approval for bilateral severe to profound loss. By this guidance, patient must have greater than 70 dB on pure tone averages, have failed amplification with aided speech discrimination scores of less than 50%, and have no anatomic contraindications to placement. However, clinical trials and expert opinion both support less strict criteria, in addition to bilateral implantation. Many endorse implantation for low-frequency losses (at 250–500 Hz) that would not, by strict criteria, meet FDA approval for implantation. Currently, several otologists use cutoffs of 70 dB loss at 1,000 Hz or above for better hearing ear with word discrimination scores of <70% (Wackym and Runge-Samuelson 2010). If imaging was not recently obtained during diagnostic evaluation, it is warranted prior to cochlear implantation to evaluate the cochlea, IAC, and middle ear to ensure anatomic suitability.
Results in case series and retrospective review articles showgood recovery of speech discrimination results for patients with profound mixed losses (Bruschini 2010). Not surprisingly, the longer the duration of severe to profound hearing loss, the worse the outcome. Additionally, cochlear implantation in patients with cochlear otosclerosis and labyrinthitis ossificans has higher rates of facial nerve stimulation following implantation (Wackym and Runge-Samuelson 2010). Several cases of BAHA users that have ultimately been transitioned to CI for progressive mixed losses show good results for this group of patients’ mixed losses (Verhaegen et al. 2009).
Middle Ear Implantable Devices
More recently therapies have included implantation of the Vibrant Soundbridge or other ▶middle ear implantable devices. These devices are similar to a cochlear implant in that they are approved to treat SNHL, but different in that they use mechanical amplification to increase movement of the ossicular chain. The Vibrant Soundbridge is one such device being used in Europe to treat conductive hearing loss. The device itself (called a floating mass transducer) is placed external to the cochlea and either attached to ossicular chain or, more recently, against the round window. Criteria for placement require word recognition scores greater than or equal to 50% with losses less than 65–85 dBs for 500–4,000 Hz frequencies, respectively (Slatterly 2005). Recent case series showed pure tone average gain at 32 dB, and 25 dB for speech recognition, with excellent subjective improvements in hearing and speech discrimination (particularly for round window insertion) (Dumon 2009; Baumgartner 2010).
The vast majority of sensorineural hearing loss is uncorrectable and progressive (rare exceptions noted above). Similarly, with most causes of mixed hearing loss, disease processes themselves are progressive. As such, patient counseling and treatment strategies are nearly all focused on protection of hearing, prevention of further loss, and capitalizing on retained hearing function, rather than curative therapies.
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