Otosclerosis 

Updated: Feb 22, 2021
Author: Jack A Shohet, MD; Chief Editor: Arlen D Meyers, MD, MBA 

Overview

Practice Essentials

Otosclerosis is an osseous dyscrasia limited to the temporal bone that results in slow, progressive conductive hearing loss. A full audiometric evaluation, including impedance testing, is required to evaluate patients for the condition. In addition, computed tomography (CT) scanning of the temporal bone can often demonstrate foci of demineralization in the otic capsule in cases of cochlear otosclerosis.[1]

Signs and symptoms of otosclerosis

Symptom onset usually occurs by the early third decade of life, but onset is not unusual in later years. Symptoms include slowly progressive hearing loss, which is bilateral in 70% of cases. Vertigo is uncommon. Tinnitus may be present and often resolves after successful surgical management.

Otosclerosis is often associated with osteogenesis imperfecta (van der Hoeve syndrome) in a classic triad of hearing loss (conductive, mixed, or sensorineural), spontaneous bone fractures, and blue sclera.

Workup in otosclerosis

A full audiometric evaluation, including impedance testing, is necessary. Computed tomography (CT) scanning of the temporal bone can often demonstrate foci of demineralization in the otic capsule in cases of cochlear otosclerosis.[1]

Management of otosclerosis

As with conductive hearing losses of other etiologies, hearing aids are usually helpful. Fluoride supplementation has met with variable response and is used sporadically for labyrinthine otosclerosis. It has also been used for postoperative medical management of obliterative otosclerosis.

Approaches to surgical management of otosclerosis include total stapedectomy, partial stapedectomy, and stapedotomy.

History of the Procedure

Ankylosis of the stapes was first described by Valsalva in 1704. Toynbee described fixation of the stapes to the margins of the oval window in 1841 and found similar cases in 136 of 1,000 temporal bone dissections.[2] The histologic features of otosclerosis were demonstrated by Politzer, who identified the problem as an otic capsule disorder characterized by abnormal new bone formation.

Otosclerosis surgery has developed through 3 distinct eras. The mobilization era began in the late 1800s when Kessel attempted stapes mobilization without ossicular chain reconstruction in cases where it was noted to be fixed. Later, Jack removed the stapes, leaving the oval window open.[3] Both techniques allowed increased transmission of sound through the oval window but did not use middle ear amplification structures. Furthermore, fatal cases of meningitis from intraoperative exposure of perilymph to bacteria occurred, and any gains in hearing frequently were temporary because any remaining stapes footplate often refixed.

The fenestration era began in 1923, when Holmgren created a fistula in the horizontal semicircular canal and sealed it immediately with periosteum.[4] This procedure allowed sound conduction preferentially through the fistula, rather than the ossicular chain. Sourdille popularized the procedure when his 3-stage technique was widely published during the 1930s.[5] Lempert developed a 1-stage technique for horizontal semicircular fenestration, which went on to gain worldwide acceptance after it proved to enhance hearing.[6] Results, however, were short-lived because the fenestra often resealed with bone.

The stapedectomy era began before the fenestration era closed. Rosen revisited stapes mobilization in 1952.[7] Later, Shea removed the stapes, sealed the oval window with an autograft vein wall, and then reconstructed the sound-conducting mechanism with an artificial prosthesis.[8]

This technique gained wide acceptance and has been improved since inception. In the 1970s, Myers conducted stapedotomy using a piston prosthesis. In the early 1980s, Perkins began using the laser for stapedotomy in a procedure in which a small hole is made in the footplate, as opposed to complete or subtotal removal. Several techniques and approaches are commonly used today, with largely excellent results. A few challenges remain, such as those patients enduring sensorineural hearing loss and unsteadiness, but many think surgical treatment for otosclerosis has reached perfection.

Evidence has recently mounted that the measles virus plays an important role in gene activation of otosclerosis. This hypothesis is supported by a declining incidence of otosclerosis since measles vaccinations became widespread as well as finding measles virus RNA in the footplate of otosclerosis specimens.

Epidemiology

Frequency

Otosclerosis affects 10% of the white population. Frequency, as mentioned above, is thought to be decreasing secondary to measles vaccination.

Otosclerosis is inherited in an autosomal dominant pattern with incomplete penetrance. Women are 2 times more likely to develop the disease than men. Otosclerosis is generally limited to the white population.

Etiology

The exact cause of otosclerosis is unknown. Measles virus RNA is found in otosclerotic foci in footplates removed during surgery. Measles virus infection may activate the gene responsible for otosclerosis. Otosclerosis, however, is not responsible for all cases of stapes ankylosis. A heterogeneous group of disorders, including other bone degenerative disorders, appears to cause stapes fixation and conductive hearing loss.

Pathophysiology

The lesion is a pleomorphic replacement of normal bone with spongiotic or sclerotic bone. The histiologic disease progresses in stages. Bony resorption and replacement with new spongy bone characterize early lesions. Osteolytic osteocytes appear at the leading edge of the lesion, and sheets of connective tissue can be observed replacing the bone. Formation of dense sclerotic bone in areas of previous resorption signifies the late phase of otosclerosis. The result is disorganized bone, increased population of osteocytes, and enlarged marrow spaces containing vessels and other connective tissue. Spaces are later replaced by dense sclerotic bone with narrow vasculature and few recognizable haversian systems. Pleomorphism is largely due to normal coexistence of both stages of otosclerosis in any single temporal bone.

Initiating lesions often neighbor the fissula ante fenestram and expand via vascular channels.[9] In 80-90% of patients, lesions are limited to the anterior oval window and affect its pathology by calcification of the annular ligament or by involving the stapes. Both processes result in characteristic conductive hearing loss. In 8% of patients, the process involves the cochlea and parts of the labyrinth (labyrinthine otosclerosis), resulting in sensorineural hearing loss. Approximately 2% of patients display both labyrinthine and ossicular chain involvement.

Explanation of the clinical portion of sensorineural loss has been difficult, but investigators theorize that enzymes from the inner ear lesion diffuse via the spiral ligament to suppress neuron and hair cell activity.

Presentation

As with any disease, a careful history and thorough physical examination are prerequisites. Often this process reveals a family history of otosclerosis. After compiling this information, audiometric evaluation helps to narrow the differential diagnoses. Pregnancy and estrogen therapy have been reported to accelerate the progression of otosclerosis.

Symptom onset usually occurs by the early third decade of life, but onset is not unusual later in life. Symptoms include slowly progressive hearing loss, which is bilateral in 70% of cases. Vertigo is uncommon. Tinnitus may be present and often resolves after successful surgical management.

Otosclerosis is often associated with osteogenesis imperfecta (van der Hoeve syndrome) in a classic triad of hearing loss (conductive, mixed, or sensorineural), spontaneous bone fractures, and blue sclera.

On physical examination, patients with conductive hearing loss often exhibit low-volume speech because they perceive their own voices louder because of the enhanced bone conduction of sound. Otoscopic examination findings are usually normal, although 10% of patients demonstrate a Schwartze sign, characterized by a reddish-blue hue over the promontory and oval window niche areas, secondary to rich vascular supply associated with immature bone. Tuning-fork examination reveals signs of conductive hearing loss.

Early in the disease course, pure-tone audiometry usually demonstrates low-frequency conductive hearing loss. High-frequency losses begin to manifest with gradual air-bone gap widening. If cochlear involvement is not present, otosclerosis is limited to maximal conductive loss of 50-65 dB across all frequencies. If cochlear involvement is present, a mixed hearing loss appears, with high frequencies more affected. In severe cases, tinnitus may interfere with pure-tone audiometry.

Stapes fixation produces an audiometric artifact known as the Carhart notch, which is characterized by elevation of bone conduction thresholds of 5 dB at 500 Hz, 10 dB at 1000 Hz, 15 dB at 2000 Hz, and 5 dB at 4000 Hz. A Carhart notch may also be seen in cases of incudostapedial joint detachment and incus or malleus fixation.[10] Cochlear otosclerosis is characterized by the presence of mixed or primary sensorineural hearing loss where air-bone gaps are minimal, and the speech discrimination scores are better than what would be anticipated with the degree of hearing loss.

Tympanometry usually reveals a type As or A tympanogram. Acoustic reflexes are often abnormal and may provide the earliest evidence of otosclerosis. Observed abnormality depends on the stage of disease. Almost half of the healthy population may show an initial increase in compliance at stimulus onset, but compliance increase observed at offset occurs only in stapedial fixation and is virtually pathognomonic. Advancing fixation affects both ipsilateral and contralateral acoustic reflexes, even in unilateral disease.

In most cases, discrimination is much better than would be expected with the level of hearing loss. Low speech discrimination scores prognosticate poorly for postsurgical hearing improvement.

In cases of well-advanced lesions and in lesions with vestibular symptoms, several presentation variations are possible in addition to the classic presentation. Patients with advanced lesions can present with no measurable hearing, in which case they are best identified by history. This far-advanced otosclerosis often presents with a family history of hearing loss or a gradually progressive hearing loss that starts early in adult life. Previously obtained audiograms help differentiate this process from other, more common, causes of profound sensorineural hearing loss, especially if measurable hearing existed with previous air-bone gaps.

History of bone-conducting hearing aid use and clinical benefit from hearing aids, even with lack of measurable hearing improvement, supports the diagnosis. Tuning-fork responses are also suggestive of this diagnosis. These patients may benefit from a stapes procedure prior to consideration of a cochlear implantation. Successful stapes surgery may improve hearing thresholds to a point at which conventional hearing aids may be beneficial.

McCabe described otosclerotic inner ear syndrome was described in 1966 as episodic vertigo and a usually unilateral conductive hearing loss.[11] Vestibular symptoms have been found in approximately 10% of patients with otosclerosis. Suggested mechanisms include end-organ or neural degeneration or biochemical derangement in the perilymph by contact of the disease process. A stapes surgery can improve the symptoms, but endolymphatic hydrops should be ruled out prior to consideration of otosclerosis surgery.

For excellent patient education resources, visit eMedicineHealth's Ear, Nose, and Throat Center. Also, see eMedicineHealth's patient education article Tinnitus.

Indications

Other causes of conductive hearing loss (eg, trauma, infection) should be considered before procession to stapes surgery. Indications for surgical management of otosclerosis include conductive hearing loss with a greater than 20 dB air-bone gap. Patients should have the ability to tolerate the procedure in supine position.

Relevant Anatomy

See Pathophysiology.

Contraindications

Coexistent Ménière disease significantly increases the possibility of residual hearing loss in the operated ear. Documented dilation of the vestibule or the vestibular aqueduct on CT scanning or MRI also negatively effects hearing outcome. A large tympanic membrane perforation should be repaired in a surgery prior to and separate from stapes surgery.

Relative contraindications include prior stapes surgery complications in the contralateral ear, such as a profound sensorineural hearing loss or protracted vertigo, as well as a medically-infirmed patient.

All patients with otosclerosis should be offered the option of wearing a hearing aid prior to proceeding with stapes surgery.

 

Workup

Imaging Studies

CT scanning of the temporal bone can often demonstrate foci of demineralization in the otic capsule in cases of cochlear otosclerosis.[1]

Other Tests

A full audiometric evaluation, including impedance testing, is necessary.

Histologic Findings

See Pathophysiology.

 

Treatment

Medical Therapy

As with conductive hearing losses of other etiologies, hearing aids are usually helpful. Fluoride supplementation has met with variable response and is used sporadically for labyrinthine otosclerosis. It has also been used for postoperative medical management of obliterative otosclerosis.

Surgical Therapy

Approaches to surgical management of otosclerosis include total stapedectomy, partial stapedectomy, and stapedotomy. Hearing improvement can be achieved through surgery for congenital stapes footplate ankylosis with a concomitant ossicular chain anomaly. Final hearing levels after surgery can be influenced by sensorineural impairment.[12]

Preoperative Details

A history of significant nausea, emesis, or severe vertigo from previous surgical procedures, including a stapes operation on the contralateral ear, is a strong indicator that the patient will have a similar reaction after a stapes operation. Use of a transderm scopolamine patch placed postauriculary on the contralateral side the night prior to surgery imparts significant antiemetic properties.

The patient should be counseled as to the surgeon's specific postoperative instructions, including avoidance of nose blowing, sneezing, and allowance of water into the ear canal. Documenting the risks of a stapes operation is helpful from a medicolegal standpoint; the risks of such surgery include, but are not limited to, permanent and complete hearing loss, facial paralysis, permanent or temporary taste alterations, tinnitus, vertigo, tympanic membrane perforation, infection, and failure of the surgery to improve hearing.

Intraoperative Details

Primary procedures may be performed under general or local anesthesia with sedation.[13] The transcanal approach is almost always used.

A tympanomeatal flap is raised along the posterior external auditory canal. The ossicles are immediately identified and palpated to verify movement. Palpation of the malleus should elicit movement all the way to the incudostapedial joint. Palpation of the lenticular process of the incus indicates fixation of the stapes superstructure. Delicate palpation prevents an inadvertent immobilization of the stapes. See the images below.

A tympanomeatal flap is incised along the posterio A tympanomeatal flap is incised along the posterior external auditory canal.
The fibrous tympanic annulus is elevated with the The fibrous tympanic annulus is elevated with the tympanomeatal flap to expose the middle ear.

Mobilizing the chorda, which usually involves careful curettage of the scutum and partially translocating the nerve out of its bony canal, is often necessary. Additional bone from the scutum is removed to allow positive identification of the pyramidal process, stapedial footplate, and the tympanic segment of the facial nerve (see image below). Attention is directed at the facial nerve to ensure that the nerve is not prolapsed onto the footplate. Presence of such an anomaly may require termination of the procedure at this point.

Scutal bone is curetted to allow visualization of Scutal bone is curetted to allow visualization of the pyramidal process, tympanic segment of the facial nerve, and the stapedial footplate.

The prosthesis is sized by measuring the distance from the footplate to the medial surface of the incus. The incudostapedial joint is disarticulated sharply in an anterior-posterior plane, using the stapedial tendon to provide stabilization of the stapes. See the image below.

The incudostapedial joint is disarticulated after The incudostapedial joint is disarticulated after measurements have been made.

The stapedial tendon is divided with either scissors or laser. See the image below.

The stapedial tendon is sectioned either with a la The stapedial tendon is sectioned either with a laser or scissors and then the posterior crus of the stapes is sectioned either with a drill as shown or a laser.

In stapedectomy operations, a control hole is made in the midportion of the footplate with a straight pick prior to disarticulation of the incudostapedial joint. The stapes superstructure may be downfractured by exerting delicate force in a superior-to-inferior plane just medial to the capitulum of the stapes. The control hole is widened and the footplate bisected with a 0.3-mm right-angle pick. The posterior one third of the footplate may be removed to perform a partial stapedectomy. A total stapedectomy involves removal of the entire footplate with a right-angle pick. A small fenestra stapedectomy involves widening the control hole to 0.6-0.8 mm. Whichever stapedectomy technique is used, the oval window opening is now covered with a tissue autograft composed of either fascia, vein, or perichondrium. A prosthesis may now be placed between the oval window and the incus. See the image below.

The stapes superstructure has been down-fractured The stapes superstructure has been down-fractured and removed. The footplate remains and may now be removed completely as in a total stapedectomy or a stapedotomy hole may be made with a laser or microdrill.

In a stapedotomy procedure, the posterior crus of the stapes is divided with either the laser or crurotomy scissors. The stapes superstructure is downfractured and removed. A hole is made in the footplate using either the laser or a microdrill. A tissue graft is placed over the oval window opening when a bucket handle prosthesis is used, or the oval window may remain uncovered when other types of prostheses are used. A prosthesis is chosen based on the surgeon's experience and comfort. A blood seal can be created by abrading the mucosa of the superior promontory and allowing blood to ooze onto the prosthesis-oval window interface. This may prevent perilymphatic fistulization. See the images below.

A stapedotomy has been performed. A prosthesis may A stapedotomy has been performed. A prosthesis may now be placed.
A piece of tissue (fascia, perichondrium, or vein) A piece of tissue (fascia, perichondrium, or vein) is placed over the stapedectomy/stapedotomy.
A Robinson bucket handle prosthesis is placed over A Robinson bucket handle prosthesis is placed over the stapedotomy and looped over the incus.
A stapedotomy procedure with a bucket handle prost A stapedotomy procedure with a bucket handle prosthesis positioned over the footplate.
Video of laser stapedotomy with SMart piston prosthesis.
Video of bucket handle prosthesis positioning.

Perilymphatic fluid suctioning should be avoided to minimize postoperative vertigo and cochlear damage. The malleus is carefully palpated to ensure movement of the ossicles all the way through the prosthesis. Visualization of a round window reflex confirms the presence of an intact ossicular mechanism.

The tympanomeatal flap is replaced over the posterior external auditory canal and the ear canal is filled with either an antibiotic ointment or antibiotic solution-treated Gelfoam.

A study by Vincent et al indicated that stapedotomy can be safely and effectively performed in children with juvenile otosclerosis. The study included 34 pediatric patients with otosclerosis (41 primary stapedotomies total), with the postoperative air-bone gap closure being 10 dB or less in 93% of patients and to within 20 dB in 98% of patients.[14]

Postoperative Details

Surgery is typically performed on an outpatient basis. Unless experiencing significant vertigo or emesis, patients are usually safe to leave the hospital within a few postoperative hours. Bedrest is necessary only in the immediate postoperative period.

Follow-up

The first postoperative visit is usually scheduled after 2 weeks, which allows time for the tympanomeatal flap to heal in place. The canal may be débrided at this time. Cotton balls are placed in the meatus 2-3 times per day to collect discharge from the canal over the first several days after the surgery. Postoperative audiometrics are typically performed 3-6 weeks after surgery.

Complications

Sensorineural hearing loss occurs in only 0.2% of cases, but patients should be informed of a 2% chance of additional hearing loss in the operated ear and less than a 1% chance of complete loss of residual hearing. This complication has an unclear etiology. A Weber tuning-fork test that lateralizes away from the operative ear in the immediate postoperative period is an indication of a sensorineural hearing loss provided that the contralateral ear does not also have a significant conductive hearing loss. If sensorineural hearing loss is suspected, steroid therapy is initiated and tapered over a 10-day course. Prednisone 1 mg/kg/d divided into 2 doses over the first 7 days with a rapid taper is the author's preferred dosing regimen.

Tinnitus may be more pronounced postoperatively, especially if hearing worsens following surgery.

Facial nerve palsy is a devastating, and fortunately rare, complication. In a series of 700 stapedectomies, only 2 patients had facial weakness; both were of delayed onset (around postoperative day [POD] 5), and both completely resolved within 2 weeks with steroid administration.

Significant vertigo affects approximately 5% of patients and usually is transient and brief, persisting only in rare cases. Management is typically supportive only, unless symptoms suggest perilymphatic fistula. (A prospective observational study by de Vilhena et al reported a higher rate of vertigo. The study, of 140 patients with otosclerosis who underwent stapedial surgery, found that, while 12 patients [8.6%] reported having vertigo prior to surgery and no vertigo after the operation, 36 patients [25.7%] who had no vertigo before surgery reported having vertigo 4 months after the procedure.[15] )

Perilymphatic fistula occurs in 3-10% of patients poststapedectomy and is characterized by fluctuating sensorineural hearing loss and vertigo. Incidence of this complication has declined concomitantly with the decline of total stapedectomies. Laser use and abandonment of absorbable gelatin sponge have further decreased this complication.

Taste disturbance occurs in about 9% of patients. Most authors advocate sectioning the chorda tympani if it is intraoperatively found stretched or otherwise injured to decrease the incidence further.

Tympanic membrane perforation is rare, and if it occurs during surgery, a fascial graft repair can be performed. This was supported by a retrospective study by Luryi et al that suggested that in patients with otosclerosis who undergo stapes surgery, a small intraoperative tympanic membrane perforation can be safely and effectively managed with concurrent tympanoplasty. Following surgery, individuals in the study who had suffered perforation did not significantly differ from the other patients in terms of the air-bone gap or change in the air-bone gap. Moreover, the perforation group was not found to be at greater risk of postoperative reparative granuloma, otitis media, or sensorineural hearing loss, although these patients did report a greater rate of postsurgical dysgeusia than did the nonperforation group (30.0% vs 5.3%, respectively).[16]

Postoperative or persistent perforations may be managed by the usual means.

Infection is rare and must be identified and treated early to avoid profound sensorineural hearing loss, meningitis, and a prolonged hospital course.

Reparative granuloma formation in the middle ear (at the oval window) is a delayed complication appearing approximately 1-6 weeks postoperatively. This formation manifests as a gray mass behind the tympanic membrane with sudden hearing loss. Therapy consists of timely surgery to remove the granuloma, often with excellent results.

A floating footplate can develop if the footplate becomes dissociated from the annular ligament and, therefore, freely floats on the perilymph of the vestibule. In this case, the footplate or portions of it can sink into the vestibule. If a floating footplate develops, the surrounding promontory margin may be removed with the microdrill or laser to allow for a gap to place a right-angled micropick and remove the footplate. Great care must be taken not to allow the instrument or footplate to penetrate deeply into the vestibule, which can cause a profound sensorineural hearing loss.

Obliterative otosclerosis denotes a footplate completely obliterated by otosclerosis. This yields a greatly thickened and poorly defined footplate identifiable only with the use of landmarks such as the stapedial crura, round window, and facial nerve. This special situation requires a drill-out of the footplate, whereby a microdrill is used to blue-line the vestibule and the endosteum is removed with a micropick. Alternatively, a laser may be used to remove the bone, which may require many passes of the laser with intermittent pauses to remove bone char and to allow cooling.

A biscuit footplate refers to a localized focus of otosclerosis within the center of a footplate that has well-defined margins. Complete resection of the entire footplate or a formal stapedectomy can achieve complete removal of the otosclerosis in these patients.

Outcome and Prognosis

Commonly quoted statistics indicate that 90% of appropriately chosen surgical candidates enjoy a significant hearing improvement. Eight percent experience no significant hearing improvement. Up to 2% (including 0.2% who may experience complete sensorineural hearing loss in the operative ear) experience additional hearing loss.

Revision stapes surgery yields less-successful results than primary surgery. Typically, the air-bone gap is closed to within 10 dB in approximately 50-70% of patients who undergo revision stapes surgery. Most surgeons prefer to use conscious sedation anesthetic technique when performing revision surgery. Manipulation of the existing prosthesis that elicits severe vertigo intraoperatively may indicate adherence of the prosthesis to the underlying vestibule. Further manipulation or removal of the prosthesis may cause a tear in the membrane and resulting profound sensorineural hearing loss. Having the patient awake enough to report vertigo during this portion of the operation is advantageous in this respect.

Residual or recurrent conductive hearing loss after stapes surgery has many causes. The most likely cause is a migration of the prosthesis out of the stapedotomy and subsequent fixation against the residual footplate or otic capsule margin. This is thought to be due to a contraction of collagen within a neomembrane created between the prosthesis and the membranous labyrinth, which lifts the prosthesis out of the oval window fenestration. This may cause complete or partial erosion of the incus due to vibration of the incus against a fixed prosthesis. Alternately, incus erosion may occur as a result of vascular compromise of the bone due to an overcrimped prosthesis. Other causes include malleus or incus fixation or incus dislocation.

A retrospective report by Shim et al indicated that in cochlear otosclerosis patients, the presence of cavitating lesions involving the internal auditory canal negatively impacts poststapedotomy audiologic results. This was reflected in the study patients’ mean air- and bone-conduction thresholds and air-bone gap values at 6-month follow-up, being 45.7 dB, 33.8 dB, and 11.8 dB, respectively, in patients with cavitating lesions, and 24.1 dB, 20.0 dB, and 4.1 dB, respectively, in those without.[17]

A study by Kishimoto et al indicated that postoperative outcomes in otosclerosis are influenced by the preoperative air-bone gap, the preoperative air conduction threshold, the presence of cochlear otosclerosis, male sex, laterality, and age. Eg, unilaterality and male sex predicted a better prognosis at 250 Hz, while cochlear otosclerosis predicted a worse prognosis at 2 kHz. The study involved 191 patients with otosclerosis (234 primary stapes operations).[18]

A prospective clinical study by Dziendziel et al of 168 otosclerosis patients with chronic tinnitus indicated that stapedotomy can reduce tinnitus severity. The investigators found that tinnitus had significantly improved in 93 patients (55%) postsurgery, with 62 patients (37%) reporting that the condition had disappeared completely.[19]

Future and Controversies

The following represent somewhat unresolved controversies. Exploration of each issue requires extensive discussion beyond the scope of this article.

  • Preoperative tuning fork examination as a prognosticator for surgical success

  • Laser stapedotomy minus prosthesis (STAMP) procedure in which the posterior footplate is detached from the diseased anterior footplate without disrupting ossicular chain continuity

  • Implantable hearing devices (Baha) that may produce adequate gain for amplification without the risks of stapedectomy

  • Endoscopic techniques now being applied to stapes surgery to further minimize morbidity[20]