To understand why otosclerosis causes hearing loss, it is important to understand how the ear works. The human ear is divided into three parts: the external, middle, and inner ear.
- The external ear consists of the part of the ear you can see (the auricle) and the ear canal.
- The middle ear includes the eardrum (tympanic membrane) and the three bones of the middle ear, the malleus (“hammer”), incus (“anvil”), and the stapes (pronounced STAY-PEAZ, “the stirrup”).
- The inner ear is a fluid-filled series of chambers. One of these chambers, the cochlea, is responsible for converting sound vibrations into nerve impulses. It is these nerve impulses that the human brain interprets as sound and what we call “hearing.”
The inner ear also contains the semicircular canals which are responsible, in part, for sensing movement and maintaining balance.
Otosclerosis is a disorder of bone that affects human beings only and involves the otic capsule (bony labyrinth) and primarily the stapes bone of the middle ear. It is the slow progressive fixation of the stapes bone in the oval window which usually causes the clinical symptoms to become noticed in the form of a conductive hearing loss. As the condition progresses a mixed conductive and sensorineural hearing loss develops. The clinical condition affects only 1% of the Caucasian population and it is more frequent in woman than in men with an approximate 2:1 ratio. It is much less common in the black population. The condition is noticed first in young adulthood (ages 20-30’s) but has been reported in individuals as young as 11 years of age. In females, often the conductive hearing loss accelerates during pregnancy perhaps due to the regulation of bone deposition by hormones. This condition may be found more commonly in certain families and is thought to be transmitted as an autosomal dominant trait with incomplete penetrance which is why it does not affect all individuals nor every generation.
The exact etiology of Otosclerosis remains a mystery, but the pathogenic features involve bone resorption, remodeling and then bone deposition. The lesions of the otic capsule transition from otospongiosis which is the highly vascular remodeling phase, to one of dense bone deposition known as the otosclerotic phase. What triggers these changes are speculative but there has been some evidence linking the disorder with early exposure to the measles virus antigen. While there are other theories suggesting that it is a metabolic bone disorder the exact cause remains elusive.
The clinical symptom that drives a patient to the doctor is the progressive conductive hearing loss that is associated with new bone depositing around the stapes footplate wedging it in the oval window so that its normal mobility is hampered. Even though sound is normally transmitted to the tympanic membrane, the ossicular chain has become stiffened by the fixation of the stapes bone. 80-90% of these otosclerotic lesions have an anterior location at the oval window in what is known as the fissula ante fenestram. However, each patient’s lesion can be different and can range from a biscuit type footplate to a sea of otosclerosis obliterating the entire footplate. Experience recognizing the position of the lesions and how to best deal with these variations is crucial in order to prevent complicating situations such as a floating footplate which makes completion of a successful procedure more difficult and hazardous. On rare occasions, the disorder is so widespread that it obliterates the entire round window which causes a non-surgically correctable conductive hearing loss.
The otologist/neurotologist (ear specialist) typically finds that the patient with clinically significant otosclerosis fits the following description:
- Hearing loss with typical onset in young adulthood
- Conductive or mixed hearing loss documented on a standard high quality audiogram showing a pathognomonic Carhart’s notch (dip in bone conduction at 2000 Hz).
- No history of trauma, ear infections, drainage, perforations etc.
- Possible positive family history
- Normal tympanic membrane and aerated middle ear on otoscopic examination and a mobile malleus seen on pneumotoscopy
- Either absent or “on-off effect” seen on the stapedial reflex testing
- Abnormal 512 Hz tuning fork test suggesting a conductive hearing loss (Rinne-negative, Weber-lateralizes to the affected ear with conductive hearing loss)
On occasion the patient may need to undergo a CT scan to rule out other causes of these presenting symptoms but this is left to the discretion of the treating physician. However, a CT scan is not required to make the presumptive diagnosis in the majority of cases, nor is it required before performing surgery as there is a small risk of radiation exposure with a high resolution CT scans required to visualize the inner and middle ears. In some advanced cases the hypervascular phase of otosclerosis can be seen on otoscopic examination by the physician looking through the tympanic membrane and seeing a reddish blush on the promontory of the cochlea- this is the so-called “Schwartze’s sign”.
There is a differential diagnosis that must be considered every time a physician sees a medical condition and this represents the other possible causes of the patient’s presenting symptoms. Similarly, in otosclerosis there is a differential diagnosis that should be considered including the following:
- Necrosis of the long process of the incus or ossicular chain discontinuity
- Fixation of the malleus, incus or stapes by tympanosclerosis or calcification
- Congenital fixation or malformations of the ossicles
- Serous otitis media
- A cholesteatomatous mass or other non-visible tumor mass interfering with ossicular chain mobility
- Superior canal dehiscence or another third window condition
- Dehiscent and prolapsed facial nerve
- Stapes superstructure fracture
- Fracture or dislocation of the ossicles
- Exostoses of the external canal impinging on the malleus
- Congenital absence of the round or oval windows
- Large persistent stapedial artery coursing through stapes superstructure
As the condition becomes clinically significant with the development of a conductive or mixed hearing loss, the patient may be significantly handicapped at school, work or in social situations. When this occurs intervention is recommended either through amplification or surgery. Patients who are poor surgical risks or have some personal or psychological reason to avoid surgery should be offered a hearing aid. Some patients with otosclerosis may develop severe vestibular symptoms with vertigo and imbalance and until this resolves they may be better off to pursue amplification with less attendant inner ear risks from surgery.
If there is no medical contraindication to surgery then a stapes procedure is a very good option because the surgery in experienced hands is highly successful, can be accomplished as an outpatient procedure and has a relatively low rate of complications or side effects.
The modern day operation has evolved over the years from what was originally conceived of as an open cavity mastoidectomy with creation of a skin covered fistula of the lateral semicircular canal known as the fenestration procedure of Julius Lempert to the stapes mobilization procedure popularized by Samuel Rosen. Because of difficulties with these procedures, John Shea developed the stapedectomy procedure in the late 1950’s which solved many of the previously noted issues.
The technique most commonly employed by Dr. Harris is as follows:
During stapedectomy (also known as stapedotomy), the top portion of the stapes bone is removed by breaking it or vaporizing it with a laser. The bottom part, or footplate, of the stapes remains fixed in place by the otosclerosis. A small hole (0.7mm) is then drilled or lasered in the footplate . A small prosthesis is then inserted into the hole and attached to the incus (the second hearing bone).
One of the most common prostheses resembles a small piston (0.4-0.6mm diameter) with a wire attached to the top. The piston goes into the hole in the stapes footplate and the wire is hooked onto the incus and tightened to prevent slipping. The piston is free to move in and out of the inner ear through the hole in the stapes footplate. This allows sound vibrations to again pass from the eardrum membrane to the internal ear fluids.
Although hearing improvement can be immediate, packing placed into the ear canal at the end of the surgery prevents patients from experiencing their new hearing for a week or so.
The development of the small hole fenestra technique minimized the potential surgical trauma to the inner ear and thus reduced the chances of hearing loss and post-operative dizziness. By far and away, the small hole technique is the most common stapes surgery practiced in throughout the world. It has been modified and improved over the years to where it is now often performed under mild sedation and entirely through the normal ear canal with the use of an operating microscope.