Otosclerosis

Anatomy
It may be pertinent to re view the anatomy of the
labyrinth and introduce the terminology often used in
describing the labyrinth.
(i) Otic labyrinth. Also called membranous labyrinth
or endolymphatic labyrinth. It consists of utricle,
saccule, cochlea, semicircular ducts, endolymphatic
duct and sac It is filled with endolymph.
(ij) Periotic labyrinth or perilymphatic labyrinth (or
space). It surrounds the otic labyrinth and is filled
with perilymph. It includes vestibule, scala tympani,
sca la vestibuli, perilymphatic space of semicircular
canals and the periotic duct, which surrounds the
erlclolymphatic duct of otic labyrinth.
(iii) Otic capsule. It is the bony labyrinth. It has three
laye rs.
Endos ~e(ll. The innermost layer. It lines the bony
labyrinth.
Enchondral. Deve lops from the cartilage ancllater oss ifies
into bone. It is in this layer that some islands of cartilage
are left unoss ified that later give rise to otosclerosis.
Periosteal. Covers the bony labyrinth.
Otic capsule or the bony labyrinth oss ifies from 14
centres, the first one appears in the region of cochlea at
16 weeks and the last one appears in the postero lateral
part of posterior semicircular canal at 20th week.
Otosclerosis, more aptly called otospongws is, is a
primary disease of the bony labyrinth. In this, one or
more foci of irregularly laid spongy bone replace part of
normally dense enchondral layer of bony otic capsule.
Most often, otosc le rotic focus involves the stapes region
leading to stapes fixation and conductive deafness.
However, it may involve certain other areas of the bony
labyrinth whe re it may cause neurosensory loss, or no
symptoms at all.
Aetiology
The exact cause of otosclerosis is not known; however the
following facts have been documented.
Anatomical basis. Bony labyrinth is made of enchondral
bone which is subject to little change in life. But
sometimes, in this hard bone there are areas of cartilage
rests which, due to certain non-specific factors, are act ivated
to form new spongy bone. One such area is the fissula.
ante fene stram lying in front of the oval window- the
site of predilection for stapedial type of otospongiosis.
Heredity. About 50% of otoscl erotics have positive
family history; rest are sporadic Genetic studies reveal
that it is an autosomal dominant trait with incomplete
penetrance and variable express ivity.
Race. White races are affected more than Negros. It is
common in Indians but rare among Chinese and Japanese.
Sex. Females are affected twice as often as males but in
our country, otosclerosis seems to predominate in males.
Age of onset. Deafness usually starts between 20 and
30 yea rs of age and is rare before 10 and after 40 years.
Effect of other factors . Deafness due to otoscle rosis may
be initiated or made worse by pregnancy. Similarly, deafness
may increase during menopause, after an accident or
a major operation.
The disease may be associated with os teogenesis imperfecta
with histo ry of multiple fracture s. The triad of
symptoms of osteogenesis imperfecta, otoscleros is and
blue sclera, is called van der Hoeve s),ndrome. Lesions of
otic capsule seen in osteogenesis impelfecta are histologically
indistinguishable from those of otosclerosis and
both are due to genes encoding type I collagen.
Viral infection. Electron microscop ic and immunohistochemical
studies have shown RNA related to measle
virus. It is likely that otosclerosis is a viral disease as has
been suggested for Page t's disease.
Types of Otosclerosis
1. Stapedial otosclerosis. Stapedial otosc lerosis
causing stapes fixation and conductive deafness is the
most common variety. Here lesion starts just in front of
the oval window in an area called 'fissu la ante fenestram'.
This is the site of predilection (anterior focus).
Lesion may start behind the oval window (p os terior
focus), around the margin of the stapes foo tplate (circumferential),
in the footpl ate but annular ligament being
free (biscuit type). Sometimes, it may completely obliterate
the oval window niche (obliterative type) (Fig. 13.1).
2. Cochlear otosclerosis. Cochlear o tosclerosis
invo lves region of round windo~' or other areas in the
otic capsule, and may cause sensorineural hearing loss
probably due to liberation of toxic materials into the
inner ear fluid.
3. Histologic otosclerosis. This type of otosclerosis
remains asymptoma tic and causes neither conductive
nor se nsorineural hearing loss .
Pathology
Grossi)., otosclerotic Lesion appears chalky white, greyish
or yellow. Sometimes, it is red in colour due to increased
vascularity, in which case, the otosclerotic focus is active
and rapidly progress ive.
Microscopicall)" spongy bone appears in th.e normally
dense enchon.dral layer of otic capsule. In immature;1ctive lesions, there are numerous marrow and vascular
paces with plenty of osteoblasts and osteoclasts and a lot
of cement substance which stains blue (blue mantles)
vith haematoxylin-eosin stain. Mature foci show less
\'ascu larity and laying of more bone and more of fibrillar
,ubstance than cementum, and is stained red.
Symptoms
1. Hearing loss. This is the presenting symptom and
usually starts in twenties. It is painless and progressive with
Isidious onset. Often it is bilateral conductive type .
2. Paracusis willisii. An otosclerotic patient hears
better in noisy than quiet surroundings. This is because a
normal person will raise his voice in noisy surroundings.
3. Tinnitus. It is more commonly seen in cochlear
toscleros is and in active lesions.
4. Vertigo. It is an uncommon symptom.
5. Speech. Patient has a monotonous, well moduted
soft speech.
Signs
Tympanic membrane is quite normal and mobile.
Sometimes, a reddish hue may be seen on the promontory
through the tympanic membrane (Schwartze
sign). This is indicative of active focus with
increased vascularity.
Eustachian tube function is normal.
Tuning fork tests show negative Rinne (i.e.
BC > AC) first for 256 Hz and then 512 Hz and
still later, when stapes fixat ion is complete, for
1026 Hz. Weber test will be lateralised to the ear with
greater conductive loss. Absolute bone conductionmay be normal. It is decreased in cochlear otosclerosis
with sensorineural loss.
Pure tone audiometry shows loss of air conduction,
more for lower frequencies.
Bone conduction is normal. In some cases, there is a
dip in bone conduction curve. It is different at different
frequencies but maximum at 2000 Hz and is called the
Carhart's notch. (5 dB at 500 Hz, 10dB at 1000Hz, 15 dB
at 2000 Hz and 5 dB at 4000 Hz) (Fig. 13.2). Carhart's
notch disappears after successful stapedectomy.
Mixed hearing loss is not uncommon in otosclerosis.
There is loss in bone conduction with air-bone gap.
Speech audiometry reveals normal discrimination
score except in those with cochlear involvement.Tympanometry may be normal in early cases but later
shows as curve due to oss icular stiffness. Stapedial reflex
becomes absent when stapes is fixed.
Differential Diagnosis
Orosclerosis should be differenti ated from other causes of
conductive deafness particularly serous oti tis media,
adhesive otitis media, tympanosclerosis, attic fixation of
head of malleus, oss icular discontinuity or congenital
stapes fixation.
Treatment
Medical. There is no medical treatment that cures
otosclerosis. Sodium fluoride has been tried to hasten the
maturity of act ive focus and arrest further cochlear loss,
but controve rsies exist and this tre atment is not recommended
generally.
Surgical. Stapedectomy with prosthesis replacement
is the treatment of choice. Here the fixed otosclerotic
stapes is removed and a prosthesis inserted between
the incus and oval window (Fig. 13.3). Prosthesis employed
may be a teflon piston, stainless steel piston, platinum
teflon or titanium teflon piston (Fig. 13.4). In 90%
of patients, there is good improvement in hearing after
stapedectomy.
Selection of patients for stapes surgery. Hearing
threshold should be 30dB or worse (It is this level wnen
patient sta rts feeling socially handicapped).
Average air-bone gap should be at least 15 dB with
Rinne negat ive for 256 and 512 Hz.
Speech discrimination score should be 60% or more.
Contraindications to stapes surgery
(i) The only hearing ear.
(ii) Associated Meniere 's disease. When there is histo ry of
vert igo with clinical evidence of Meniere's d isease
in an otosclerotic patient, there are more chances of
sensorineural hearing loss after stapedectomy.(iii) Young children . Recurrent eustachian tube dysfunction
is common in children. It can displace the prosthesis
or cause acute otit is media. Also the growth of otoscle
rotic focus is faster in children leading to reclosure
of oval window.
(iv) Professional athletes, high construction workers, divers,
and frequent air-travellers. Stapes surgery has the risk
to cause post-operati ve ve rtigo and/or dizziness and
thus interfere with their profess ion; or frequent air
pressure changes may damage the hearing or cause
severe vertigo.
(v) Those who work in noisy surroundings . After stapedectomy,
they would be more vulnerable to get sensorineural
hearing loss due to noise trauma.
(v i) Otitis externa, tympanic membrane perforation and
exostosis are relative contra indications. Stapedectomy
can be done after they have been treated first for
above conditions. S imilarly, stapedectomy is avoided
during pregnancy.
The operation is preferably done under loca l anaesthesia.
Steps of stapedectomy (Fig. 13.5) include:
1. Meatal incision and elevation of the tympanomeatal
flap.
2. Exposure of stapes area . This may requ ire removal of
posterosuper ior bony overhang of the canaL
3. Removal of stapes superstructure.
4. Creation of a hole in the stapes footplate (stapedotomy)
or removal of a part offootplate (stapedectomy).
5. Placement of prosthesis.
6. Repositioning the tympanomeatal flap .
Two percent of patients undergoing this operation
may suffer sensorineural loss. Slowly progressive high frequency
loss is seen in long-term follow up. One in 200
patients may get a totally "dead" ear.
Stapes mobilisation is no longer done these days as it
gives temporary results; refixation being quite common.Lempert's fenestration operation is almost outdated n ow.
t-fere all. alternative window is created in the lateral
-.e micircular canal to function for the obliterated oval
" indow. It has the disadvantage of a post-operativemastoid cavity and an inherent hearing loss of 25 dB
which cannot be corrected.
Hearing aid. Patients who refuse surgery or are unfit for
surgery can use hearing a id. It is an effective alternative.