Otosclerosis
By
Dr. T. Balasubramanian M.S. D.L.O.
Synonyms: Otospongiosis, Ankylosis of foot plate of
stapes.
Definition: Otosclerosis is a hereditary localised
disease of the bone derived from the otic capsule characterised by
alternating phases of bone resorption and new bone formation. The
mature lamellar bone is removed by osteoclasis and replaced by woven
bone of greater thickness, cellularity and vascularity.
History: In 1741 Antonio Valsalva described ankylosis of stapes while
doing a postmortem on the body of a deaf patient.
In 1894, Adam Politzer introduced the term "otosclerosis" and described
the histopathology of the disease for the first time.
In 1912 Siebenmann introduced the term otospongiosis to denote active
otosclerotic foci.
Pathophysiology:
The primary pathological change occurs in the
bony labyrinth with secondary effects upon middle ear and inner ear
function. The otosclerotic focus may be asymptomatic, or if
present in the area of foot plate of stapes it may give rise to
ankylosis of foot plate with resultant conductive deafness.
Otosclerotic foci may involve other portions of labyrinth causing
sensori neural hearing loss and vestibular abnormalities.
A combination of
effects are possible in otosclerosis. They are:
Histological otosclerosis: Otosclerotic foci doesnot cause any symptoms
and hence known as histological otosclerosis.
Stapedial otosclerosis: is the classical otosclerosis with
fixation of stapedial foot plate causing conductive deafness.
Cochlear otosclerosis: The foci involves the cochlea causing
sensorineural deafness.
Combined otosclerosis: Here in addition to fixation of foot plate
of stapes there is also associated sensorineural hearing loss due to
involvement of cochlea.
Otospongiosis: European otologists prefer to use this term to
indicate the active phase of otosclerosis.
Incidence: Otosclerosis is common in caucasian races. It is
rarely found in Mongoloid and Negro population.
Sex incidence: In clinical practice otosclerosis is seen more
often in women than in men. The ratio was found to be 2:1.
Nowadays the authors believe the apparent female preponderance may be
due to the fact that unilateral otosclerotic deafness is less common in
women than in men. Noticeable deterioration in hearing also occur
during pregnancy due to hormonal changes. Deafness due to
otosclerosis may be initiated or made worse by pregnancy.Causative
factors / etiology: Many theories have been proposed to explain
the etiological factors of otosclerosis. They are:
1. Metabolic
2. Immune disorders
3. Vascular disease
4. Infection (Measles) currently accepted
5. Trauma : The petrous bone doesnot have regenerative capacity.
This is because of the fact that the enzymes released during reparative
phase are very toxic to the inner ear hair cells.
Pockets of tissue capable of regeneration
may be sequestered in various portions of labyrinthine bone. These
tissue could be activated by the presence of regenerative enzymes in
the blood following bone fracture elsewhere in the body.
6. Temporal bone abnormalities
(congenital)
Genetic factors predisposing to otosclerosis: The tendency for
otosclerosis to run in families has been documented. Authors have
postulated an autosomal dominant mode of inheritance with varying
degrees of penetration.
Otosclerosis is associated with osteogenesis imperfecta in 0.15 %
of cases. This is known as Van der Hoeve syndrome or Adair -
Dighton syndrome.
Sites affected by otosclerosis: The commonest site for apperance
of otosclerotic bone is fissula ante fenestrum. This fissula
is constantly seen in the labyrinthine capsule lying in front of
the oval window. This area may contain unossified cartilage
persisting even in adults. This area was referred to as
Cozzolino's zone by Perozzi in memory of his teacher.
Otosclerosis may occur in this area due to bony ossification of the
cartilage.
Residual cartilage may be present in the following areas of labyrinth:
1. Fissula ante fenestram
2. Fissula post fenestram
3. Intracochlear
4. Round window
5. Semicirular canals
6. Petrosquamous suture
7. Base of styloid process
In normal development the fissula
appears as fibrous connective tissue bundle joining the vestibule with
the tympanic cavity. This fibrous tissue is encased in primary
cartilage which later gets replaced by bone. From the fissula the
bone acquires a connective tissue lining which later becomes converted
into perichondrium. The fissula is reduced in size by the
production of new secondary cartilage from the perichondrium.
These changes are completed by birth. The secondary
cartilage remains throughout life as a stable, dormant cartilage and
hence may even be considered as normal structure. It is only when
this secondary cartilage gets ossified otosclerosis occur (Bast &
Anson).
Diagramatic representation of fissula ante fenestram
Otosclerotic changes may appear as a result of
interaction between bone growth promoting substances circulating in the
blood stream, and the unstable cartilagenous elements in the capsule of
the labyrinth. Otosclerosis is often seen at times when the bone
growth promoting substances are circulating in the blood as in
pregnancy and following fractures of other bones.
Histopathology of otosclerosis:
The normal endochondral bone of labyrinthine
capsule in which otosclerotic focus begins is compact in type.
Ultrastructurally, lamellae composed of fine fibrils lying in a ground
substance are concentrically disposed around haversian canals
containing blood vessels and connective tissue. In otosclerosis
there is sharply defined new bone formations that could be
differentiated from normal bone by their deep carmine stain and by
marked enlargement of bone spaces and haversian canals. The
following are the changes which occur in a otosclerotic foci:
1. Focal / diffuse replacement of normal compact bone by
irregular, loose cancellous bone with more deeply staining lamellae.
2. There is an associated increase in size of Haversian canals,
cell spaces and marrow spaces with corresponding increase in
vascularity. The blood vessels are frequently surrounded by a
narrow margin of blue staining material that Manassee described first
as Blue Mantle zone.
3. Increase in osteocytes, and appearance of osteoblasts and
osteoclast cells.
Histologically otosclerosis may be classified into:
1. Early focal otosclerosis
2. Diffuse active otosclerosis
3. Quiescent otosclerosis
4. Cochlear otosclerosis
Early focal otosclerosis: In this type the
abnormalities are localised to one or two small areas of an otherwise
normal foot plate section. The abnormal areas show an enlarged
marrow space surrounded by a blue staining area on H&E staining.
Diffuse active otosclerosis: In this type there is abnormal
vascularity with a great increase in size and number of marrow
spaces. Most of these spaces are lined by osteoblasts. In
places around the circumference of the marrow spaces there is a
scalloped appearance where bone has been recently absorbed. The
number of osteocytes are greatly increased.
Quiescent otosclerosis: Here eventhough there may be some
increase in the size and number of marrow spaces there is no evidence
of bone formation or bone destruction. Osteoblasts and
osteoclasts are only occasionally seen. This could be considered
as a burnt out phase of the disease spectrum.
Cochlear otosclerosis: This condition causes pure sensorineural
deafness without stapes fixation. Otosclerotic foci may occur in
the otic capsule without the involvement of stapedial foot plate.
The process of bone erosion and new bone formation which occur in
otosclerosis releases enzymes like amylase, SGOT, SGPT etc which can
enter into the endolymph via the round window membrane. These
enzymes are toxic to the sensitive hair cells of the cochlea causing
sensorineural hearing loss.
Clinical types of otosclerosis: Classification of various
clinical types of otosclerosis is based on microscopic appearances of
the diseased foot plate.
Rim fixation: Here the otosclerotic foci starts from the anterior
portion of the oval window niche. It gradually expands to involve
the anterior portion of the foot plate causing fixation of the anterior
portion of the foot plate only leaving the centre of the plate free.
Diagramatic representation of various clinical
types of otosclerosis
Biscuit foot plate: This type occur less
frequently. The focus originates in the foot plate itself and as
it expands it gives rise to the biscuit or rice grain foot plate with
delineated margins.
Obliterative otosclerosis: Rarely a large mass of otosclerotic new bone
fills up the oval window niche obscuring the entire foot plate.
This condition is known as obliterative otosclerosis. It is a
difficult condition to manage surgically.
Clinical features:
Deafness: Typically deafness in otosclerosis is bilateral and
gradually increasing in nature. Unilateral otosclerosis
occur in 15% of patients. Frequently it occurs between
third and fifth decades of life. In majority of cases the
deafness is conductive in nature. The deafness will not be
noticed by the patient till the loss reaches 30 dB level. This is
the deafness level in which understanding speech becomes
difficult. These patients may hear better in noisy environment
because the speaker has a tendency to raise his voice because of
excessive ambient noise. This phenomenon a feature of
otosclerosis is known as Paracusis Willisii.
In cochlear otosclerosis the deafness is
purely sensorineural in nature. Some patients may have both
conductive and sensorineural hearing loss (mixed deafness) because of
the tendency of bone reparative enzymes to damage the inner hair
cells.
Patients with otosclerosis have
characteristically quiet voice with good tone and the change in speech
pattern may be detected only by close relatives.
Tinnitus: is a common symptom and occasionally could be the only
presenting feature. The presence of tinnitus should alert the
physician about the presence of cochlear otosclerosis. It could
also be seen in some patients without cochlear degeneration due to
abnormally increased vascularity of the otosclerotic bone. Mostly
tinnitus indicates sensorineural degeneration. Tinnitus may be
unilateral or bilateral. It is usually roaring in nature.
Vertigo: Transient attacks of vertigo is not uncommon in patients
with otoslerosis. This could be due to the action of toxic
enzymes released by the lesion into the vestibular labyrinth.
These patients may even have coexisting Meniere's disease.
Clinical examination: The ear drum in these patients is normal
(mint condition). Rarely during active phase of the disease the
increased vascularity of the promontory may be seen through the ear
drum. This sign is known as Flemingo's flush sign or Schwartz's
sign. This indicates otospongiosis (active otosclerosis).
Hearing assessment can be done using tuning forks. For detailed
description of tuning fork tests read the chapter titled clinical
examination of the ear.
Pure tone auditometry will show precisely the amount and type of
hearing loss. The presence of Carhart's notch is a classic
audiometric feature in these patients. This Carhart's notch
is present in bone condution. There is a dip centered around 2000
Hz. This is actually an artifact. In cochlear otosclerosis
audiometry reveals sensorineural hearing loss.
Stapes fixation causes an elevation in the bone conduction thresholds
of 5dB at 500Hz, 10dB at 1000 Hz, 15 dB at 2000 Hz, and 5 dB at 4000
Hz. In the audiogram this creates a peculiar pattern known as
Cookie bite audiogram. The bone conduction audiogram appears like
a cookie having been bitten.
Fig showing a cookie bite audiogram
Fig showing audiogram with carhart's notch
Impedence audiometry is an useful investigation to
diagnose otosclerosis. Middle ear compliance is often
reduced. When stapes is fixed stapedial reflex is absent.
The typical impedence curve is As curve.
All these patients with pure conductive deafness have excellent speech
discrimination thresholds.
Management:
Medical: The aim of medical management is to convert an active
otosclerotic foci into an inactive or quiscent foci. Fluride is
the drug of choice.
Indications of fluride therapy:
1. Patients with surgically confirmed otosclerosis who show
progressive sensorineural deafness disproportionate to age.
2. Patients with pure sensorineural loss with family history, age of
onset, audiometric pattern and good auditory discrimination indicate
the possibility of cochlear otosclerosis.
3. Patients with radiological demonstration by CT scan of spongiotic
changes in the cochlear capsule
4. Patients with positive Schwartz sign.
5. Post op treatment: If patients are found to have an active focus
during surgery, fluride therapy is prescribed for 2 years.
Contraindications of fluride therapy:
1. Patients with chronic nephritis and nitrogen retention
2. Patients with chronic rheumatoid arthritis
3. Patient who are pregnant / lactating
4. In children before skeletal growth has been completed
5. Patients who show allergy for the drug
6. Patients with skeletal flurosis
Flurides act on otosclerotic foci by reducing
osteoclastic bone resorption with a corresponding increase in
osteoblastic bone formation. Fluride also has antienzymatic
action thereby it can neutralise the toxic enzymes released from the
otospongiotic foci.
Dose: A daily dose of 50 mg of sodium fluride is given for a
period of 2 years. In patients with positive Schwartz's sign the
dose can be increased up to 75 mg per day.
Adverse effects of sodium fluride therapy:
1. Gastric disturbance
2. Arthritis
3. Skeletal flurosis
Surgical treatment: Stapedectomy
Hearing aids: These patients will benefit from the use of hearing
aids if surgery is not acceptable to the patient or if it is
risky. There is always a 1% risk of producing a dead ear during
surgery even in the best of hands.
Copyright drtbalu 2007
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