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seen with conductive, sensorineural and mixed patterns
of hearing loss. While the otosclerotic lesions are histo-
logically similar to those seen in otosclerosis, in OI they
are more extensive.
Computed tomography (CT) scans of the tempo-
ral bones in patients with OI had shown that the bone
surrounding the cochlea was thicker with much lower
attenuation than that typically found in a normal otic
capsule.
11
In addition, there was a proliferation of the
bony labyrinth which contributed to a narrowing of
the middle ear cavity, envelopment of the stapes foot-
plate, and covering of the oval and round window.
11
Bone density loss has been shown to correlate with
bone conduction thresholds in otosclerosis
12
and in
other disorders of the bone associated with hearing
loss.
13
Recently, in 56 adult patients with OI it has been
shown that lower bone mineral density was associated
with conductive and mixed but not sensorineural hear-
ing loss.
8
The authors suggested the possibility that
reduced bone mineral density might lead to greater sus-
ceptibility to microfractures and thinning of the ossicles
which ultimately may contribute to the development of
conductive hearing loss in OI.
8
Examination with a scanning electron microscope
using the Ca/P ratio as the criterion based on character-
istic X-ray fluorescence shows that the stapes in OI has
a higher Ca/P ratio (2.6:1) as compared to the normal
stapes 2:1 ratio. The stapes in OI are poorly mineralized
with low calcium salt and apparent increase in phos-
phates. This finding suggests a possible change from
hydroxyapatite (or apatite) to brushite, which implies
an acidification of bone.
14
RADIOLOGY
Computerized tomography (CT) of the temporal bone
in OI shows the predicted demineralization of bone
found in other parts of the skeleton.
15,6
There has been a
correlation between the progression of the hearing loss
and the degree of demineralization found on CT.
6
Magnetic resonance imaging (MRI) has also shown a
similar finding with pericochlear lesions, increased soft
tissue signal intensity and contrast enhancement of the
otic capsule in type I OI.
16,17
The differential diagnosis
includes otosclerosis, Paget's disease, otosyphilis and
Camurati-Englemann disease.
M
EASUREMENT OF HEARIN
G
For most circumstances the measurement of hearing
loss in OI is the same as for any other individual. There
are a few exceptions though that should be noted. When
audiometry is undertaken use of insert phones is rec-
ommended to avoid collapse of the ear canal leading to
spurious findings. The use of bone conduction thresh-
olds while vital in determination of hearing loss etiology
is potentially confounded by the presence of middle ear
pathology. Fixation of the footplate may alter cochlear
function enough to elevate bone conduction thresholds.
This is seen in bone conduction thresholds in otosclero-
sis especially at 2000 Hz and also in OI. When the sta-
pes footplate is immobilized the resulting mechanical
restriction of the perilymph may alter cochlear function
sufficiently to elevate bone conduction thresholds.
18
On the other hand, bone conduction thresholds may be
atypically reduced in the frequencies below 2000 Hz in
patients with dehiscence of the superior semicircular
canal.
19
Air-bone gaps in the low-frequency region are
frequently reported in patients with enlarged vestibu-
lar aqueduct syndrome.
20
In patients with OI, tympano-
metric findings may be normal despite the presence of
significant conductive involvement.
21,22
For example, in
one study 35% of ears with conductive hearing loss and
52% of ears with mixed hearing loss had normal tym-
panograms.
22
Measurement of acoustic reflexes in those
cases may assist in clarification of the presence of con-
ductive involvement, as acoustic reflexes are typically
absent in OI in the presence of conductive hearing loss
secondary to ossicular dysfunction accompanied with
normal tympanograms.
22
In one study, abnormal acous-
tic reflexes were reported in 57% of patients greater than
20 years of age.
22
Some investigators have reported on
the temporal properties of the acoustic reflex in patients
with OI.
21,23
However, those measures are of limited util-
ity given the frequency with which acoustic reflexes are
absent in patients with OI, the need for relatively high-
end instrumentation in order to observe the acoustic
FIGURE 32.2
A hematoxylin and eosin-stained temporal bone of
a patient with type II OI. The cochlea and structures of the inner ear
are normal. In this section delayed patterns of ossicle formation are
noted. The stapes footplate is separated into two layers and the incus
has a higher content of marrow (asterix) than predicted for age.
