w8 Flashcards
where are the windows?
Temporal bone, perilymphatic space, membranous labyrinth containing endolymph
- Either high or low impedance
What are these windows?
Small holes, channels and ducts in inner ear that may be covered (by membrane)
low impedance windows
oval and round window
- Connect perilymph to air in ME cavity
- Large in area & mobile
- Low impedance allows transmission of sound from ME air space to inner ear fluids
- Movement of these 2 windows allow for bulk movement of inner ear fluids with sound vibrations
oval window
covered by stapes footplate & ligament
round window
covered by membrane
high impedance windows
- Perilymphatic space is embedded within temporal bone but not entirely enclosed
- Small openings and channels in temporal bone
o Connect perilymph to the cranial cavity containing CSF (cochlear aqueduct)
o Allow blood vessels and nerves to connect inner ear & brain
- Habenula perforate:
holes” allow nerve fibers & blood vessels to pass into cochlea
Abnormal windows can develop and cause inner ear dysfunction when:
- High impedance windows/channels/ducts become enlarged
a. High impedance window change & become low impedance - New holes or weaknesses develop in temporal bone or membranous labyrinth
a. New holes/channels appear in inner ear forming extra low impedance windows
b. Normally there are only 2 low impedance windows: oval and round
Low impedance 3rd window mechanisms
Characteristic low frequency air-bone gap due to
- Decreased (Worse) air conduction
- Increased (better than normal) bone conduction
Unusual types of tinnitus: increased sensitivy to internal sounds
Normal air conduction with normal ear 2 low impedance windows
difference in vibration between the oval & round window generates a pressure gradient across the BM activating haircells and creating perception of sound
Third window air conduction
incoming acoustic energy from oval window is shunted away decreasing transmission to round window and decreasing pressure gradient across the BM, resulting in reduced air conduction thresholds
Normal BC mechanism
Bc vibrations produce outward motion of oval and round windows (round window is greater) this is due to unequal impedances oval window is higher impedance bc covered by bony footplate of stapes
Normal ear 2 low impedance windows: vibrations produce unequal outward motion of oval and round windows, pressure difference across BM creates BM vibrations and perception of sound by BC
underlying mechanisms: tinnitus
- Tinnitus rupture in bone or enlargement of high impedance windows creates low impedance connections b/w inner ear fluids and CSF surrounding bone
- Pressure changes in cerebral spinal fluid with changes to heartbeat
- If connection between perilymph and CSF the pressure changes can flow into cochlea and into BM causing pulsatile tinnitus or ringing in ear, likely as pulsatile and rhythmic to heartbeat,
- Increase in pressure differential so vibrations are being detected by cochlea whereas in normal patients it is not
Perilymphatic Fistula
rupture in oval or round window creating an abnormal connection b/w inner ear perilymph and ME space
- can be aquired, congenital or ideopathic
Abnormal ear with enlarged window (3rd window BC)
energy shunting across 3rd window decreases motion of higher impedance oval window, motion of round window unchanged, elevating pressure difference across BM, increasing BM displacement and improving BC sound perception
acquired perilymphatic fistula
surgery (stapedectomy) head trauma, barotrauma, erosion in the bony labyrinth (cholesteatoma)
congenital perilymphatic fistula
TB malformation
Perilymphatic fistula: idiopathic
- No specific diagnostic test
- Reliability of observed leaks on exploration of ME space (OW or RW)
- Without inciting event (surgery, barotrauma) does it even exist
pathologic 3rd window
- Defect in the bony labyrinth can create new low impedance window
- Disruption of high impedance window (converted to a low impedance window, new fistulas (holes) or weakness can reduce impedance)
pathologic third window symptoms
variable depending on size and location in membranous labyrinth; extra low impedance window renders the membranous labyrinth to sound and pressure changes
Pathological 3rd window clinical presentation
often mimics ME disoreders (ex., otosclerosis)
- Auditory & vestibular symptoms
- Sensitive to loud sound & pressure change
- HL with conductive component (normal tests of ME function)
If no genetic deafness or congenital malformation may be otologic mysteries
Hypothesis: inner ear lesions induce inner ear CHL, caused by defects in otic capsules of inner ear
Superior semicircular canal dehiscence (SSCD)
- Temporal bone around SSCC is unusually absence
- SSCD forms a 3rd window into inner ear
Superior semicircular canal dehiscence (SSCD) prevelance
20-50% bilateral
Etiology: idiopathic
0.5-0.6%
Superior semicircular canal dehiscence (SSCD) mean age
45 years old
