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LESSON 71

Ossicular discontinuity and trauma

Year 1
Spring semester
Otorhinolaryngology
Outer and middle ear disorders
Duration: 9:59

In this lesson, we look at ossicular chain discontinuity and middle ear trauma: incudostapedial joint separation, a large air-bone gap, the type Ad tympanogram and acoustic reflex findings. We also discuss barotrauma, hemotympanum, temporal bone fractures and follow-up after trauma.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson, we learn about conditions in which the ossicular chain loses its connection and about injuries to the middle ear.
We will read audiogram, tympanogram and acoustic reflex findings together and also look at barotrauma and hemotympanum.
The ossicular chain is made up of the malleus, or hammer, the incus, or anvil, and the stapes, or stirrup, linked by two joints.
The incudomalleolar joint lies between the malleus and incus, and the incudostapedial joint between the incus and stapes.
Ossicular discontinuity is a loss of connection at any point along this chain.
The most common form is separation of the incudostapedial joint.
The incus hangs between two joints and is the least supported by ligaments, so it is the ossicle most easily dislocated in trauma.
In chronic inflammation, the thin long process of the incus can erode and break the chain.
The causes of ossicular discontinuity can be grouped into trauma and disease.
In head injury, especially with temporal bone fractures, the incus can be dislocated.
An object entering the canal, such as a cotton swab or a hairpin, can perforate the eardrum and damage the chain directly.
Chronic otitis media and cholesteatoma can break the chain by eroding bone.
A nearby blast or a sudden pressure wave can also affect the chain along with the eardrum.
Less often, congenital anomalies or a displaced prosthesis after ear surgery also cause discontinuity.
In this example, right air conduction thresholds are between fifty five and sixty five decibels HL, with a fairly flat curve.
Masked bone conduction is within normal limits, so the loss is conductive.
The air-bone gap is about fifty decibels, which is a large gap among conductive losses.
A complete break in the chain behind an intact eardrum causes one of the largest conductive losses; the gap can reach sixty decibels.
This is because the intact eardrum cannot pass sound to the ossicles and also largely blocks sound from reaching the windows directly.
If thin fibrous tissue remains between the broken ends, the gap may be smaller and sometimes more marked at high frequencies.
When the chain breaks, the tympanic membrane is freed from the load of the cochlea and moves much more easily than normal.
So the tympanogram may show normal peak pressure with a clearly increased peak height; this is a type Ad curve.
However, if the eardrum is thickened or scarred, the curve can look normal despite the discontinuity.
Conversely, a thin or flaccid eardrum alone can produce a type Ad curve when hearing is normal.
So a type Ad curve is always read with the audiogram: if there is a large air-bone gap, think of ossicular discontinuity.
With the probe in the affected ear, the acoustic reflex usually cannot be measured.
The break is usually lateral to the attachment of the stapedius tendon, for example at the incudostapedial joint.
The stapedius muscle may contract, but the movement does not reach the tympanic membrane, so the probe measures no change.
If the break is medial to this point, for example in the stapes crura, the reflex can sometimes be measured.
When the stimulus goes to the affected ear, the conductive loss weakens it, so the reflex may be elevated or absent.
The picture that suggests ossicular discontinuity comes from a combination of findings, not from a single test.
The audiogram shows a large and often flat air-bone gap.
The tympanogram is type Ad, and with the probe in the affected ear the acoustic reflex is absent.
On otoscopy, the eardrum is often intact; right after trauma, blood may be seen behind it.
Computed tomography and surgical evaluation for a definite diagnosis belong to the physician.
Barotrauma occurs when middle ear pressure cannot keep up with a rapid change in outside pressure.
It is most common when diving and when a plane descends, if the Eustachian tube cannot open to equalize the rising outside pressure.
Relative negative pressure forms; the eardrum retracts, the mucosa swells, fluid or blood can collect, and the eardrum may tear.
The person feels pain and fullness in the ear; the audiogram usually shows a conductive loss.
A cold, allergies or known Eustachian tube dysfunction increase the risk.
Barotrauma can also affect the inner ear; with sudden sensorineural loss, tinnitus or vertigo, the person is referred urgently.
Hemotympanum is a collection of blood in the middle ear space.
On otoscopy, a dark red, purple or bluish color is seen behind an intact eardrum.
Its most common causes are temporal bone fracture and barotrauma.
Like fluid, blood in the middle ear reduces eardrum movement and causes a conductive loss; the tympanogram may be flat.
The blood usually clears within a few weeks and hearing recovers; a loss that does not recover suggests chain damage.
Temporal bone fractures are classically classified by the direction of the fracture line relative to the long axis of the bone.
Longitudinal fractures are more common and usually affect the middle ear, eardrum and ossicular chain.
With these fractures, bleeding from the ear canal, hemotympanum and conductive loss are common.
Transverse fractures are rarer; they can cut across the inner ear and cause sensorineural loss and vertigo.
Facial palsy is more common with transverse fractures.
Today, a classification based on whether the fracture involves the bony shell of the inner ear, the otic capsule, is also common.
After middle ear trauma, the first evaluation is done by a physician.
On the first audiogram, bone conduction is especially important; it shows whether the inner ear is affected.
Hemotympanum and fluid usually clear within a few weeks, and the conductive loss may lessen on its own during this time.
After the eardrum heals and the blood clears, usually a few weeks after the trauma, a follow-up audiogram is done.
If a clear air-bone gap remains at this check, chain damage is suspected and the physician plans further workup.
Surgical rebuilding of the chain is called ossiculoplasty.
The surgeon can reshape the patient's own incus and place it in the gap, or use a partial or total ossicular prosthesis.
Which method is chosen depends on which parts of the ossicles remain intact.
For people who do not want or cannot have surgery, a hearing aid is a good option because bone conduction is normal.
At check-ups after surgery, closure of the air-bone gap and preservation of bone conduction are evaluated.
A tip from practice: when you see a type Ad tympanogram, do not write down ossicular discontinuity right away.
In a person with normal hearing and a thin or scarred eardrum, a type Ad curve is a common and harmless finding.
If the same curve comes with a large air-bone gap and an absent acoustic reflex, ossicular discontinuity becomes a strong possibility.
After trauma, wait until the blood clears and the eardrum heals, and then repeat the audiogram.
The most common form of discontinuity is incudostapedial joint separation; the incus is the most easily dislocated ossicle.
A complete break behind an intact eardrum causes a large, flat conductive loss.
The tympanogram is usually type Ad, and with the probe in the affected ear the acoustic reflex is absent.
Hemotympanum clears within a few weeks; a gap that remains after the blood clears suggests chain damage.
Test yourself: What do a large air-bone gap, a type Ad tympanogram and an absent acoustic reflex suggest together?
The correct answer is B: ossicular chain discontinuity.
The reflex is also absent in otosclerosis, but the stiff system usually gives a type A or As curve; a type Ad curve suggests discontinuity.
In the next lesson, we will look at Eustachian tube dysfunction and the effects of negative middle ear pressure.
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Prerequisites

LESSON 70 Otosclerosis : Stapes Fixation and the Carhart Notch

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