Last month’s post was an introduction to Third Mobile Window Syndromes (TMWS) in general. This month’s post will focus on the diagnostic techniques that may be helpful in the diagnosis of a TMWS.
Diagnosis is often reached through a combination of measures including vestibular and hearing tests, imaging with CT scan or MRI, as well as correlation with consistent symptoms. This approach is taken because some asymptomatic individuals may have diagnostic evidence of a TMWS but are asymptomatic.
Adaptations to existing test protocols in many clinical settings must be made in order to identify a TMWS.
The most widely performed vestibular function tests, the VNG, would not identify the most common TMWS’s without modifications. A hearing tests that does not include both air and bone conduction testing would not identify most TMWS’s.
Additionally, CT scans that are poor resolution or an inappropriate slice thickness have the potential to miss many TMWS’s. There must be some degree of clinical awareness on the behalf of the provider or the patient in order to know what symptoms to inquire about as well as what measures can assist in diagnosis.
Typical Symptoms of a Third Mobile Window Syndrome
The most common symptoms associated with a TMWS can include but are not limited to:
- Dizziness provoked by loud sounds (Tullio phenomenon)
- Dizziness provoked by physical strain or exertion (Hennebert sign)
- One’s own voice sounding unnatural or strange in one or both ears (autophony)
- Pulsatile tinnitus
- Conductive hearing loss
- Hearing internally generated sounds abnormally loud such as eye movements, chewing, footsteps, or belching
- Fullness or pressure in the affected ear(s)
- Chronic imbalance
- Mental fogginess
Semicircular Canal Dehiscence
The condition of superior semicircular canal dehiscence syndrome (SCDS) is the most common and well studied and as such, the assessment protocols to reach a diagnosis for this condition are the best understood.
The assessment protocols are largely the same for posterior semicircular canal dehiscence (PCDS) and lateral semicircular canal dehiscence (LCDS), with the exception of assessing for a concomitant perilymph fistula in the case of lateral canal dehiscence as these are often associated with cholesteatoma or surgical alterations. Perilymph fistula will be discussed in greater detail later.
A hearing test or audiogram should consist of both air conduction and bone conduction testing. A study by Dr. Lloyd Minor in 2005 reported the following air/bone gap findings by frequency in patients with known SCDS: 250 Hz (70%), 500 Hz (64%), 1000 Hz (64%), and 2000 Hz (21%). Because air/bone gaps can be caused by a number of conditions, this finding should be cross checked with tympanometry and acoustic reflex testing. These cross check measures can help determine whether the air/bone gap on an audiogram is related a middle ear abnormality.
Tuning fork tests can also be helpful as a cross check measure to determine whether the hearing loss is conductive or sensorineural. With SCDS and PCDS, one would expect a normal tympanogram and present acoustic reflexes, as these are not related to middle ear abnormality. In the case of LCDS, there may also be a true middle-ear-related conductive hearing loss due to the frequent association with cholesteatoma.
The VNG can be supplemented with additional measures to include stimulating the ear with brief loud sounds, Valsalva, vibration, and insufflation while monitoring for nystagmus. The same study by Dr. Minor showed eye movements evoked by loud sounds in 82%, while 75% had eye movements with Valsalva, and 45% had eye movement with insufflation. Additionally, skull vibration can elicit nystagmus or eye movement in the majority of individuals with SCDS. Skull vibration is a quick and easy way to assess ones peripheral vestibular function status and requires minimal additional equipment.
Vestibular evoked myogenic potentials (VEMP) have been shown to be very useful for identifying SCDS with roughly 80-100% sensitivity and specificity dependent on the VEMP (oVEMP or cVEMP) as well as the protocol that is utilized. Typically the VEMP thresholds are reduced, the responses are asymmetric in the case of unilateral canal dehiscence, and can show altered frequency tuning. Similar findings have been reported in the instance of PCDS. VEMP testing with air conduction could be adversely impacted in cases of LCDS due to an existing middle ear related hearing loss secondary to cholesteatoma or surgical changes. VEMP testing can be performed using standard evoked potentials equipment that many otolaryngology clinics likely already have, the settings in the software just need to be adjusted to allow for this.
Additional vestibular function tests such as rotary chair, video head impulse test, and caloric irrigations are normal in most instances. These additional measures can be useful to rule out other inner ear conditions.
CT scans that have incorrect slice thickness can over estimate SCDS. Experts feel that a slice thickness of 0.625 mm or less is ideal when investigating for SCDS.
- There is currently a scarcity of data detailing the assessment protocols for some of the less common TMWS’s and as such, we currently rely largely on case reports or small-scale studies. With single case studies or small-scale studies, it is hard to draw conclusions on the typical pattern of hearing loss that might be expected. Also, many of these case studies did not implement any sort of vestibular testing and as such it is hard to draw conclusions on the utility of vestibular testing for some of these TMWS’s. More studies are needed to better understand the best assessment protocols for patients with these conditions. A common theme with these less common TMWS’s is that they are most often detected with cranial imaging MRI or CT, most cases have hearing loss, but the utility of vestibular function tests for diagnosis is less clear at this time.
Cochlear Facial Dehiscence
A case report of two adults with cochlear facial dehiscence showed that they both had mixed conductive and sensorineural hearing loss.
Another study showed that 78% of participants had milder degrees of conductive hearing loss, while 22% had more significant degrees of conductive hearing loss. A high resolution CT scan seems to be the most useful measure to identify a cochlear facial dehiscence.
Cochlear Carotid and Cochlear Internal Auditory Canal Dehiscence
A single participant case study showed a moderate sensorineural hearing loss in one ear and a severe sensorineural hearing loss in the other ear in a patient with cochlear carotid dehiscence. Other single participant case studies have shown a mild, bilateral, high frequency sensorineural hearing loss and a conductive hearing loss in those with cochlear carotid dehiscence.
I could not find any studies investigating isolated cochlear internal auditory canal dehiscence with hearing and vestibular function tests. A case study of a patient that had both cochlear carotid dehiscence and cochlear internal auditory canal dehiscence was found to have severe to profound sensorineural hearing loss bilaterally. This same patient was previously diagnosed with bilateral Meniere’s disease which can cause sensorineural hearing loss, making the etiology of the hearing loss less clear.
High resolution CT scans of the temporal bone seem to be the most useful measures to detect cochlear carotid and cochlear internal auditory canal dehiscence.
Enlarged Vestibular Aqueduct (EVA)
It is generally agreed upon that those with EVA have hearing loss that can be exacerbated with head trauma. This hearing loss type can be sensorineural, conductive or mixed. An audiogram including air and bone conduction, as well as cross check measures such as tympanometry and acoustic reflexes should be completed.
The dizziness symptoms and vestibular function test findings are sparser with those that have EVA. One study showed that 45% of participants with known EVA reported symptoms of dizziness, imbalance, or nausea. This same study showed that 32% had abnormal caloric responses, 41% had abnormal rotational chair findings and only 22% had abnormal cVEMP measures. Interestingly, one study showed that 92% of participants had reduced cVEMP thresholds and yet another study showed that 100% of participants had cVEMP abnormalities. Clearly these studies conflict with one another on the VEMP abnormalities and more data is required in order to better understand the role of vestibular testing in assessing EVA.
Cranial CT or MRI are both often used for diagnosing EVA. One study showed 100% sensitivity and 99% specificity for detecting EVA with cranial MRI.
Perilymph Fistula
A fistula is often caused by barotrauma, physical trauma, or following surgical procedures to an ear. Traditionally, this diagnosis was made by direct intra-operative observation of perilymph leaking from the inner ear. This was likely a difficult and highly subjective task.
Diagnosis of the condition is still somewhat controversial. Some studies have shown that perilymph fistula can often be identified with CT or MRI by observing air within the cochlea or fluid in the oval or round windows, while other experts feel that CT and MRI have limited clinical utility in identifying a fistula.
Recent advances have allowed for possible testing for biomarkers suggestive of perilymph leakage which would minimize the subjective component to making this diagnosis.
Clinical assessment normally includes a fistula test which involves observation for nystagmus with the introduction of air pressure into the canal with pneumatic otoscopy or with tympanometry. The sensitivity of the fistula test can often times be poor and is somewhat subjective.
VEMP measures are often normal in those with fistula but can be helpful in differentiating fistula from a canal dehiscence. Additional vestibular function tests are also likely to be normal, which can assist by ruling out other inner ear conditions. Most individuals with perilymph fistula have some degree of hearing loss in the affected ear.
Summary
A combination of consistent clinical symptoms, hearing and vestibular test findings, as well as imaging of the temporal bones are useful to identify most TMWS’s. Some of these TMWS’s have only recently been discovered and as such, the ideal assessment protocols are not fully understood at this time.
Many providers are still unaware of these conditions and due to this lack of awareness, many people with a TMWS go undiagnosed or are misdiagnosed. Increased awareness of these conditions is essential so that appropriate assessments can be made to identify these disorders.
More research is required to better understand the best assessment protocols for most TMWS’s.
