A 49-year-old woman came in with a complaint of hearing loss in her left ear. She had a stapedectomy operation on her left ear a year ago, but her hearing did not improve significantly as she had expected. She said there was clearly an improvement in her hearing after surgery but it was not to the level of the other side. She denied having vertigo, pulsatile tinnitus, or any family history of ear problems or ear surgery. Her Weber goes to the left, with equal bone and air conduction. Her audiogram is on the right. What is your diagnosis?
Diagnosis: Superior Canal Dehiscence
Conductive hearing loss in the presence of a normal tympanic membrane can be caused by a multitude of etiologies. The most common is stapes fixation which is usually caused by otosclerosis. Other causes of conductive hearing loss in patients with a normal ear exam include malleus fixation, incus fixation, erosion of the incus, anomalies of the ossicles, and third windows into the inner ear such as superior or posterior canal dehiscence. Malleus fixation can be present since birth, or may develop later in life. Congenital malleus fixation generally occurs between the attic wall and the malleus head. Acquired malleus fixation commonly results from the calcification of the anterior malleal ligament. Malleus fixation can occur as a result of tympanosclerosis, a form of middle ear scarring and calcification that immobilizes the ossicles. Tympanosclerosis can occur in the mastoid and involve the incus. It is usually seen on the tympanic membrane, in which case it is called myringosclerosis. Though uncommon, myringosclerosis can cause conductive hearing loss if the entire tympanic membrane is involved or if it fixes the malleus anterior ligament. Small islands of myringosclerosis on the tympanic membrane usually have very minimal effects on a patient's hearing.
Incus fixation can be congenital or caused by a fixation of the short process of the incus due to a bony growth or tympanosclerosis in the mastoid. Among patients with normal tympanic membranes, erosion of the incus usually occurs in the long process likely due to its relatively poor blood supply. Also, patients with eustachian tube dysfunction may have tympanic membranes that retracted in the incus long process, compromising the blood supply and resulting in erosion.
Superior or posterior canal dehiscence can cause an inner ear conductive hearing loss by wasting some of the energy delivered to the inner ear through the third window of the canal dehiscence. Normally, all the energy that enters the vestibule via the oval window is delivered to the cochlea. However, in the presence of a dehiscence, some of the sound energy will travel into the vestibule but not reach the cochlea. Patients with this condition experience conductive hearing loss often because of the lost sound energy that goes into the vestibule instead of the cochlea. The most common spontaneously dehiscent canal is the superior canal followed by the posterior canal. In cholesteatoma, the horizontal canal is the most commonly dehiscent canal.
Patients with conductive hearing loss and normal tympanic membranes may exhibit some signs that suggest a stapes fixation. They may have a greater conductive hearing loss in the low frequencies and a lesser conductive loss in the higher frequencies. Conductive hearing loss at 2,000 Hz is usually much less due to the Carhart notch, a drop in the bone conduction threshold at 2,000 Hz. A conductive hearing loss where the air-bone gap is the same across all frequencies is usually not caused by stapes fixation. The absence of acoustic reflex also indicates stapes fixation.
Persistent conductive hearing loss after stapedectomy surgery can be due to a multitude of causes. One possibility is the presence of malleus fixation that was not recognized during the operation. The incidence of malleus fixation is generally low and found in less than five percent of patients with stapes fixation. Other causes of conductive hearing loss after stapedectomy include not taking down the anterior crus of the stapes, which can be hidden behind the incus long process. Intraoperatively, the posterior crus of the stapes is taken down using an instrument or laser; however, the anterior crus remains in place and will maintain the fixation of the incus against the anterior of the foot plate.
Another possible cause is prosthesis displacement at the oval window. When the prosthesis is placed, a slightly short prosthesis may sit at the edge of the oval window's opening. In the final steps of the operation, the short prosthesis may get pulled to the edge of the oval window, thereby blocking the full transmission of sound information into the cochlea. Conductive hearing loss may later develop (after an initial improvement) due to the erosion of the incus or the development of scar tissue.
Conductive hearing loss may also be caused by an issue with the surgical technique or an oversight during the operation like missing to check the mobility of the malleus. Investigating persistent conductive hearing loss after surgery involves obtaining a CT of the temporal bones to check the position of the prosthesis and for other anatomical abnormalities.
In the case of this patient, the CT scan of her temporal bones showed that the prosthesis was in place (Fig. 2). However, a closer look of the CT showed that the patient had a significant superior canal dehiscence which was visible on multiple slices of the imaging (Figs. 3A & 3B). While surgical repair of the superior canal dehiscence could improve the conductive hearing loss, the patient declined this option. A retrospective look at the initial pre-operative CT showed that the patient had a 1 mm slice scan, which precluded the diagnosis of superior canal dehiscence on pre-operative imaging.
BONUS VIDEOS: VISUAL DIAGNOSIS
Read this month's Clinical Consultation case, then watch the accompanying videos from Hamid R. Djalilian, MD, to review the patient's imaging for yourself.
• Video 1. Axial CT (horizontal) of the temporal bone showing the prosthesis position in the axial plane on the patient's left side (right side of images).
• Video 2. Coronal CT (vertical) of the temporal bone showing the prosthesis position in the coronal plane and the dehiscent superior canal on the patient's left side (right side of images).
• Video 3. Sagittal CT (looking from outside in from left to right) of the temporal bone showing the dehiscent superior canal on the patient's left side (initial side shown on the video; right side is shown towards the end).
• Video 4. Axial CT (horizontal) of the temporal bone before surgery showing the otosclerotic plaque anterior to the oval window in axial plane on the patient's left side (right side of images).
• Video 5. Coronal CT (vertical) of the temporal bone showing the superior canal dehiscence before the stapedectomy surgery. The patient's right side (left side of images) shows the superior canal to be thin but not fully dehiscent.
• Video 6. Sagittal CT (looking from outside in from left to right) of the temporal bone demonstrating the dehiscent superior canal on the patient's left side (initial side shown on the video; right side is shown towards the end).
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