The MRI image in Figure 2 shows a very small lesion on the left side anteriorly and laterally in the internal auditory canal. Lesions that are this small in the internal auditory canal may or may not necessarily represent an acoustic neuroma. Sometimes a small blood vessel with contrast inside may appear like an enhanced mass in the lateral internal auditory canal. Additionally, a hemangioma in the bone or a meningioma may create the same appearance on an MRI. Considering the very small size of the lesion, we elected to observe this with serial MRI at six months. And since the lesion appeared to involve the bone, we also obtained a CT scan of the temporal bones to evaluate the internal auditory canal bone. The follow-up MRI showed no change in the lesion in question; however, the CT scan of the temporal bones was far more interesting. Figures 3 and 4 show the CT of the temporal bone on the left side where the lesion appears to be a cochlear otosclerosis plaque that involves the anterior wall of the internal auditory canal. Evaluation of the CT on the right side shows the classic halo sign of cochlear otosclerosis around the cochlea (Fig. 5).
All bones in the body continually undergo a process of remodeling, which involves the very slow removal and re-creation of bones. The speed of this bone reabsorption and formation is slow and always in balance. However, the bone remodeling process in the otic capsule is different from that of the other bones in the body. This process is very slow and minimal. However, in the case of otosclerosis, the production of bone is faster than reabsorption, which leads to extra bone formation. In the early phase, the new bone formed is vascular with less calcium, allowing the bone to be more visible on CT scans. These appear less dense (less white and more gray) compared with the surrounding otic capsule bone, which is the densest bone in the body (will appear very white on CT). On MRI, the bone, due to its vascularity, would appear hyperintense (white), instead of the typically dark appearance of the temporal bone on T1 post-contrast MRI.
CLINICAL AND COCHLEAR OTOSCLEROSIS
Clinical otosclerosis, which primarily affects the otic capsule bone around the oval window, manifests as conductive hearing loss, whereas cochlear otosclerosis manifests as sensorineural or mixed hearing loss. While clinical otosclerosis has been found to have an autosomal dominant pattern, the genetics of cochlear otosclerosis have not been elucidated. Most patients with cochlear otosclerosis typically have some level of clinical otosclerosis, but they do not always experience conductive hearing loss. Cochlear otosclerosis that involves the cochlear endosteum and causes SNHL without stapes fixation is uncommon. It is thought to be the cause of progressive sensorineural hearing loss with no conductive loss in about one percent of patients with progressive hearing loss.
The measles virus has been theorized to be the potential cause of otosclerosis in some patients. Additionally, antibodies to the measles virus have been found in the perilymph of patients with otosclerosis. While progression of otosclerosis in pregnancy has been seen clinically, it has not been observed universally. This patient's increased hearing loss during pregnancy is likely due to otosclerosis progression.
The clinical course of the disease depends on the progression of the plaques and the level of otic capsule involvement. Some patients may develop vestibular symptoms that can be recurrent, positional, or spontaneous. These symptoms may be due to the biochemical changes in the perilymph caused by the otosclerotic involvement of the vestibular organs. This patient's otosclerotic plaque involved the anterior internal auditory canal (IAC) where the facial nerve and cochlear nerves travel. The vestibular nerves traverse the posterior half of the IAC, and have been undisturbed in this patient. Although this patient has significant, otosclerosis on the right side, there is no endosteal involvement, and therefore no hearing loss or vestibular symptoms. The presence of an active focus of otosclerosis on the promontory causes a reddish appearance on otoscopy called the Schwartz's sign.
IMPAIRED BONE MINERALIZATION
CT imaging shows a hypodense (gray) double ring around the cochlea due to the impaired mineralization of the bones during the active phase of cochlear otosclerosis. This phase results in bones with less calcium that appear gray on CT scans, which only show active otosclerosic plaques, as those areas have impaired mineralization. Inactive otosclerosis is not as clear on CT and may be hyperintense (bright white). MRI may show a ring of intermediate signal in the areas around the cochlea or vestibular organs on T1-weighted images. Some enhancement after gadolinium administration may be seen due to the hypervascular nature of the bone. T2-weighted images may show increased signals due to the higher water content of active otosclerosis plaques, unlike the dense cochlear bone that has low water content.
Most clinicians use sodium fluoride to reduce the activity of the otosclerotic bone. The benefit of fluoride was noted when it was observed that otosclerosis was commonly seen in areas in northern Europe where the population primarily drank non-fluorinated water. It was found that city dwellers in the same country had slower otosclerosis progression compared with those who lived outside the city and regularly drank from non-fluorinated well water. It has been theorized that fluoride reduces the activity of enzymes that contribute to the destruction of the native otic capsule bone and its replacement with active vascular otosclerosis bone. The primary side effects of flouride include the development of joint pain, osteoporosis, and gastrointestinal issues. In our practice, the patients are screened with bone densitometry before the initiation of fluoride treatment and a year later.
Osteoporosis is treated with bisphosphonates, a class of drugs believed to help reduce the breakdown of bone seen in otosclerosis. However, several studies have reported varied results using this treatment. A prospective clinical trial is ongoing to determine if bisphosphonates can slow the progression of cochlear otosclerosis. Given the potentially severe side effects of bisphosphonates, such as osteonecrosis of the mandible (loss of the jaw bone from dental work or infection), we have refrained from prescribing these drugs to our patients until we see the results of the said clinical trial.
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 T1-weighted post-gadolinium MRI showing the hyperintense area in the anterior IAC.
- Video 2. Axial CT of the left temporal bone showing the areas of otosclerosis around the IAC and cochlea.
- Video 3. Axial CT of the right temporal bone demonstrating the cochlear otosclerosis involving the basal turn and apical turns.
- Video 4. Coronal (vertical) CT of the left temporal bone showing the various areas of otosclerosis involvement in the coronal plane.
- Video 5. Sagittal (outside in) CT of the left temporal bone showing otosclerotic plaques near the facial nerve.
- Video 6. Coronal (vertical) CT of the right temporal bone demonstrating involvement of the areas around the superior semicircular canal.
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