Clinical Characterization of Thin Bone Overlying the Superior Semicircular Canal

Superior semicircular canal dehiscence (SSCD) is a rare disorder that causes vestibular and auditory symptoms resulting from a dehiscence in the middle cranial fossa floor overlying the superior semicircular canal (SSC). The symptoms that result from this include vertigo, autophony, hearing loss, hearing distortion, ear fullness, and tinnitus. It has been found that patients with thinning of the bone overlying the canal without true dehiscence can also experience these symptoms, but there has not been much work done yet to characterize thin bone patients’ presentation. It is therefore important to establish diagnostic criteria for classifying thin bone patients and study what symptoms these patients present with.

Currently, many institutions do not perform surgeries on symptomatic patients with thin bone and will only consider surgical intervention with a true dehiscence. Our aim is to provide more data on thin bone patients to characterize their symptoms and explore possible surgical intervention to alleviate these symptoms.

This retrospective study involves examining coronal CT scans of temporal bones from 256 SSCD patients and determining whether they had a true dehiscence or thin bone overlying the SSC. A threshold of 0.5 mm perpendicular distance was used for thin bone classification. An example of a thin bone patient (Fig. 1) compared with a dehiscence patient (Fig. 2) is shown. There were 37 patients with bilateral thin bone (bilat-thin), 33 patients with unilateral thin bone and normal bone (thin-normal), and 73 patients with unilateral thin bone and dehiscence (thin-dehisc). We then examined the thin bone patients’ consult notes pre- and postoperatively to better characterize the clinical presentation of these patients.

In bilat-thin patients, recovery of pulsatile tinnitus in postsurgical thin bone patients is affected by the thinness of the bone presurgery (p = 0.02). Additionally, the sex of bilat-thin patients affects whether they present with hearing loss presurgery (p = 0.03) and postsurgery (p = 0.001). The age of bilat-thin patients has a role in presenting with dizziness (p = 0.05) and headache (p = 0.05) presurgically. In thin-normal patients, autophony is more likely to present on the left ear presurgically (p = 0.04). Bone thinness also affects the presentation of autophony (p = 0.02) and vertigo (p = 0.07) in thin-normal patients.

In this study, bilat-thin and thin-normal patients were highlighted under the assumption that their symptoms were caused by the thin bone itself without confounding by the presence of a true dehiscence. These results show that there are a variety of different symptoms that are present in thin bone patients. Pulsatile tinnitus, autophony, and vertigo are directly affected by how thin the bone is compared with normal patients. Thus, the thinness of the bone can be used as a relative indication of symptom severity. The results also show that there are other factors such as age and sex that could affect thin bone patient presentation. This could be taken into account when assessing whether a patient would benefit from surgical correction of their thin bone. Future studies will include a comparison of symptoms and patient vestibular testing between true dehiscence and thin bone patients.