Optical Coherence Tomography Imaging and Angiography of Skull Base Tumors Presenting as a Middle Ear Mass in Clinic

<b>Background</b>: Skull base tumors can extend into the temporal bone and occasionally even be visible through the tympanic membrane (TM) if they grow into the middle ear cavity. The differential diagnosis of a skull base mass is extensive and ranges from non-tumorous lesions like cholesteatoma to benign tumors like schwannoma and to malignant lesions like metastatic cancer. Optical coherence tomography (OCT) is a noninvasive imaging technique that can image tissue with high resolution in three dimensions, including through structures such as the TM and bone. OCT angiography is also able to assess tissue vascularity. We hypothesized that OCT could help shrink the differential diagnosis in clinic on the day of initial presentation. Specifically, we thought that OCT angiography could help distinguish between highly vascular skull base tumors such as glomus jugulare and other less vascular tumors and middle ear pathologies such as cholesteatoma and schwannoma. <b>Objectives</b>: We sought to determine whether OCT can image through the TM in clinic to distinguish a normal ear from an ear with a mass behind the tympanic membrane. Furthermore, we sought to assess whether OCT angiography can detect vascularity in these masses to help inform the diagnosis. <b>Methods</b>: We designed and built a custom handheld OCT system that can be used like an otoscope in clinic. It is based off a 200 kHz swept-source laser with a center wavelength of 1310 nm and a bandwidth of 39 nm. It provides a 33.4 μm axial and 38 μm lateral resolution. Cross-sectional images of the middle ear space, including OCT angiography, were captured in an academic neurotology clinic. Patients with normal ear exams, glomus tumors, cholesteatomas, and facial nerve schwannoma were imaged. <b>Results</b>: OCT images revealed key structures within the middle ear space, including the TM, ossicles (malleus and incudostapedial joint), chorda tympani, and cochlear promontory. OCT also identified middle ear pathology (using pixel intensity ratio in the middle ear normalized to the TM) when compared with patients with normal ear exams (mean 0.082, <i>n</i> = 6), in all patients with a glomus tumor (mean 0.620, <i>n</i> = 6, <i>p</i> < 0.001), cholesteatoma (mean 0.153, <i>n</i> = 4, <i>p</i> < 0.01), and facial nerve schwannoma (0.573, <i>n</i> = 1). OCT angiography revealed significant vascularity within glomus tumors (mean 1.881, <i>n</i> = 3), but minimal vascularity was found in normal ears (mean 0.615, <i>n</i> = 3, <i>p</i> < 0.05) and ears with cholesteatoma (mean 0.709, <i>n</i> = 3, <i>p</i> < 0.01), as expected. <b>Conclusions</b>: OCT is able to image through the TM and detect middle ear masses. OCT angiography correctly assesses the vascularity within these masses. Thus, OCT permits the clinician to have additional point-of-care data that can help make the correct diagnosis.