The Thickness of the facial Nerve Sheath Consistently Varies by Region in Its Intra-Tympanic Course on Cadaveric Study

Abstract

Objectives: Iatrogenic removal of intra-temporal disease processes, such as cholesteatoma and keratosis obturans, can be challenging when the facial nerve (FN) is involved. Despite this concern about possible FN injury during these procedures, our clinical observation has been that the diseased growth can be cleaned quite easily from the vertical FN epineurium. Therefore, we designed a cadaveric protocol to measure thickness of the FN sheath (epineurium) in horizontal, second genu and vertical FN segments and to correlate these measurements with surgical management of FN disorders. Methods: Fifty non-fixated (wet) cadaveric temporal bones were dissected over 1 year’s time. The intra-temporal FN sheath epineurium was harvested from the mid-horizontal, second genu, and mid-vertical segments. Using a digital micrometric technique, the thickness of each sample was measured. Data analysis was performed using student’s two-tailed, dependent t-test. Results: Epineurial nerve sheath thickness was the least in the horizontal segment (mean 0.9 mm, range 0.040-0.140 mm), greater at the second genu (mean 0.19 mm, range 0.010-0.280 mm), and greatest in the vertical segment (mean 0.29 mm, range 0.170-0.570 mm). These differences were statistically significant. Conclusion: In cases of cholesteatoma and keratosis obturans involving the vertical FN, the disease process can be separated from the FN sheath because of the sheath thickness in this region. Disease in the horizontal segment involves a thinner sheath and separating the disease process from the nerve is more difficult in this area.