Styloid and hyoid bone proximity is a risk factor for cervical carotid artery dissection.

BACKGROUND AND PURPOSE Carotid artery dissection (CAD) is more common with increased styloid process length. Our goal was to determine whether proximity of the styloid process and the hyoid bone to the internal carotid artery (ICA) was a risk factor for CAD. METHODS We studied axial slices on computed tomography angiograms of 88 patients with nonaneurysmal CAD, from 88 age- and sex-matched controls without dissection, and from 32 nonage-/sex-matched nonaneurysmal vertebral artery dissection control patients. We measured the nearest distance between the ICA and both the styloid and the hyoid bones, blinded to clinical information and radiological reports. RESULTS Styloid-ICA and hyoid-ICA distances were significantly shorter on the side of the CAD as compared with nondissection control patients (P<0.0001 for the styloid-ICA distance; and P=0.0037 for the hyoid-ICA distance). Styloid-ICA distances, regardless of the side of the dissection, were shorter in CAD patients compared with the nondissection control group (right side, P=0.001; left side, P=0.0002) and the vertebral artery dissection control group (right side, P=0.0031; left side, P=0.0067). Direct mechanical contact of the styloid with the ICA was more common in CAD patients. CONCLUSIONS Shorter distances between the styloid and ICA (and possibly also the hyoid and the ICA) are important risk factors for CAD. Further study is needed to determine whether dissections result from direct injury to the outer vessel wall of the carotid artery.