Anatomical Reasons for an Impaired Internal Jugular Flow.

The internal jugular vein (IJV) is of utmost importance during various surgical and endovascular approaches, including central access. It descends through the parapharyngeal space, carotid triangle, and sternocleidomastoid region. The anatomical variables of the IJV are mainly related to its calibre and dominance, number of venous channels (i.e., duplications and fenestrations), and compression sites. Specific compressions of the IJV are not exclusively due to the jugular nutcracker between the styloid process (SP) of the temporal bone and the C1 transverse process, which, in turn, should not be granted the eponym of Eagle. The possible morphologies of the SP and ossified stylohyoid chain are discussed here. Additionally, the digastric and sternocleidomastoid muscles, the hyoid, and the distorted carotid arteries may compress the IJV, thereby raising intracranial pressure. Here, a case is documented with a long inferior petrosal sinus adjacent to the IJV, both compressed into the C1-styloid nutcracker, which is an absolute novelty. Multiple compression sites of the IJV are supported here with original evidence. All anatomical variables of the IJV are relevant, as they may lead to stenoses or interfere with IJV cannulation. In rare cases of IJV agenesis, multiple compression sites on the opposite side may significantly alter bilateral cerebral drainage. Different methods may be used to decompress a stenotic IJV, including styloidectomy. In conclusion, the anatomical variables of the IJV should be acknowledged by practitioners and documented on a case-by-case basis.