Inferior petrosal sinus (IPS) anatomical variants. Type I consist of an IPS (P) anastomosing with the internal jugular vein (J). The anterior condylar vein (C) is absent or joins the IPS at a defined point. Type II has a common origin of the IPS and the anterior condylar vein from the internal jugular vein. Type III consist of an IPS with several channels communicating with the internal jugular vein. In Type IV, the IPS communicates with the anterior condylar vein and not the internal jugular. In Type III, sampling is difficult, while it is impossible in Type IV anatomy.