Sinus pericranii, petrosquamosal sinus and extracranial sigmoid sinus: Anatomical variations to consider during a retroauricular approach

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The retroauricular surgical approach is the most commonly practiced approach for exploration of the middle ear. It serves well to perform all kinds of middle ear related surgeries as well as inner ear surgeries, such as cochlear implantation. In some cases, rare anatomic variations of the temporal bone related sinuses, such as the sinus pericranii, might transform this common surgical pathway into a real life-threatening problem.

The term sinus pericranii refers to a cranial venous anomaly, which presents as a fluctuating, compressible venous scalp mass that connects directly to the intracranial dural sinuses through dilated diploic and emissary veins, which may or may not become varicous. This anomaly provides a direct, abnormal communication between the intracranial dural sinuses and the extracranial veins [1–3]. The petrosquamosal sinus is one of these rare emissary veins, and in humans it typically regresses during fetal and early post-natal life. As noted by Mortazavi
et al. in 2012, it serves as the primary cerebral drainage site in many lesser primates and quadrupeds, contrary to humans, in whom the internal jugular veins and vertebral veins represent the major outflow pathways. When present, its diameter ranges from 2 to 4 mm, and it originates at the junction of the transverse and sigmoid sinuses, coursing laterally above the superior border of the temporal bone [4]. The petrosquamosal sinus may be confused with a large mastoid emissary vein, since in many described cases, it presents as an extracranial vein varix in the vicinity of the mastoid region, which may be covered by a thin bone layer or only by subcutaneous tissue.

This vascular anomaly demands a correct preoperative diagnosis since its damage during surgery may be life
threatening, leading to troublesome bleeding or even fatal ischemic consequences, especially because it sometimes
represents the major or only drainage route of the transverse sinus, which connects the dural sinuses with the external jugular venous system [5].

Another venous anomaly of the temporal bone that surgeons should have in mind is the extratemporal course of the sigmoid sinus. When encountered, the sinus is anteriorly and laterally displaced with no cortical bone cover. This uncommon course of the sigmoid sinus may increase the risk of damage and troublesome bleeding immediately after a common retroauricular skin incision is performed [6,7].

It is important to consider that venous malformations of the temporal bone are a more common feature when associated with CHARGE syndrome. Besides inner and middle ear anomalies associated with this syndrome, patients with CHARGE may present a number of collateral emissary veins in the temporal bone including posterior condylar veins, mastoid emissary veins connecting occipital or post-auricular veins with the sigmoid sinus, and the aforementioned petrosquamosal sinus [8].

Even though most ear surgeons often have temporal bone CT images available prior to surgery, these vascular anomalies may not be easily detected. A thorough analysis of CT scans, as well as contrast-enhanced MRI images, is useful to better diagnose these anomalies and to avoid surgical complications [9]. We report three cases of temporal bone venous sinus anomalies seen in patients who underwent cochlear implant surgery.

The persistent petrosquamosal sinus, the sinus pericranii in its different anatomical presentations and the extracranial sigmoid sinus are rare anatomical variations that the surgeon must have in consideration to avoid complications, which may be life threatening, during a retroauricular approach.