We read the article by Marsot-Dupuch et al (1), “The Petrosquamosal Sinus: CT and MR Findings of a Rare Emissary Vein,” with great interest. We would like to take this opportunity to emphasize the two following points.
First, our evaluation of 13 anatomic corrosion casts of the cerebral venous system corroborates the existence of a petrosquamosal venous channel, commonly referred to as the petrosquamosal sinus (PSS). This structure was present in five of the 26 corrosion cast sides we studied (Figs 1 and 2). In three instances, the PSS had the typical appearance of a diploic channel (ie, rounded irregular contours and a tortuous course), which seems to correlate with the CT scans presented in the article by Marsot-Dupuch et al. However, we found no association with venous anomalies such as described by these authors. Considering that a petrosquamosal sinus was observed in five of 26 specimen sides (19%) and that not all the corrosion casts indicated complete filling of the cerebral and meningeal venous system, we think that the petrosquamosal sinus can not be considered a rare anatomic entity. Although no investigational procedure is perfect, endovascular moldings obtained with the corrosion cast technique remain a powerful anatomic tool for the study of vascular structures, particularly when they are difficult to access, as is the case for the venous system at the skull base.
Second, in their discussion, Marsot-Dupuch et al. mention that emissary veins connected to the sigmoid sinuses (ie, the posterior condylar vein, the mastoid emissary vein, and the petrosquamosal sinus) play a minor role in healthy persons, acting as a safety valve. This conclusion is based on the assumption that the only normal drainage pathway for encephalic blood flowing through the sigmoid sinuses occurs via the internal jugular veins. This assumption is correct for a person who is lying supine. However, in the upright position, encephalic drainage occurs mainly through the internal and external vertebral venous plexuses (2, 3). This drainage pattern is rendered possible by connections linking the vertebral venous plexuses to the transverse and sigmoid sinuses and to the bulb of the internal jugular vein. These connections occur via the anterior, lateral, and posterior condylar veins and via the mastoid and occipital emissary veins (4). Emissary veins, therefore, play a major role in encephalic and neurocranial venous drainage. We agree, however, that the petrosquamosal sinus most likely plays a minor role in encephalic drainage because it is not connected to the vertebral venous plexus.
Left lateral corrosion cast of the cerebral venous system shows the PSS (arrows), the transverse sinus (1), the sigmoid sinus (2), the cavernous sinus (3), the pterygoid plexus (4), and the mastoid emissary vein (asterisk).
Right lateral corrosion cast of the cerebral venous system (different specimen). Note the presence of the superior petrosal sinus (arrowheads) and the medial connection of the petrosquamosal sinus to an emissary vein of the middle cranial fossa (double arrow). 1 indicates the transverse sinus; 2, sigmoid sinus; 3, cavernous sinus.
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