In the early embryo at the 5 to 8 mm stage, the brain is covered by a continuous plexus which drains both laterally and ventrally through three stems, the anterior, the middle, and the posterior. By the 11 to 14 mm embryonic stage, each subdivision of the brain is drained laterally into the dural plexuses by primitive pia-arachnoidal vessels. During the 17 to 20 mm stage of development, middle and posterior plexuses have joined to form the sigmoid sinus while the two parts of the anterior dural plexus; the tentorial sinus, and the marginal sinus border the expanding hemisphere caudally. At the 22 to 24 mm embryonic stage, the supraorbital vein no longer drains into the stem of the anterior dural sinus, which has diminished and is continuous with the prootic sinus. The prootic sinus is a stem of the middle dural plexus which incorporates into the stem of the posterior dural plexus and empties into the sigmoid sinus. Growth of the brain has elongated the tentorial sinus, and the marginal sinus now constitutes the medial end of the primitive transverse sinus.

By the 40 mm stage, the cavernous sinuses are developing, and are medial extensions of the prootic sinus which is now continuous with the newly developed inferior petrosal sinus. A new lateral tributary of the prootic sinus now anastomoses with a primitive temporal emissary vein derived from the sigmoid sinus to form the petrosquamous sinus. At the 60 to 80 mm stage, cerebral growth and the development of the otic capsule has led to the formation of the superior petrosal sinus. The primitive transverse sinus has moved backward on the sigmoid sinus and has anastomosed with the tentorial sinus. The prootic sinus is now continuous with the petrosquamous sinus. In the adult, remnants of the tentorial sinus may persist and anastomose with the cavernous sinus which is now joined with the superior petrosal sinus. It would be at this stage that one would expect to find the sinus of Kelch being formed (Fig. 3). The petrosquamous sinus and the remnants of the prootic sinus become diploic veins and drain the dura and bone when the middle meningeal sinus drain through the foramen ovale and into the sagittal sinus [3].

In primates, there are large evolutionary differences in the orbitotemporal venous sinuses. In primitive primates, these sinuses are large and drain the brain. The cranio-orbital sinus empties into the postglenoid emissary vein which also receives drainage from the petrosquamous sinus. In humans, this network of orbitotemporal sinuses is modified and persists as part of the middle meningeal vein system which no longer drains the brain, the orbit, or the temporal fossa, and the petrosquamous sinus is attenuated or non-existent [4]. Due to these evolutionary differences, the presence of a venous sinus of Kelch may indicate the remnant of the cranio-orbital sinus in humans [5].

Reports of a venous sinus of Kelch are rare in the literature. Knott [6] briefly described such a sinus as an accessory sinus which passes backward from the superior orbital fissure over the upper border of the petrous part of the temporal bone to terminate into the transverse sinus. Hollinshead [7] stated similarities between the sinus of Hyrtl and the sinus of Kelch and indicated that they may actually describe the same structure. More recently, unilaterally present sinus of Hyrtl have been described in the literature and bilateral occurrences appear to have not been previously reported [8-10]. We previously reported a unilateral variant of the sinus of Kelch found in an 88-year-old at death female cadaver [11]. In this specimen, the sinus of Kelch, which was approximately 3 mm in diameter posteriorly and 6 mm in diameter anteriorly, drained posteriorly into the superior petrosal sinus rather than into the transverse sinus. On the ipsilateral side of this same specimen, an enlarged arachnoid granulation was identified protruding through the foramen rotundum. However, in the case described herein, the only intracranial anatomical variations identified were the two venous sinuses of Kelch.

In conclusion, knowledge of the variant anatomy of the intradural venous sinuses is important to anatomists and clinicians alike. Clinically, such knowledge can minimize misdiagnoses such as when anatomical variations are noted on imaging. Surgically, such an understanding can avoid intraoperative complications such as iatrogenic hemorrhage.