Two or three meningeal sinuses may join to form a vestibule just before reaching the superior sagittal sinus. There is a tendency for the veins draining the lateral surface of the anterior frontal and posterior parietal regions to join the meningeal sinus in the dura mater lateral to the superior sagittal sinus. The veins from the posterior frontal and parietal region most commonly dip beneath the venous lacunae and pass directly to the superior sagittal sinus. The veins from the medial surface of the hemisphere enter the inferior border of the sinus or turn laterally onto the superior margin of the hemisphere to join the veins from the lateral surface before entering the sinus.
The segment of the superior sagittal sinus in the frontal region above the genu of the corpus callosum receives fewer bridging veins than any other area except the 4 to 6 cm proximal to the torcular herophili, where bridging veins infrequently enter the superior sagittal sinus.
Inferior Sagittal Sinus
- The inferior sagittal sinus (Fig 2) courses in the inferior edge of the falx(16).
- It originates above the anterior portion of the corpus callosum and enlarges as it courses posteriorly to join the straight sinus.
- It arises from the union of veins from the adjacent part of the falx, corpus callosum, and cingulated gyrus. The junction of the veins from the cingulate gyrus and corpus callosum with the sinus often forms an acute hook-like bend, with the apex directed forward.
- The largest tributaries of the inferior sagittal sinus are the anterior pericallosal veins.
- The superior sagittal sinus may communicate through a venous channel in the falx with the inferior sagittal sinus. This connection may infrequently be so large that the superior sagittal sinus drains predominantly into the inferior sagittal and straight sinuses.
- Straight Sinus
- The straight sinus (Fig 5) originates behind the selenium of the corpus callosum at the union of the inferior sagittal sinus and the great vein.(16)
- It continues posteriorly and downward in the junction of the tentorium and falx.
- It may drain into either transverse sinus, but most commonly drains predominantly into the left transverse sinus.
Transverse and Sigmoid Sinuses
- The right and left transverse sinuses originate at the torcular herophili and course laterally from the internal occipital protuberance in a shallow groove between the attachments of the tentorium to the inner surface of the occipital bone. (16)
- The transverse sinus exits the tentorial attachments to become the sigmoid sinus at the site just behind the petrous ridge, where the transverse and superior petrosal sinuses meet.
- Although the superior sagittal sinus may drain equally to the left and right transverse sinus or predominantly or wholly to either side, it is the right transverse sinus that is usually larger and receives the majority of the drainage from the superior sagittal sinus.
- The left transverse sinus is usually smaller and receives predominantly the drainage of the straight sinus. Thus, the right transverse sinus, right sigmoid sinus, and right jugular vein contain blood from the superficial parts of the brain, and the left transverse sinus, left sigmoid sinus, and left internal jugular vein contain blood mainly from the deep parts of the brain drained by the internal cerebral, basal, and great veins.
- The difference in symptoms caused by blockage of the venous drainage on one side or the other and the differences in Queckenstedt’s sign with compression of the jugular veins on either the left or right side have been explained by the differences in drainage on each side.The cortical veins from the lateral surface of the temporal lobe may drain into the transverse sinus, but before entering it, they commonly pass medially below the hemisphere to join a short sinus in the tentorium, which courses within the tento-rium for approximately 1 cm before draining into the terminal part of the transverse sinus.
The cortical veins from the basal surface of the temporal and occipital lobes usually join the lateral tentorial sinus. The vein of Labbé commonly ends in the transverse sinus, but may curve around the inferior margin of the hemisphere to join the lateral tentorial sinus. The transverse sinus may communicate through emissary veins in the occipital bone with the extracranial veins.
The occipital sinus (fig 7) is the smallest of the cranial sinuses. It is situated in the attached margin of the falx cerebelli, and is generally single, but occasionally There are two.
It commences around the margin of the foramen magnum by several small venous channels, one of which joins the terminal part of the transverse sinus; it communicates with the posterior internal vertebral venous plexuses and ends in the confluence of the sinuses. (21)
Each half of the tentorium has two constant but rarely symmetrical venous channels
- The medial and
- Lateral tentorial sinuses.(16)
- The medial tentorial sinuses are formed by the convergence of veins from the superior surface of the cerebellum, The medial tentorial sinuses course medially to empty into the straight sinus or the junction of the straight and Transverse sinuses.
- The lateral tentorial sinuses are formed by the convergence of veins from the basal and lateral surfaces of the temporal and occipital lobes. The lateral tentorial sinuses arise within the lateral part of the tentorium and Course laterally to drain into the terminal portion of the transverse sinus.
The paired cavernous sinuses are situated on each side of the sella turcica and are connected across the midline by the anterior and posterior intercavernous sinuses, which course in the junction of the diaphragma sellae with the dura lining the sella. (16)
- Anteriorly, each cavernous sinus communicates with the sphenoparietal sinus and the ophthalmic veins.
- Its middle portion communicates through a lateral extension on the inner surface of the greater sphenoid wing with the pterygoid plexus via small veins that pass through the foramina spinosum and ovale.
- Posteriorly, the cavernous sinus opens directly into the basilar sinus, which sits on the clivus. It communicates through the superior petrosal sinus with the junction of the transverse and sigmoid sinuses and through the inferior petrosal sinus with the sigmoid sinus.
Superior Petrosal Sinus
- The superior petrosal sinus (fig 12) courses within the attachment of the tentorium to the petrous ridge (16).
- Its medial end connects with the posterior end of the cavernous sinus. ü its lateral end joins the junction of the transverse and sigmoid sinuses.
- The bridging veins that join it usually arise from the cerebellum and brainstem, not the cerebrum.
- The sinus may course over, under, or around the posterior root of the trigeminal nerve.
- The superficial sylvian veins may empty into an infrequent tributary of the superior petrosal sinus called the sphenopetrosal sinus.
Inferior petrosal sinus
- Inferior petrosal sinuses (fig 12) are small sinuses situated on the inferior border of the petrous part of the temporal bone on each side (16).
- Each inferior petrosal sinus drains the cavernous sinus into the internal jugular vein.
- The inferior petrosal sinus is situated in the inferior petrosal sulcus, formed by the junction of the petrous part of the temporal bone with the basilar part of the occipital bone.
- It begins in the postero-inferior part of the cavernous sinus and, passing through the anterior part of the jugular foramen, ends in the superior bulb of the internal jugular vein.
- The inferior petrosal sinus receives the internal auditory veins and also veins from the medulla oblongata, pons, and under surface of the cerebellum.
Sphenoparietal, Sphenobasal, and Sphenopetrosal Sinuses
The sphenoparietal sinus is the largest of the meningeal channels coursing with the meningeal arteries. Fig (9)
It accompanies the anterior branch of the middle meningeal artery above the level of the pterion. Below this level, it deviates from the artery and courses in the dura mater just below the sphenoid ridge to empty into the anterior part of the Cavernous sinus. Its upper end communicates through the meningeal veins with the superior sagittal sinus. The sinus coursing along the sphenoid ridge may turn inferiorly to reach the floor of the middle cranial fossa rather than emptying into the anterior part of the cavernous sinus. From here, it courses posteriorly to empty into a lateral extension of the cavernous sinus on the greater sphenoid wing or joins the sphenoidal emissary veins, which pass through the floor of the middle fossa to reach the pterygoid plexus. It also may pass further posteriorly to join the superior petrosal or lateral sinuses.
- The variant in which the sinus exits the cranium by joining the sphenoidal emissary veins and the pterygoid plexus is referred to as the sphenobasal sinus,
- The variant in which the sinus courses further posteriorly along the floor of the middle fossa and drains into the superior petrosal or lateral sinus is called the sphenopetrosal sinus.
The superficial sylvian veins commonly empty into the sphenoparietal sinus. If the sphenoparietal sinus is absent or poorly developed, the sylvian veins may drain directly into the cavernous sinus or they may turn inferiorly around the anterior pole and inferior surface of the temporal lobe to empty into the sphenobasal or sphenopetrosal sinuses.