Superior Sagittal Sinus
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Direct answer
One midline venous channel drains most of the blood from the brain's outer surface: the superior sagittal sinus runs in the attached upper border of the falx cerebri, from the foramen caecum of the frontal bone to the internal occipital protuberance, and continues as the right transverse sinus in most people. It receives the superior cerebral veins, diploic veins and parietal emissary veins, and its walls bear arachnoid granulations that return cerebrospinal fluid to the blood. Because it is the principal site of CSF absorption, obstruction of this sinus produces raised intracranial pressure without any mass lesion.
What you must remember
- Triangular on cross-section, the sinus grooves the inner surface of the frontal, two parietal and occipital bones along the sagittal suture.
- Arachnoid granulations project into its walls and into lateral venous lacunae (frontal, parietal and occipital); this is where bulk CSF resorption happens.
- Superior cerebral veins are bridging veins — they cross the subdural space before entering the sinus, and tearing them is the classic cause of a subdural haematoma.
- Parietal emissary veins connect it to scalp veins, so infection from the "danger area" of the scalp can seed the sinus.
- At the foramen caecum in children, an emissary vein may link it to nasal veins, a route for spread of infection from the nose.
- It usually becomes the right transverse sinus; the straight sinus usually drains to the left; their meeting point at the internal occipital protuberance is the confluence of sinuses (torcular Herophili).
- Classical settings for thrombosis: dehydrated or marasmic infants, head injury, puerperium and thrombophilias.
How the bedside story of sagittal sinus thrombosis unfolds
Consider an eight-month-old brought in with gastroenteritis and two days of vomiting, now convulsing, with a tense bulging anterior fontanelle and a right hemiparesis. The anatomy explains every step. Dehydration sludges flow in a dependent-free, low-pressure venous channel; a thrombus forms, occludes the sinus and blocks the arachnoid granulations, so CSF can no longer be absorbed. Intracranial pressure climbs, producing seizures and a bulging fontanelle, while back-pressure into the superior cerebral veins infarcts the parasagittal motor cortex, explaining the hemiparesis.
Work through the confirmation in order. A contrast-enhanced CT shows the empty delta sign — the clot sits as a dark triangle inside the enhancing sinus wall. MR or CT venography demonstrates the filling defect in the sagittal sinus itself and any extension into the right transverse sinus. Management then follows current practice: correct the dehydration aggressively, control seizures, and start anticoagulation, which remains standard care even when a venous infarct is already visible, because it prevents clot propagation. Asked in a viva why the child has no focal mass on imaging, the answer is pure anatomy: the lesion is an obstructed vein, not a tumour, so pressure rises diffusely.
Where students slip
The commonest error is stating that the superior sagittal sinus "becomes the confluence of sinuses" as a fixed rule. In reality the drainage is asymmetric in most people — right transverse sinus dominant as the continuation of the superior sagittal, left dominant from the straight sinus — and examiners love asking which sinus continues which channel. The second slip is attributing subdural haemorrhage to middle meningeal artery injury; that artery is extradural, whereas the veins entering this sinus are the ones that traverse the subdural space. Third, students forget that the granulations protrude both into the sinus and into its lateral lacunae, so a question on "where exactly is CSF absorbed" deserves both answers, with the scalloped pits they leave on the inner parietal bone as the tell-tale proof.
Frequently asked questions
Which landmark marks the beginning of the superior sagittal sinus?
The foramen caecum, just in front of the crista galli on the frontal bone; in the young, an emissary vein passing through it may connect the sinus to nasal veins.
Which veins, when torn, produce a subdural haematoma?
The superior cerebral veins, because they bridge the subdural space to reach the sinus; they tear when the head decelerates and the brain shifts within the skull.
What is the empty delta sign?
On contrast-enhanced CT, the thrombosed sinus fails to enhance centrally and appears as a dark triangle surrounded by the enhanced dura — the imaging hallmark of sinus thrombosis.
Why is a dehydrated infant the textbook candidate for this thrombosis?
Sluggish, concentrated blood in a low-pressure sinus during hyperosmolar dehydration favours clot formation, and occlusion rapidly raises intracranial pressure in a child whose sutures may still be open.
How does CSF return to the bloodstream through this sinus?
Arachnoid granulations, herniating from the subarachnoid space through the dura, project into the sinus and its lateral lacunae, returning CSF directly into the venous blood.
Which sinus usually continues from the superior sagittal sinus?
The right transverse sinus in most individuals, with the straight sinus usually draining into the left, meeting at the torcular Herophili.