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Neuroradiology

Subarachnoid haemorrhage

acute extravasation of blood into the subarachnoid space, usually following rupture of an aneurysm of the circle of Willis (see aneurysm cerebral).

The onset is characterized by sudden headache, usually associated with acute nausea, vomiting, vertigo, photophobia and neck stiffness. Less than 15% of patients have symptoms prior to rupture and these usually consist of premonitory headaches different from the "usual" headache. Lumbar puncture should not be performed; a CT scan should be performed instead.

CT scan clearly shows the hyperdensity of the subarachnoid spaces filled with blood (Fig.1). There may be relatively specific patterns of distribution or pooling of blood according to the location of the aneurysm. A septal haematoma associated with prevalence of blood in the suprasellar cisterns and interhemispheric frontal fissure is highly suggestive of an anterior communicating artery aneurysm but may also be encountered in pericallosal aneurysms. A "sylvian haematoma" is almost pathognomonic of a middle cerebral artery aneurysm. Prevalence of blood in one crural cistern may be indicative of a posterior communicating artery on that side. Not infrequently intraventricular blood is found at time of the first diagnosis of subarachnoid haemorrhage. This may be due to direct rupture of the aneurysm within the ventricular system but this is a rare event. More commonly early ventricular blood, frequently confined to the occipital horns with fluidfluid levels, reflects some degree of CSF circulation impairment with reversed flow.

Subacute or chronic subarachnoid haemorrhage is more difficult to detect on CT scans, since blood is cleared from the subarachnoid spaces within 1 week. If subarachnoid hyperdensity is seen after 1 week from initial symptoms, this most likely reflects rebleeding. Repeated chronic subarachnoid or intraventricular haemorrhage may cause deposition in the leptomeninges (see siderosis superficial).

Immediately after CT selective catheter angiography should be performed to find the cause of haemorrhage. Less than 15% of haemorrhages are due to rupture of an arteriovenous malformation. About 15% of cases are negative for vascular anomalies at the first angiographic study. The remaining patients usually have a ruptured intracranial aneursym (see aneurysm cerebral). Arterial spasm is a dreadful complication that may cause very severe deficits due to brain ischaemia in about one third of patients. Angiography and MR angiography may show vasospasm and cerebral signs of infarction.

GS

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Fig.1

a, b, c, d. CT, axial images without gadolinium. Marked hyperdensity in the subarachnoid spaces due to extravasated blood. A more pronounced collection of blood is seen in the inferior part of the frontal interhemispheric cistern, indicating the source of haemorrhage from a ruptured anterior communicating artery aneurysm. Intraventricular blood is also seen in the third and lateral ventricles.
Subarachnoid haemorrhage, Fig.1 (a)
Subarachnoid haemorrhage, Fig.1 (b)
Subarachnoid haemorrhage, Fig.1 (c)
Subarachnoid haemorrhage, Fig.1 (d)