Monday 28 November 2011

Imaging in Sub Arachnoid H'rhage

SAH, An extravasation of blood into the subarachnoid space between pia and arachnoid.

Divided as Spontaneous and Post traumatic. 
Spontaneous SAH include Aneurysmal and Non-aneurysmal causes SAH. 
- Aneurysmal SAH 
The most common cause of Spontaneous SAH.
Rupture of a saccular (berry) aneurysm (80%)
Rupture of an arteriovenous malformation (AVM) (10%).
Rarely in the setting of mycotic aneurysms and congenital disorders, including coarctation of the aorta, Marfan's syndrome, Ehlers-Danlos syndrome, fibromuscular dysplasia and polycystic kidney disease.
- Non Aneurysmal SAH: include CVT, amyloid angiopathy, blood dyscrasias, fibromuscular dysplasia, Moyamoya disease, and vasculitis (10%).

Rupture is related to the tension on the aneurysm wall. Tension on the wall is proportional to the diameter. Thus, the rate of rupture is directly related to the size of the aneurysm.

Symptoms and signs:
A sudden onset of severe headache ("thunderclap headache"), often described as the “worst headache of my life" .... most common.
Nuchal pain and rigidity
A sudden loss of consciousness (occurs in half of patients at bleeding onset; it is usually transient)

The most common and specific locations of intracranial aneurysms are at the middle cerebral artery bifurcation and along the anterior communicating artery. These 2 locations account for approximately 70% of all intracranial aneurysms.
Other common sites are at the origins of the posterior communicating and ophthalmic arteries.
Approximately 10-20% of aneurysms arise from the vertebral and basilar arteries. The tip of the basilar artery is the most common location of aneurysm formation in the posterior circulation.

Preferred examinations:
Non contrast CT
MRI with MRA (Non contrast 3 D TOF)

Non contrast CT
A preferred initial diagnostic study.
On CT, subarachnoid haemorrhage appears as a high-attenuating, amorphous substance that fills the normally dark, CSF-filled subarachnoid spaces around the brain, appear white in acute haemorrhage, most evident in the largest subarachnoid spaces, such as the supra sellar cistern and Sylvian fissures.
Prediction of site of bleed on CT, is possible. If bleed is marked in anterior inter hemispheric fissure, an Acom aneurysm is possible and if marked only in one side sylvian fissure corresponding MCA aneurysm is possible.

Magnetic Resonance Imaging
Fluid Attenuated Inversion Recovery (FLAIR) is the most sensitive sequence.
On FLAIR images, SAH appears as high signal-intensity (white) in normally low signal-intensity (black) CSF spaces.
MRA for evaluating aneurysms and other vascular lesions that cause SAH. The  sensitivity is low for aneurysms smaller than 5 mm.

DSA (Digital Substraction Angiography)
Considered the standard imaging technique for the detection of intracranial aneurysms.
Detected as focal areas of out pouching or dilatation of the arterial wall, frequently occur at arterial branching points in characteristic locations within or near the circle of Willis.
Gives valuable information about aneurysm location, shape, neck size, and neck-to-maximal diameter ratio  which are crucial in determining whether the aneurysm is better treated with open craniotomy or with an endovascular technique.

Sentinel bleed
Sentinel or "warning" leaks that produce minor blood leakage, reported to occur in 30-50% of cases, precedes the aneurysm rupture and major episode of SAH by a few hours to a few months, so should not be overlooked. MRI FLAIR is the best sequence. Lumbar puncture is confirmatory.

Complications of Subarachnoid Haemorrhage
Hydrocephalus, Communicating and Non-Communicating Hydrocephalus.

Communicating Hydrocephalus
SAH with dilatation of the anterior and temporal horns, as well as the third and fourth ventricles. Flow is most likely blocked at the arachnoid granulations resulting in communicating hydrocephalus. 
Requires emergency ventricular shunting.

Non-Communicating Hydrocephalus
Non-communicating (obstructive) hydrocephalus occurs when the ventricular system is not in continuity with the subarachnoid space due to blood clots. Most often, the site of the blockage in non-communicating hydrocephalus is at the cerebral aqueduct, but rarely can occur at the foramen of Monro, the third ventricle, or the outlet of the fourth ventricle.
Important to recognize as it is potentially treatable by shunting.

Vasospasm following Subarachnoid Hemorrhage. 
A known delayed complication of subarachnoid hemorrhage.
Reported to occur in as many as 70% of patients with SAH.
Most commonly occurs 4-14 days after the onset of bleeding.
If severe enough, may lead to progressive ischemia and stroke.
Imp to recognize as typically treated with the calcium channel blocker nimodipine, volume expansion, mild elevation of blood pressure.

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