Basilar Tip Aneurysms
The basilar artery forms at the confluence of the two vertebral arteries at the base of the medulla oblongata. The basilar artery then runs up the front of the brainstem giving off several branches along the way (see schematic drawing below). At the top of the brainstem (ie: midbrain), it divides into the two posterior cerebral arteries, specifically the "P1" segments. Basilar tip aneurysms form at this division point.
Basilar tip aneurysms are not common. They compromise about 5% of all aneurysms within the confines of the skull. However, they are the most common aneurysm of the vertebrobasilar system.
Patients at risk for developing cerebral aneurysms include those with atherosclerosis, those with a family history of intracranial aneurysms, those with a history of hypertension or collagen vascular disease, and those with polycystic kidney disease. Smokers are also at a higher risk of developing aneurysms.
Basilar tip aneurysms form when the lining of the vessel wall is thinned. Typically the muscular layer of the wall - the tunica media - is weakened as a result of the aforementioned reasons.
This thinning allows turbulent blood flow to form outpouchings in the vessel wall. Typically these outpouchings occur at branch points within arterial trees (ie: the branch point of the basilar artery into the posterior cerebral arteries).
The most common symptoms of a basilar tip aneurysm occur after it ruptures. The resulting subarachnoid hemorrhage can cause a variety of signs and symptoms. The most common being a severe headache, although cranial nerve dysfunction, stroke, coma, and death can also occur.
Less commonly, basilar tip aneurysms enlarge to a point where they put pressure on adjacent nerves. Pressure on the oculomotor nerve can cause the eyeball to deviate downwards and outwards; this is a result of paralysis in several muscles of the eye sub served by this nerve. An enlarging basilar tip aneurysm can also push on the optic chiasm causing a bitemporal hemianopsia (ie: loss of peripheral vision in both eyes).
Basilar tip aneurysms are usually diagnosed after a subarachnoid hemorrhage or workup for cranial nerve dysfunction. The best methods for diagnosing basilar tip aneurysms are with CT angiograms and formal cerebral angiograms. Non-contrasted head CT scans can illustrate blood in the subarachnoid space if the aneurysm has ruptured.
Like other intracranial aneurysms, basilar tip aneurysms may be clipped or coiled. Clipping of an aneurysm involves an open surgical procedure where the surgeon dissects down to the aneurysm and places a clip across its neck. This effectively excludes it from the circulation and prevents it from rupturing.
Aneurysms may also be treated from inside the blood vessel. In this procedure a catheter is threaded from the femoral artery in the groin up into the basilar artery. At this point the aneurysm is located and small metallic coils are placed within the dome of the aneurysm.
Regardless of how the aneurysm is treated - either with clipping or coiling - the end result is that the aneurysm is excluded from the normal circulation. This prevents it from rupturing.
The merits of clipping versus coiling are still under debate. Ultimately, the treatment depends on the size and location of the aneurysm, as well as other medical problems that the patient may have.
Basilar artery aneurysms are uncommon, but can be devastating if they rupture. They are diagnosed using CT angiograms or formal cerebral angiography. Treatment is with clipping and/or coiling.
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