An aneurysm is an abnormal
focal dilatation of an artery, the main complications being rupture and
predisposition to thrombosis.
Although it is the most frequent cause, atherosclerosis is not the only disease
that produces aneurysms; any abnormality that weakens the media may produce an
aneurysm, although permanent or transient hypertension is also an important
factor in their enlargement and rupture
'Dissecting aneurysm' is not a
true aneurysm
In dissecting aneurysm, a tear
in the intima leads to tracking of blood into the arterial media, which splits.
The most common site for this is the aorta, where the split forms a false
channel, usually between the inner two-thirds and outer third of the medial
thickness.
Among the predisposing factors
are hypertension, present in 70% of cases, and degenerative changes in the
aortic media (medial mucoid degeneration), which sometimes arise as part of a
recognized hereditary disorder of support tissues, e.g. Marfan's syndrome due to
defective fibrillin, and Ehlers-Danlos syndrome.
Atherosclerosis is also a predisposing factor, the original intimal tear
occasionally occurring at the edge of an atheromatous plaque.
This pattern is particularly important in the distal aorta.
Dissection due to
instrumentation of an artery wall is a rare complication of arterial puncture or
cannulation.
It often heals spontaneously without rupture, since the medial wall is usually
healthy, limiting the extent of blood tracking.
External rupture, usually
lower down the thoracic aorta, with massive fatal haemorrhage into the thoracic
cavity.
Retrograde spread (back towards the heart) with rupture into the pericardial
cavity, with fatal haemopericardium.
Internal rupture, with blood tracking back into the lumen by rupturing through
the inner media and intima, to produce a double-barelled aorta. This is rare.
