Dissecting aneurysm. (Holmes.)
Hernial aneurysm; the sac formed by the inner coat only. (Holmes.)
Sacculated aneurysm of ascending aorta. Death by pressure. (Erichsen.)
The formation of an aneurysm implies previous disease of the bloodvessel or traumatism, by either of which its coats must have been weakened or divided. The previous disease which leads to this change is either of syphilitic or other toxic origin, and usually of the type of the endarteritis already alluded to, or its continuation into atheroma. A so-called atheromatous ulcer may lead to giving way of the intima and the passage of blood between the coats of the vessel. It is in this way that most dissecting aneurysms are formed. On the other hand, violent strain may stretch the vessels already weakened by increasing blood pressure, or those conditions which induce abnormally high blood pressure may produce it by slow processes. Lastly a vessel may be partly divided, as by a bullet or stab wound, or its adjoining supports may have been weakened by disease or by accident to such an extent that it constitutes a weakening of the arterial wall. The result of this will be expansion in the direction of least resistance and the formation of a sacculated aneurysm.
As a morbid condition spontaneous aneurysm seems to be less frequent now than in the past. Certain features pertain to all cases, the most essential being a pulsating tumor, giving physical signs of its presence by pressure, which causes pain, sometimes paralysis, and nearly always absorption of surrounding tissues as the tumor expands. Pulsation is characteristic and pathognomonic of aneurysm, but an aneurysmal sac may have become so filled with clots as to minimize the prominence of this symptom. The same is true of the aneurysmal bruit or murmur which is heard on auscultation. This sound and pulsation, especially of the expansile type, when present will rarely deceive. They may, however, be simulated by a solid tumor which overlies a large vessel and transmits its pulsation or even some of its murmur. Even in this case the significant expansile character of the pulsation will be lacking.
The progress of an aneurysm may be checked by spontaneous or surgical processes, but no vessel involved in this way can return to its previous condition. As the vessel expands the tendency is to fortification of its weakened walls by coagulation of the blood around the periphery of the sac. This process may be a continuous one or may occur at intervals in such a way as to produce laminated coats of blood clot, complete or incomplete, which in certain specimens can be peeled off, one after another, much as an onion can be peeled, the innermost portion representing the most recent coagulum. In this way an aneurysm is strengthened and thickened, and rupture postponed for an indefinite period. On the other hand, as the aneurysmal tumor grows slowly but steadily it tends to make way for itself at the expense of every other tissue in the body. The hardest bone will disappear before the constant advance of such a growth, and this permits aneurysms which have had their origin in the thorax to develop into large extrathoracic tumors whose walls, lacking resistance, become thinner and finally give way, death from hemorrhage being the result. In fact, rupture is the natural tendency of such lesion, the question being whether it may be averted by spontaneous or non-operative methods, or whether it should be subjected to operation ([Fig. 146]).
Fig. 146
Thoracic (aortic) aneurysm. Death from external rupture.
Aneurysms may be minute and multiple, or single and large. The former are seen in the brain in connection with syphilis, and in the mesentery ([Fig. 147]). No artery in the body is necessarily exempt, though obviously the larger arterial trunks are the more frequent sufferers.