An aneurysm is a localized dilatation of an artery. Taking this as the definition, it will follow that in the aneurysm the coats of the artery are stretched, and, to some extent, retained as the covering of it. We shall afterwards see that there are aneurysms in which this is hardly the case, in which the coats rather give way and rupture. Such aneurysms are often designated Spurious aneurysms, of which it will be necessary afterwards to describe several varieties. Even of the aneurysms which come under the above definition there are several varieties, but we have to consider here, in the first place, those which arise by a limited dilatation of an artery. The artery may be dilated in a considerable part or the whole of its circumference, thus forming a spindle shaped or Fusiform aneurysm. The dilatation may affect such a considerable portion of a vessel as scarcely to come under the designation of an aneurysm. This is especially the case in the aorta where Dilatation of the arch (as in Fig. 250) is not infrequent. On the other hand, the dilatation may be such as to form a pouch, and we have then a Sacculated aneurysm.
A general survey of the conditions under which aneurysms occur leads to the conclusion that two chief agents have to do with their production, namely, some condition which weakens or injures the wall of the vessel, more especially its middle coat, and increase of the blood-pressure. The injury to the media is local, the increase of blood-pressure is in the general arterial system and depends on general circumstances.
Atheroma is to be mentioned as the most potent local agent in the production of aneurysm. The connection of atheroma and aneurysm is at once indicated by the fact that virtually in all cases of aneurysm of the aorta, atheroma is present, and often in a very marked form. The manner in which atheroma conduces to the causation of aneurysm has been made the subject of study by the author in association with Dr. Auld. It may be remarked that, for purposes of resistance to the force of the blood, the media is the essential part of the vessel-wall. In the aorta it forms a thick mantle composed chiefly of elastic and muscular tissues. The intima is a thin layer, and the adventitia, which is composed of connective and elastic tissue, has neither the resistance nor the clastic recoil of the media.
Fig. 250. - Dilatation of aortic arch. The vessel is shown laid open from the valve onwards.
The relations of atheroma and aneurysm are best studied in instances where there are the first beginnings in small depressions or pouches visible in scanning the internal surface. In most cases of actual aneurysm, and in some cases of atheroma without aneurysm, such small beginnings are visible. When such small depressions are examined in microscopic section the conditions visible are those shown in Fig. 251. Here, it will be noted, the depression coincides with an atheromatous patch or node. The depression is largely at the expense of the middle Boat, which tapers off towards the centre, and is entirely awanting at one point near the bottom of the depression. The actual dilatation is evidently referable to destruction of the media, and this has been occasioned by the impingement of the patch or node upon it. It has been already explained under atheroma that the patch often leads to injury and rupture of the media, and in the beginnings of aneurysms this is regularly present.
We have already seen also, under atheroma, that the injury to the media leads secondarily to inflammatory processes in the wall generally. These are partly reparatory and are continued after the formation of the aneurysm, whose wall, as will appear afterwards, is ultimately composed of connective tissue formed by a process of inflammation.
Fig. 251. - Beginnings of an aneurysm in aorta, a, Media; b, intima. The intima is greatly thickened by atheroma, and has impinged on media, causing great atrophy of the latter, x 8.
Whilst atheroma is a frequent element in the causation, other conditions which injure or rupture the middle coat are also causes. Thus embolism in the cerebral arteries, when it does not cause complete obstruction, is liable to produce aneurysm by injuring the wall. Examination of an aneurysm of this kind showed that there was virtually a gap in the middle coat, the intima and adventitia being pushed outwards. Calcification of the media, by rendering the media brittle, may also be regarded as an agent in the causation of aneurysms. A blow over a vessel may extend deeply enough to injure its coats, or the fractured end of a bone may do so. (See under Traumatic Aneurvsm, p. 500).
The influence of Increase of blood-pressure is shown by a number of considerations. In severe continued exertion we find the most frequent cause of increased blood-pressure. The engineer who has to manipulate a piece of hot iron while wielding a heavy hammer, or the football player who has a succession of fast runs to make, or the mountaineer who forces the pace too much, or the soldier who has to perform long marches with heavy accoutrements, must each put a strain on his heart and larger vessels which ordinary persons are not liable to. In this connection the Age at which aneurysms are most common is of importance. This coincides with the latter part of the most active period of life. Aneurysms are most frequent between the ages of thirty and forty. On the other hand, atheroma, although not infrequent between these ages, is much more common after forty. It will be seen that aneurysms coincide with the time of life when the period of greatest bodily vigour overlaps the beginnings of the period of occurrence of atheroma. Another circumstance is the great preponderance of aneurysm in the male sex. Atheroma shows a similar predominance, but not nearly to such an extent. As has been pointed out by M'Crorie, statistics bearing on this point are not usually reliable, unless it be remembered that males are more frequently subjected to post-mortem examinations than females. But still when the differences are very great this factor cannot be the determining one. Thus Bizot. found in 189 cases of aneurysm 171 in males and 18 in females, and Hodgson found in 63 cases 56 in males and 7 in females. The explanation of this difference is chiefly that men are more exposed to extremes of physical exertion than are women. On the same principle we explain the greater frequency of aneurysm in some countries as compared with others. The excessive stress to which workmen in our engineering and shipbuilding establishments and sailors in our ships are frequently put in this country goes far to explain the frequency of aneurysm here.