Thus, the close proximity of the popliteal artery and vein, together with the particularly firm adhesion which exists between the vessels, probably favours the formation of a varix; again, a varix more readily forms if the femoral artery and vein are wounded in Hunter's canal than if the injury is situated high in Scarpa's triangle, where the vessels lie in a large areolar space. The passage of a bullet between an artery and vein may perhaps produce either condition, but wide separation of the two vessels, as for instance of the subclavian artery and vein, renders an aneurismal sac almost a certainty. These suggestions seem borne out by the cases recounted below, since the pure varices are one femoral, one popliteal, and one axillary. I cannot include the calf and forearm cases, as the existence of a small sac could not be disproved.
To these anatomical factors certain others must be added. In most cases a false sac exists at first, which tends to undergo contraction and spontaneous cure, as is observed in some of the ordinary traumatic sacs. This history of development is moreover supported by the observation that proximal ligature of the artery usually converts an arterio-venous aneurism into an aneurismal varix. The process is no doubt favoured by cleanness and small size of the perforation, moderation in the amount of primary hæmorrhage, the tone and resistance of the surrounding tissues, special points in the circulatory force and condition of the blood, and the possibility of maintaining the part at rest after the injury.
Aneurismal varix, when pure, was evidenced by the presence of purring thrill and machinery murmur alone. In none of the cases I saw was pain or swelling of the limb present. In one popliteal varix, slight varicosity of the superficial veins of the leg was present, but it was not certain that the development of this was not antecedent to the injury, as the patient did not notice it until his attention was drawn to its existence. In none of the cases under observation in South Africa had enough time elapsed for sufficient dilatation of the artery above the point of communication to give rise to any confusion from this cause as to the presence of a sac.
When an arterio-venous sac has once formed, clinical observation shows that the general tendency is towards extension in the direction of least resistance. This direction of course varies with the situation of the aneurism, and also with the nature of the wound track.
Speaking generally the direction of least resistance in a typically pure perforation is towards the vein. Initial flow of blood from the wounded artery is naturally favoured towards the potential space afforded by a canal occupied by blood flowing at a lower degree of pressure. The partial collapse of the vein dependent on the wound in its wall also probably helps in determining the initial flow in its direction. Examples are afforded by the carotid aneurisms (cases 10, 11, and 14), and here it must be borne in mind that the outer limits of the cervical vascular cleft are those least likely to offer resistance to extension of the sac. In each the aneurisms mainly occupied the exit segment of the track; this is the general rule, as in the case of external hæmorrhage, and is determined by the same cause.
The latter rule however finds exceptions when the entry segment is so situated as to cross a region of lesser resistance, and case 12 illustrates this point with regard to the cervical vascular cleft. Examples of the tendency to spread in the anatomical direction of least resistance are also offered by the cases of aneurism at the root of the neck, where extension was into the posterior triangle.
The further clinical history and signs are as follows. A local swelling is found, usually at first diffuse, often commencing to develop with cessation of the external hæmorrhage. It increases, for the first few days maintaining its diffuse character. If near the surface, it may be superficially ecchymosed. At the end of this time a tendency to localisation, as evidenced by increasing firmness and more definite margination, takes place, and this is followed by general contraction and rounding off of the tumour. The latter process may be continuous, and eventually the sac may become small and stationary or ultimately disappear and a pure varix be the result. The latter is only likely to be the case under the most satisfactory of the conditions enumerated above. Occasionally an opposite course may be followed, and fresh extension take place, as evidenced by enlargement of the tumour, disappearance of sharp definition, softening, and pain. The natural termination of such cases in the absence of interference would no doubt be rupture, and possibly death in some positions, loss of the limb in others. The former I never saw.
Purring thrill.—This, the pathognomonic sign of either condition, was always present in the fully developed stage, and is probably present from the first unless a temporary thrombosis obstructs the vascular openings. It was noted as early as the third day in case 13. In many of the other patients it was palpable only with the subsidence of the primary swelling attendant on the injury. In some of the forearm and calf aneurisms, and in some of the popliteal, it was only discovered by accident some weeks even after the injury, but this often because no serious vascular lesion had been suspected. The thrill was widely conducted, often apparently superficial on palpation, and much more pronounced with light than with forcible digital pressure.
In case 10 the visible vibration in consonance with the thrill when the vein was exposed during the operation of ligature of the carotid was a novel experience to me.
Murmur.—The typical 'bee in the bag,' or 'machinery' murmur was present in every case, and was often very widely distributed, especially over the thorax. (Cases 13, 14, and 20.)