Abstract
In twenty-four or more instances a circumscribed dilatation of the subclavian artery has been observed in cases of cervical rib. The dilatation in these cases is distal to the site of pressure made by the rib.
As to the cause of these aneurisms there has been considerable conjecture, usually prefaced by the comment that their occurrence would be comprehensible if they presented on the proximal instead of on the distal side of the compression.
Weakening of the wall of the artery from erosion or trauma, variable or intermittent pulse pressure, and vasomotor disturbances in nutrition are the suggestions which have been offered to explain the phenomenon.
For several years my experiments in arterial compression have had more or less in view the determination of the cause of this dilatation. For the past year they have been continued by Dr. Mont Reid and myself almost exclusively with the object of shedding light on this problem. In 1906 we (Dr. Richardson, Dr. Dawson and myself) made the observation 1 that after partial occlusion of the thoracic aorta the maximum pressure may be permanently lowered as much as 46 mm. Hg, and the minimum pressure actually increased distal to the constricting band of metal.
The dilatation of the artery observed in arterio-venous fistula, might, it seemed to me, have a bearing on the interpretation of the aneurisms in cases of cervical rib. Might not both phenomena, I asked myself, be due to degenerative changes in the arterial wall consequent upon lowered pressure-in the case of the cervicalribaneurisms, upon lowering of the pulse pressure.
Now, inasmuch as dilatation of the subclavian artery has relatively so seldom been observed with cervical rib (perhaps 24 times in about 400 cases) it seemed to me that if it were due merely to the lowered pulse pressure then only a very definite absolute or relative amount of reduction of the systolic pressure would suffice to produce it.
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