A dissecting aneurysm in the bulbus arteriosus of a brook stickleback, Culaea inconstans

In the bulbus arteriosus of an adult, male brook stickleback, C. inconstans, one of 100 caught in July 1982 from Lake 200, near Erickson, Manitoba, Canada, there was a dissecting aneurysm similar to those in the human aorta. The vessel also showed the classic signs of Erdheim's "idiopathic cystic medionecrosis", but its lumen was dilated and the lesion could thus also be regarded as a fusiform aneurysm. In the proximal half of the bulbus the endothelial lining was torn transversely and blood had extended into the middle layer of the wall to create 2 concentric and longitudinal channels which re-entered the lumen distally at approximately the mid-point of the bulbus. Except near the bulbar-aortic junction, the amount of elastica and (probably) smooth muscle was drastically reduced, the wall was infiltrated with amorphous deposits of mucoid substances, there were focal, cyst-like areas, and the normally tall and intensely PAS-positive, endothelial cells were flattened and poorly staining. There was no evidence of cystic medionecrosis in the bulbus arteriosus of other fish without dissecting aneurysms. This is thought to be the first report of a dissecting aneurysm in any lower vertebrate. Evidently, such an aneurysm can occur in a vessel without vasa vasorum, and in a very small animal (48 mm long), where the wall tension in its blood vessels must be extremely low compared with that in the human aorta.