Atypical cotton-wool spots.

Atypical cotton-wool spots in four selected cases of diabetic retinopathy are discussed. The most striking characteristic distinguishing them from typical cotton-wool spots is their size, which ranges from about 2 to 4 disc diameters. They develop after stenosis or a complete obstruction of a first order arteriole at the point at which it emerges from the parent arterial branch. The size of the lesion corresponds with the size of the affected arteriole. Restoration of local circulation is attempted by neighbouring arterioles and venules, the latter carrying a reversed blood flow. Arterio-venous communications along the border of the infarcted area are a prominent feature, most likely attributable to the necessity of free drainage of the arterial influx. The same haemodynamic principle applies to the coarsening of the adjacent capillary bed. Secondary changes of the bordering venous branches include aneurysm formation and the staining of the venous wall.