The influence of mechanical forces and flow mechanisms on vessel occlusion.

The exact mechanism of irradiation-induced occlusion of normal or abnormal retinal vessels is still to a large extent obscure. Blood flow acts as a powerful heat dissipation mechanism. Therefore, a procedure which reduces flow velocity as rapidly as possible after the beginning of irradiation may work with higher efficiency. A theory based on a simplified assumption states that vessel narrowing techniques, which involve only a short length of the vessel to be occluded by a small focus diameter irradiation beam, are less efficient in bringing blood velocity down than a procedure which narrows the vessel over a greater length by a larger focus diameter irradiation beam. The thrombus building up over a greater length of the affected part of the vessel may also better resist the pressure head building up after thrombogenesis is complete because friction between thrombus and vessel wall is larger, and occlusion may, therefore, be more durable. One has to assess, therefore, whether a small-spot technique, which is presumed to be less efficient and may need repetitions, is in fact less innocuous for the surrounding retina--as is generally assumed--than a technique using a larger beam diameter, which, because it is more efficient, may need fewer or even no repetitions.