What could be the cause of tissue necrosis? I've had no previous sclerotherapy reaction with sotradecol

The most common cause of tissue necrosis after sclerotherapy is extravasation of the solution outside the vein being treated. This can happen with any kind of solution- sotradecol, polidocanol, saline- and rarely related to the way in which the solution was compounded, or prepared. One other possibility is using a solution that is too concentrated (too strong) for the size vein (eg using 3% sotradecol in a spider vein), or injecting the solution with too much force. The doctor was only preparing the solution in front of you by mixing it with saline (0.9%) or sterile water to dilute the solution.

Sclerotherapy with any solution can cause tissue necrosis and this is usually due to escape of the sclerosing solution into the skin and outside of the vein.  The stronger the concentration of the solution the greater the risk of a problem. Fortunately tissue necrosis and ulceration is not a common problem.