Piriformis trapped nerve dangerous?

See below: The piriformis is a muscle in your hip that helps to externally rotate your leg. (turn your leg out). The sciatic nerve runs behind this muscle and sometimes actually runs through the muscle. Occasionally this muscle can become tight or contracted which may increase pressure on the sciatic nerve. This is called piriformis syndrome. It is typically treated conservatively with physical therapy.

Piriformis sciatica: I'm not sure what you mean by "dangerous". Most cases of piriformis syndrome with intermittent sciatica are painful, but not at all what i'd consider dangerous. As you can see from the image attached, the sciatic nerve can pass behind the muscle or split the muscle (just butt pain then), or one or both parts of the nerve can pierce the muscle which can cause the shooting pain down the leg.

Piriformis Pain: The sciatic nerve runs underneath the piriformis muscle and typically does not irritate the nerve. However if the muscle is irritated or the nerve runs through the belly of the muscle (5-10% of people) it can press on the sciatic nerve and cause the sciatica type pain in the buttock region but also radiate into the leg as well. Typically stretching and nsaids are mainstay for treatment.