One possible but uncommon side effect of ovarian cysts is ovarian torsion (also called adnexal torsion or tubo-ovarian torsion). This occurs when the ovary twists on its axis, twisting its venous and lymphatic vessels and thus shuts down its own blood supply.
It is the cause of severe abdominal pain as well as nausea and vomiting, similar to appendicitis. Diagnosis is difficult because of this, but it is also crucial that a correct diagnosis is made quickly, to avoid loss of the ovary and thus loss of fertility. Blood tests, urinalysis, and imaging are necessary for a timely confirmation of the diagnosis.
Emergency surgery is called for to correct the twisting (detorsion) to save the ovary.