The majority of the underlying tumors are "null cell" or nonsecretory tumors, which do not produce excessive amounts of hormones; this might explain why the tumor has often gone undetected prior to an episode of apoplexy.
Most primary tumors are "null cell" or nonsecretory types that do not secrete large amounts of hormones. This could be the reason why such tumors have frequently gone undetected until a symptom like apoplexy arises.