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 of the primary tumors are "null cell" or nonsecretory types that do not secrete significant amounts of hormones. This could be why such tumors have frequently remained undetected until a symptom like apoplexy occurs.