<qa>  What is the main reason for TRD?Medication resistance to antidepressant therapy, or Treatment-Resistant Depression TRD, is the main reason, if not main indication, for which patients are presently referred for electroconvulsive therapy ECT in hopes of a rapid, definitive response APA 2001 ECT Task Force Report. While the causes of TRD remain unclear, this category of patients includes those who have incomplete remission, those who relapse rapidly and frequently, those who have psychotic depression, those who develop chronic depression, and those who often have comorbid medical and psychiatric illness Sackeim 2001, Sharma 2001, Fava and Davidson 1996, Thase 1995. Moreover, while increased age per se is not a barrier to antidepressant response, many elderly, especially those with comorbid medical or neurological illness, are unable to tolerate courses of aggressive pharmacotherapy or seem to have a less robust response to pharmacotherapy, and possibly to ECT alone, with significant residual functional and cognitive impairment Reynolds 1998, Stoudemire 1998, Lebowitz 1997, Gurland 1991. 