tell me the total dose of desitin dosage that patient 13841 had been prescribed since 160 months ago?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 13841 ) and prescriptions.drug = 'desitin' and datetime(prescriptions.startdate) >= datetime(current_time,'-160 month')