When do you recommend lumbar puncture in patients with concern for memory loss?  

Rapidly progressive memory loss/dementia- because you obviously want to exclude diagnoses like CJD, infectious/autoimmune/paraneoplastic encephalitis, lymphoma or other malignancy, etc.  When you need to know the patient's "brain amyloid status". E.g. a) Patients with amnestic MCI and atypic...