"limbic ADD is where ADD and depression intersect." THey have the classic symptoms of ADD (distraction, short attention spans, disorganization, impulsiveness) Dr. Amen describes sufferers of limbic ADD as "inattentive, experience chronic low-grade depression, are negative thinkers (glass half empty), have low energy, and have frequent feelings of hopelessness and worthlessness."
They do not have hyperactivity and they do not respond to stimulent medications. They respond better to antidepressants espeacially Welbutrin.
I too did not respond to stimulents and the only antidepressant I have responded to has been welbutrin.
Under a SPECT scan both Depressed and ADD Limbic show too much activity in the limbic center but when a depressed person tries to concentrate activity in the prefrontal cortex increases but someone with ADD L activity actually decreases.
My problems are also similar to Sluggish cognitive tempo or ADHD primarily inattentive and I think Limbic ADD may be similar in some ways to those as well.
As with SCT, I often feel as though I am drifting, introspective and daydreamy, and feel as if "in the fog"
When I read or do any activity that requires brain power my brain seems to fall asleep and I get this feeling of being half brain dead and out of it, like I just woke up.

Here is another description of Limbic ADD
Symptoms: primary ADD symptoms plus chronic mild sadness, negativity, low energy, low self-esteem, irritability, social isolation, and poor appetite and sleep patterns. Stimulants, by themselves, usually cause problems with rebound or cause depressive symptoms.
SPECT: usually high deep limbic activity plus low prefrontal cortex at rest and with concentration
Supplements: multiple vitamin, NeurOmega fish oil, SAMe or DL-phenylalanine
Medications: Stimulating antidepressants, such as Wellbutrin.

If you have ADD problems similar to Limbic ADD and stimulants have not worked for you let me know what has?