The 2 can look similar during an episode, so it can be hard to distinguish, and both can co-occur. One difference is that the person with bipolar will have symptom free periods when the mood is stable, or the symptoms can be thought to be caused by swings in mood, while the person with BPD will have symptoms at any times, and the mood swings are in response to other symptoms. Stress can trigger an episode in bipolar, but once a person is in an episode, external events will not tend to alter the mood. External events will affect the shifting moods in BPD.

Here's a description of the 2 disorders:

Criteria for BPD:

Identity disturbance/unstable sense of self
impulsicivity
suicidal behavio/self harm
mood instability in reaction to events, often fluctuating within the same day
chronic feelings of emptiness
inappropriate anger
transient psychotic-like symptoms such as paranoia, depersonalization, and dissociation

A theory of the cause is a reaction to a traumatic or invalidating environment, so it can be thought of as a kind of post traumatic stress reaction. The treatment is typically psychotherapy, often DBT, with or without medication. People with BPD view the world in black and white, and they hold themselves and others to this standard. So a person can be all good, but then they do something causing dissapointment or hurt, and they swing to being all bad. This is called idealization and devaluation. BPD is a very painful condition.

I'm going to describe bipolar in the next window, because I tend to loose what I"m typing, and I don't want to loose all of this and have to start again - so see the next window.