Q:  I was just wondering how do you tell the differance between PTSD and bipolar disorder if the symptoms can be alike
thanks
 

Dear Tracey -- 
Look at the PTSD symptoms (the official ones for the diagnosis):  hypervigilance, hyperarousal, avoidance, numbing.  Hypervigilance can look like the agitation/anxiety that can be part of bipolar disorder (see my essay on anxiety and bipolar).  Avoidance is a pretty common behavior in many situations and not very specific for anything (e.g. it's part of panic disorder, as agoraphobia; it's part of depression, as withdrawal from activities and relationships; it's part of social phobia, as avoidance of particular situations).  And numbing (physical and especially mental) is also somewhat similarly non-specific, as it's kind of the mental internal equivalent of avoidance in some respects. 

How about bipolar disorder, which can be viewed broadly as "cyclic changes in mood and energy"....  I think this is a little more distinct, in some respects.  There should be some history of mood symptoms, something suggestive of cyclic changes, some family history usually of something like mood problems or anger problems or sometimes just florid alcohol problems.  There should be some problems with sleep, although this overlaps completely with PTSD, except that phases of sleeping 12 or 14or 16 hours a day is not characteristic of PTSD. 

And of course, there must be history of trauma (something life-threatening) in PTSD and this is not required in bipolar (but people with bipolar disorder probably have an excess exposure, through their family history as well as their potential for sexual acting out and other risk taking, to life-threatening trauma).  So this is only useful in the diagnostic distinction if you didn't have the history of trauma, in which case you probably wouldn't be asking this question, right.  

Other than that, I'd probably approach this by saying "okay, the trauma history is there, maybe PTSD is probably there; but is "bipolar" there?  I.e. you can have the one but do you also have the other?  Because the treatment for PTSD would focus on therapy and probably trials of antidepressants, at least to start (coming around to "mood stabilizers" later, sometimes).  But the treatment for bipolar disorder would specifically avoid starting with antidepressants, which can worsen bipolar disorder.  So it becomes more a question of "ruling out bipolar" before you proceed to treat as though PTSD is the only target. If it was looking more like possibly both, I'd treat from the bipolar perspective first.  

In other words, you're right: there's a lot of overlap.  If you haven't been there already, you could read my "diagnosis" section on bipolar II for more on the subtler forms of bipolar disorder.  

Dr. Phelps