Lexapro and ALL of the SSRI antidepressants (Prozac, Effexor, Lexapro, Celexa, Zoloft, etc.) is they are generally not recommended for bipolar depression. They almost always cause mania in an otherwise stable or depressed bipolar person. Sometimes, if you take a mood stabilizer and a very small dose of them, it won't happen, but you are not on a mood stabilizer, an antipsychotic (Risperdal) won't stop mania. If you are on a very large dose of Risperdal and a very small dose of Lexapro, you may have better luck, although at that combination, the Lexapro will probably be ineffective.
If you do decide to take it, watch very closely for signs of mania, usually starting with insomnia and increasing from there. Also, panic and anxiety can occur as well. We are wired differently than regular depression, and often those meds backfire and just make things worse. I don't know if you read the thread about Lexapro and Effexor previously, but the opinions largely supported this.
The problem with going hypomanic or manic means you have to stop the medicine and you will probably crash and your depression will be even worse. Ask your doctor about trying a mood stabilizer and Welbutrin; it doesn't seem to cause the mania. Plus, there are no sexual side effects, and no weight gain. Many times weight loss, as it can reduce your appetite. Risperdal causes weight gain. Seroquel is another medicine like Risperdal and doesn't have the weight gain side effect. Lamictal is an excellent choice for bipolar depression; it is the least likely to cause mania and is also a mild mood stabilizer, many bipolars are put on that with welbutrin and it keeps you from going too low. (One for stopping highs, one for stopping lows). It also has no sexual or weight gain or sleepy side effects. It is a fairly new treatment, so many docs don't use it as much.
As bipolars, since all of us are so different, need to learn as much as we can about the latest treatments, because they are getting better and with less side effects all the time.
I hope this helps.