Self injury is not related to Bipolar, it is a feature of Borderline Personality disorder. However some Bipolar Sufferers turn to self harm when hopelessness and sadness overwhelm them. Self mutilation or cutting is used to help them with anger or anxiety.
There are various ways of self injury ,burning, scratching,biting, Picking of skin, pulling out hair to name a few. It is related to stressful events or it can be related to other people.
Self harm can be done regularly, it can be impulsive or at the spur of the moment. Regulating emotions is important and psychiatric medication and psychologists are needed to find out reasons and how to stop this behaviour.
Often these self harm episodes are hidden and hard to control. Professional help is needed to examine the cause.