Some attempted to inform valid symptomatic boundaries by proposing a differentiation of narrow versus broad pediatric bipolar phenotypes. The narrow phenotype was defined by strict DSM criteria for mania or hypomania, including discrete episodes of grandiosity and euphoria. In contrast, the broad phenotype, also referred to as severe mood dysregulation (SMD), was defined by chronic, non-episodic illness lacking hallmark symptoms of grandiosity and euphoria, but typified by severe irritability and hyperarousal. Work at the National Institute of Mental Health (NIMH) demonstrated that patterns of adolescent irritability are stable and distinct, with episodic irritability leading to mania and simple phobia, and chronic irritability leading to diagnosed depression and ADHD. On structured assessment, children with SMD were highly comorbid for major depression (20%), anxiety (64%), oppositional defiant disorder (83%), and ADHD (87%). Others also found increased rates of ADHD and anxiety. In addition, the broad and narrow phenotypes could be differentiated according to electroencephalography (EEG) measures. SMD youth have impaired face emotion recognition deficits correlated with dysfunctional family relationships and were less influenced by emotional distracters during tasks of attention. A recent study revealed patterns of amygdala hypoactivation similar to depression, further supporting links between chronic irritability and subsequent depressive episodes.