There are two distinct forms of delirium,
hypoactive and agitated (or hyperactive). When
individual patients intermittently have both forms,
it is termed mixed delirium. The hypoactive
form is characterized by inattention, disordered
thinking, and a decreased level of consciousness
without agitation. Pure agitated delirium affects
less than 2% of patients with delirium in the ICU.34
Patients with hypoactive delirium are the least
likely to survive, but those who do survive may
have better long-term function than those with
agitated or mixed delirium.33 Separating the effects
of delirium status from those of illness
severity with respect to the risk of death is difficult,
since patients with more serious illnesses
are at increased risk for both delirium and death.
Association studies typically adjust for illness
severity on admission to the ICU rather than at
the time that delirium is diagnosed. Although the
association between delirium and a worse outcome
is clear, a causal relationship has not been
established. Currently, the evidence that specific
treatment of delirium may improve outcomes is
tenuous.