Despite the high prevalence of delirium in palliative care settings, this diagnosis is frequently missed, particularly in patients with hypoactive delirium. These patients are also commonly misdiagnosed with depression because of the overlap in symptoms between the two diagnoses. Failure to promptly diagnose delirium can have significant ramifications in terms of delirium reversal, subsequent patient involvement in end-of-life decision making, and the recognition and treatment of other symptoms.

We report a case of a 63-year-old French-speaking woman admitted to our inpatient palliative care unit with colorectal cancer and a history of depression. This case report highlights the major challenges associated with making the diagnosis of delirium in a patient with a complex medical history, including depression.

The patient presented with symptoms of depressed mood and fluctuation in psychomotor activity, but failed to respond to an increase in her fluoxetine treatment in addition to methylphenidate and treatment of her hypothyroidism. A psychiatric assessment in her own language detected features of inattention and confirmed a diagnosis of delirium that was multifactorial, secondary to a combination of posterior reversible encephalopathy syndrome (PRES), hypothyroidism, hepatic dysfunction, and medication.

Subsyndromal delirium may present with mood lability, and as delirium and depression can coexist, clinicians should perform a delirium screen for all patients presenting with symptoms of depression, preferably in the patient's first language. Cognitive testing can be particularly helpful in distinguishing delirium, especially hypoactive delirium, from depression.