This patient most likely has delirium. Tests such as spelling a word backwards or reciting the days of the week in reverse are rapid ways to measure attention at the bedside. Frequent redirection during the course of an interview is another indicator of inattention. A tangential thought process is often misinterpreted as part of normal aging, but it actually is an indicator of disorganized thinking. Acute onset of cognitive dysfunction over hours to days, impairment of attention, disorganized thinking, and fluctuating mental status are core features of delirium. Increased or decreased psychomotor activity, disorientation, and perceptual disturbances are other supportive features. The use of a screening instrument (such as the Confusion Assessment Method) allows for improved recognition and diagnosis of delirium.

Delirium is an underrecognized disorder in older patients who are hospitalized and may result from various causes, including organ failure (such as the worsening kidney function in this patient), metabolic disturbances, medications, or infection. A key to the likely cause of delirium in this patient is myoclonus seen on physical examination. Myoclonus is a sudden involuntary muscle contraction (positive myoclonus) or sudden brief loss of muscle activity (negative myoclonus, or asterixis); this patient's examination shows asterixis, a common finding in metabolic disturbances (uremia, liver failure, or hypoglycemia) and toxic encephalopathy (due to antibiotics, pain medications, and immunosuppressants).

Although the presence of delirium significantly increases the risk of developing dementia and, conversely, dementia is a significant risk factor for developing delirium, this patient was previously functioning independently. The onset of dementia is typically insidious. The diagnosis of dementia requires 6 months of progressive cognitive decline.

Nonconvulsive status epilepticus (NCSE), or alteration in mental status without overt convulsive activity as a result of continuous or near continuous epileptiform discharges, is often unrecognized in older patients with mental status changes and should be considered as part of the differential diagnosis of acute confusional state if the cause remains unknown. However, the cause of delirium is often identifiable by a careful history, physical examination, and review of medical conditions or interventions that may be contributing to a change in mental status. Additionally, the negative myoclonus seen in this patient would be unlikely in a patient with NCSE without a history of preexisting epilepsy.

Although stroke presents with an abrupt onset, and this patient is at higher risk for stroke because of his recent myocardial infarction, this patient does not have any focal neurologic signs, such as dysarthria, facial droop, hemiparesis, or dysmetria, to suggest that a stroke has occurred.