This patient most likely has dementia with Lewy bodies, the second most common cause of degenerative dementia. Core features of dementia with Lewy bodies are fluctuating attention and alertness; recurrent, well-formed visual hallucinations; and spontaneous parkinsonism. Sensitivity to neuroleptic medications also is commonly seen. Suggestive features include rapid eye movement sleep behavior disorder and low dopamine transporter uptake in the basal ganglia on single-photon emission CT or PET scans. The patient's features of cognitive fluctuations (episodic disorganized speech and functional disability, daytime sleepiness), parkinsonism, visual hallucinations, and severe sensitivity to haloperidol all support a diagnosis of dementia with Lewy bodies.

Although visual hallucinations and parkinsonism can occur in Alzheimer disease, these features generally present relatively late in the disease course. Severe sensitivity to neuroleptic medications also is less common in Alzheimer disease than dementia with Lewy bodies. Alzheimer disease is predominantly characterized by a decline in memory that can be accompanied by deficits in other cognitive domains. Typical behavioral changes in the earlier stages of Alzheimer disease include irritability, anxiety, and depression.

Delirium is characterized by disturbances of attention, cognition, and perception, with fluctuations in symptoms during the day. These disturbances typically develop over the course of hours to days, rather than over 1 year as occurred in this patient. A careful history obtained from a reliable informant often easily distinguishes delirium from dementia with Lewy bodies.

Major depression with psychotic features is characterized by hallucinations, more often auditory than visual, and delusions that are typically mood congruent, such as guilt, nihilism, or deserved punishment. Although the patient is being treated for depression, no evidence that supports a diagnosis of major depression, such as suicidal ideation, hopelessness, or guilt, has been reported.