From an article in U.S. News & World Reports:
The researchers autopsied the brains of 426 Japanese American residents of Hawaii who had been, on average, 87 years old when they died. About half had been diagnosed with some form of dementia, typically Alzheimer's.
Among those whose brain lesion profile did not confirm an Alzheimer's diagnosis, the investigators did uncover evidence of other forms of dementia, including Lewy body dementia, vascular dementia, generalized brain atrophy and a range of non-specific forms.
The chances of a misdiagnosis, the researchers found, appeared to rise with the patient's age.
Here's a great primer on dementia from the National Instistute of Neurological Disorders and Stroke: Dementia Information Page