Parkinson vs Alzheimer Dementia: A Clinical Sign


The finger displacement sign is a simple bedside test that can help distinguish Parkinson dementia from Alzheimer dementia.



Aman Deep, MD, a neurology resident at the University of Tennessee Health Science Center, Memphis, TN, presented an interesting poster at AAN 2018,1 which illustrates a clinical sign that helps distinguish Parkinson disease dementia from Alzheimer dementia.

Deep and colleagues examined 56 patients (42 male, 14 female, mean age 75) with Parkinson disease dementia (PDD) and 35 (21 male, 14 female, mean age 77) with Alzheimer disease dementia (ADD). The mean onset of dementia was 3.1 years in the PDD group and 3.9 years in the Alzheimer group. The mean Mini-Mental State Examination (MMSE) score was ≤24 in the PDD group and 17.8 in the ADD group.

The finger displacement test required patients to point both of their index fingers at a ruler for 15 seconds, then maintain that position for an additional 15 seconds with their eyes closed. The amount of downward drift was measured.

Among the PDD patients, 53/56 (95%) had bilateral downward drift of their finger of at least 5 cm. Only 3 (5%) had <5 cm of drift. The mean downward finger drift of the PDD patients was 6.8 cm.

Among the ADD patients, only one patient had mild downward drift, and none of the patients had downward drift of more than 5 cm.

Overall, the finger displacement test has a sensitivity of 100% and specificity of 92.1% in distinguishing patients with Parkinson dementia from Alzheimer dementia.

Dr. Deep observed that the extent of drift correlated with the degree of dementia: the “worse the score, the more the drift.” Dr. Deep added that none of these patients had a proprioceptive loss that might have contributed to the downward drift. The pathophysiology accounting for the difference of performance on this test between the Parkinson and Alzheimer patients remains unknown.

Comment: The finger displacement test can be performed in 30 seconds and has a very high sensitivity and specificity. This paper demonstrates that despite the formidable value of neuroimaging and other modern tests, a careful clinical examination at the bedside can offer additional insight to the observant neurologist in distinguishing patients with Parkinson dementia from those with Alzheimer dementia.


1. Deep A, Lieberman A, Aboud T, et al. Bilateral downward finger displacement in Parkinson disease may be a sign of worsening dementia and a bedside test to distinguish it from Alzheimer disease. Poster P1.049. Sunday, April 22, 2018.

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