Parkinson's vs. Essential Tremor

My wife's grandmother has been experiencing what we all assumed to be the beginnings of Parkinson's for about 3-4 years now.  However, after a recent study of Shingles in my Microbiology class, I was curious if nerve damage during her bouts of this disease could be causing her tremor symptoms.

My professor agreed that it could indeed be responsible, so I did a bit more digging.  Apparently, there is a disease called Essential Tremor.  This disease, while 10 times more common than Parkinson's, is frequently overlooked and is often misdiagnosed as Parkinson's.

So, what piqued my curiosity?  First, it struck me that her symptoms had not progressed much since we first noticed them.  Then, the symptoms themselves are not consistent with Parkinson's.  Usually, Parkinson's sufferers experience tremors while at rest.  The most obvious is the tell-tale "pill rolling" motion in the hands.  Her hands are quite still at rest.  Further, the tremor that is most obvious is a rhythmic, "no no" shaking of the head.  This is atypical of Parkinson's as well.

Her symptoms seem much more characteristic of essential tremor, particulary something called Postural Tremor, or tremors that occur as a result of opposition to gravity.  Her hands exhibit some trembling, but not at rest.  So far, I have not mentioned my suspicions to her because I don't want to prematurely negate her doctor's opinion.  However, given my research into the subject so far, I am inclined to bring it up and see if she will allow me to observe some specific actions that may confirm my suspicions.

First, the manner in which the hand tremors manifest themselves is important.  Adduction and abduction of the digits are characteristic of Essential Tremor, while supination/pronation is not.  Imagine holding an imaginary glass of water in your hand out in front of your body.  If the hands tremor in a way that is similar to turning a doorknob back and forth, it is characteristic of Parkinson's.

Now imagine holding your hand out flat (palm down and parallel with the floor) in front of you, and spreading your fingers out slightly.  If the fingers tend to be "jumpy/jittery" as they oscillate toward and away from each other (think of the scissor motion in rock-paper-scissors), this is characteristic of Essential Tremor.

Anyway, I will let you know how it goes.

FYI:  I have linked the relevant diseases here to the MayoClinic website for more info.  Also, the following video is a great starting point for understanding the subtle differences between Parkinson's and Essential Tremor: