ET and Parkinson’s are two separate, quite
different conditions. In contrast to ET
and its postural or kinetic tremor coming on simultaneously in both
upper limbs, Parkinson’s tremor is usually unilateral (i.e. one side only) and,
in the majority of cases, starts in the hand as a rest tremor . That
is, a tremor that occurs in a body part which is not voluntarily activated and is completely supported against
gravity - for example, when sitting watching television with the arm resting at
the side and the hand is seen to shake usually until the arm is moved
intentionally.
Another difference between ET and Parkinson’s
is that head tremor is found in more than 30% of people with ET, yet is
extremely rare in Parkinson’s. However, it
is important to note that, if shaking in Parkinson’s becomes violent, the head
can manifest a secondary tremor as a passive consequence of such movements.
Parkinson’s tremor is also considered to be symptomatic in that it reflects known changes
in the nervous system and, unlike ET, the disease is not generally inherited.
Because ET is not associated with any
degenerative process in the brain itself, accompanying features of Parkinson’s,
such as rigidity, bradykinesia or gait disturbance, are not seen. For example, when an individual with a
Parkinson’s tremor of the upper limb is observed to walk, invariably there is
loss of normal arm swing. This does not
occur in ET. Confusingly, however,
cogwheel rigidity (that is the interruption of passive movement of a body part
around a joint) is a feature of both Parkinson’s and ET.
Whilst careful clinical assessment usually
allows a confident diagnostic separation between ET and Parkinson’s,
similarities between the two conditions, such as cogwheel rigidity, often
result in misdiagnosis. Furthermore, as ET is relatively common, the two conditions
can occur in the same individual. Not surprisingly, there have been occasions
when ET has been diagnosed ET as Parkinson’s and vice versa. In particular, some American doctors are
prone to call the postural tremor
found in Parkinson’s ET.
It is vital to distinguish ET from Parkinson’s,
both for prognostic purposes and because the treatments of Parkinson’s are not
effective in ET.
