Cerebral Causes and Consequences of Parkinsonian Resting Tremor

A Tale of Two Circuits?

Rick C. Helmich; Mark Hallett; Günther Deuschl; Ivan Toni; Bastiaan R. Bloem

Disclosures

Brain. 2012;135(11):3206-3226. 

In This Article

The Tremors of Parkinson's Disease

Tremor is characterized clinically by involuntary, rhythmic and alternating movements of one or more body parts (Abdo et al., 2010). Parkinson's disease harbours many different tremors. These tremors can vary according to the circumstances under which they occur, the body part that is involved and the frequency at which the tremor occurs. For example, tremor may occur at rest, during postural holding or during voluntary movements; it can be seen in the hands, feet or other body parts; and tremor frequency can vary from low (4–5 Hz) to high (8–10 Hz). A consensus statement of the Movement Disorder Society includes a parallel classification scheme that categorizes three tremor syndromes associated with Parkinson's disease (Deuschl et al., 1998). This classification is still used widely today (Fahn, 2011). First, the most common or classical Parkinson's disease tremor is defined as a resting tremor, or rest and postural/kinetic tremor with the same frequency. This tremor is inhibited during movement and may reoccur with the same frequency when adopting a posture or even when moving. When recurring with posture, it has been called re-emergent tremor. Second, some patients with Parkinson's disease develop rest and postural/kinetic tremors of different frequencies, with the postural/kinetic tremor displaying a higher (>1.5 Hz) and non-harmonically related frequency to the resting tremor. This form occurs in <10% of patients with Parkinson's disease. Some consider it to be an incidental combination of an essential tremor with Parkinson's disease (Louis and Frucht, 2007), but it appears more plausible that postural tremor is a manifestation of Parkinson's disease. Third, isolated postural and kinetic tremors do occur in Parkinson's disease. The frequency of these tremors may vary between 4 and 9 Hz. A specific form of (position-dependent) postural tremor is orthostatic tremor, which may occur in Parkinson's disease at different frequencies (4–6, 8–9 or 13–18 Hz), with or without co-existent resting tremor (Leu-Semenescu et al., 2007). Since this tremor type occurs at a higher age of onset than primary orthostatic tremor, and since it may respond to dopaminergic treatment, it has been argued that it is a manifestation of Parkinson's disease rather than a chance association of two tremor syndromes (Leu-Semenescu et al., 2007).

The distinction between these different tremors is not always visible to the naked eye. For example, resting tremor can re-emerge during postural holding, making it difficult to clinically distinguish it from essential tremor. This distinction can be made by focusing on the delay between adopting a posture and the emergence of tremor: in essential tremor there is no delay, while Parkinson's disease resting tremor re-emerges after a few seconds (on average ± 10 s) (Jankovic et al., 1999). Since the frequency of re-emergent and resting tremor can be similar, it has been hypothesized that both tremors share a similar pathophysiological mechanism. One interesting patient with Parkinson's disease had no resting tremor, but a marked 3–6 Hz postural tremor that occurred after a delay of 2–4 s following postural holding (Louis et al., 2008), thus resembling re-emergent tremor. Such observations point to heterogeneity in the circumstances under which the classical Parkinson's disease 'resting' tremor occurs.

In the following sections, we will mainly focus on the classic resting tremor in Parkinson's disease. We will first describe the clinical and cerebral differences between patients with tremor-dominant and non-tremor Parkinson's disease. Then we will detail how these differences may inform us about the causes and consequences of Parkinson's disease resting tremor.

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