Tremor is defined as “an involuntary, rhythmic, oscillatory movement of a body part.” Tremor is typically assessed when the patient is relaxed and distracted, with arms outstretched in posture, and when performing finger-to-nose testing to assess components of resting, postural, and intention tremors. Other movement disorders can resemble tremor, including myoclonus, polyminimyoclonus, and athetosis. Myoclonus is a fast, jerky movement that can arise spontaneously, with activity, or in response to a stimulus. Trains of myoclonus can mimic fast, low-amplitude tremor. Electrophysiologic studies (e.g., electromyogram/nerve conduction studies [EMG/NCS]) are necessary at times to establish an accurate diagnosis. Polyminimyoclonus is seen often in patients with multiple system atrophy and is characterized by irregular, small-amplitude myoclonic movements of the hands and/or fingers in posture. Athetosis can also involve the fingers, but it is slower and characterized by an inability to keep them in a certain position. Pseudoathetosis, which has an identical phenomenology, is seen in patients with impaired joint position sense. Functional movement disorders should also be considered and are distinguished based on clinical examination.

Both cerebellar tremor and rubral tremor are often of sudden onset and high amplitude, and present in a combination of rest and posture. Cerebellar tremors are slower (< 4 Hz), worse with intention, and are accompanied by other signs of ataxia. Rubral tremor is also slow, often irregular, and can occur at rest, posture, and intention. These patients should undergo brain magnetic resonance imaging as focal lesions, such as stroke or multiple sclerosis, are frequent causes. If the tremor is distractible or entrains to the frequency of distracting maneuvers, functional tremor should be considered.

Parkinsonism is defined by the presence of bradykinesia and one or more of the following symptoms: rigidity, tremor, or postural instability. The most common form of tremor in these patients is rest tremor (a tremor that emerges seconds after a limb is allowed to rest completely and disappears with activation). Patient with Parkinsonian rest tremors may also exhibit tremor with posture and/or intention, but the resting component is often the most prominent. Resting tremors can also be seen in patients with dystonia or advanced essential tremor (ET), so identifying bradykinesia and other features of Parkinsonism is necessary to establish a diagnosis. In patients with subtle or questionable bradykinesia, an [I-123] ioflupane dopamine transporter (DAT) single proton emission computerized tomography (SPECT) scan can identify the nigrostriatal dopaminergic deficit associated with Parkinsonian syndromes. The ligand is directed against the dopamine reuptake transporter on the presynaptic side of the terminal end of the nigrostriatal pathway.

Patients with rest tremor without other signs of Parkinsonism typically have a dystonic tremor, defined as tremor in a limb affected by dystonia. Dystonic tremors are often jerky and irregular and can resemble myoclonus or coarse essential tremor. Distinguishing dystonic tremor from other tremor types can be challenging, but presence of the following can be helpful: (1) directionality—a tendency of the affected limb to move in a certain direction upon positioning of the limb in the opposite direction; (2) null point—significant tremor improvement when the limb is positioned in the direction in which it tends to move; and (3) the presence of a sensory trick—improvement in tremor severity by light tactile stimulation of the affected body part.

All patients with a prominent postural tremor should be screened for causes of tremor, including exposure to medications or substances, comorbid medical diseases, or anxiety. Patients with ET may also find the same factors exacerbate their tremor. It can be difficult to distinguish enhanced physiologic tremor from ET, particularly early in the course of the disease.

Most patients with isolated prominent postural tremor are thought to have an enhanced physiologic tremor. These patients have a tremor that is fast frequency, low amplitude, and most visible when holding the arms outstretched.

Patients with ET may exhibit additional subtle neurologic signs of uncertain significance (such as impaired tandem gait, questionable dystonic posturing, or memory impairment); these patients have been recently defined as having “ET plus”. Patients with action tremor for < 3 years who do not have signs of dystonia or Parkinsonism are defined as having an indeterminate tremor. Patients with prominent postural and intention tremor in ET often have the appearance of slight resting tremor, which may be due to incomplete relaxation. Occasionally DAT scan may be necessary to exclude a developing Parkinsonian disorder.