In every healthy subject, physiologic tremor is present in the joint or muscle that is free to oscillate. It has low amplitude and high frequency. Usually, it is not visible, except for intermittent finger tremors.

Enhanced physiologic tremor (EPT) has the same frequency characteristics as physiologic tremor but is easily visible. It is mainly postural. The diagnosis should not be made in the presence of an underlying neurologic pathology. This form of tremor overlaps the category of drug- or toxin-induced tremor. Screening for potential metabolic derangements associated with tremor should be performed (Table 11-2). Distinction between EPT and mild forms of ET is arbitrary and usually is based on the presence of functional disability with the latter.

The diagnosis of ET is based on the presence of bilateral, largely symmetric, postural or kinetic tremor that involves hands and forearms and that is visible and persistent (6,8). Tremor can involve head and voice; however, chin or leg involvement is atypical. In cases of severe ET, resting tremor can be present; however, the possibility of coexisting Parkinson’s disease (PD) has to be ruled out. Tremor frequency is 4 to 12 Hz. Some patients, especially women, can present with head tremor as an isolated manifestation of ET. Caution should be taken to rule out an underlying cervical dystonia with a prominent tremor component. The possibility of an underlying symptomatic cause, such as drugs, toxins, or metabolic abnormalities, should be excluded before making the diagnosis of ET. A distinction should be made between ET as the causative entity, also referred to as benign familial essential tremor, versus ET as a clinical syndrome, which defines tremor of a particular type that potentially can be associated with other neurologic findings. The former is labeled as definite ET in the Tremor Investigation Group (TRIG) nomenclature, whereas the latter is labeled as probable versus possible,
depending on the type of associated neurologic findings (2). Definite ET is a monosymptomatic disease, meaning that no other abnormal neurologic findings should be present. It has strong familial predisposition, with about 50% of patients having a positive family history for tremor, pointing to likely autosomal dominant inheritance, although the gene has yet to be identified (9). Conversely, lack of family history does not preclude the diagnosis of ET. Symptoms have an insidious onset with a variable rate of progression. The most disabling feature of ET is action or kinetic tremor of the arms, which interferes with the patient’s ability to perform the simplest daily activities (e.g., eating, drinking, writing). Patients with severe ET can be completely disabled, so the label of benign is a misnomer. A number of patients have a combination of ET and other movement disorders (e.g., dystonia, parkinsonism, myoclonus, restless legs syndrome). Those patients are classified as possible ET according to TRIG criteria (2).

Primary orthostatic tremor is a unique syndrome characterized by the presence of high-frequency (13 to 18 Hz) tremor of the trunk and legs that occurs only with stance (4). The patients are asymptomatic when lying down or sitting. Rarely, symptoms persist with walking. Subjectively, patients report a feeling of unsteadiness when standing up. Constant change of the position of the feet relieves the symptoms; thus, patients quickly learn to march in one spot. The clinical examination finding is benign, except for minimally visible and sometimes only palpable fine-amplitude rippling of the leg muscles. The diagnosis can be confirmed by an electromyographic (EMG) study of the quadriceps muscles, which records a typical tremor pattern of 13 to 18 Hz. Symptoms respond to low-dose clonazepam. A recent study also demonstrated the benefit of gabapentin, although the number of study subjects was small (29).