Essential Tremor

Differential Diagnosis

The differential diagnosis of ET includes a number of hereditary and idiopathic disorders, metabolic conditions, cerebral diseases, and peripheral neuropathies that may be characterized by postural, intention, or rest tremors or combinations of such tremor elements. (For a listing of such conditions, please refer to table 4.)

Table 4. Differential Diagnosis of ET

Parkinson's disease (PD) Multiple system atrophy (MSA) (e.g., olivopontocerebellar atrophy [OPCA], striatonigral degeneration) Huntington's disease (HD) Benign hereditary chorea Wilson's disease Fahr's disease Paroxysmal dystonic choreoathetosis (PDC) Ataxia-telangiectasia (AT) Familial intention tremor and lipofuscinosis Ramsay-Hunt syndrome (progressive myoclonic ataxia) Dystonia musculorum deformans DOPA-responsive dystonia (Segawa syndrome) Spasmodic torticollis (cervical dystonia) Meige syndrome Task- or position-specific tremors (e.g., primary writing tremor, isolated voice tremor) Space-occupying lesions of the brain (e.g., cerebrovascular insults, trauma, cysts, tumors, hematomas) Various metabolic disturbances (e.g., hepatic encephalopathy, hypoglycemia, hyperthyroidism, hyperparathyroidism, hypocalcemia, etc.) Peripheral neuropathies (e.g., Charcot-Marie-Tooth disease, Guillain-Barré syndrome, Roussy-Levy syndrome, dysgammaglobulinemic neuropathies, etc.)

ET must also be differentiated from other specific tremor types. (Please refer to "Syndromic classification of tremors.") These include enhanced physiologic, drug-induced, toxic, dystonic, or parkinsonian tremors. For example, isolated head tremor in ET must be excluded from head tremor seen in up to 40% of patients with cervical dystonia. In ET patients, head tremor is characterized by rhythmic, regular oscillations, whereas that associated with cervical dystonia tends to be irregular, occurs with tilting of the head or chin, and varies in intensity with position changes. ET and parkinsonian tremor may be characterized by postural, kinetic, and resting tremor components. However, traditionally, PD is primarily characterized by rest tremor that dampens with action, whereas ET is generally a postural/kinetic tremor with dampening upon rest. In addition, PD almost never involves tremor of the head or voice yet may involve the chin and perioral structures.

Also included in the differential diagnosis are a number of syndromes that may be misinterpreted as tremor. These may include:

• clonus or rhythmic, uniphasic contractions and relaxations of muscle groups
• rhythmic myoclonus, characterized by irregular or rhythmic, shock-like contractions of a muscle group due to CNS disease
• epilepsia partialis continua, a focal motor epilepsy associated with recurrent, rhythmic clonic movements of a specific muscle group.
• asterixis, a condition in which sudden, periodic interruptions in muscle contraction lead to arrhythmic lapses of sustained posture associated with EMG pauses ranging from about 35 to 200 milliseconds or greater. This condition is sometimes referred to as "negative myoclonus."

ET is sufficiently common that some patients will have coexistent tremorogenic or other neurologic disorders. Mild extrapyramidal signs, such as a masked face or balance difficulty, may be observed in some patients, particularly those of advanced age. Some studies have also supported a possible association between ET and dystonia and ET and parkinsonism, demonstrating that members of some kindreds may have ET whereas other family members have parkinsonism, dystonia, or signs of all three conditions. In addition, some patients with ET later develop concomitant dystonia, parkinsonism, or both. Some researchers contend that the coexistence of ET and PD may represent the chance occurrence of two common conditions. However, others indicate that there appears to be a higher frequency of PD in ET than occurs in the general population. Concerning dystonia, although the condition is frequently associated with ET, the DYT1 gene on chromosome 9 has been excluded in hereditary ET through linkage analysis, suggesting different genetic loci or a physiologic rather than a genetic relationship between the two conditions.