ORIGIN

These are typically postural or kinetic tremor syndromes.  There is ongoing debate whether these tremors are similar in pathophysiology to ET or dystonia.

• Isolated focal tremors are specifically excluded from the diagnosis of ET, although, with time, ET may be sometimes be recognized as having been the correct diagnosis.
• By contrast, some patients may later develop focal or segmental dystonia that is idiopathic or attributed to genetic abnormalities, such as mutations in the anoctamin 3 (ANO3) gene¹.

CLINICAL FEATURES

Isolated task- and position-specific tremors may occur during a specific task or posture. Task-specific tremors, typically focal, are not uncommon in persons who perform the affected motor task repetitively and frequently.  

As with focal tremors in general, there is uncertainty as to whether these task-specific tremors represent variants of dystonia, overuse syndromes, or simply unique isolated tremor syndromes. Isolated task- and position-specific tremors should be distinguished from similar syndromes that occur in combination with other neurological signs, such as dystonia (e.g., writer’s cramp) and parkinson- ism (e.g., young-onset PD with dystonia).

Task-specific tremors of the hand or mouth occur in musicians and sportsmen. Yips in golfers is mostly considered a task-related dystonia, sometimes manifesting with tremor as the main symptom.

Rare focal tremors that may occur in the absence of other neurological signs:

• Hereditary geniospasm,
• Isolated jaw tremor
• Isolated tongue tremor
• Rabbit syndrome
• Tremor during smiling

Hand

Primary writing tremor (PWT) is likely the commonest form of task-specific tremor, and occurs only when writing or attempting to write; typically, there is either a pronation/supination or a flexion/extension tremor at the wrist.  PWT is, per definition, task specific, and is elicited either by writing itself (type A: task‐induced tremor) or by adopting hand position as if writing (type B: positional sensitive tremor). 

Age at onset varies, and cases manifesting during childhood have been reported. The disorder begins slowly, progresses for years, and then stabilizes. Family history is generally unremarkable.

No other neurological signs are evident except for slight postural and terminal kinetic tremor. Writing tremor has been variably classified as an independent entity, an ET variant, a focal dystonia, or a bridging entity.

Voice  Link to Dystonia Section on Spasmodic Dysphonia

Patients with no signs of dystonia in the vocal apparatus and no tremor, dystonia, or other neurological signs elsewhere are considered to have isolated voice tremor. Isolated voice tremor is a visible and/or audible tremor of the vocal apparatus.

• Enhanced physiological tremor is an extremely rare cause of voice tremor.
• Parkinson’s disease is characterized by constant hypophonia due to incomplete glottic closure and poor breath support resulting from chest wall rigidity, sometimes exacerbated by articulatory difficulties and cognitive problems.
• Voice tremor in ET occurs, by definition, in combination with hand tremor. However, this is potentially controversial, since some patients may not have a prominent arm tremor². The vast majority of patients are female, and the condition typically begins in the 6^th decade, although there may be a bimodal distribution as seen in other forms of ET². Misdiagnosis with dystonia of vocal cords is common².

The most prominent voice symptom and diagnostic indicator for ET of the voice is a periodic or rhythmic modulation of either frequency (i.e., pitch) or intensity (i.e., loudness) in the voice, best heard during sustained vowel phonation. The salient feature of essential voice tremor is kinetic (action-induced) tremor of the muscles of the larynx, pharynx, palate and/or tongue, and an absence of rigidity, bradykinesia, and spasms typical of other disorders of involuntary motion.

Normal conversational speech (connected speech) does not mask the abnormality, but often make its periodicity less obvious. Acoustic variations had two basic patterns:

1. Most commonly, variations remained more or less constant during sustained vowel phonation. In severe cases, these could be so marked as to create sharp soundless voice breaks that do not suggest tremor².
2. Secondly, variations increased in amplitude until voice production stopped.

Dystonic voice tremor: Cases with hyper-abduction or -adduction of the vocal cords, as observed in laryngeal dystonia, or with dystonia in other body parts, are classified as dystonic voice tremor of known or idiopathic aetiology (see dystonic tremor syndromes). Essential voice tremor does not result in the sphincteric glottic closure of adductor spasmodic dysphonia, which produces the characteristic ‘‘strain-strangle’’ phonation of laryngeal dystonia. The voice of patients with essential voice tremor is affected across all voice tasks, unlike adductor spasmodic dysphonia, for example, which is provoked by phrases containing many voiced onsets (e.g., counting from 80 to 90; "the hunter hides in the hut"). Modulation of voice due to ET is most noticeable during prolongation of a vowel: tremor will be clearest when someone is speaking vowel laden sentences (i.e., “you were away all year”).

Voice tremor may be observed in dystonia 6 and ANO3 mutation carriers without clinical dystonia³. As noted (see Dystonic Tremor), patients with dystonic vocal tremors cannot change pitch during vocalization, and performing this manoeuvre can aid in the bedside diagnosis. Ask the patient to make a sustained sound (“ahh” or “eee”) for 7 seconds, while listening for breaks in the voice.

Head

Isolated head tremor is a shaking of the head in yes-yes, no-no, or variable directions. Head tremor is common in the context of ET, but is also a common manifestation of tremulous dystonia. The relationship between isolated head tremor and focal tremulous cervical dystonia is a topic of ongoing controversy; the contrast between head tremor due to ET and dystonia is reviewed in the section on dystonic tremor.

As noted, PD may also rarely be associated with head tremor.

DISTRIBUTION

Isolated rest tremor syndromes most commonly occur in the upper limbs, or lower limb, or as a hemitremor, but may occur elsewhere (e.g., lips, jaw, voice, face or tongue).

DIFFERENTIAL DIAGNOSIS

Isolated task- and position-specific tremors should be distinguished from similar syndromes that occur in combination with other neurological signs, such as dystonia (e.g., writer’s cramp) and parkinsonism (e.g., young-onset PD with dystonia).

DIAGNOSTIC TESTS

Even if the tremor is isolated, dopamine transporter imaging (e.g., F-Dopa PET; DaTscan]) may be needed to exclude a parkinsonian condition, given that some early tremor-dominant PD patients (monosymptomatic tremor at rest) may not have evident bradykinesia or rigidity.

Fibreoptic nasolaryngoscopic examination of pharyngeal and laryngeal musculature, supplemented by acoustic measures may be of benefit: the presence of periodic, oscillatory motion affecting the vocal apparatus globally (palate, pharynx and/or larynx) is considered diagnostic for voice tremor due to ET².