Table 4

Pseudorhythmic myoclonus in the differential diagnosis of tremor

Focal myoclonusMyoclonic jerks with low frequency (usually below 5 Hz) and topographically limited to segmental levels, often the consequence of a spinal lesion (spinal myoclonus; Halliday, 1967).
Oscillatory myoclonusRhythmic jerks that occur in a burst and then fade, described in patients with ‘essential myoclonus’ (Fahn and Singh, 1981).
OMIt is probably a poorly recognised condition to consider in the differential diagnosis of orthostatic tremors (Leu-Semenescu et al, 2007). It is characterised by short-lived, shock-like and relatively ample movements affecting the trunk or legs soon after the maintenance of a standing position. Some participants also show myoclonus in the upper limbs. Burst duration is brief (20–100 ms) and the frequency ranges between 1 and 14 Hz. Like orthostatic tremors, the most frequent complaint of these patients is unexplained unsteadiness during orthostatism or gait. OM is usually associated with neurodegenerative diseases, such as parkinsonism, mild cognitive impairment, Alzheimer's disease and normal pressure hydrocephalus (Gasca-Salas et al, 2013).
Tongue myoclonusIt is a rare condition to be differentiated from isolated tongue tremor. In contrast with the latter it is generally characterised by the shortening and ‘U’ shape folding of the tongue with a pseudorhythmic pattern on EMG studies (Bettoni et al, 1999).
Negative myoclonusAsterixis (also known as a ‘flapping tremor’*) is a typical example of negative myoclonus commonly resulting from endocrine dysfunction, intoxication and liver or kidney diseases. Negative myoclonus also occurs in the Lance-Adams syndrome (posthypoxic myoclonus) and when it involves axial or legs muscles, it manifests as trunk bobbling when the patient stands
Cortical tremorA misnomer that describes the 8–18 Hz irregular, tremor-like, typically bilateral, action myoclonus originating from a hyperexcitable cerebral cortex (cortical reflex myoclonus). It can be classified as:

  1. ‘Acquired’, as in the context of systemic disease (eg, coeliac disease; Fung et al, 2000), medication toxicity (valproate, lithium, serotoninergic drugs, narcotics), focal lesions (Wang et al, 1999), or autoimmune disorders involving the CNS, usually of paraneoplastic aetiology.

  2. ‘Primary’, as in FCMTE.† FCMTE is a rare autosomal dominant syndrome of adult onset, which manifests with low-amplitude myoclonus at 7–15 Hz distally affecting the upper limbs. FCMTE is variably associated with generalised tonic-clonic seizures, myoclonic status (often triggered by photic stimulation, sleep deprivation or emotional stress) or other neurological signs (intellectual disability, progressive gait impairment, dementia or psychosis). No clear causative genes have been identified and still debated is the role of ACMSD or CNTN2 (alias TAG-1) genes (Stogmann et al, 2013).

  • *This is another misnomer used to describe jerking movements that, by definition, cannot be present at rest because the movements are caused by gravity pulling the hands or feet down while holding of a posture.

  • †Many other acronyms, for example, autosomal dominant cortical tremor (ADCME) or benign adult myoclonic epilepsy (BAFME), have been proposed. Please see online supplementary material for references listed in the table.

  • EMG, electromyography; FCMTE, familial cortical myoclonic tremor and epilepsy; OM, orthostatic myoclonus.