Sudden, brief, shocklike, involuntary movements result from brief contractions (or lapse in contractions) of a muscle or group of muscles.¹

      When the movement results from a contraction of muscles, it is also termed positive myoclonus.

      The movement is called negative myoclonus when it results from a transient loss of muscle contraction.

      Friedreich first described myoclonus as a separate entity in a case report in 1881, but it was in 1883 that Lowenfeld used the term myoclonus for the first time.

      In 1903, Lundborg proposed a classification scheme for myoclonus: symptomatic, essential, and familial.

      Lindenmulder described the first family with essential myoclonus and drew attention to its occurrence in nondegenerative conditions without metabolic derangements or epilepsy.²

      Although myoclonus is quite distinctive in its presentation with “sudden shocklike movements,” it may sometimes still be confused with other movement disorders, such as tics, chorea, dystonia, and tremor. It then becomes important to make sure that the phenomenology is consistent with myoclonus (Table 7.1).

      There is no single best way to approach a patient with myoclonus.

      Here is a suggested sequence to follow (Figures 7.1–7.3).

              Myoclonus may be classified in several ways: according to distribution, clinical presentation, pathophysiology, or etiology (Table 7.2).

              Identifying myoclonus by its distribution and/or type may assist in localizing the pathology (Table 7.3A, B).

      Myoclonus with ataxia and epilepsy

      Drug-induced myoclonus

              Myoclonus secondary to underlying conditions accounts for 72% of cases (Figure 7.4).³

              A detailed drug history with identification of all current and recent medications is imperative. Drug-induced myoclonus is especially common in hospitalized patients (Table 7.5).⁴

      Inherited disorders presenting with myoclonus (Table 7.6A–C)⁵

      Essential myoclonus

              Essential myoclonus can be hereditary or sporadic (Figure 7.5).

      Opsoclonus-myoclonus-ataxia syndrome (Table 7.7)^3,6–11

      Palatal myoclonus (Figure 7.6)

      Myoclonus in neurodegenerative extrapyramidal disorders (Figure 7.7)

      Physiologic myoclonus

              This occurs as the body’s physiologic response to certain external or internal triggers in otherwise healthy individuals and includes hiccups (singultus), hypnic jerks that are abundant in the facial and distal limb muscles, and physiologic startle.

              These symptoms do not usually need to be treated.

      Orthostatic myoclonus (OM)

              A relatively recent observation of myoclonus that occurs on standing^12,13 and in subjects who may or may not have neurodegenerative conditions like Parkinson’s disease (PD). It may be even more common than orthostatic tremor.¹⁴

              The most frequent symptom was unexplained unsteadiness in a series of patients referred for gait imbalance and tested. OM was diagnosed in 17.2% of the patients.¹⁴

              The most frequent comorbid neurological condition was parkinsonism.¹⁴