What are myoclonic seizures?

Myoclonic Seizures: Definition, Characteristics, and Management

Myoclonic seizures are characterized by sudden, brief, involuntary jerks caused by muscular contractions or inhibitions, typically lasting only a fraction of a second. 1, 2

Clinical Characteristics

• Myoclonic jerks are sudden, brief, shock-like contractions that can vary in distribution and intensity 3
• They can occur as isolated events or in repetitive patterns, affecting a specific body part or multiple body regions simultaneously 1, 4
• Myoclonic seizures can be symmetrical and synchronous (more typical of epilepsy) or asymmetrical and asynchronous (more common in syncope but can occur in epilepsy) 1
• The movements typically begin at or shortly after the onset of loss of consciousness in epileptic myoclonic seizures 1
• Myoclonic jerks can involve the face, arms, hands, or other body parts, with common patterns of propagation from lower face to upper face and distal hand to proximal arm 4

Electrophysiological Features

• Electromyographic studies can determine whether the jerk is caused by muscular contraction (positive myoclonus) or interruption of muscular activity (negative myoclonus) 5
• Myoclonic seizures are considered epileptic when combined with epileptiform discharges on EEG 5
• Surface EMG recordings show synchronous brief tetanic contractions of agonist and antagonist muscles, alternating with synchronous silent periods 4
• The most common EEG seizure pattern is paroxysmal rhythmic monomorphic activity 4

Types and Syndromes

• Myoclonic seizures can occur in different epilepsy syndromes, including:

  • Idiopathic generalized epilepsy 3
  • Juvenile myoclonic epilepsy 6, 7
  • Benign myoclonic epilepsy of infancy 8
  • Epileptic encephalopathies 3
  • Progressive myoclonus epilepsies 3, 7
  • Familial adult myoclonic epilepsy (FAME) 7

• Status myoclonus is a prolonged period of continuous and generalized myoclonic jerks, with a minimum reported duration of 30 minutes in prognostication studies 1

Differential Diagnosis

• Important to distinguish myoclonic seizures from:

  • Clonic seizures (repetitive rhythmic myoclonic contractions at 0.2-5 Hz) 4
  • Tonic seizures (sustained muscle contractions) 1
  • Syncope-related movements (typically fewer, asymmetrical, and occurring after loss of consciousness) 1
  • Psychogenic non-epileptic seizures (PNES) 1

• The term "myoclonus" is used for movements in syncope as well as for certain types of epilepsy, while "clonic" is typically restricted to epilepsy 1

Treatment

• First-line pharmacological treatment options include:

  • Valproate (widely used for most myoclonic seizures) 3, 7, 8
  • Levetiracetam (indicated as adjunctive therapy for myoclonic seizures in patients 12 years and older with juvenile myoclonic epilepsy) 6, 7
  • Clonazepam and other benzodiazepines (effective for most forms of myoclonus) 2, 3, 7

• For juvenile myoclonic epilepsy, levetiracetam treatment should be initiated with a dose of 1000 mg/day (500 mg twice daily) and increased by 1000 mg/day every 2 weeks to the recommended daily dose of 3000 mg 6

• Dosing must be adjusted for patients with impaired renal function 6

Prognosis

• Prognosis varies depending on the underlying syndrome:
  • Benign myoclonic epilepsy of infancy has good outcomes with early treatment 8
  • Juvenile myoclonic epilepsy and familial adult myoclonic epilepsy have favorable responses to medication but may require lifelong treatment 7
  • Status myoclonus starting within 48 hours after cardiac arrest is associated with poor neurological outcomes (FPR 0% [0-5%]), though some patients can recover 1, 2

Clinical Pearls and Caveats

• Myoclonic seizures can be triggered by photic stimulation, emotional stress, and sleep deprivation 7
• Some antiepileptic drugs may exacerbate or even induce myoclonus in certain patients 3
• Patients with post-arrest status myoclonus should be evaluated off sedation whenever possible; EEG recording can help identify awareness and reactivity 1
• Despite the typical association of status myoclonus with poor outcomes after cardiac arrest, several case reports document good neurological recovery despite early-onset, prolonged, and generalized myoclonus 1