Pediatrics

Essential myoclonus

OVERVIEW: What every practitioner needs to know

Are you sure your patient has essential myoclonus? What are the typical findings for this disease?

Myoclonus is an involuntary, sudden, shock-like, and brief contraction of a muscle or group of muscles. It can occur repetitively but is usually not rhythmic. Myoclonus can be induced by movement (action myoclonus) or by specific stimuli such as light or sound (reflex myoclonus). Positive myoclonus is when muscle contraction occurs. Negative myoclonus involves muscle relaxation (as seen in asterixis).

Essential myoclonus is considered a form of primary myoclonus, meaning that no proximal cause is identified or that genetic causes are suspected or identified.

Myoclonus can be classified in several ways:

Distribution: focal, multifocal, segmental, or generalized

Etiology: physiologic, primary (idiopathic or hereditary), epileptic, symptomatic (secondary to an underlying disorder), psychogenic

Time of occurrence: at rest, action or reflexive

Primary myoclonus subtypes

Essential myoclonus

  • Myoclonus is multifocal and arrhythmic

Benign myoclonus of infancy

  • May mimic infantile spasms

  • Myoclonus appears daily, usually in clusters

  • Symptoms diminish and resolve in childhood

Myoclonus dystonia syndrome

  • Myoclonus may dominate and dystonia may be apparent during activities

What other disease/condition shares some of these symptoms?

The critical element to consider is whether the myoclonus is epileptic in nature or nonepileptic. Types of myoclonus include the following:

Epileptic (without encephalopathy)

Juvenile myoclonic epilepsy

Myoclonic-astatic epilepsy

Epileptic (with encephalopathy)

Lennox-Gastaut syndrome

Neuronal ceroid lipofuscinosis

Lafora body disease

Mitochondrial encephalopathy

Nonepileptic

Primary

Benign myoclonus of infancy

Hereditary essential myoclonus

Myoclonus dystonia syndrome

Physiologic

Hiccups

Hypnic jerks (sleep myoclonus)

Exaggerated startle

Metabolic

Hyperammonemia (asterixis)

Uremia

Neurodegenerative

Huntington disease

Wilson disease

Whipple disease

Toxic

Serotonin syndrome

Hypoxic

Oncologic

Opsoclonus-myoclonus (neuroblastoma)

Psychogenic

What caused this disease to develop at this time?

Onset is typically in the first or second decade of life.

For benign myoclonus of infancy, onset is in the first year of life.

What laboratory studies should you request to help confirm the diagnosis? How should you interpret the results?

Electroencephalography is essential to rule out epileptic myoclonus.

Electromyography (EMG) can be considered but is typically not indicated. EMG can help differentiate "positive" myoclonus from "negative" myoclonus.

Laboratory studies should be tailored to findings in the history and physical examination. If the history is consistent with essential myoclonus, no additional testing is needed.

Would imaging studies be helpful? If so, which ones?

Magnetic resonance imaging (MRI) is indicated if there are focal findings on examination or concern for a neurodegenerative process.

Computed tomography of the abdomen and pelvis is indicated if opsoclonus is present with myoclonus to rule out neuroblastoma.

If you are able to confirm that the patient has essential myoclonus, what treatment should be initiated?

For most children with essential myoclonus, reassurance alone is indicated. Treatment with medication is typically reserved for individuals whose myoclonus is significantly impacting their quality of life.

Anticonvulsants: levetiracetam (10 mg/kg/d divided twice daily) valproic acid, clonazepam, zonisamide, piracetam, phenobarbital

Do not use oxcarbazepine, carbamazepine, or phenytoin because they are promyoclonic

What are the adverse effects associated with each treatment option?

Anticonvulsants: sedation, cognitive dulling, rash, weight gain (valproic acid), wieight loss (zonisamide)

What are the possible outcomes of essential myoclonus?

Essential myoclonus is typically benign and nonprogressive.

What causes this condition and how frequent is it?

Overall incidence and prevalence is not known, but it is felt to be rare

Affects men and women equally

Hereditary forms are typically autosomal dominant

Mutations in the epsilon sarcoglycan gene ( SGCE) have been linked to myoclonus dystonia syndrome

How do these pathogens/genes/exposures cause the disease?

Gamma-aminobutyric acid (GABA), glycine, and serotonin are all purported to play a role in the genesis of myoclonus.

What complications might you expect from the disease or treatment of the disease?

Essential myoclonus typically has a benign course.

How can essential myoclonus be prevented?

Essential myoclonus cannot be prevented.

What is the evidence?

Maydell, BV, Berenson, F, Rothner, AD. "Benign myoclonus of early infancy: an imitator of West's syndrome". J Child Neurol. vol. 16. 2001. pp. 109-12.

Shibasaki, H, Thompson, PD. "Milestones in myoclonus". Mov Disord. vol. 26. 2011. pp. 1142-8.

Caviness, JN, Truong, DD. "Myoclonus". Handb Clinc Neurol. vol. 100. 2011. pp. 399-420.

Dijk, JM, Tijssen, MA. "Management of patients with myoclonus: available therapies and the need for an evidence-based approach". Lancet Neurol. vol. 9. 2010. pp. 1028-36.

Mink, JW, Zinner, SH. "Movement disorders ii: chorea, dystonia, myoclonus and tremor". Pediatr Rev. vol. 31. 2010. pp. 287-94.

Kaddurah, AK, Holmes, Gl. "Benign neonatal sleep myoclonus: history and semiology". Pediatr Neurol. vol. 40. 2009. pp. 343-6.

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