eMedicine Specialties > Neurology > Pediatric Neurology

Myoclonic Epilepsy Beginning in Infancy or Early Childhood

Author: Raj D Sheth, MD, Division Chief, Division of Pediatric Neurology, Department of Pediatrics, Nemours Alfred I duPont Hospital for Children
Contributor Information and Disclosures

Updated: Aug 21, 2008

Introduction

Background

Myoclonic seizures can occur in many types of epilepsy; however, in infancy and early childhood, they may occur as the dominant seizure type. The outlook and treatment of this condition differ from those of the more severe Lennox-Gastaut syndrome, which also may have myoclonic seizures as an important component.

Myoclonic epilepsies with onset in infancy and childhood are clinically and etiologically heterogeneous. At times in this heterogenous group, nonmyoclonic seizures may dominate the clinical picture.¹

The International League Against Epilepsy classified early myoclonic encephalopathy and early infantile epileptic encephalopathy with burst suppression as a generalized symptomatic epilepsy of nonspecific etiology.²  

For more information, see Medscape's Epilepsy Resource Center. For a CME/CE activity, see Maternal Infections During Pregnancy May Increase Risk for Childhood Epilepsy.

Pathophysiology

Myoclonic seizures are produced via a cortical or a subcortical generator that utilizes a polysynaptic mechanism acting on muscles rather than a monosynaptic corticospinal pathway. 

Seizures associated with early myoclonic encephalopathy can be heterogenous in etiology. A genetic basis has been suggested in some patients with a familial pattern.¹ Other cases are related to neurodegenerative disorders. 

Frequency

United States

The incidence is approximately 1 case in 40,000 children.

Mortality/Morbidity

Typically, patients do not die of myoclonic seizures but of the pathophysiological condition underlying the myoclonic epilepsy. Aspiration pneumonia is common in this population and results in frequent hospitalization.

Race

Myoclonic seizures are reported in all races.

Sex

No sex preponderance is observed in myoclonic seizures.

Age

Typically, the onset of these disorders is during the first 3 years of life.

Clinical

History

Syndromes of myoclonic epilepsy may be divided into the following:

• Symptomatic myoclonic epilepsy
  • This is less frequent than the idiopathic (ie, cryptogenic) variety, and the age of onset is usually between a few months and 2-3 years. Clinical features include psychomotor retardation, and neuroimaging frequently demonstrates brain atrophy.
  • Myoclonic jerks can occur alone, but they more commonly are associated with generalized clonic seizures. Rhythmic jerks can occur during sleep, with associated dystonic posturing during wakefulness.
  • The prognosis for meaningful cognitive function is poor, although the myoclonic jerks may be controlled medically.
• Cryptogenic myoclonic epilepsy
  • This group includes all patients with idiopathic seizures who display primarily recurrent myoclonic attacks. Some patients have infrequent generalized tonic-clonic seizures. Most myoclonic seizures are axial, which sometimes results in falls.
  • The outcome in these patients is usually favorable, although about half the patients may have behavioral or cognitive dysfunction. This has sometimes been termed "benign myoclonic epilepsy."
• Myoclonic epilepsy with other types of brief, recurrent seizures
  • In this group, all patients have myoclonic seizures associated with recurrent brief attacks of varying types.
    • Examples of these attacks include atonic seizures, atypical absence, partial seizures, and brief tonic seizures.
    • This is sometimes termed the myoclonic variant of Lennox-Gastaut syndrome.
  • The outcome is worse than in the cryptogenic myoclonic epilepsy group; more patients have cognitive dysfunction, and a significant proportion of those have severe mental retardation. However, the outcome is better than in patients with Lennox-Gastaut syndrome.
• Myoclonic epilepsy associated with clonic seizures and multiple seizure types
  • This condition is sometimes termed "severe myoclonic epilepsy" and occurs in a significant proportion of severe childhood epilepsies. The age of onset of seizures is between 4 and 11 months, and the seizures are initially unilateral or generalized clonic movements (or rarely, generalized tonic-clonic movements).
    • Seizures are usually long lasting, from 10-90 minutes in duration, and mostly associated with fever or minor infections.
    • Myoclonic seizures appear by the second or third year and are often photosensitive.
    • Atypical absence seizures may be seen without an electrographic correlate.
  • Initially, development may be normal; later, cognitive delays become evident and are usually moderate to severe. Most patients have fluctuating ataxia and erratic myoclonus.
  • The myoclonic seizures may resolve after a few years, but other seizures tend to be persistent. Nonconvulsive status is common.
  • EEG findings are normal initially, despite frequent seizures.
    • Later, generalized bursts of spike-wave and polyspike-wave complexes are seen.
    • Multifocal spikes also may be seen and may be unilateral.
  • Most patients are photosensitive and this feature may be diagnostic if present in children younger than 1 year.
  • MRI and CT scans are usually normal and do not show focal lesions or atrophy. Generally, the outlook is poor; most children are markedly dependent on care or institutionalized.
• Neonatal myoclonic epilepsy
  • This syndrome presents in the first 4 weeks of life with prominent myoclonic seizures.
  • The seizures usually result from a severe metabolic disorder, including that associated with elevated glycine levels in the cerebrospinal fluid (CSF), although they may be associated with any condition that produces severe brain dysfunction.
  • Distinguishing neonatal myoclonic epilepsy from benign neonatal sleep myoclonus is important. The latter condition is seen in healthy infants and occurs only in sleep.
  • EEG is very helpful in distinguishing between these 2 conditions. In benign sleep myoclonus, EEG findings are normal even when the infant is having myoclonus. In neonatal myoclonic epilepsy, EEG findings are always markedly abnormal in background and may even show a burst-suppression pattern.

Physical

• Abrupt and brief myoclonic jerks occur several times a day. They can manifest as head nodding, abrupt abduction of arms, or sudden falls.
• Eyelid or facial muscles are affected commonly, although axial myoclonic jerks are most common and occur in 90% of patients.
• Myoclonic seizures commonly occur on awakening, and some may be precipitated by photic stimuli. Rarely, myoclonic seizures occur continuously as myoclonic status epilepticus with partial preservation of consciousness.
• Although myoclonic seizures may occur as the sole type of seizures in some patients, they more commonly are associated with other seizures.
  • Generalized tonic-clonic seizures are the most common associated type. Brief generalized clonic seizures or unilateral clonic seizures also may be seen.
  • Atypical absence seizures occur in 40%, and pure atonic seizures may also occur.
  • Pure tonic seizures are not seen (this is an important distinguishing feature from Lennox-Gastaut syndrome).
• During the physical examination, particular attention should be given to conditions that may mimic myoclonic seizures. In the case of isolated myoclonic jerks, tics or mannerisms may be identified mistakenly as myoclonic attacks; usually, however, the diagnosis becomes clear over time.
• At times, absence seizures may be associated with myoclonic or clonic attacks. EEG is useful in differentiating myoclonic attacks from absence seizures.
• Progressive neurodegenerative diseases frequently present with myoclonic activity and other seizures types.
  • Neuronal ceroid lipofuscinosis (NCL) is important to consider in patients aged 2-5 years.
  • EEG is useful in NCL, since photic stimulation at 1 Hz produces giant evoked potentials in the form of a spike followed by a slow wave.
• Patients with Lennox-Gastaut syndrome have a mixed seizure disorder, including tonic seizures and drop attacks; however, myoclonic attacks are not a prominent feature. Additionally, EEG in Lennox-Gastaut syndrome has a more uniform slow spike and wave discharge pattern.
• Some severe cases of myoclonic epilepsy may be confused with febrile seizures. Prolonged duration, rapid recurrence, and occurrence of afebrile seizures differentiates myoclonic epilepsy from febrile seizures.

Causes

• Perinatal insults
• Inborn errors of metabolism
• Brain malformations
• Cryptogenic causes: Children with this condition have no identifiable cause for their syndrome and an appropriate workup for the seizures is negative.

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