Myoclonus epilepsy with ragged-red fibers
Jun. 10, 2021
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Myoclonic epilepsy in infancy is a rare self-limited idiopathic generalized epilepsy that typically appears between 6 months and 2 years of age. It is characterized by the occurrence of myoclonic seizures as the unique type of seizure (expect simple febrile seizure) occurring in normal infants, either spontaneously or induced by unexpected acoustic or tactile stimuli (reflex variant). The long-term outcome is usually favorable, with disappearance of seizures and normal cognitive functioning. However, reports of protracted evolution show that about 20% of patients have other seizure types, mainly at adolescence, and that about 39% have mainly mild to severe cognitive impairment. The reflex variant of this syndrome has a better prognosis, with nearly all patients becoming seizure-free within weeks or months from onset even without treatment. The syndrome should be differentiated from other epileptic and nonepileptic conditions presented with myoclonic jerks. The author presents clinical and EEG manifestations, epidemiology, pathophysiology, differential diagnosis, and management of this rather benign early onset idiopathic myoclonic epilepsy.
• Myoclonic epilepsy in infancy (formerly named “benign myoclonic epilepsy in infancy”) encompasses 2 forms: 1 with predominant spontaneous seizures and 1 with predominant reflex seizures elicited by unexpected acoustic and tactile stimulation. Spontaneous and reflex seizures may occur in the same infant.
• Myoclonic epilepsy in infancy is an early form of possibly genetic idiopathic generalized epilepsy.
• Seizures are self-limiting, pharmacoresponsive, and may remit without treatment.
• In spite of complete remission of seizures, long-term cognitive outcome is abnormal in around 30% of patients, usually in the range of mild impairment and rarely severe. The factors of this unfavorable outcome remain unknown.
• The reflex variant is truly benign, with early cessation of seizures and normal neurocognitive development.
Dravet and Bureau in 1981 described “benign myoclonic epilepsy in infancy” in 7 normal children with onset of myoclonic seizures in the first 3 years of life (13). The syndrome was defined as including myoclonic seizures only, except rare simple febrile seizures, with good prognosis regarding response to therapy and cognitive functions. Prior to this, early benign myoclonic epilepsy was reported in 3 infants in a study of early-onset epilepsies (11), but without known follow-up. In 1977 Jeavons gave the name ‘‘myoclonic epilepsy of childhood’’ to a similar type of syndrome, beginning at 3 years of age (21). Approximately 200 cases have been reported to date. The benignity of the syndrome has been questioned, leading to a change in its name to “myoclonic epilepsy in infancy” (16; 40). “Idiopathic myoclonic epilepsy in infancy” has been proposed (14). The ILAE Commission on Classification and Terminology initially proposed replacing “idiopathic” with “genetic” (05) but more recently recommended that either idiopathic or genetic can be used (31).
Ricci and associates described 6 normal infants with reflex myoclonic jerks provoked by unexpected (startle) auditory or tactile stimuli (29). Seizure onset was before the age of 2 years with rapid remission after 3 to 12 months. They proposed that this constitutes a new “reflex myoclonic epilepsy of infancy,” which is an age-dependent idiopathic generalized epilepsy syndrome, with an apparently good prognosis (29). To date, about 80 cases have been described by other authors (37). It is debated whether this reflex form is a variant of the myoclonic epilepsy in infancy or an independent stimulus-sensitive syndrome.
In the ILAE proposal, myoclonic epilepsy in infancy including the reflex variant is classified among neonatal/infantile epileptic syndromes (05). The following is a complete description from the ILAE epilepsy manual (Commission on Classification and Terminology of the International League Against Epilepsy 2014):
Overview. This epilepsy syndrome is uncommon. Myoclonic seizures are the only seizure type seen at onset. Infrequent febrile seizures may also occur. Myoclonic seizures may be activated by photic stimulation in one fifth of the patients, whereas others may have myoclonic seizures that are induced by sudden noise or touch. Cognitive, behavioral, and motor difficulties may exist. Seizures are self-limiting, ceasing within 6 months to 5 years from onset. Generalized convulsive seizures may be seen infrequently during the teen years.
Note. Self-limiting refers to seizures having a high likelihood of spontaneously remitting at a predictable age.
Clinical context. This syndrome is characterized by the onset of myoclonic seizures between the ages of 6 months and 2 years, and in some cases, earlier (4 months) or later (2 to 4 years) onset has been reported. Myoclonic seizures may be induced by photic stimulation in some patients or by sudden noise or touch in others. Infrequent febrile seizures may be seen in approximately 10% of patients. Seizures remit within 6 months to 5 years from onset, but generalized convulsions may be seen in teenage years in 10% to 20% of patients. Patients with photosensitivity may have seizures that are more difficult to control. Males are twice as likely to be affected as females. Antecedent and birth history is unremarkable. Head size and neurologic examination are normal. Cognitive, motor, and behavioral difficulties are reported, especially if seizures are not controlled.
Mandatory seizures. Myoclonic seizures are seen. These are mainly of the head (causing nodding), eyeballs (which roll upwards), upper extremities (causing the arms to fling up and out), and the diaphragm (resulting in vocalization). Rarely, the lower limbs are affected, causing falls. Jerks can be singular or can occur in a series and may vary in severity. Responsiveness is preserved; however, it may be reduced if there are clusters of jerks in a series. Jerks may occur in all states (alert, drowsiness, slow sleep, and on wakening).
Patients may have:
• Simple febrile convulsions (seen in 10%, infrequent)
• Generalized convulsive seizures (seen in 10% to 20% of patients in teens, infrequent)
• Early-onset absence seizures have been reported in up to 20% of myoclonic epilepsy in infancy patients.
• Evolution to juvenile myoclonic epilepsy may occur.
• Evolution to drug-resistant epilepsy has been reported in case reports.
• Evolution to myoclonic-astatic epilepsy has been reported in case reports.
Exclusionary. Other seizure types are exclusionary.
EEG Background. The background EEG is normal.
Interictal EEG. The interictal EEG is normal.
Activation. Sleep may activate the EEG, and generalized spike-and-wave and polyspike-and-wave may occur, with or without accompanying myoclonic jerks clinically. One fifth of patients have photosensitivity, with myoclonic jerks precipitated by photic stimulation. In 10% of patients, myoclonic jerks may be activated by sudden noise or touch in either awake or sleep states.
Ictal EEG. Myoclonic jerks are associated with generalized spike-and-wave or polyspike-and-wave discharges.
Imaging. Neuroimaging is normal.
Genetics. The pattern of inheritance is unknown but is likely genetic.
Family history of seizures/epilepsy. A family history of febrile seizures or epilepsy is seen in one third of patients.
• Dravet syndrome: myoclonic seizures are frequent, however typically occur in the second year of life and are preceded by a period of susceptibility to febrile convulsions.
• Hypnogogic jerks
• Benign myoclonus of early infancy or benign spasms of infancy
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