Myoclonus epilepsy with ragged-red fibers
Jun. 18, 2022
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Startle epilepsy is characterized by seizures triggered by unexpected sudden sensory stimuli, usually sound or touch. Most patients have intractable seizures, evident static encephalopathy, and neurologic deficits (infantile hemiplegia is common) with corresponding abnormal brain imaging. The startle response consists of axial tonic posturing, frequently causing falls that can often be traumatic. Concurrent symptoms, such as marked autonomic manifestations, automatisms, laughter, and jerks, may occur. Less commonly, startle-induced seizures may be atonic or myoclonic, particularly in patients with cerebral anoxia. Seizures are frequent, occurring many times a day, and sometimes progress to status epilepticus. The prognosis is often poor, particularly for those with severe preexisting encephalopathies. Differentiation between startle-induced seizures and exaggerated startle responses can be challenging. Startle epilepsy should be differentiated from other nonepileptic disorders with abnormal responses to startling events and mainly hyperekplexia. Startle epilepsy is often resistant to medical treatment. Epilepsy surgery may be beneficial to those with focal etiology of seizures. The author details clinical manifestations, etiology (including inborn errors of metabolism), investigative procedures, pathophysiology, and management issues (including neurosurgery) of startle-induced epileptic seizures.
• Startle epilepsy is characterized by seizures precipitated by sudden, unexpected stimuli, usually sound or touch.
• Startle-induced seizures mainly consist of axial tonic posturing, frequently causing falls, which can often be traumatic.
• Typically it affects children with severe neurologic deficits, brain anoxia, infantile hemiparesis, and Down syndrome.
• Startle epilepsy should be differentiated from nonepileptic, startle-inducing movement disorders and mainly hyperekplexia.
• The prognosis is often poor, particularly for those with severe pre-existing encephalopathies.
• Total control of the seizures is almost impossible though certain antiepileptic drugs may be beneficial, and neurosurgery in some patients with focal lesions may be therapeutic.
Modern descriptions of startle epilepsy date from the 1950s, but older literature suggests that some confusion may have existed between startle epilepsy and nonepileptic disorders such as hyperekplexia and Tourette syndrome, and confusion with nonepileptic paroxysmal movement disorders may persist.
The 1989 International Classification of Epilepsies and Epileptic Syndromes defines startle-induced epileptic seizures as follows: “Epileptic seizures may also be precipitated by sudden arousal (startle epilepsy); the stimulus is unexpected in nature. The seizures are usually generalized tonic but may be partial and are usually symptomatic” (16).
The ILAE diagnostic scheme considered “startle epilepsy” to be a reflex epileptic syndrome although most realistically this is a type of startle-induced seizures that occurs in a heterogeneous group of patients of variable etiologies and EEG correlates. Startle-induced seizures or startle epilepsy are not considered in the most recent position reports of the ILAE Commission for Classification and Terminology (25; 53) or in the ILAE glossary (09). However, in the new EpilepsyDiagnosis.org ILAE online diagnostic manual of the epilepsies, startle epilepsy is recognized and is detailed as shown below (15).
Startle epilepsy. Startle epilepsy is characterized by seizures that may start in childhood or early adolescence (1 to 16 years). Both sexes are affected. Most patients have underlying structural brain abnormality with neurologic (eg, hemiplegia) and intellectual disability. Seizures are often difficult to control.
Clinical context. Seizures are exclusively induced by startle with a sudden brief tonic seizure, typically affecting the trunk and causing a fall. Tonic seizures often have focal features with asymmetry. There may be associated autonomic features or automatisms. Other seizure types may occur, including atonic and myoclonic seizures. Seizures are typically frequent.
Interictal EEG. Interictal EEG findings reflect the underlying etiology (eg, structural brain abnormality).
Ictal EEG. Ictal EEG shows an initial midline vertex discharge followed by diffuse attenuation or low voltage fast activity (which may have onset or emphasis in areas of structural brain abnormality, and then spread). Ictal EEG may be obscured by myogenic artifact.
Neuroimaging is usually abnormal, with acquired or developmental structural brain abnormality, typically involving lateral sensorimotor cortex.
Genetics. Startle epilepsy is usually due to structural brain abnormality and is not inherited, and it may occur in children with Down syndrome.
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