CHAPTER 76 Seizure Disorders (Epilepsy)
Epilepsy is defined by the International League Against Epilepsy (ILAE) as a “condition characterized by recurrent (two or more) epileptic seizures, unprovoked by any immediate identified cause.” An epileptic seizure is “a clinical manifestation presumed to result from an abnormal and excessive discharge of a set of neurons in the brain.”1
Epilepsy can be disabling due to many domains, including physical effects of seizures and associated psychiatric and neuropsychological manifestations, as well as from the social stigmata. Management needs to encompass all of these domains, and is frequently challenging.
The word epilepsy is derived from the Greek word epilambanein, meaning “to lay hold of or to seize.” This definition accurately reflects the prevalent belief in antiquity that epilepsy was caused by a deity or by demonic possession. This identification of epilepsy as “the sacred disease” is mirrored in the nature of its treatments at the time, which involved supernatural or religious mechanisms.
The first recorded challenge of this interpretation appears at about 400 b.c. in the Hippocratic text, “On the Sacred Disease,” in which the author wrote that epilepsy was a disease involving the brain. However, the brain was not considered to be the site of origin of all epileptic seizures. Galen and others believed that although epilepsy involved the brain, involvement of other systems of the body was not seen as a secondary effect, but as the source of seizures themselves. For example, eclamptic seizures were believed to originate from the uterus.2
John Hughlings Jackson carried out observations crucial to the development of modern epileptology in the 1860s. In “Study of Convulsions,” he noted a certain order in the onset and spread of unilateral convulsions and concluded that focal origin of seizures was due to local pathology of a particular region of the brain.3 In the 1950s, a second model was proposed by a Canadian neurosurgeon, Wilder Penfield. Penfield found that some of his epileptic patients had seizures in which the electroencephalogram (EEG) revealed bilateral and symmetrical patterns. He proposed the centrencephalic model of epilepsy in which seizures originated in the central area of the brainstem.4
Epilepsy is the most common neurological disorder after strokes. Its incidence varies widely from country to country, but is estimated to be between 40 and 70 per 100,000 person-years in developed countries. Incidence of epilepsy is high during infancy, decreases to adulthood, but increases with advancing age.5,6
Most structural brain lesions increase the risk for seizures and epilepsy. Known risk factors for epilepsy include head trauma, cerebrovascular diseases, brain tumor, congenital or genetic abnormalities, infectious diseases, alcohol/drug use, and dementia. The risk of seizures from marijuana use is unclear.5,7,8
Epileptic seizures are caused by abnormal, repetitive firing of neurons. Although multiple etiologies may result in these discharges, it is believed that three key elements are con tributory: neuronal membrane and ion channel characteristics; reduced action of the inhibitory neurotransmitter γ-aminobutyric acid (GABA); and increased excitation through excitatory circuits through glutamate or other excitatory neurotransmitters.9,10 Absence seizures are believed to involve an abnormality in the circuitry between the thalamus and the cerebral cortex.11
Certain epileptic syndromes, both partial and generalized, follow single gene mendelian inheritance genetics. Since 1995, genetic discoveries have linked idiopathic epilepsies to mutations of voltage-gated channels, ligand-gated ion channels, and neurotransmitter receptors.12 Many other epilep-sies with genetic predispositions are likely due to complex multiple-gene inheritance patterns, and targeted pharmacological strategies have not yet emerged from these findings.
Hippocampal sclerosis is the most common cause of recurrent partial epilepsy in adults. This is characterized macroscopically by shrinkage and induration, and histopathologically by pyramidal neuron loss and gliosis, most severely found in certain subregions of the hippocampus.13 Hippocampal sclerosis can be seen on magnetic resonance imaging (MRI) scans, and the presence of this abnormality portends good outcomes after epilepsy surgery in patients with temporal lobe epilepsy (TLE).
Malformation of cortical development is the most common structural pathological abnormality seen in pediatric epilepsy. Malformation may occur during neuroglial proliferation (e.g., as in cortical dysplasia), migration (heterotopia), or organization (polymicrogyria) of the cortex.14 These conditions are frequently associated with severe seizures and with developmental delays.
The classification of seizures provides the basis for the classification of epileptic syndromes. Pharmacological therapies and prognoses are dependent on the proper classification of seizures and epileptic syndromes. The two models (focal and centrencephalic) provide the basis for the current international classification of seizures and epileptic syndromes.
The ILAE broadly classifies epileptic seizures into two groups (Table 76-1)15: focal (partial) seizures (i.e., those with an initial onset limited to one part of the brain); and generalized seizures (i.e., those with no discernible focus of onset). A third category consists of seizures that cannot be classified in these two categories.
I. Partial (focal, local) seizures
A. Simple partial seizures
1. With motor signs
2. With somatosensory or special-sensory symptoms
B. Complex partial seizures
1. Simple partial onset followed by impairment of consciousness
Modified from Commission on Classification and Terminology of the International League Against Epilepsy: Proposal for revised clinical and electro-encephalographic classification of epileptic seizures, Epilepsia 22(4): 489-501, 1981. Raven Press LTD, New York. © International League Against Epilepsy.
An epileptic syndrome is an “epileptic disorder characterized by a cluster of signs and symptoms customarily occurring together” (Table 76-2).16 Epileptic syndromes are defined by a variety of characteristics, including the types of seizures encountered and the findings on the EEG and on neuroimaging tests. Syndromes are classified into localization-related (or focal, partial) epilepsies, generalized epilepsies, special syndromes, and epilepsies that do not fall into the preceding groups.
1. Localization-related (focal, local, partial) epilepsies and syndromes
1.1 Idiopathic (with age-related onset)
Modified from Commission on Classification and Terminology of the International League Against Epilepsy: Proposal for revised classification of epilepsies and epileptic syndromes, Epilepsia 30(4):389-399, 1989. Raven Press LTD, New York. © International League Against Epilepsy.
Each category is subdivided into idiopathic, symptomatic, and cryptogenic subgroups. Idiopathic epilepsies are associated with epileptic seizures without other neurological or structural abnormalities of the brain. They tend to have a genetic predisposition, an age-related onset, and are generally benign. Symptomatic epilepsies are secondary to a specific cerebral abnormality, either genetic (e.g., tuberous sclerosis) or acquired (e.g., trauma). A third category, the cryptogenic group, refers to epilepsies that are not idiopathic, but in which the underlying condition causing the symptomatic epilepsy cannot be detected.
Generalized tonic-clonic (GTC) seizures (also called “grand mal,” generalized convulsive, or major motor seizures) are the most common type of generalized seizure; they also occur when a partial seizure becomes secondarily generalized.
In a purely GTC seizure, the first component is a loss of consciousness; the patient will be unaware of what has happened. The second component is the tonic stage, characterized by contraction of the skeletal muscles, extension of the axial musculature, upward deviation of the eyes, and paralysis of the respiratory muscles due to thoracoabdominal contractions. This stage is brief, ranging from only about 3 seconds to a maximum of 30 seconds, although it may seem longer because of its dramatic appearance. The most striking feature is extension of the upper and lower extremities into a semi-opisthotonic posture. Sudden spasm of the respiratory muscles results in forced exhalation that may sound like a scream, the so-called epileptic cry. Although contraction of the respiratory muscles causes the patient to stop breathing, it is not a cause for concern, since the tonic stage lasts only a few seconds.
As the muscles of mastication go into spasm, the patient may bite down hard. Tongue biting is seen frequently, but contrary to popular belief, the patient will not swallow his or her tongue, so objects such as a spoon or tongue depressor should not be inserted forcefully into the patient’s mouth. In the young patient, this action could also dislodge a loose tooth, which could be aspirated.
Eye movements that occur during the tonic stage can provide clues as to the nature of the seizure. In a generalized seizure, the eyes deviate directly upward, whereas in focal seizures, particularly those involving the frontal eye fields, the eyes deviate to the side contralateral to the lesion or focus. The direction of eye movement does not reliably lateralize in temporal seizures.
Once the tonic stage ends, the patient enters the clonic phase, which is characterized by rhythmic jerking movements. Clonic movements have a high amplitude and a low frequency, unlike myoclonic movements, which are very brief, or tremors, which have a low amplitude and high frequency. In a generalized seizure, clonic movements are symmetrical, with the arms and legs moving in unison. Clonic arm movements generally have greater amplitude than clonic leg movements, and the trunk is usually uninvolved.
The clonic stage generally lasts between 3 and 7 minutes, after which time the patient is usually conscious, but confused. If after 7 minutes the patient either does not wake up or has another seizure, the diagnosis of status epilepticus should be considered.
GTC seizures are also characterized by symptoms involving the autonomic nervous system. Hippus, in which the pupils alternately contract and dilate in a rhythmic pattern, is common, but occasionally the pupils may either contract or dilate. It is often useful to examine the pupils during a seizure, even in a patient on a respirator, since hippus can be a sign of seizures. Other common autonomic signs include changes in facial color (to either pallid or flushed), excessive salivation (to the point of drooling), increased heart rate and blood pressure, increasing intravesicular pressure, and relaxation of the urinary and anal sphincters (resulting in incontinence or defecation).
Absence (“petit mal”) seizures are another form of generalized seizures. Absence seizures occur mainly during childhood and are less frequent after puberty. They are characterized by the arrest or suspension of consciousness for 5 to 10 seconds. Parents may not notice the typical brief seizures in an otherwise healthy child, but teachers will report that the child stares absently for short intervals throughout the day. Without treatment, absence seizures may occur up to 70 to 100 times a day, and such frequent blackouts can seriously impair a child’s school performance. The physician can usually confirm the diagnosis by asking the child to hyperventilate, since this maneuver will precipitate an attack. Other signs include rhythmic blinking (at a rate of 3 blinks per second) and rudimentary motor behaviors, called automatisms, which also occur in adult TLE. Absence seizures are the easiest seizure disorder to diagnose because of its pathognomic EEG (a spike-and-wave pattern that occurs at a frequency of 3 cycles per second), especially when the child hyperventilates.
Myoclonic seizures are characterized by sudden, brief muscular contractions that may occur singly or repetitively. Myoclonic seizures may be present in devastating childhood epileptic syndromes, such as West syndrome, which is often accompanied by developmental delay. Myoclonic seizures may also occur in adolescents with gray matter disease and in adults with infections, such as viral encephalitis, and with prion disease. Myoclonic seizures may also occur in association with dialysis dementia and after severe hypoxic ischemic brain damage that has resulted from a cardiac arrest.
Unlike myoclonic seizures, akinetic seizures are characterized by a loss of muscle tone. The appearance depends on the muscles affected. “Head bobbing” occurs if the neck loses muscle tone, “bending seizures” occur if the upper extremities are affected, and “drop attacks” occur if the lower extremities are involved. However, it is much more common for children to faint than to have akinetic seizures. Although not diagnostic, the EEG is often helpful in identifying akinetic seizures. A typical EEG pattern consists of slow spikes and waves (or polyspikes and waves), even during the periods between seizures.