Atlas of pathophysiology, 2 Edition

Part II - Disorders

Neurologic disorders

Epilepsy

Epilepsy is a condition of the brain characterized by susceptibility to recurrent seizures—paroxysmal events associated with abnormal electrical discharges of neurons in the brain.

Generalized seizures originate from multiple areas of the brain simultaneously. Partial seizures originate from a single focus but can spread or be generalized.

Causes

·         Half of all cases are idiopathic

·         Birth trauma

·         Anoxia

·         Perinatal infection

·         Genetic abnormalities, such as tuberous sclerosis and phenylketonuria

·         Perinatal injuries

·         Metabolic abnormalities, such as hypoglycemia, pyridoxine deficiency, or hypoparathyroidism

·         Brain tumors or other space-occupying lesions

·         Meningitis, encephalitis, or brain abscess

·         Traumatic injury ingestion of toxins, such as mercury, lead, or carbon monoxide

·         Stroke

·         Apparent familial incidence in some seizure disorders

Pathophysiology

Some neurons in the brain may depolarize easily or be hyperexcitable; this epileptogenic focus fires more readily than normal when stimulated. In these neurons, the membrane potential at rest is less negative or inhibitory connections are missing, possibly because of decreased gamma-aminobutyric acid activity or localized shifts in electrolytes.

On stimulation, the epileptogenic focus fires and spreads electrical current toward the synapse and surrounding cells. These cells fire in turn, and the impulse cascades to one side of the brain (a partial seizure), both sides of the brain (a generalized seizure), or cortical, subcortical, or brain stem areas.

The brain's metabolic demand for oxygen increases dramatically during a seizure. If this demand isn't met, hypoxia and brain damage ensue. Firing of inhibitory neurons causes the excitatory neurons to slow their firing and eventually stop.

If this inhibitory action doesn't occur, the result is status epilepticus: one prolonged seizure or one seizure occurring right after another and another. Without treatment, this may be fatal.

Signs and symptoms

Generalized tonic-clonic seizures

·         Altered consciousness

·         Tonic stiffening followed by clonic muscular contractions

·         Tongue biting

·         Incontinence

·         Labored breathing

·         Apnea

·         Cyanosis

Absence seizures

·         Change in level of awareness

·         Blank stare

·         Automatisms (purposeless motor activity)

Atonic seizures

·         Sudden loss of postural tone

·         Temporary loss of alertness

Myoclonic seizures

·         Brief muscle contractions that appear as jerks or twitching

Diagnostic test results

·         EEG reveals paroxysmal abnormalities to confirm the diagnosis and provide evidence of the continuing tendency to have seizures. In tonic-clonic seizures, high, fast voltage spikes are present in all leads; in absence seizures, rounded spike wave complexes are diagnostic. A negative EEG doesn't rule out epilepsy because the abnormalities occur intermittently.

·         Computed tomography scan and magnetic resonance imaging show abnormalities in internal structures.

·         Skull X-ray shows evidence of fractures or shifting of the pineal gland, bony erosion, or separated sutures.

·         Brain scan reveals malignant lesions when X-ray findings are normal or questionable.

·         Cerebral angiography shows cerebrovascular abnormalities, such as aneurysm or tumor.

·         Serum chemistry blood studies show hypoglycemia, electrolyte imbalances, elevated liver enzymes, and elevated alcohol levels, providing clues to underlying conditions that increase the risk of seizure activity.

Treatment

·         Drug therapy specific to the type of seizure, including phenytoin, carbamazepine, phenobarbital, gabapentin, and primidone for generalized tonic-clonic seizures and complex partial seizures

·         Valproic acid, clonazepam, and ethosuximide for absence seizures

·         Gabapentin and felbamate and other anticonvulsant drugs

·         Surgical removal of a demonstrated focal lesion, if drug therapy is ineffective

·         Surgery to remove the underlying cause, such as a tumor, abscess, or vascular problem

·         Vagus nerve stimulator implant

·         I.V. diazepam, lorazepam, phenytoin, or phenobarbital for status epilepticus

·         Administration of dextrose (when seizures are secondary to hypoglycemia) or thiamine (in chronic alcoholism or withdrawal)

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TYPES OF SEIZURES

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