(USMLE topics) Types of seizures, signs and symptoms, pathology (mechanism) of epilepsy, causes and treatments.
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Epilepsy is a group of neurological diseases characterized by recurrent seizures. Seizures happen as a result of a sudden surge in the brain’s electrical activities. Depending on which part of the brain is affected, a seizure may manifest as loss of awareness, unusual behaviors or sensations, uncontrollable movements or loss of consciousness. The brain is a complex network of billions of neurons. Neurons can be excitatory or inhibitory. Excitatory neurons stimulate others to fire action potentials and transmit electrical messages, while inhibitory neurons SUPPRESS this process, preventing EXCESSIVE firing. A balance between excitation and inhibition is essential for normal brain functions. In epilepsy, there is an UP-regulation of excitation and/or DOWN-regulation of inhibition, causing lots of neurons to fire SYNCHRONOUSLY at the same time. If this abnormal electrical surge happens within a limited area of the brain, it causes PARTIAL or FOCAL seizures. If the entire brain is involved, GENERALIZED seizures will result. Partial seizures subdivide further to: - Simple partial: - Complex partial Generalized seizures subdivide further to: - Absence seizures: - Tonic seizures - Atonic seizures, drop attacks - Clonic seizures - Myoclonic seizures - The most common and also most dramatic are tonic-clonic seizures, also known as convulsive seizures, which are combinations of muscle stiffening and jerking. This type is what most people relate to when they think of a seizure. It also involves sudden loss of consciousness and sometimes loss of bladder control. A tonic-clonic seizure that lasts longer than 5min requires immediate medical treatment. Epilepsy may develop as a result of a brain injury, tumor, stroke, previous infection or a birth defect. Generalized seizures that start in childhood are likely to involve genetic factors. Epilepsy due to a single gene mutation is rare. More often, an interaction of multiple genes and environmental factors is responsible. Hundreds genes have been implicated. Examples include genes encoding for GABA receptors – major components of the inhibitory circuit, and ion channels. Many genetic disorders that cause brain abnormalities or metabolic conditions have epilepsy as a primary symptom. The cause of epilepsy is unknown in about half of cases. Diagnosis is based on observation of symptoms, medical history, and an electroencephalogram, or EEG, to look for abnormal brain waves. An EEG may also help in differentiating between partial and generalized seizures. Genetic testing maybe helpful when genetic factors are suspected. There is no cure for epilepsy but various treatments are available to control seizures. Medication successfully controls seizures for about 70% of cases. Many anti-epileptic drugs are available which target sodium channels, GABA receptors, and other components involved in neuronal transmission. Different medicines help with different types of seizures. Patients may need to try several drugs to find the most suitable. Dietary therapy: ketogenic diet has been shown to reduce or prevent seizures in many children whose seizures could not be controlled with medication. Ketogenic diet is a special high-fat, low-carbohydrate diet that must be prescribed and followed strictly. With this diet, the body uses fat as the major source of energy instead of carbohydrates. The reason why this helps control epilepsy is unclear. Nerve stimulation therapies such as vagus nerve stimulation in which a device placed under the skin is programmed to stimulate the vagus nerve at a certain rate. The device acts as a pacemaker for the brain. The underlying mechanism is poorly understood but it has been shown to reduce seizures significantly. Finally, a surgery may be performed to remove part of the brain that causes seizure. This is usually done when tests show that seizures are originated from a small area that does not have any vital function.