Traditionally, focal and generalized epilepsy are classified as distinct types of seizure disorders. However, generalized epilepsy and focal epilepsy can overlap, and this interaction can blur the line between the two disorders. This makes diagnosis challenging for people experiencing these types of episodes. What is the difference between these two seizure types? How do they interact? Finally, why does this interaction occur?
The types of seizures that define focal and generalized epilepsy are focal or generalized seizures.
Focal seizures start in a single area of the brain (on one side). They can involve movement or lack of movement. A person may remain aware during these episodes or may lose awareness.
Generalized seizures start in both halves of the brain (hemispheres) at the same time. These are tonic-clonic seizures. A person loses awareness during generalized seizures. This category includes absence seizures.
Therefore, the primary difference between generalized epilepsy and focal epilepsy is how and where seizure activity starts. Specialists used this principle to reclassify seizure disorders in 2017 to give them more accurate names and stress the importance of the seizure’s origin.
If a focal seizure spreads to involve a large enough area (or enough nodes) of the brain, it can turn into a generalized seizure. This is different from a primary generalized seizure, which starts on both sides of the brain at once.
The evolution of a focal seizure to a generalized seizure can happen very quickly, often taking only seconds. The person may initially experience a sudden uncontrollable movement or change in sensation like smell. They may be confused. The person may then progress into a classic tonic-clonic seizure, with loss of consciousness, jerking uncontrollable movements, and loss of bowel and bladder control. After the seizure finishes, the person may experience a period of confusion, drowsiness, change in mood, headache, and nausea.
The reverse is also possible. Research has shown that someone can have seizures that have a generalized onset but then evolve into a focal seizure. This has been proven clinically and also shown on electroencephalogram (EEG) testing, which measures brain waves.
It is also becoming more widely known that focal seizures can occur in cases of idiopathic generalized epilepsy. This group of disorders includes generalized seizure disorders of unknown causes that are thought to be largely driven by genetic factors. Unfortunately, the presence of focal seizures in idiopathic generalized epilepsy often leads doctors to misdiagnose it as focal epilepsy. Recently, people have also argued to classify generalized onset seizures with focal evolution as a unique seizure type.
Someone can have both types of seizures one after the other, and this can present a diagnostic dilemma.
The interaction between focal seizures and generalized seizures creates a problem because it complicates the diagnostic process. Not only does this interaction often lead to misdiagnosis, but it lengthens the time until the most appropriate treatment methods can start.
When treating seizure disorder, one of the first considerations is the type of seizure the person has. This will help dictate which medication is used. Broad-spectrum anticonvulsants are typically used first and for generalized seizures. Narrow-spectrum anticonvulsants are normally used for the treatment of partial seizures.
The identification of the correct seizure type in an individual will help neurologists avoid narrow-spectrum drugs that can worsen generalized seizures. This can also help people avoid some negative side effects of anticonvulsants, by ensuring they receive the right drugs earlier in their treatment process.
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