In order to make an accurate diagnosis of epilepsy and assign the most appropriate treatment, epileptologists must be able to detect epileptic activity in the brain. They also need to locate the region in which seizures originate (the seizure focus) and if/where they spread.
In practice, a common test used (amongst others) is electroencephalography (EEG), in which electrical waves in the brain are recorded via electrodes placed on the scalp. This is a non-invasive and relatively simple procedure; however it has a number of limitations. For example, the number of electrodes used in a standard EEG is relatively small, and they are widely spaced over the scalp, meaning that large areas of potentially important activity are missed. In addition, a positive diagnosis usually relies upon a person actually having a seizure at the time of assessment, which is not always the case, even if the EEG period is extended. This can lead to significant delays in diagnosis and treatment (sometimes up to several years) and more invasive diagnostic methods are often required.
Researchers at the University of Minnesota and the Mayo Clinic (also inMinnesota) have now found a different method that might overcome these hurdles. Earlier studies suggest that specific electrical signals, known as a cortical slow waves (CSW), increase in number shortly after a temporal lobe seizure. The group in Minnesota wondered if by monitoring these signals they could obtain important information about a person’s seizures, without a seizure actually having to occur.
In the current study, the scientists recruited 28 people with temporal lobe epilepsy who suffered a range of seizure types – simple partial (SPS), complex partial (CPS) and secondary generalised (SGS). They used a specialised form of scalp EEG known as dense array EEG (daEEG) to monitor the subjects’ brains immediately after a seizure (known as the post-ictal period), and focused on CSW activity in particular. One of the main advantages of daEEG over standard scalp EEG (and indeed more invasive forms of EEG), is that it involves many more recording electrodes, meaning that a greater area of the brain can be covered. Advanced imaging techniques can be used to ‘translate’ this information and produce a much clearer picture of the brain’s activity.
The team discovered that all three types of seizure were followed by the appearance of CSW, and that the number of CSW increased with seizure severity (most in SGS, followed closely by CPS and least in SPS). They also noticed that the CSW tended to spread from the temporal lobe in which the seizure originated to the frontal lobe, and often to the temporal lobe on the other side of the brain. Again this was more apparent in SGS andCPS than in SPS.
These findings need to be explored further (and in other types of epilepsy); however they indicate that a seizure doesn’t have to be taking place in order for important information about a person’s epilepsy to be obtained. They also suggest that temporal lobe seizures, including SPS, may have wider reaching effects than previously thought.
Having examined the patterns of CSW activity following SGS, CPS and SPS, the researchers suggest that CSW may be responsible for the cognitive and behavioural changes that accompany these types of seizure. If this is the case, it may become possible to manipulate CSW and prevent these changes in the future.
In the shorter term daEEG could potentially become a useful non-invasive tool, helping neurologists to diagnose epilepsy more quickly and accurately, thus reducing delays to treatment.
http://www.epilepsyresearch.org.uk/
