The temporal lobes of the brain play a crucial role in memory function; including episodic memory (memory for events) and semantic memory (memory for facts and concepts). In most people the left temporal lobe is the more dominant for verbal memories (involving words), and the right temporal lobe is more dominant for visual memories (involving pictures); but this so called ‘lateralisation’ can be verified using a technique called functional MRI. Evidence shows that adults who undergo surgery for drug-resistant temporal lobe epilepsy (TLE) have a high risk of memory problems, especially when the operation is on the left side.
It is possible that children benefit most from TLE surgery, a) because their brains are adaptable and can undergo ‘reorganisation’, and b) because the stopping/reduction of seizures may lead to a higher degree of cognitive development. However, the long-term impact of surgery in children is not yet known. In this Epilepsy Research UK-funded project, Dr Baldeweg and colleagues, at University College London, set out to explore this. Their results have been published in this month’s edition of Brain.
The Researchers recruited 42 children who had undergone one-sided TLE surgery (after in-depth investigation), and examined them for post-surgical changes in visual and verbal episodic and semantic memory. They then analysed each child’s MRI scans from before and after surgery, to assess the state of the hippocampus (an important memory structure) in the operated side, and measured the volumes of tissue that had been removed and remained; to see whether this impacted memory outcome. The overall follow-up period between surgery and assessment was an average of nine years.
The results showed no significant decrease in memory function as a result of surgery. Interestingly though, the children who had right TLE surgery showed an improvement in verbal episodic memory, and those who had left TLE surgery showed an improvement in visual episodic memory. This suggests that the unoperated temporal lobe had a surge in function after seizures stopped/were reduced.
When the MRI scans were incorporated in to the analysis, there was no link between the relevant measurements and change in memory function after surgery. However the team did notice that, in left TLE surgery patients in particular, the greater the volume of hippocampus left behind after surgery, the better the child’s verbal memory was at follow-up. They also observed (again particularly after left-sided surgery) better semantic memory scores when smaller amounts of tissue were removed, and when a specific area known as the temporal pole was intact. These findings were independent of post-surgical IQ and language lateralisation (the side that was dominant for language).
These findings suggest that, although there is an increase in the memory function of the intact lobe following epilepsy surgery, the degree to which a child’s brain can compensate for the loss of tissue from the operated side is limited by the volume that remains, and how intact specific areas of the lobe are.
By using this information to tailor the surgery of children with TLE, neurologists will hopefully be able to enhance memory outcomes in the future.
You can read the full paper here: http://brain.oxfordjournals.org/content/early/2014/11/06/brain.awu313
Source: Epilepsy Research UK