July 06, 2007
Virus linked to temporal lobe epilepsy:
Open-access science journal PLoS Medicine published a recent study that suggests that infection with the herpes virus might cause temporal lobe epilepsy in some people.
The study found the virus in the brains of 11 out of 16 patients with temporal lobe epilepsy but not in those with other forms of epilepsy.
Studies that test brain tissue are often done post-mortem, on people who have died, because brain surgery is just too risky for the sake of removing samples for research.
This study is particularly impressive because it studied brain tissue from live patients.
In severe cases of epilepsy that don't respond to medication, one option is to find which bit of the brain triggers the seizures (the 'foci') and surgically remove it.
This is particularly effective in people with mesial temporal lobe epilepsy, a type in which the foci is deep within the temporal lobes (mesial means 'towards the midline'), usually stemming from disturbance in the hippocampus.
The team examined brain tissue removed in operations on 22 patients, and tested it for the presence of the human herpesvirus 6B (HHV-6B).
This type of herpes infection is incredibly common, more than 90% of the population have it. Normally, it's completely harmless and just lies dormant in the body.
We don't really know why, but in some people, it seems to reactivate, and is linked to neurological disorders like multiple sclerosis.
The researchers found that it was present in brain cells called astrocytes from 11 out of 16 patients with mesial temporal lobe epilepsy, but wasn't present at all in patients with other types of epilepsy.
The image on the right is of a herpes infected astrocyte, the infection is visible due to a green marker.
They also studied one patient in more detail. He had four operations in a row, each of which reduced his seizures, until the final one left him seizure-free.
They found that the herpes virus was present most strongly in the temporal lobe tissue from the first operation, was weakly present in later operations, and wasn't present in other brain areas.
They also found that infected brain tissue didn't produce very much of a chemical that transports the key neurotransmitter glutamate across the brain.
If it doesn't get transported properly, it 'hangs around', and because glutamate tends to make brain cells more active, too much could lead to overactivity and seizures.
To test the herpes - glutamate link, the team deliberately infected brain tissue taken from a patient without a previous infection.
In the lab, they discovered that herpes slowed the creation of the transporter chemical for glutamate, providing strong evidence for the link.
The evidence from the lab tests, the single case study, and the 22 patients, provides strong evidence that herpes infection could lead to temporal lobe epilepsy in some people.
This is an important finding because it suggests a cause for the disorder in some people, and provides a clear target which could lead to better treatments and prevention strategies.
What is still not clear is why this usually harmless infection might cause some people severe neurological problems, and remain dormant in others.
Link to PLoS Medicine paper.
