**2005 Report submitted by Benchmark Steward:**

Jerome Engel, M.D., Ph.D. (UCLA School of Medicine)

**Background of the benchmark goal:**

Epilepsy is a chronic disorder characterized by recurrent epileptic
seizures, resulting from a persistent localized or diffuse disturbance
of brain function. Although interictal (between seizures) EEG recordings
can reveal epileptiform abnormalities, such as spikes, which help to
diagnose the existence of an epileptic disorder, and to localize the
epileptogenic region, these interictal electrographic events are
relatively nonspecific: they do not occur in all people with epilepsy;
they do occur in some people without epilepsy; their location does not
accurately coincide with the region generating spontaneous seizures; no
features of these events have been demonstrated to correlate with the
severity of the epileptic condition; and they can not predict when an
epileptic seizure will occur. There is a great need for noninvasive,
interictal electrophysiological and/or neuroimaging surrogate markers
that reliably indicate the existence of an epileptic disorder, the
location and degree of epileptogenicity, and predict when and whether a
next seizure will occur.

A noninvasive, interictal surrogate marker that reliably indicates the
location and degree of epileptogenicity would: 1) greatly enhance our
ability to screen potential epileptic compounds in experimental models
of epilepsy (Benchmark IC3); 2) permit the effectiveness of
antiepileptic drugs to be determined in individual patients without the
need to wait for another seizure to occur (Benchmark IIA); 3) provide a
substrate for evaluating new approaches to the treatment of epilepsy
(IIIA); 4) provide an accurate and cost-effective means of localizing
the epileptogenic region for resective surgical therapy (IIIF), and 5)
if also indicative of epileptogenesis, allow determination of the risk
for developing epilepsy, so that preventive treatments can be introduced
(IIA). A surrogate marker that reliably predicts when seizures will
occur will not only enhance patients' ability to protect themselves from
injury during these events and perhaps institute self-abortive measures,
but will also provide opportunities to devise automated interventional
feed-back systems to prevent spontaneous seizure occurrence (Benchmark
IIIE).

**Current status of field:**

*Overview of Interictal Surrogate Markers of Epileptogenicity and
Epileptogenesis.* Until recently, very little research was devoted to
identifying reliable surrogate markers of epileptogenicity. Initial
research focused on the interictal EEG epileptiform spike. Spike mapping
during intraoperative electrocorticography was a common technique for
determining the size of brain resections to treat intractable focal
epilepsy, but the high degree of false localization based on this method
was well-recognized. Theodore Rasmussen, a prominent neurosurgeon at the
Montreal Neurologic Institute, noted that spike mapping should provide
more reliable information because there are red spikes and green spikes;
the problem is that electrophysiologists are not able to tell them
apart. Attempts to distinguish red from green spikes based on aspects of
spike morphology, such as rise time, sharpness, or aftercoming slow
waves, by patterns of recurrence such a frequency, or by response to
suppression by drugs such as methohexitol, were generally unsuccessful.

Recent studies suggest that EEG spikes with high frequency oscillations
(250-600 Hz), termed Fast Ripples, may be unique to structures capable
of generating spontaneous seizures in patients with mesial temporal lobe
epilepsy, and also in animal models of this disorder. There has been
considerable interest in this phenomenon in the past fourteen months,
elucidating the specificity and potential neuronal substrates of
epileptiform high frequency oscillations (1-14). Some slower
oscillations in the "ripple" frequency (100-200 Hz) may also be abnormal
epileptiform events with a similar significance with respect to
epileptogenicity and these epileptiform high frequency oscillations
occur early after an epileptogenic insult, long before seizures begin.
High frequency oscillations, therefore, appear to be putative surrogate
markers not only for epileptogenicity, but also for epileptogenesis (1).

Although these putative red spikes can so far only be detected by direct
brain recordings, which require depth electrode implantation, they
indicate that the pathophysiological mechanisms responsible for
interictal EEG spikes originating in tissue capable of generating
spontaneous seizures is different from those of propagated spikes, or
spikes originating in secondary epileptogenic areas that are not capable
of generating spontaneous seizures. Consequently, research has now begun
to determine whether higher resolution noninvasive EEG and/or
magnetoencephalography (MEG) can identify these high frequency
epileptogenic oscillations. Other techniques such as functional magnetic
resonance imaging (fMRI) have a high likelihood of detecting the
metabolic consequences of these red spikes. A number of laboratories
have successfully used EEG spike-triggered fMRI averaging to
anatomically localize the source of interictal EEG spikes, and studies
in the past 14 months continue to provide hope that the fMRI image
produced by spikes originating in primary epileptogenic regions will be
different from those that are propagated, or originate in secondary
epileptogenic regions (15-31).

Positron emission tomography (PET) may provide another novel approach to
identification of seizure-generating brain tissue. In patients with
tuberous sclerosis who have multiple brain tubers and epileptic
seizures, alpha methyl tryptophan (AMT) was found to accumulate
preferentially in the tubers that generated spontaneous seizures (32).
AMT uptake appears to detect the presence of increased activity in the
kynurenine pathway, which may be important for the epileptogenicity of
this condition. Two studies on AMT-PET were published in the past 14
months (33-34). In addition, 5-HT1A receptor ligands have been
identified as showing changes with epilepsy. \[18F\]FCWAY binding was
lower in the epileptic hippocampus of patients with mesial temporal lobe
epilepsy (35), and \[18F\]MPPF also showed decreased binding in
epileptic hippocampus (36). If these binding changes reflect dynamic
disturbances, rather than merely cell loss, it is possible that labeling
with a shorter half-life radionuclide would permit continuous studies
over time to demonstrate changes in epileptogenicity.

Structural imaging does not indicate epileptogenicity and does not, per
se, provide information that could be considered surrogate markers of
epileptogenicity or epileptogenesis; however, MRI of rats following
lithium-pilocarpine-induced status epilepticus suggests that changes
identified in piriform and entorhinal cortices may predict which animals
will develop epilepsy (37). I am unaware of any further work in this
area since the last benchmark report.

A potentially exciting new development for functional MRI is the use of
magnetized nanoparticles (MNP), extremely small magnetized particles
that are visible on MRI, and that can be attached to a wide variety of
bioactive molecules (38-42). Consequently, MNP could be used to measure
localized alterations in neurotransmitter activities that reflect brain
excitation and inhibition, cerebral metabolism, immune responses, and
drug distribution with extremely high spatial resolution. The potential
for this new technology to be used to identify surrogate markers of
epileptogenesis and epileptogenicity is enormous.

Transcranial magnetic stimulation (TMS) is currently being investigated
as a possible diagnostic, as well as therapeutic, tool for epilepsy at a
number of laboratories. TMS, particularly paired-pulse stimulation, can
be used to measure cortical excitability, a potential surrogate marker
of epileptogenicity (43-52). It is conceivable that such TMS measures
could eventually be employed to assess, for instance, the effect of an
antiepileptic drug on reducing epileptogenicity without the need to wait
for another seizure to occur.

Many investigators are now using gene microarray technology to examine
alterations in genetic profile of animal and human brain regions that
may correlate with an epileptic disturbance. Gene expression profiles
reflect dynamic processes that could cause, or result from, changes in
epileptogenicity and serve as surrogate markers not only of epilepsy,
but of the severity of an epileptic condition. Genetic expression
fingerprints of brain disorders can also be measured in peripheral blood
(53-55), suggesting the possibility that genomic surrogate markers for
epileptogenesis and epileptogenicity may eventually be monitored from a
finger stick. If gene expression profile surrogate markers are
identified in the brain, but not in peripheral blood, there may be ways
to image them noninvasively with PET or MNP-MRI.

Optical intrinsic signal (OIS) imaging is an invasive technique for
measuring changes in brain associated with neuronal activity. Several
investigators are applying this technique to animal models of epilepsy
and in patients during surgery, and it may yield results in future that
could become surrogate markers of epileptogenicity. Only modest progress
has been made in the past year (56-58).

Surrogate markers of epileptogenicity specifically for the purpose of
screening potential antiepileptic compounds that are more relevant to
current medically refractory epilepsy syndromes, and sufficiently
cost-effective to permit rapid throughput of tens of thousands of
compounds, would revolutionize drug discovery. Preliminary efforts in
this direction might include the development of assays in small mammals
such as mice, using Fast Ripples, but more innovative approaches are
also being investigated with zebrafish and fruit flies. Ideally, drug
screening ought to be accomplished using integrated circuits constructed
from cultured neurons.

*Overview of Surrogate Markers for Seizure Prediction*. The use of
depth-recorded epileptiform EEG events to predict the onset of epileptic
seizures and automatically activate seizure-aborting interventions has a
long history, and was popularized by the Michael Crichton novel and film
"The Terminal Man." Nevertheless, such efforts remained in the realm of
science fiction until recent publications from several laboratories
demonstrating that non-linear dynamic analysis of ongoing depth-recorded
EEG activity can reliably identify changes in neuronal synchronization
that precede epileptic seizure occurrence by several minutes. One of
these laboratories has successfully applied this technique to scalp EEG
recordings, demonstrating that noninvasive seizure prediction and
automatic seizure abortion is feasible. Work in this area over the past
14 months provides additional evidence for dynamical surrogate markers
defining the preictal state (59-69).

**Activities update:**

*Interictal Surrogate Markers of Epileptogenicity and Epileptogenesis:*
NINDS research grants currently support work on Fast Ripples, PET with
AMT, and OIS, while various sources, including private foundations and
venture capital, are supporting MEG and fMRI-EEG spike imaging studies.

*Surrogate Markers for Seizure Prediction:* In addition to research
grants from government and private foundations, at least two medical
instrument companies (NeuroPace and Medtronics) are interested in a
feed-back approach to seizure control and are currently funding
investigations, not only to refine techniques for seizure prediction,
but to create effective seizure-aborting interventions.

**Top priorities for next 5-10 years:**

The recent evidence that identification of reliable, noninvasive,
surrogate markers is feasible should stimulate an increased interest in
both basic and clinical research on these important topics. The NINDS
could further this research direction by encouraging investigators to
consider how their ongoing research may contribute to these goals and
objectives. The time is now right for an international workshop on
surrogate markers.

The most fruitful areas of investigations over the next five to ten
years would include work to: 1) further establish and characterize the
role of Fast Ripples as an interictal surrogate marker of
epileptogenicity; 2) determine if EEG, MEG and/or fMRI can identify
these events noninvasively; 3) develop novel PET tracers to measure
compounds in the brain that are associated with epileptogenicity; 4)
develop MNP technology in order to identify surrogate markers that can
be imaged with MRI; 5) search for gene expression fingerprints
measurable in peripheral blood that reflect epileptogenicity; and 6)
pursue noninvasive computerized EEG analysis to further define pattern
changes that precede seizure onset.

**Roadblocks to progress:**

The major roadblock to progress is the high risk/high payoff nature of
these projects, which makes it extremely difficult to get NIH funding to
pursue the most creative concepts.

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