Role of Nuclear Medicine in Drug-Resistant Focal Epilepsy

Neurology · 3 min read

In drug-resistant focal epilepsy, interictal FDG PET and ictal perfusion SPECT are what move the surgical decision — and ictal SPECT lives or dies by the injection time.

About a third of people with focal epilepsy never become seizure-free on medication, and for them surgery offers the best chance of cure. That only works if the epileptogenic zone can be found — and when the MRI is normal, or points somewhere the EEG does not, functional imaging is usually what breaks the deadlock.

Interictal FDG PET: look for the cold spot

Between seizures the epileptogenic zone is hypometabolic, and FDG PET shows this as a region of reduced cortical uptake. In temporal-lobe epilepsy it localises reliably enough to support a decision to operate. Extratemporal foci are harder: the hypometabolism is subtler and less focal.

Two caveats matter at the reporting desk. The hypometabolic region is usually larger than the tissue that needs to come out, so PET regionalises rather than defining a resection margin. And “interictal” has to be true — a seizure during or shortly before the uptake period can reverse the pattern, so check the seizure history for that day before you call it.

Ictal SPECT: a picture the tracer keeps

^99mTc-HMPAO and ^99mTc-ECD are extracted in proportion to perfusion within about a minute and then stay fixed. That one property is what makes ictal imaging possible: inject during the seizure and the brain holds that perfusion pattern, so the patient can be scanned calmly afterwards, once it has ended.

The catch is timing, and it is unforgiving:

• At onset — hyperperfusion marks the focus.
• Late — you map where the seizure spread to, not where it started.
• After it stops — the picture flips to post-ictal hypoperfusion.

Seconds decide which of those three you get. That is why ictal SPECT belongs in a video-EEG unit, with the dose drawn up at the bedside and staff trained to inject on the alarm.

SISCOM: subtract, then co-register

Reading ictal and interictal studies side by side asks a great deal of the eye. SISCOM — subtracting the interictal scan from the ictal one and co-registering the difference to the patient’s MRI — makes the change explicit and puts it in anatomical context, improving localisation over visual comparison alone. Where it is available, it is how these studies should be read.

Where it fits

None of this decides anything on its own. Functional imaging joins video-EEG, MRI, semiology and neuropsychology in the multidisciplinary meeting, and what carries weight is concordance: an interictal cold spot and an ictal hot spot in the same place, agreeing with the EEG. Where they disagree, the next step is usually intracranial EEG — and PET and SPECT then help decide where to put the electrodes.

Fuller version, with performance and pitfalls: Epilepsy Imaging (SPECT & PET).