“Your EEG was negative.” Don’t bet on it.Posted: 07/20/2011
A couple of weeks ago I mentioned at the end of my post that most physicians are limited in what they can offer a person experiencing visually induced seizures. If a doctor investigates possible seizures, an EEG (electroencephalogram) is usually the first diagnostic procedure. EEGs can (sometimes) detect problems with the electrical function in the brain.
If you’re told that an EEG “didn’t find anything,” don’t assume there was nothing to be found. Many issues can and do routinely interfere with the accuracy of EEG results, an open secret that clinicians rarely share with patients. In today’s post, I’ll present some of the shortcomings of EEG when used for seizure detection, as described by experts.
Let’s start with this:
“The main limitation with EEG is its poor sensitivity for epilepsy. The generally accepted numbers are that the yield of a single routine EEG in epilepsy is ~ 50% and increases with repeated EEG recordings to reach about 80% by the third recording.”
Source: Selim R. Benbadis, “Introduction to Sleep Electroencephalography” in Sleep: A Comprehensive Handbook, edited by T. Lee-Chiong, John Wiley & Sons, Inc., 2006
Translation: EEG is more useful in assessing conditions such as coma, stroke, tumors, and brain death than it is in accurately detecting seizure activity. Half of routine EEGs don’t pick up seizure activity during the first recording. In cases proven to involve seizure activity, if the EEG is repeated a couple of times, 80 percent of them will capture evidence of seizures.
“While…often adequate for the appropriate diagnosis of a seizure disorder, patients with epilepsy can have persistently normal EEGs.”
Source: Gregory A. Worrell, Terrence D. Lagerlund, and Jeffrey R. Buchhalter,“Role and Limitations of Routine and Ambulatory Scalp Electroencephalography in Diagnosing and Managing Seizures.” Mayo Clinic Proceedings September 2002**
Translation: A seizure disorder should be diagnosed on the basis of the clinician’s judgment, using information from the patient’s history, the clinician’s own observations/examination, and diagnostic tests. Doctors should not allow EEG results alone to drive the diagnosis, and patients should question a diagnosis that is based primarily or exclusively on the EEG.
“The human electroencephalogram (EEG) was discovered by the German psychiatrist, Hans Berger, in 1929…EEG continues to play a central role in diagnosis and management of patients with seizure disorders—in conjunction with the now remarkable variety of other diagnostic techniques developed over the last 30 or so years—because it is a convenient and relatively inexpensive way to demonstrate the physiological manifestations of abnormal cortical excitability that underlie epilepsy.”
Source: Shelagh J.M. Smith, “EEG in the diagnosis, classification, and management of patients with epilepsy.” Journal of Neurology, Neurosurgery, and Psychiatry June 2005
Translation: Folks, EEG is pretty old technology. It was updated to incorporate digital technology, so that instead of recording the data on paper (think of seismographs), results are now viewed and stored on computers. However, the capture of electrical signals from inside the brain using electrodes on the scalp is much the same as it was in 1929. More specifically,
- There are other diagnostic procedures that have been developed more recently. (You may find it difficult to convince a neurologist to order one of these others, though, without having some evidence of seizures showing on the EEG. A real Catch-22.)
- EEG is economical for providers and risk-free/non-invasive for patients.
Ready to get just a little technical? Remember, a translation follows:
“Despite the importance and widespread use of scalp EEG epileptiform discharges, the cortical EEG substrates underlying these spikes and seizure discharges are mostly speculative…the cortical area of epileptiform discharges required for the scalp recording is considerably larger than commonly thought. A cortical area of 10 to 20 cm is often required to generate a scalp recognizable interictal spike or ictal rhythm. Sufficient cortical source area and synchrony are mandatory factors for the corresponding scalp EEG epileptiform recording.”
Source: J.X. Tao, M. Baldwin, S. Hawes-Ebersole, J.S. Ebersole, “Cortical substrates of scalp EEG epileptiform discharges.” Journal of Clinical Neurophysiology April 2007
Translation: Data retrieved via EEG requires a lot of guesswork to figure out what is really going on inside the brain, and where.
- EEG electrodes glued onto the scalp do a so-so job of picking up the brainwaves that signal a seizure. For one thing, the skull gets in the way…for another, brain tissue.
- Scalp electrodes aren’t great at correctly identifying the location of abnormal discharges, either.
- Seizures involving only a small portion of the brain may not generate a strong enough electrical charge to pass through the skull for detection on the scalp.
- The spike and wave patterns on an EEG that characterize seizure discharges must arrive at the scalp undistorted by the intervening structures (many do not reach the scalp without distortion), or the waves will not clearly show seizure activity occurring.
Are you developing some healthy skepticism? Good! Next up: Using EEG to test for sensitivity to flash/flicker carries a whole set of additional challenges that further limit EEG’s usefulness.
**full text available here
Update, 7/26/2014: Please read here about a new study that found only 6.2% of patients with a history of visually induced seizures showed a positive result on EEG during photic stimulation.