Electrode artifact or Spike?

A paediatrician telephoned me about a nine-year-old who had a generalized tonic-clonic seizure in sleep, wanting to know whether an MRI and EEG should be requested. The child is intellectually and developmentally normal. Given the probabilities of different seizure syndromes/causes, I recommended an EEG, which, if diagnostic, would obviate the need for an MRI scan.

As you can see, there are electrode artifacts at F3 and C3, in particular. Hence, one's instinct is to view the sharply contoured waves at C3 during the second and eighth second of the page as artefacts and It is good to have a conservative instinct like this. However, notice that there is a slow wave after each of these sharply contoured waves. Furthermore, there is a very low amplitude wave at T3-T5 that is synchronous with the above sharply contoured waves. This markedly changes the probabilities, as the discharge then has a "biological field", as I like to term it, a characteristic morphology and it cannot be explained entirely by an electrode artifact. It is then best to view the discharge on another montage.

In addition to the above three spike-and-wave discharges, the recording demonstrated numerous such discharges, with a normal background and no focal, independent slow waves. The diagnosis of Self-limiting Epilepsy with Centrotemporal Spikes, also known as Benign Rolandic Epilepsy of Childhood, is highly likely and there is no need for an MRI scan at present.

The lessons?

1. Spikes may occur precisely where you don't want them, namely in an artifactual electrode.

2. Hence, while one must be extraordinarily careful of considering waves in an artifactual electrode as spikes or sharp waves, there are exceptions.

3. Technologists need to identify electrode artifacts and remedy these as quickly as possible. I would suggest that the most important responsibility of a technologist, by a country mile, is to produce an artefact-free recording. It is important to discuss electrode artifacts regularly with technologists so that they never become sloppy about these. It is all too easy simply let these go.

4. I would suggest that recordings, wherever possible, should be recorded on bipolar montages, as electrode artifacts are most easily identified on these.