Bilateral Lesions Yield Breach Rhythm Without Skull Defect

A scalp EEG waveform long treated as a benign skull-defect artifact has surfaced in a patient with bilateral cerebral lesions and no cranial bone loss, in a case report published ahead of print in Clinical EEG and Neuroscience.

The article, "Breach Rhythm Beyond Skull Defects: Clinical and Functional Insights from a Bilateral Lesion Case," describes the patient and aims to refine the differential diagnosis of sharply contoured fast activity in adult EEG reading.

What is breach rhythm?

Breach effect has appeared in EEG textbooks for decades, described alongside artifacts such as eye movement, sweat, and muscle activity. Academic interest in the phenomenon predates digital EEG, when paper traces made the sharply contoured activity easily visible. Modern digital review has not eliminated the problem; higher sampling rates and sharper filters can make the visual contrast even more striking.

Breach rhythm (BR) is a well-recognized scalp EEG pattern. It is defined by high-amplitude, sharply contoured fast activity recorded over an area of skull defect, most often the site of a prior craniotomy or a traumatic cranial window.

The waveform is physiological, not epileptiform. The underlying cortex sits closer to the recording electrode once bone is absent, and that proximity amplifies faster rhythms such as alpha, beta, and mu activity into sharply peaked transients. Removing a segment of skull reduces the impedance between cortex and electrode, which is the mechanical explanation behind every reading rule about breach effect.

In routine reading, BR is best known for the trouble it causes at the differential diagnosis step. Its morphology overlaps with interictal epileptiform discharges, with ictal onset patterns, and with sharply contoured normal variants such as wicket spikes and small sharp spikes.

Why does this case extend the differential?

The report's title positions its contribution as an extension of BR into tissue without skull defects. The bilateral lesion presentation, paired with no documented cranial bone loss, suggests that impedance changes from parenchymal disease can generate a surface waveform indistinguishable from a classic post-craniotomy breach rhythm.

For decades, breach effect has been taught as a post-surgical or post-traumatic finding, identifiable because the bone loss is visible on head CT or at the bedside. The present case moves that teaching outside the craniotomy ward and into the general neurology reading room.

The authors pair the scalp EEG with "clinical and functional" insights, indicating at least one additional measurement was added to the report, although the abstract excerpt does not specify which modality.

How should EEG readers act on the finding?

For epilepsy monitoring units and general EEG laboratories, the practical message is straightforward. Technologists should continue to mark prior craniotomies, burr holes, and skull fractures on the EEG requisition, and now also flag known structural lesions, even when the skull is intact.

The single published case does not establish prevalence. It does, however, justify a low threshold for cross-referencing any focal sharply contoured fast activity with the patient's most recent neuroimaging before a clinical epilepsy diagnosis is made.

Automated spike-detection software, which has become standard in long-term monitoring, does not currently distinguish breach rhythm from epileptiform spikes. The case therefore raises a tractable question for vendors and academic tool developers. Could a CT-derived or MRI-derived skull-attenuation map be folded into detection algorithms to flag probable breach sites in advance?

Where did the case appear?

Clinical EEG and Neuroscience is a peer-reviewed clinical neurophysiology journal that covers scalp EEG, intracranial recording, and related functional measures. Ahead-of-print placement means the article is fully citable now, with a digital object identifier that resolves to the publisher's full-text version.

The case reinforces a basic EEG reading rule. High-amplitude sharply contoured activity can be set aside as a benign breach effect only after skull and brain imaging have been reviewed side by side. The new report widens that imaging checklist to include bilateral parenchymal lesions, even when no cranial defect is visible. Subsequent case series, or any quantitative reanalysis with dipole source modeling, would help the field define the phenomenon more precisely.