The breach rhythm, also known as the breach effect, is a benign EEG pattern with an epileptiform morphology, caused by a skull abnormality. A focal skull defect, like after a craniotomy, can cause an increase in the amplitude of alpha, beta, and mu rhythms, leading to the breach effect. Breach rhythm and epileptiform activity in the same area can be challenging to differentiate. Breach rhythm typically has sharp contours and irregular morphology, usually have a frequency of 6 to 11 Hz but may be associated with faster or slower wave activity. It may sometimes be hard to differentiate from actual epileptiform activity in the same area, which can lead to under or overdiagnosis of epilepsy and thus can have significant clinical consequences.[1][2]

EEG Features of Breach Rhythm

An electroencephalogram (EEG) is a noninvasive test for providing neurophysiological activity of the brain. It is usually done by placing electrodes over the scalp which measures the activity of neurons.[3] It is very important to understand how to differentiate between normal/benign variants of EEG vs pathological variants of EEG. The benign variants of EEG are considered normal but can be confused with epileptiform activity. Accurate recognition of benign variants in the EEG is crucial to avoid an errant diagnosis of epilepsy.[4]

Breach rhythm typically is a focal, asymmetrical, high-voltage activity. It often has arch-like waveforms, sometimes having spiky morphology. It can manifest as an irregular rhythm sometimes associated with sharp activity. Breach rhythm is most prominent when recorded over central and temporal regions. Breach rhythm can be identified easily when it occurs in serial trains. Single, spike-like, or sharp-contoured waveforms are more likely to be mistaken for epileptiform activity. These waveforms should be compared with waveforms of the rhythmic activity constituting breach rhythm to determine if these are distinct waveforms or similar to those constituting the breach rhythm. The absence of after coming slow-wave components and lack of spread to other areas is characteristic of breach rhythm. Brain damage should be suspected if breach rhythm is associated with polymorphic delta activity. If the breach rhythm is located over the central region, it can be blocked by physical movement due to the presence of an underlying normal mu activity. Breach rhythm can sometimes persist during sleep. It can manifest as a voltage increase in spindles in stage 2 of sleep.[2]

The breach effect occurs due to an increase in the amplitude of alpha, beta, and mu rhythm activity over or near an area of a bony defect on the skull, usually post-surgical craniotomy site. Skullbone normally acts as a filter for brain electrical activity and attenuates the higher frequencies. An increase in amplitude and frequency of the breach effect is due to the decrease in electrical impedance caused by reduced filtering from the skull defect. Small skull defects, for example, a burr hole, is unlikely to produce a breach effect, as the field detected by each electrode is larger than the hole.[1][2]  There were also reports of breach rhythm being related to the functional state of underlying brain tissue.[5] Lyudmilov et al noticed that after a seizure event in a patient with craniotomy, EEG revealed subclinical seizure patterns until these were suppressed by an antiseizure medication, then the breach rhythm was seen. So they mentioned that breach rhythm could be suppressed due to todd's paralysis/post-seizure subclinical activity [5].

In certain cases like astrocytoma, there has been a case that reported either attenuation or loss of the breach rhythm can be an initial sign of tumor progression.[6] There are also reports of breach rhythm being developed over a solitary skull lesion due to multiple myeloma.[7]

Breach rhythm is a rare benign variant of EEG. The epidemiology of breach rhythm is not known.

Breach rhythm was described in 1979 by Cobb and associates. Breach rhythm refers to a change in the transmission of EEG waves through a skull defect. Bone offers major resistance between cortex and scalp electrodes. One cm2 of the skull provides about 40,000-ohms resistance, dura mater about 12,000 ohms, and scalp about 1000 ohm resistance. Therefore, there is a significant increase in electrical activity from cortex to scalp when bone between them is absent. These events are spiky, sharply-contoured waveforms. Breach rhythm is most prominent in the central and temporal regions. [1][2][8]

Breach rhythm is not due to the abnormality of the brain; it is due to a skull defect.