Breach Rhythm

Etiology

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 a postsurgical craniotomy site. The skullbone normally acts as a filter for brain electrical activity, and it selectively attenuates the higher frequencies. An increase in amplitude and frequency of the breach effect is due to decreased 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 are also reports of breach rhythm being related to the functional state of underlying brain tissue.[7] Lyudmilov et al noticed that after a seizure event in a patient with a craniotomy, the EEG revealed subclinical seizure patterns until an antiseizure medication suppressed these, and then a breach rhythm was observed. It was noted that a breach rhythm could be suppressed due to Todd's paralysis/postseizure subclinical activity [7].

In some instances of astrocytoma, there have been reports that either attenuation or loss of the breach rhythm can be an initial sign of tumor progression.[8] There are also reports of breach rhythm being developed over a solitary skull lesion due to multiple myeloma.[9]

Epidemiology

Breach rhythm is a rare benign variant of EEG. The epidemiology of breach rhythm is unknown.

Pathophysiology

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 the cortex and scalp electrodes. One square centimeter of the skull provides approximately 40,000 ohms of resistance, the dura mater about 12,000 ohms, and the scalp about 1000 ohms of resistance. Therefore, there is a significant increase in electrical activity from the cortex to the scalp when these resistors are absent. The final results of the absence of these resistors are the spiky, sharply-contoured waveforms of the breach effect. Breach rhythm is most prominent in the central and temporal regions.[1][2][10]

Breach rhythm is not due to abnormality of the brain; it is due to a skull defect. The defect can be the result of surgery or, rarely, a large osteolytic bony skull lesion, such as in multiple myeloma.[9][11]

History and Physical

Obtaining a surgical history relative to the skull and a history of brain trauma is essential. Skull defects can be missed on palpation because an acrylic material may have replaced a skull defect. Bipolar montage may better identify the breach rhythm due to its higher spatial resolution.[2]

Evaluation

Identifying breach rhythm while interpreting EEG is vital to avoid misinterpretation of epileptiform activity.

It is important to inquire about the patient's surgical history and any history of head trauma to evaluate if a breach rhythm is likely to be present. Palpation of the scalp may not be sufficient. Breach rhythm is seen over skull defects and abruptly diminishes beyond the margin of the defect. It is best appreciated by bipolar montage due to its higher spatial resolution.[2]

Breach rhythms typically are high voltage waveforms between 6 to 11 Hz, often mixed with waves of higher and lower frequencies, including beta, mu, or theta activity. They often appear as a high-amplitude spiky or sharply contoured activity similar to epileptiform activity. Pure breach rhythms do not have an after-going slow wave. There is no spread of activity to other areas of the brain. Sleep recordings can aid in the differentiation between breach rhythm and epileptiform activity, although breach rhythms can affect both awake and sleep rhythms.[1][2]

In general, interictal epileptiform abnormalities are facilitated during NREM sleep and inhibited during REM sleep. If breach rhythm is constituted mostly by enhanced underlying rhythms, breach rhythm will attenuate or disappear during drowsiness and sleep. It may linger into drowsiness and light NREM sleep if the breach rhythm is mixed with a mid-temporal, alpha-like rhythm. Mu rhythm within breach rhythm can be tested for reactivity by asking the patient to move the contralateral limb. Attenuation suggests reactivity of breach rhythm and is indicative of a normal pattern; however, lack of reactivity does not rule out the presence of normal rhythm.[2][1]

EEG filters should be used carefully. Filters to reduce too-high frequency may filter out the higher frequencies of breach rhythm, but the left-out activity could resemble spikes or sharp waves. Muscle artifacts usually can be easily identified by their much higher frequency component, provided an appropriate high-frequency filter is chosen. Filtering out muscle activity will attenuate the signals and may be confused for epileptiform abnormalities within the breach rhythm.[1][10][2]

Treatment / Management

Breach rhythm is a benign variant and does not warrant treatment. The attenuation of preexisting high amplitude and/or the presence of focal slow waves on serial EEG recordings may suggest progression of the lesion beneath the skull defect. On the other hand, it is also important not to miss actual epileptiform activity on top of a breach rhythm. The presence of slow waves in the delta range and an electrical field associated with the abnormality must be recognized as real underlying pathology.

A "conservative" reading is encouraged by most specialists since most believe that over-reading is likely more harmful than under-reading.[2] When in doubt, brain imaging with CT and/or MRI of the head can be used to determine if any focal lesion is causing seizure activity.

Differential Diagnosis

It is important to differentiate the breach rhythm from interictal epileptiform discharges by scrutinizing the surrounding background activity.[12][13] Polyphasic interictal discharges are complex with a slow wave and have a characteristic morphology that does not resemble the background activity.

Interictal epileptiform discharges include the following:

• Paroxysmal fast activity
• Beta frequency activity
• Electromyographic (EMG) artifact [14]

The breach effect has a clearly circumscribed region of increased amplitude. The paroxysmal fast activity appears in bursts with an intervening return to the asymmetric baseline.

Breach rhythm can be differentiated from beta activity by its distribution. Normal beta activity is bilateral but may vary in distribution. 

EMG artifacts occur in regions with overlying muscle, most commonly frontal and temporal regions. EMG artifacts can be differentiated by their inconsistent presence and significantly higher components.[10][2]

Prognosis

As breach rhythm is a benign variant, the prognosis is excellent.

Complications

The failure to distinguish breach rhythm from epileptiform abnormalities can lead to misinterpretations and complications. The breach effect's sharply contoured morphology may be mistakenly perceived as an epileptiform abnormality, resulting in a misdiagnosis of epilepsy and unnecessary antiepileptic drug treatment. Conversely, epileptiform abnormalities occurring in the same area as breach rhythm may be overlooked, leading to undiagnosed cases. Hence, a precise and accurate interpretation is crucial due to its significant clinical implications.