Rhythmic Variant Patterns

There are six main types of rhythmic variant EEG patterns:

1. Alpha variant.

3. Rhythmic temporal theta burst of drowsiness ("psychomotor variant").

4. Subclinical rhythmic electrographic (theta) discharges in adults (SREDA).

5. Midline theta rhythm.

6. Frontal arousal rhythm (FAR).

1.1. Alpha Variant

This pattern was described first by Goodwin in 1947. There are two types of alpha variants, "slow" and "fast." The slow (subharmonic) alpha variant appears as an abrupt rhythm usually at half the frequency of the patient's more typical waking background rhythm, and often of greater voltage (Fig. 1). The fast (harmonic) alpha variant may appear as a notched or bifurcated form of the patient's usual waking background rhythm, so that a superimposed harmonic rhythm of twice the alpha frequency occurs. Alpha variants are blocked with eye opening and exhibit a posterior predominance, just as with normal alpha rhythms. Alpha variants vary in their prevalence within a subject's tracing, alternating with periods of normal-appearing alpha

From: The Clinical Neurophysiology Primer Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ

Fast Alpha Variant
Fig. 1. Slow alpha. This subject is 46 yr of age. Note how the posterior background rhythm shifts from a bilateral 11-Hz rhythm earlier in the figure to a somewhat notched biposterior 5- to 6-Hz rhythm late in the figure.

activity. Alpha variants have been regarded as a physiological variation of the more familiar posterior background alpha activity, and do not predict increased convulsive tendency.

1.2. Mu Rhythm

Numerous terms have been applied to the mu rhythm, including arcade, comb, and wicket rhythms, owing to its morphology. Individual waveforms have an arciform morphology. This occurs in waking over the central regions, especially the C3, Cz, and C4 contacts (Fig. 2). It is closely associated with the sensorimotor cortex, hence the term "mu," for motor. Mu exhibits a frequency in the alpha range, typically at 9 to 11 Hz. Niedermeyer observed this rhythm in approx 14% of adolescents' EEG tracings, and less often in younger children and the elderly. Similar to the alpha activity of the occipital cortex, it exhibits physiological reactivity. It attenuates with contralateral limb movement or just planned movement of the contralateral limb. With direct cortical recording methods, a 20-Hz beta activity may be observed from the sensorimotor cortex, with similar reactivity. Thus, the scalp-recorded mu is likely a subhar-monic of this underlying rhythm.

Mu is usually observed bilaterally with shifting predominance; it may, however, be asymmetrical and asynchronous. Mu activity is augmented in the setting of a focal skull breach. This could, for instance, explain some instances of highly lateralized mu rhythms. Exclusively lateralized mu should raise a suspicion of an abnormality in the hemisphere lacking mu activity. Sometimes, focal mu activity in the setting of a bony defect of the skull may be so sharp and of higher voltage as to falsely mimic an epileptogenic focus.

1.3. Rhythmic Temporal Theta Bursts of Drowsiness ("Psychomotor Variant")

Gibbs et al. called this pattern the "psychomotor variant" because it was thought to represent a temporal lobe or psychomotor seizure. This concept has been more recently discarded because this pattern is observed in asymptomatic healthy individuals and exhibits poor correlation with patients with true temporal lobe or psychomotor seizures. This pattern has also been called "rhythmic mid-temporal discharges" describing its character, location, and frequency.

This pattern may be present in waking or early drowsiness and usually in tracings of adults and adolescents. It wanes with deepening sleep. As its name implies, this particular pattern is found in the mid-temporal head regions, but can spread parasagittally. It is comprised of 5- to 7-Hz rhythms in bursts or trains lasting often longer than 10 s and sometimes beyond a minute (Fig. 3). This variant rhythm can exhibit variable morphologies, but is often sharply contoured. It is usually monomorphic; it does not evolve significantly in frequency or amplitude, as occurs in most ictal patterns. Rhythmic mid-temporal discharges can occur bilaterally or independently with shifting hemispheric predominance. This pattern is uncommon; its incidence is approx 0.5%, according to Gibbs et al.

1.4. Subclinical Rhythmic Electrographic Discharges in Adults

This variant pattern involves sharply contoured 5- to 7-Hz activities with a wide distribution, mainly over temporo-parietal derivations. It is usually bilateral, but can be asymmetrically disposed. SREDA can appear as repetitive monophasic sharp waves or as a single discharge followed seconds later by sharp waves that gradually accelerate to form a sustained, rhythmic train of theta activity. This may last from 20 s to several minutes, usually 40 to 80 s. Because of its duration and evolution, SREDA can easily be misinterpreted as an ictal pattern, even by experienced readers. Nevertheless, SREDA has not been shown to have any consistent

Rhythm Eeg
Fig. 2. Mu. This subject is 27 yr old. Note the prominent mu rhythm over the C3 contact. This blocks efficiently when the subject is asked to make a fist with the contralateral hand, as indicated.
Midline Theta Rhythm
Fig. 3. Psychomotor variant. The subject is 20 yr of age. Note the run of monomorphic sharply contoured theta activity lasting 2 to 3 s over the left temporal channels.

correlation with seizures. SREDA is more typically seen in older adults, and more common at rest, drowsiness, or during hyperventilation.

1.5. Midline Theta Rhythm

The midline theta rhythm is most prominent at Cz but may spread to nearby contacts. This 5- to 7-Hz frequency exhibits either a smooth, arc-shaped (mu-like) or spiky appearance. The duration is variable and it tends to wax and wane. It is more common in wakeful and drowsy states and reacts variably to limb movements, alerting, and/or eye opening. This rhythm is now regarded to be a nonspecific variant, although it once was considered a marker of an underlying epileptic tendency.

1.6. Frontal Arousal Rhythm

FAR involves trains of 7- to 10-Hz activities in the frontal head regions. These rhythms may be notched and may last up to 20 s. FAR has been described as an uncommon rhythm that appears during sleep-to-wake transitions, especially in children. FAR disappears once the subject is fully awake. This pattern was, at one time, associated with children with minimal cerebral dysfunction, but this specific association has been subsequently doubted. This pattern is still considered to be a nonspecific finding without pathological significance.

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