Mu rhythm

This is a normal variant in EEG which some time it is misinterpreted.

1. By: Mohibullah Fazli Kakar Trainee of Neurophysiology French Medical Hospital MU Rhythm

2. Mu rhythm Physiologic EEG finding of no clinical significance . Central rhythm of an alpha frequency band (8–10 Hz) with an arciforme configuration , intermix or alternate with beta activity. Occurs in less than 5% of children younger than 4 years of age and in 18–20% between the age of 8 and 16 years. Not blocked with eye opening, but blocked by touch, movement of limbs (especially contralateral limbs) or thought of movement. Usually asymmetric, asynchronous and independent in the two hemispheres.

3. Mu rhythm Consistent asymmetry of amplitude or frequency of mu suggests an abnormality on the side of lower amplitude or frequency. Believed to be the rhythm of the sensorimotor cortex at rest. Prominent in the patients with underneath skull defect (breach rhythm). Paradoxical mu rhythm – induced by contralateral movement or touch after the mu rhythm has dropped out in drowsiness. Paradoxical AR may be induced at the same time.

4. Mu Rhythm Mu is a central rhythm of an alpha frequency band (8–10 Hz) with an aciform configuration. Mu rhythm occurs in less than 5% of children younger than 4 years and has the adult incidence of 18–20% between the age of 8 and 16 years. It is not blocked with eye opening, but blocked by touch, movement of limbs (especially contralateral limbs), or thought of movement. Mu rhythm is usually asymmetric, asynchronous, and seen independently in the two hemispheres. It may present on only one side .

5. Mu Rhythm Mu rhythm is believed to be the rhythm of the sensorimotor cortex at rest . Mu rhythm is prominent in the patients with underneath skull defect (breach rhythm). Mu is a physiologic EEG finding of no clinical significance

6. Mu Rhythm

7. Mu Rhythm in Young Child A 3-year-old girl who underwent epilepsy surgery in the right frontal-temporal region. EEG shows breach rhythm with higher amplitude of mu rhythm at the C4 electrode . In infants and young children whose alpha rhythm is still less than 6 Hz, mu rhythm can lack its characteristic waveform

8. Mu Rhythm in Young Child

9. Breach Rhythm; Prominent Mu Rhythm During Photic Stimulation EEG of a 12-year-old boy with recurrent syncope shows prominent mu during photic stimulation. Mu rhythm is enhanced during photic stimulation and pattern vision

10. Breach Rhythm; Prominent Mu Rhythm During Photic Stimulation

11. Breach Rhythm; Prominent Mu Rhythm A 9-year-old boy with a history of right occipital ganglioglioma resection . T1-weighted coronal and sagittal MRIs with GAD reveal skull defects and an area of previous surgical resection (arrow head ). EEG during wakefulness with eye opening shows frequent runs of asymmetrical rhythmic 9 Hz arc-like activity in the central regions which is higher in amplitude on the right side

12. Breach Rhythm; Prominent Mu Rhythm

13. Breach Rhythm; Prominent Mu Rhythm The arc-like activity in the central regions is attenuated by left hand movement . This is consistent with mu rhythm which can become very prominent beneath a skull defect. Sometimes , mu rhythm can simulate ictal EEG activity especially in patients with skull defects; therefore, activation procedures such as limb movement are very important in their differentiation

14. Breach Rhythm; Prominent Mu Rhythm

15. Mu Rhythm activated by Intermittent Photic Stimulation Mu rhythm is enhanced during intermittent photic stimulation77 and pattern vision78 . It is maximally expressed at C3 and C4 electrodes, and occasionally at Cz . Some spread to the parietal region is not uncommon . In older children and adults, the most common frequency is 10 Hz, slightly higher than alpha frequencies

16. Mu Rhythm activated by Intermittent Photic Stimulation

17. Mu Rhythm in Subdural EEG Monitoring Subdural recording during wakefulness showing a run of mu rhythm. Harmonic of the mu rhythm is more frequently seen in intracranial EEG . In addition, the mu rhythm usually shows sharper morphology, as the scalp and skull act as a high-frequency filter passing lower frequencies more than higher frequencies . Disappearance during limb movement helps to differentiate the mu rhythm from a spike run .

18. Mu Rhythm in Subdural EEG Monitoring

19. Mu Rhythm in Depth EEG (Bipolar ) Mu rhythm during bipolar run of the combined depth electrode-subdural EEG recording during wakefulness. The wave disappeared during sleep and was attenuated by limb movement (not shown).

20. Mu Rhythm in Depth EEG (Bipolar )

21. Mu Rhythm in Depth EEG Implantation During Seizure Intracranial EEG shows mu rhythm (in the box) during the seizure with the epileptogenic focus at DC3 (depth electrode contact #3) (arrow). Mu rhythm disappears during the burst of ictal EEG activity. Recognition of mu rhythm prevents misinterpretation of mu as an ictal EEG activity in the DD3 electrode which can cause false localization of epileptogenic onset

22. Mu Rhythm in Depth EEG Implantation During Seizure

23. Alpha and Mu Rhythm Eye opening (open arrow) attenuates the alpha rhythm but reveals a prominent mu rhythm (C3 and C4) at the same frequency (11 Hz). Note lateral eye movement (X) after the eye opening. Mu is an arc-like central rhythm with negative sharp component and positive slow component. The frequency is similar to alpha rhythm and it is intermixed with 20-Hz beta activity. It is located at the C3, C4, and Cz electrodes. It is not blocked by eye opening but is attenuated by movement of extremities or thinking about moving with greater eff ect on opposite hand. The apiculate phase may resemble spikes.

24. Alpha and Mu Rhythm Arcade, comb, and wicket rhythms, owing to its morphology…. have an arciform morphology. 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….. Niedermeyer observed this rhythm in approx 14% of adolescents’ EEG tracings, and less often in younger children and the elderly. Reactivity…attenuates with …. With direct cortical recording methods (skull breach), a 20-Hz beta activity may be observed from the sensorimotor cortex, with similar reactivity. Thus, the scalp-recorded mu is likely a subharmonic of this underlying rhythm. Mu is usually observed bilaterally with shifting predominance; it may, however, be asymmetrical and asynchronous… explain some instances of highly lateralized mu rhythms. Exclusively lateralized mu should raise a suspicion of an abnormality in the hemisphere …… 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.

25. Alpha and Mu Rhythm

26. Alpha and Mu Rhythm

27. Reference: Atlas of Pediatric EEG