The document discusses the mu rhythm, which is a central rhythm seen on EEG with an alpha frequency band of 8-10 Hz. It has an arciform configuration and occurs in less than 5% of children under age 4 and 18-20% of children ages 8-16. The mu rhythm is not blocked by eye opening but is blocked by touch, limb movement, or thought of movement. It is usually asymmetric and independent between hemispheres. The mu rhythm is believed to originate from the sensorimotor cortex at rest and can be prominent in patients with skull defects.

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