NCS-EMG (ABNORMAL RESTING MUSCLE ACTIVITY)

SUMMARY

ABNORMAL RESTING MUSCLE ACTIVITY

Pathologic waveforms can be generated from resting muscle fiber/motor unit due to many reasons.

1. Fibrillation potentials: regular, spontaneously firing AP originating from denervated single muscle fibers secondary to uncontrolled ACh release.
- Causes: benign, thyrotoxicosis, anti-cholinesterase use, radiculopathy, MND.

2. Positive sharp waves: needle recording of an AP of a single muscle fiber that has not propagated far.
- Causes: benign, thyrotoxicosis, anti-cholinesterase use, radiculopathy, MND.

3. Complex repetitive discharge (CRD): “bizarre high frequency discharges” are regular polyphasic/serrated APs originating from a principal pacemaker, initiating a group of single muscle fibers to fire in near synchrony. - Causes: chronic neuropathies/myopathies.

4. Myotonia: biphasic single muscle fiber APs triggered by needle movement, percussion, or voluntary contraction, due to abnormal chloride conductance.
- Causes: myotonic dystrophy, chronic radiculopathy, peripheral neuropathy.

Reference(s)

Wilkinson, I., Furmedge, D. and Sinharay, R. (2017). Oxford handbook of clinical medicine. Oxford: Oxford University Press. Get it on Amazon.
Feather, A., Randall, D. and Waterhouse, M. (2020). Kumar And Clark’s Clinical Medicine. 10th ed. S.L.: Elsevier Health Sciences. Get it on Amazon.
Hannaman, R. A., Bullock, L., Hatchell, C. A., & Yoffe, M. (2016). Internal medicine review core curriculum, 2017-2018. CO Springs, CO: MedStudy.
Therapeutic Guidelines. Melbourne: Therapeutic Guidelines Limited. https://www.tg.org.au [Accessed 2021].