Myotonic discharges and its EMG significance

Generator :  of Myotonic discharges  is Muscle membrane

Sound : It is identified by the Diving Bonder / motorcycle rewing sound while doing EMG

Firing pattern: Myotonic discharges  has Waxing and waning firing pattern

What are the conditions where you see Myotonic discharges?

1.Myotonic discharges in  neurogenic disorders

• Chronic or longstanding axonal peripheral neuropathies (foot/distal muscles)
• Chronic or old, severe radiculopathies

Usually long, change of firing rate (waxing/waning) is slow

Increase in firing rate  can distinguishes it from a rapidly waning fibrillation potential

2.Myotonic discharges – along with clinical myotonia

• Myotonic dystrophy   

         Type 1
         Type 2
In this condition Myotonia less prominent with repetitive contraction

• Myotonia congenita
• Paramyotonia congenita-In this condition Myotonia increase following repetitive contraction

3.Myotonic discharges without clinical Myotonia

•  Hyperkalemic periodic paralysis
•  Emery-Dreifuss muscular dystrophy

Infrequently in:

• Inflammatory myopathies
• Medication induced (statins)