EMG Flashcards
Definition and cause:
Insertion activity
Spontaneous EMG activity
Definition: Brief bursts of electrical activity (+ or -), high frequency spikes in a cluster.
Abrupt onset and termination without waxing and waning.
Few hundred milliseconds
Cause: mechanically stimulated injured myofibers (disruption)
Definition and cause:
Miniature end plate potentials (MEPPs)
Spontaneous EMG activity
Definition: Monophasic negative. Variable frequency (irregular). 5-50 µV / 1-2 ms
Recorded when the needle is near the end-plate
Cause: spontaneous, nonpropagated, subthreshold end-plate potential caused by the normal random exocytosis of a single quantum of acetylcholine that cross the NMJ
diminution in denervation, dim amplitude in myasthenia, dim freq botulism
Definition and cause:
End plate spikes
Spontaneous EMG activity
Definition: Biphasic negative/positive. Intermittent and irregular firing rate of 5-50 Hz.
100-300 µV / 2-4 ms
Recorded when the needle is near the end-plate
Cause: discharge of a single muscle fiber that is excited by activity in the nerve terminals
Definition and cause:
Motor unit action potentials (MUAPs)
Spontaneous EMG activity
Definition: Bi- or triphasic with initial negative phase.
Semirhythmic with slowly increasing then decreasing interspike interval during constant contraction
100-3000 µV / 1-12 ms
Cause: associated with voluntary muscle contraction. Consist of isolated discharges of one or rarely few motor units
Definition and cause:
Fibrillation potentials
Abnormal EMG activity
Definition:
Bi- or triphasic waves appearing in bursts
10-200 µV / 0.5-3 ms
Cause: spontaneous depolarization / action potential of a single myofiber (hypersensitive denervated)
Definition and cause:
Positive sharp waves
Abnormal EMG activity
Definition:
Initial positive spike followed by much shorter, slow and small negative potential
50-4000 µV / < 5 ms / 2-50Hz
Cause: from irritated muscle membrane with the potential stopping at an area immediately adjacent to the needle electrode (depends on the needle position : if the needle deforms the fiber and makes the membrane inexcitable, conduction failure, and then PSW)
Definition and cause:
Complex repetitive discharges
Abnormal EMG activity
Definition:
Polyphasic or serrated action potential with a uniform frequency, shape, and amplitude
Abrupt onset, cessation or change
100 µV-1mV / 5-100 Hz
Cause: group of myofibers firing in near synchrony (single myofiber followed by ephaptic spread to adjacent denervated fibers) each spike represents a different single muscle fiber
Chronic denervating conditions
Some myopathies (HYPP, MD, myositis)
Definition and cause:
Myotonic potentials
Abnormal EMG activity
Definition:
Sustained run of waves (5-20ms) resembling Fibs and PSWs but is differentiated by its characteristic waxing and waning of both amplitude and frequency
Positive wave or brief spike
Amplitude (10-1000 µV) and frequency (20-80 Hz)
Cause: independent repetitive discharges of single injured myofibers
