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Clinical Neurophysiology

EMG: Generalities

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Brief practical demonstration at Landspitali

Students Dr Paolo Gargiulo

Bjarni Guðmundsson, Eysteinn Ívarsson, Aron Jónasson and Ovidiu C. Banea

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Neurology Department, National University Hospital of Iceland

6th of February 2020

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Tauga og Vöðvarit or EMG test

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When should be used these tests ?

Suspected neuromuscular disorders:

  • Radiculopathies
  • Plexopathies
  • Neuropathies(focal, peripheral, poly)
  • Myopathy
  • Widespread denervation seen in Amyotrophic Lateral Sclerosis (Motor Neuron Disease)
  • NMJ (neuromuscular junction) disorders (Miastenia Gravis)
  • Do NOT diagnose CNS diseases, NOT for musculoskeletal problems, but may help to exclude them.

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What are nerve conduction studies (NCS)?

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Electrophysiologic tests to evaluate the function and the capacity of the peripheral nervous system.

External Stimulus is applied to the nerve and a response is recorded from same nerve or a muscle.

Stimulator, Amplifier, Surface Electrodes, amplified signal

We can record both sensory and motor components

- Sensory response is measured in microvolts (uV) SNAP Sensory Nerve Action Potential�- Motor: measured in mV - much bigger response! CMAP Compound Motor Action Potential

- Decrease in amplitude represents loss of axons! Reduced CV = demyelination!��

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Types of Nerve conduction studies

Peripheral nerve is stimulated with controlled electrical stimulus�- Extremity nerves are generally used:�- Upper lims - Median nerve + Ulnar nerve (wrist, then proximal at the elbow)�- Lower limbs - Tibial nerve + Peroneal nerve + Sural nerve (ankle, then at fibula head or popliteal fossa)�- On specific occasions we can measure radialis, facialis, musculocutaneous, accesorius, etc.

TYPES

  1. Compound Motor Evoked Potentials (CMAP)
  2. Sensory Nerve Action Potentials (SNAP)
  3. Somatosensory Evoked Potentials (SSEP)
  4. Brainstem Auditory Evoked Potentials (BAEP)
  5. Visual Evoked Potentials (VEP)
  6. Event Related Potentials

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Vöðvarit or Needle EMG

We can insert a needle into a muscle to measure a resting skeletal muscle activity:�MUP(motor unit potential): Sum of Action Potentials from a muscle�- Has a duration, # of phases, amplitude and rate of rise of first component

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In the lab, nerve conduction studies (NCS)

  1. Sensory neurography
  2. Motor neurography
  3. Reflexively evoked field potentials synchronous firing (H reflex) (HERE)
  4. Surface EMG asynchronous firing. The EMG measured over the skin reflects the general behavior of motor units (MUs) or spinal motoneurons (MNs).
  5. TMS to abductor pollicis brevis (if time)

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Surface EMG (here)

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Wang et al 2006 (here)

Neurophysiology Plus Iceland February 2020

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