Abbreviations

NCSs:   nerve conduction studies
EMG:   needle electromyography
MUP:   motor unit potential
MAP:   muscle action potential
EPP:   end plate potential
NAP:   nerve action potential (summated potential of all nerve fibers stimulated
NCS/EMG:   valuable test to localize the site of a nerve lesion along a peripheral nerve or at the lower motor neuron

Potential Nerve Lesions

• Radiculopathy
• Traumatic nerve injury
• Plexopathy
• Polyneuropathy
• Entrapment neuropathy neuromuscular junction defect
• Myopathy

Anatomy and Physiology

Motor unit

• Motor neuron, axon, all muscle fibers innervated by the motor neuron
• Stimulation of the motor neuron results in contraction of all the muscle fibers
• The motor action potential is the action potential caused by voluntary contraction of muscle in the motor unit

Nerve action potential

• Resting membrane potential is negative
• Depolarization occurs with stimulation, membrane potential turns positive
• With a threshold depolarization, an action potential occurs
• Speed of conduction is dependent on nerve fiber diameter and myelin sheath
• Unmyelinated fibers are slow - 1 m/s
• Myelinated fibers are fast - 3 m/s to 80 m/s
• Salutatory conduction - potential travels from node of Ranvier to next node of Ranvier
• Segmental demyelination
• Focal neuropathies result in segmental demyelination
• Carpal tunnel
• Cubital tunnel
• Axonal degeneration - speed of conduction is only minimally affected
• When the EPP reaches threshold, acetyl choline is released and a muscle action potential is generated

Presynaptic inhibition

• Lambert-Eaton myasthenic syndrome - insufficient release of acetyl choline (antibody to the voltage gated calcium channel)
• Botulinum toxin inhibits release of acetyl choline

Postsynaptic inhibition

• Antibody binds to acetyl choline receptor

Chronic denervation processes

• High amplitude-long duration MUPs - surviving muscle fibers increase their motor unit territory and fiber density

Myopathy

• Small amplitude-short duration MUPs - motor unit territory and fiber density decrease

Nerve Compression

With focal nerve compression, focal demyelination is present. Note that the axon remains intact but, with myelin loss, slowing of conduction occurs and there can be a conduction block.

The following are common findings with focal nerve compression:

• Focal demyelination
• Nerve conduction velocity slowing
• Conduction block across the site
• Fibrillation potentials
• Positive sharp waves (PSWs)
• High amplitude-long duration MUPs (chronic denervation)

Traumatic Nerve Injury

At 6 to 8 weeks following the injury, a clinician should perform an electrodiagnostic study to detect reinnervation. A needle electromyogram will show small amplitude polyphasic motor unit potentials.

The following is the typical of sequence of electrodiagnostic findings:

• Severance of the nerve
• Positive sharp waves and fibrillation potentials at 2 weeks
• Incomplete nerve injury
• Small amplitude motor unit potentials at 6 to 8 weeks

Electrodiagnostic Testing

With electrodiagnostic testing, a clinician may find several characteristic features in different disorders:

Denervation

• Fibrillation
• Positive sharp waves
• Fasciculations

Neurogenic lesions

• Fasciculations
• Myokymic potentials

Myopathies

• Complex repetitive discharges
• Myotonic discharges

Bibliography

Lee DH, Claussen GC, Oh S. Clinical nerve conduction and needle electromyography studies. J Am Acad Orthop Surg. 2004; 12:276-287.