Updated: 5/23/2021

Nerve Conduction Studies

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  • Introduction
    • Definition
      • comprises nerve conduction velocity (NCV) studies and electromyography (EMG)
      • used to localize areas of compression and neuropathy
      • distinguish
        • lower vs upper motor neuron lesions
        • spinal root, trunk, division, cord or peripheral nerve lesion
      • determine severity and prognosis
        • neuropraxia has good prognosis
        • axonotmesis/neurotmesis has poor prognosis
      • demonstrate denervation, reinnervation, aberrant reinnervation, motor end plate lesion
      • valuable in worker's compensation patients with secondary gain issues
    • Indications
      • carpal tunnel syndrome
      • cubital tunnel syndrom
      • cervical radiculopathy
      • lumbar radiculopathy
      • nerve dysfunction of the shoulder (e.g., scapular winging)
  • Nerve Conduction Velocity
    • Definition
      • tests performed on peripheral nerves to determine their response to electrical stimuli
    • Technique
      • constant voltage electric stimulator evokes a response from muscle (motor nerve study) or along the nerve (sensory nerve study)
        • standard stimulus is 0.1 to 0.2ms square wave
      • for motor nerve studies, an additional stimulus is measured along the proximal segment between 2 points on the nerve
        • to overcome inherent delay across neuromuscular junction if the recording electrode were placed on the muscle
    • Measures
      • NCV = distance divided by latency
        • distance traveled is from the cathode of the stimulating electrode to the recording electrode
        • latency is the time from the onset of stimulus to the onset of response
          • onset latency = time from site of stimulation + time to activate postynaptic terminal (neuromuscular transmission time) + time for action potential to propagate along muscle membrane to recording potential
        • NCV is determined by
          • myelin thickness
          • internode distance
          • temperature
          • age
            • NCV in newborns are 50% of adult values
            • NCV in 1 year olds are 75% of adult values
            • NCV in 5year olds are 100% of adult values
      • Amplitude
        • from baseline to negative peak (in mV)
        • area under peak is proportional to number of muscle fibers depolarized
        • provides estimate of number of functioning axons and muscles
      • Duration
        • reflects range of conduction velocities and synchrony of contraction of muscle fibers
        • if there are axons with different CVs (acute demyelination), duration will be greater
      • Late responses evaluate proximal nerve lesions (near spinal cord, e.g. Guillain-Barre syndrome)
        • F-wave amplitude
        • H-reflex
          • stimulate Iα fibers at knee, with recording at the soleus (S1 root)
          • affected by sensory neuropathies, motor neuropathies of the tibial or sciatic nerves, and S1 root lesions
    • Demyelination leads to
      • increase latencies (slowing) of NCV
        • distal sensory latency of > 3.2 ms are abnormal for CTS
        • motor latencies > 4.3 ms are abnormal for CTS
      • decreased conduction velocities less specific than latencies
        • velocity of < 52 m/sec is abnormal
      • Findings on NCV
      • Condition
      • Latency
      • Conduction Velocity
      • Amplitude
      • Evoked Response
      • Normal
      • Normal
      • Upper limb (>45m/s)
        Lower limb (>40m/s)
      • Normal
      • Normal
      • Axonal
      • Normal
      • Normal
      • Decreased
      • Prolonged
      • Demyelinating
      • Increased
      • Decreased
      • Normal/decreaased
      • Absent/prolonged
      • Anterior horn cell disease
      • Normal
      • Normal
      • Decreased
      • Normal or polyphasic, prolonged duration
      • Myopathy
      • Normal
      • Normal
      • Decreased
      • Normal
      • Neuromuscular junction
      • Normal
      • Normal
      • Decreased
      • Normal
      • Neuropraxia proximal to lesion
      • Absent
      • Absent
      • Absent
      • Absent
      • Neuropraxia distal to lesion
      • Normal
      • Normal
      • Normal
      • Normal
      • Axonotmesis proximal to lesion
      • Absent
      • Absent
      • Absent
      • Absent
      • Axonotmesis distal to lesion
      • Absent
      • Absent
      • Absent
      • Normal
      • Neurotmesis proximal to lesion
      • Absent
      • Absent
      • Absent
      • Absent
      • Neurotmesis distal to lesion
      • Absent
      • Absent
      • Absent
      • Absent
  • Electromyography
    • Definition
      • to study electrical activity of individual muscle fibers and motor units
      • differentiate between diseases of nerve roots, peripheral nerves or skeletal muscles
      • determine if disease is acute or chronic, and if there is reinnervation
      • determine if there is nerve continuity
    • Technique
      • insert needle electrode through the skin into muscle to determine insertional and spontaneous activity
    • Types of activity
      • insertional activity
        • shows state of muscle and innervating nerve as needle is inserted
        • normal muscle has baseline electrical activity
        • abnormal insertional activity (>300-500ms) shows early denervation
          • polymyositis
          • myotonic disorders
          • myopathies
        • reduced insertional activity occurs after prolonged denervation
          • muscle undergoes fibrosis
      • contraction activity
        • patient is asked to contract muscle and shape/size/frequency of motor unit potentials are recorded
      • spontaneous activity
        • normal spontaneous activity includes end plate potentials and end plate spikes
        • abnormal spontaneous activity indicates some nerve/muscle damage
          • sharp waves
          • fibrillations
            • spontaneous action potentials from single muscle fibers caused by oscillations in resting membrane potential of denervated fibers
            • seen 3-5wk after nerve lesion begins, and stays until it resolves or muscle becomes fibrotic
            • also seen in muscle disorders e.g. muscular dystrophy
          • fasciculations
            • spontaneous discharge of group of muscle fibers
            • found in amyotrophic lateral sclerosis, progressive spinal muscle atrophy and anterior horn degenerative diseases e.g. polio, syringomyelia
            • seen as "undulating bag of worms" on physical exam
          • complex repetitive discharges
          • myokimic discharges
      • Findings on EMG
      • Condition
      • Insertional Activity
      • Spontaneous Activity
      • Minimal Activity
      • Interference
      • Normal
      • Normal
      • Silent
      • Biphasic/triphasic potentials
      • Complete
      • Axonal neuropathy
      • Increased
      • Fibrillations/positive sharp waves
      • Biphasic/triphasic potentials
      • Incomplete
      • Demyelinating neuropathy
      • Normal
      • Silent
      • Biphasic/triphasic potentials
      • Incomplete
      • Anterior horn cell disease
      • Increased
      • Fibrillations/fasciculations, positive sharp waves
      • Large polyphasic potentials
      • Incomplete
      • Inflammatory Myopathy
      • Increased
      • Fibrillations, myotonia
      • Small polyphasic potentials
      • Early
      • Noninflammatory
      • Normal
      • Normal
      • Small polyphasic potentials
      • Early
      • Neuromuscular junction disorder
      • Normal
      • Normal
      • Biphasic/triphasic potentials (decreased amplitude/duration)
      • Early/normal
      • Neurapraxia
      • Normal
      • Silent
      • None
      • None
      • Axonotmesis
      • Increased
      • Fibrillations/positive sharp waves
      • None
      • None
      • Neurotmesis
      • Increased
      • Fibrillations/positive sharp waves
      • None
      • None
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