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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