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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/decreased 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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