Sports Medicine: Just the Facts

CHAPTER 19 • ELECTRODIAGNOSTIC TESTING 113

• Runners often experience neurapraxic injury of the
  tibial nerve branches with putative tarsal tunnel syn-
  drome, possibly due to repeated traction injury with
  the foot in pronation. (Jackson and Haglund, 1991)

AXONOTMESIS AND NEUROTMESIS

• Axonotmesis and neurotmesis refer to axonal injury
  with wallerian degeneration of nerve fibers discon-
  nected to their cell bodies. There is a loss of nerve
  conduction at the injury site and distally. Axonotmetic
  injuries involve damage to the axon with preservation
  of the endoneurium (Dimitru, 1995).


• Neurotmetic injuries imply a complete disruption of
  the enveloping nerve sheath (Dimitru, 1995).


• EDX studies typically cannot separate axonotmesis
  from neurotmesis.


• Athletes can often experience axonal injury with con-
  ditions such as a radiculopathy.

SPECIFIC EDX STUDIES

NERVE CONDUCTION STUDIES

•Nerve conduction studies may be performed on motor,
sensory, or mixed nerves.

• There are numerous pitfalls with NCS (Table 19-2),
  therefore it is imperative that a well-trained consultant
  performs EDX studies (Robinson and Stop-Smith,
  1999).


• Both motor and sensory NCS test only the fastest,
  myelinated axons of a nerve, thus, the lightly myeli-
  nated or unmyelinated fibers (C pain fibers) are not
  examined with EDX (Wilbourn and Shields, 1998).


• Motor nerves are stimulated at accessible sites, and
  the compound muscle action potential (CMAP) is
  recorded over the motor points of innervated muscles.


• Deep motor nerves and deep proximal muscles are
  more difficult to study and interpret (Feinberg, 1999).
  
  
  • Sensory nerves can be studied along the physiologic
    direction of the nerve impulse (orthodromic) or oppo-
    site the physiologic direction of the afferent input
    (antidromic).
  
  
  • Stimulated and recorded sensory nerves produce a
    sensory nerve action potential(SNAP).
  
  
  • Frequently, sensory axons are tested within mixed
    nerves, such as the plantar nerves, and produce a
    mixed nerve action potential(MNAP).
  
  
  • CMAP, SNAP, and MNAP waveforms are analyzed
    and interpreted by the clinician.
    •Waveform parameters include amplitude, latency, and
    conduction velocity.
  
  
  • Amplitude evaluates the number of functioning axons
    in a given nerve, and for motor studies, the number of
    muscle fibers activated.
  
  
  • Latency refers to the time from the stimulus to the
    recorded action potential.
    •With motor NCS, latency takes into account the
    peripheral nerve conduction (distal to the site of stim-
    ulation), neuromuscular junction transmission time,
    and muscle fiber activation time (Robinson and Stop-
    Smith, 1999).
    •With sensory nerves, latency measures only the con-
    duction of the segment of nerve stimulated.
    •Conduction velocity across a segment of nerve can
    also be calculated when nerves are stimulated at a
    distal and proximal site.
  
  
  
  

LATE RESPONSES

H REFLEX

• The H reflex is the electrophysiologic analog to the
  ankle stretch reflex. It measures afferent and efferent
  conduction mainly along the S1 nerve root pathway
  (Fisher, 1992).
  •A latency difference of at least 1.5 ms is significant in
  most laboratories.


• Amplitude of <50%compared with the uninvolved
  side is also significant.


• Since the amplitude of this reflex is sensitive to con-
  traction of the plantar flexor muscles, amplitude
  changes without significant latency abnormalities
  should be interpreted with caution (Press and Young,
  1997).
  •The H reflex looks at the afferent and efferent
  pathways, thus, it gives information about the sen-
  sory pathway that is not tested by standard needle
  EMG.


• The S1 nerve injury can be due to S1 radiculopathy
  from a herniated lumbar disc or lumbar stenosis,
  peripheral neuropathy (usually with bilaterally abnor-
  mal H reflexes), or sciatic/tibial nerve injuries.

TABLE 19-2 Sources of Error in Nerve
Conduction Studies

•Temperature

• Inadequate or excessive stimulation


• Improper placement of electrodes
  •Tape measurement error


• Age


• Anomalous innervation
  •Volume conduction of impulse to nearby nerve


• Improper filter settings


• Improper electrode montage setup
  •Involuntary muscle contractions