Axonotmesis
may, in rare cases, indicate that the nerve was severed; however, it
generally indicates that regeneration can occur because the endoneurial
coverings maintain their proper alignment, indicating that this patient
will recover with conservative care. Neuropraxis however is found when
conduction is locally blocked (for example focal demyelination), and
recovery is relatively rapid. With neuropraxia think of some local,
focal blockage to nerve conduction, such as a carpal tunnel syndrome.
Finally neurotmesis can be distinguished for the other two states
because when distally stimulated, normal conduction is maintained.4
The most
sophisticated of all Neurodiagnostic tests is Somatosensory evoked
potentials, also called Somatosensory evoked responses, short-latency
Somatosensory evoked potential, Somatosensory EPs, SL-Eps, SEPs, SSEPs,
SSERs or SERs 6-17.
Functional
impairment is uniformly assessed by the performance of a quality
physical, orthopedic and neurological examination and special tests, and
obtaining the patient’s history then forming the consistent patient
status. Imaging procedures allow you to actually study a picture
revealing structural evidence, which is then correlated to determine
functional changes. The search for a relatively noninvasive, simple
test of brain function has brought about the development of sensory
evoked potentials recorded from the scalp as low-risk,
clinically-applicable procedures capable or providing new and objective
evidence about a variety of nervous system functions. Generally,
Somatosensory evoked responses are used to test the visual system, the
auditory system and somatosensory tracts and pathways
7,14.
Somatosensory evoked response specificity concerns posterior column
disease, such as posterolateral column disease or sclerosis of the
spine, to test the integrity of the rostral projections of the central
nervous system, the cervical roots, the lumbar roots, the brachial
plexes, specific upper extremity nerves such as the median nerve and
cervical and lumbar radiculopathy.
7,8,14,21
This examiner previously
discussed the fact that we could gain tremendous insight into the
integrity of the afferent pathways by use of Neurodiagnostic procedures,
yet we were unable to test the stretch and vibratory receptors. With
the use of SL-SEPs, we can now test those stretch and vibratory afferent
receptors. Sensory evoked potentials provide an objective measurement
of the functional state of the afferent sensory pathways including the
receptors, primary afferent sensory neurons into the central nervous
system, ascending sensory pathways within the nervous system, specific
sensory cortex, and nonspecific sensory cortex functions.
7,21 In fact,
it has been determined that sensory evoked potentials can a) determine
whether or not our patient has a disorder of sensory function, b)
localize the anatomical site of the patient’s disorder, c) implicate
specific etiologies for sensory impairment, and d) provide an objective
index of the efficacy of various therapies.
7
Neuroscientists have been exploring additional uses for SSEPs. Desmedt,
in 1971, revealed that peripheral nerve function can be determined by a
neurologist by measurement of the change in latency of the initial
scalp-derived negative component form stimulation at various points
along a peripheral nerve, which will provide a measure of the nerve’s
conduction velocity. 2,8
This application of somatosensory evoked potentials is particularly
applicable to individuals with advanced peripheral neuropathies in whom
compound nerve-action potentials may be difficult to detect by other
means.
Cracco (1975)
proved that SSEPs could be used to localize the level of spinal cord
pathology in infants.9,10
Perot used SSEPs as a rapid and objective clinical measure of spinal
cord function in individuals rendered unconscious, or who were
uncooperative from trauma.14
The presences of
potentials were recorded from the scalp following stimulation of nerves
in the legs. Perot determined that normal SSEPs indicate integrity of
dorsal column function, whereas their absence, prolonged latency, or
diminished amplitude alerts the clinician to the presence of a spinal
cord lesion.14
Starr
14 discussed the
use of SSEPs to monitor spinal cord function in the operating room in
individuals undergoing correction of spinal column curvature.
Synek and
Cowen12 proved that
somatosensory-evoked potentials could be effectively used in the
evaluation of traumatic lesions of the brachial plexus, especially in
young people. They studied 12 patients with traumatic lesions of the
brachial plexus using SSEPs by simulation of the median and radial
nerves, cervical spinal cord and contralateral cerebral cortex. Their
results revealed that patients with C5-C6
root avulsion had either normal, delayed or absent responses at
the cervical cord and cortex, depending on the involvement of C7
roots that were abnormal after radial nerve stimulation.
In-patients with multiple root avulsions and flail anesthetic arms, no
potentials could be recorded from the cervical cord or contralateral
cortex, regardless of which nerve was stimulated. To determine relevant
information, it was important to stimulate the nerves having roots near
the anatomic site of the lesion, as determined clinically and
electromyographically. Thus SSEPs are effective in the evaluation of
peripheral injuries to the brachial plexus, median and radial nerves,
and cervical spinal cord.
12,13,21
SSER’s have
also demonstrated slowed conduction in somatosensory pathways of the
spinal cord and the brain stem in diseases such as demyelinating
disease, multiple sclerosis, and vascular lesions of the brainstem and
infiltrating tumors that affect the ascending pathways.7,8
Cerebral
hemisphere lesions which result in a loss of sensations such as pin and
positional sense as well as touch have been associated with a loss of
evoked potentials from both the affected and normal hemispheres if the
stimulus is applied to the limbs with decreased sensibility.
15
Cervical
spondyloslysis has demonstrated an abnormal latency difference between
the brachial plexus and lower medullary components following upper limb
stimulation. 8,16
SSEPs have demonstrated abnormality in-patients with vitamin B12
deficiency, Friedreich’s ataxia, and hereditary sensorimotor
neuropathies and in children with degenerative CNS diseases such as
juvenile diabetes and renal diseases. Even intrinsic brain stem tumors,
infarctions and hemorrhages of the brain have been found by
short-latency SSEPs.7
Separating
fact from fiction relative to cervical and lumbar radiculopathy with
actual case studies using SSEPs, Electroencephalographhy, Stroboscopic
examination, Biofeedback, surface EMG’s etc will be discussed in
parts 4 and 5
(e.g.
Latest 2004 rewrite with 2004 cutting edge neuroscience by request only.
Good luck. Coming soon to Subscribing Physician members in good
standing, 2004-2005 Understanding Orthopaedic Medicine from start to
finish by our team of medical government consulting authors. Past
issues of Understanding Ear Nose and Throat, Pediatric Otolaryngology by
our past Academy President Art Weiss MD and by other .
