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dystrophy, myositis, etc.), a myelin‑related acquired or Many other spontaneous potential can be recorded from
congenital disorders (chronic inflammatory demyelinating muscles, but their discussion is beyond the intent of this
polyneuropathy, Charcot‑Marie‑tooth disease) or presynaptic review.
[22]
neuromuscular junction disorders (Eaton‑Lambert syndrome, The following step in the neurophysiological examination
botulism).
is the analysis of MUAP and their activation and
In neurapraxia, nerve conduction is either slowed or recruitment patterns during voluntary contraction.
blocked secondary to demyelination. With stimulation
proximal to the lesion, the conduction velocity will be In acute axonal loss and pure demyelinating nerve
reduced (conduction slowing), or the evoked potential injuries with conduction block, not all motor units can be
amplitude will drop with respect to the normal potential recruited; the remaining MUAPs have normal morphology
obtained by distal stimulation (conduction block). When but fire with high frequency in order to obtain sufficient
nerve remyelination completes, these abnormalities contraction, and the recruitment pattern results in
progressively disappear, with eventual complete recovery. incomplete interference. Note that denervation potentials
will appear only in case of axonal damage.
In the case of axonotmesis and neurotmesis, after distal
axonal degeneration (which completes in 3‑5 days for In chronic axonal loss and denervation, early collateral
motor fibers and in 6‑10 days for sensory fibers), CMAP sprouting from re‑innervation of orphan muscle fibers
and SNAP are reduced in amplitude when stimulating by surviving axons is recorded on EMG as small satellite
distally to the injury; the ratio between CMAP/SNAP potentials of the MUAP’s. Later, as the number of muscle
amplitudes on the injured side to the CMAP/SNAP of the fibers per motor unit increases with re‑innervation,
normal side is a good estimate of the degree of axonal MUAP’s become higher in amplitude, prolonged in
loss. The higher the axonal loss, the lower the odds of duration, and polyphasic; these are the typical neurogenic
recovery. MUAP’s representing the pattern of denervation and
reinnervation.
For technical reasons, exploration of the proximal
peripheral nervous system is more complex; late responses Incomplete nerve transection and in late stages of partial
such as F waves and the H reflex can be obtained for axonal loss, if regrowing axons from the site of injury
further information and somatosensory or motor evoked eventually reach the target, very small low‑voltage nascent
potentials can be explored. [23,24] MUAP potentials will be recorded. As reinnervation occurs,
denervation potentials will gradually disappear.
Electromyography
This examination requires the active participation of the NERVE IMAGING TECHNIQUES
patient. Needle EMG provides information on the function
of the muscles function and their minimal functional Neurophysiological investigation offers information on the
units. It explores both the quantity and quality of motor pathophysiology of the nerve deficit, the grade of severity,
unit action potentials (MUAP), their spatial‑temporal and prognosis. Although it is a fundamental tool in clinical
recruitment in order to generate adequate movements, evaluation, it does not provide precise information on the
the presence of denervation, and the onset of morphology, etiology or the extent of focal peripheral
re‑innervation. In partial or gradual denervation, nerve injuries versus the focal involvement of only few
[18]
reinnervation occurs early through collateral sprouting by fascicles.
adjacent surviving axons. In nerve transection, the only
mechanism available for re‑innervation is axonal regrowth In severe cases with unexcitable nerves and in
from the proximal stump of the injury site. This regrowth postoperative patients who do not shows signs of
is slow (1 mm/day) and may take months to years to improvement, EMG and conduction velocities cannot
reach the target muscles, depends on the distance to be provide conclusive information on the presence of
covered. neurotmesis, nerve transection, the distance between
nerve stumps, and the presence of multiple sites of
The first step in EMG of nerve injuries is the evaluation of injury. Imaging assessment, in particular high‑resolution
[25]
pathological potentials at rest. Fibrillation potentials and ultrasound (HRU) and magnetic resonance imaging (MRI),
positive sharp waves are the most common potentials and may overcome these problems by providing information
appear 10‑21 days after injury, while complex repetitive on nerve morphology and its surrounding tissues; these
discharges indicate chronic and ongoing denervation. are becoming popular instruments for planning nerve
Although all these potentials are a sign of muscle fiber reconstruction and the surgical approach.
denervation, they can also be found in myopathies and
myositis, which also induce hyposthenia. Fasciculation High‑resolution ultrasound
potentials occur from the spontaneous activation of Although MRI is still more commonly used, based on
motor units (all muscle fibers innervated by one neuron), our experience and on a review of the recent literature,
which can be visualized directly as minor muscle the authors believe that HRU currently represents the
twitches. Cramps are a painful involuntary contraction of most easily available and practical imaging technique for
the muscle which tend to occur when a muscle is in the investigation of peripheral nerve pathology [Figures 1 and 2].
shortened position and contracting, and can be recorded These machines are widely available and, when associated
as a firing of motor unit potentials at high frequency. with high frequency transducers (7‑18 MHz), reach up
152 Plast Aesthet Res || Vol 2 || Issue 4 || Jul 15, 2015