Peripheral Nerve Injury:
Incidence: Frequent
The most common cause of monoplegia is injury to a nerve plexus or peripheral nerve.
The traumatic insult produces immediate neurologic deficits, which either improve or stay the
same over time.
Neurapraxia is a transient loss of nerve function following injury, with no resultant nerve
degeneration. Neurapraxia is analogous to concussion in the brain and spinal shock in the spinal
cord, and is a physiologic dysfunction of the nerve. Because neurapraxia has a better prognosis
than does structural damage, the two must be differentiated. The duration of neurapraxia in animals is unknown, but in man it is thought to last from 3 to 12 weeks. Serial neurologic examinations
and EMG can be used to differentiate neurapraxia from neurotmesis and aid in
forming an accurate prognosis for recovery of function.
Neurotmesis is the complete severance of a nerve. The nerve function is never recovered
unless surgical repair is performed.
Axonotmesis is a rupture or severance of axons within a
nerve but with the supporting structures of the nerve spared. Ruptured axons may regenerate and
eventually reinnervate the muscles.
Most nerve injuries are caused by stretching, direct blows, excessive pressure, or
injections and are a combination of neurapraxia and axonotmesis. Associated local hemorrhage
and edema also contribute to the loss of nerve function.
When the axon is ruptured, the portion detached from the cell body completely
degenerates, a process referred to as Wallerian degeneration. The portion still attached to the
cell body may degenerate toward the cell body one or two nodes of Ranvier. After about I week,
regeneration begins. Distally, the axon and myelin degenerate but the Schwann cells proliferate
to form a neurolemmal tube through which regrowing axons can find their way back to the
appropriate muscle to reinnervate it. The rate of axon growth is about 1 to 4 mm per day, or an
average of 1 inch a month. The distance an axon can regrow is limited by continual shrinking of
the neurolemmal tube. Function is also inhibited by fibrosis of denervated muscles fibers, which
occurs after time.
The closer the nerve injury is to the muscle it must reinnervate, the better the prognosis
for anatomic contact and reinnervation of muscle before fibrosis occurs. Any injury over 12 inch
from a muscle will probably be unable to make anatomic contact with the muscle before the
neurolemmal tube closes. If anatomic contact can be made, the neurolemmal tube may be so
small that proper myelinization of the new axons is impossible. Slow axonal conduction time
and muscle fibrosis severely retard function. When an injury can be localized to a certain
portion of the nerve, the distance from the injury to the muscle to be reinnervated may be
measured and time for regeneration may be estimated using the I inch per month as a guide. The
minimal recovery time is usually several months.
Positive waves and fibrillation potentials are seen in denervated muscles 5 to 7 days
following the nerve injury. The presence of motor unit action potentials (MUAP) indicate that
some axons are still intact, even though no function may be found on the neurologic
examination.
Nerve stimulation and the ability to elicit an evoked response indicate that some axons
within the nerve are still intact. The amplitude of the evoked response may be a guide to the
prognosis for recovery. A small amplitude, between 100 to 200 :V, indicates a poorer prognosis
than a 1,000 to 5,000 :V or greater response. Serial evaluations of motor nerve conduction
velocities may aid in determining the prognosis. If the initial motor nerve conduction velocity is
slow and remains slow, the prognosis is poorer than if the initial motor nerve conduction velocity
is slow and returns to normal. A severed nerve responds to electric stimulation distal to the site
of injury for about 72 hours, but loses the response to electric stimulation immediately proximal
to the site of injury. With brachial plexus avulsions, it is often difficult to place the electrode
proximal to the lesion site; therefore, if there is no response to electric stimulation distal to the
injury site 72 hours or more after the trauma, the nerve most likely is not intact.
Serial examinations, noting improvement in voluntary movements, sensory levels, spinal
reflexes, and serial EMG studies, are the greatest aids in determining an accurate prognosis for
peripheral nerve injuries.
Therapy:
If the nerve has intact axons following an injury, if the distance that the ruptured axons
have to regenerate is not prohibitive, and if the owner is willing to make the commitment for
daily nursing care, then a physical therapy program may be outlined. Most physical therapy programs
combine heat, massage, and joint manipulation to keep the circulation in the limb as good
as possible to prevent stasis and local hypoxia, which contribute further to muscle atrophy and
fibrosis. Joints develop tendon contractures because of the decrease in tendon movements.
The
carpus and tarsus are often the last joints to be reinnervated, so they commonly contract and the
animal walks on the dorsum of the paw.
The elbow joint may also become contracted in a flexed
position in brachial plexus injuries that spare the musculocutaneous nerve.
The limb is often
carried flexed at the elbow; the tendons contract and hold it in this position.
Hot towels may be placed around the denervated limb and the muscles massaged or a
whirlpool of warm water used to increase circulation to the muscles. A regimen of 15 minutes
twice daily is preferred. Then the affected carpus, tarsus, or elbow should be stretched in
extension twice daily for 10 to 15 minutes to keep the tendons supple. A spoon splint may be
applied to keep the digits, carpus, or tarsus extended and to aid the animal in using the limb
without these joints collapsing. The splint should be placed on the limb for only a few hours a
day between physical therapy sessions, as it restricts circulation to the muscles. The splint
should be removed overnight.
The animal may drag the dorsum of the paw on the ground or rough surface and because
of the loss of sensation may develop severe abrasions. The lesions can become infected and
osteomyelitis of the digits can result. A protective stocking or boot should be placed over the
digits to prevent abrasions. If abrasions occur, they should be kept clean and treated with topical
antibiotics and further protection provided. The animal should be kept from licking the wounds,
as this only further abrades the denervated skin.
During certain stages of the regeneration period, the animal may begin to mutilate the
paw. This may be caused by a regeneration of sensory nerves and a tingling or itching sensation.
This period is usually transient, but can be very frustrating to the owner and veterinarian, as the
animal may produce severe lesions by selfmutilation, regardless of attempts to bandage and
protect the foot. Elizabethan collars, muzzles, wire-mesh foot guards, and leather boots are
among the many things that have been tried in individual cases. Prednisone 1 mg/kg orally
divided every 12 hours, then reduced by half every 3 to 5 days may be tried during this period,
but often has little effect.
The overall prognosis for most nerve injuries depends on the severity of the injury, how
much of the dysfunction is caused by neurapraxia, and how much by axonotmesis, how far the
injury occurs from the muscles denervated by it and on the owner's commitment to provide
months of physical therapy.
If no change has occurred over 1 month or if there is no response to electric stimulation,
surgical exploration of the nerve may be made for possible repair if the distance from the nerve
injury to the muscles the nerve needs to innervate is less than 5 to 6 inches.
Peripheral nerve surgery techniques are well described elsewhere.
If electromyography is not available, serial neurologic examinations should be performed
over several months. If there has been no change for several months and surgical repair is
impossible, then surgery for joint fusion or tendon transplants of the carpus or tarsus may be
considered. Amputation of the limb is a last resort, considered only when no improvement
occurs after several months of critical evaluation.
afectación del nervio ciático
afectación del nervio radial
avulsión del plexo braquial
tumores del plexo braquial
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miles de casos clínicos
