Carvell, PhD, PTUniversity
PERIPHERAL NERVE DYSFUNCTION III:
Ben.fig 13-31p 421
are diseases of the motor neuron. Lower Motor Neuron (LMN) Disorders have a
classic constellation of
signs and symptoms, see LMN/UMN REVIEW. A LMN Disorder is characterized by a
flaccid paresis or paralysis with
no sensory system involvement. Severe atrophy of the muscle is due to denervation of the
involved motor unit's muscle
fibers. Deep Tendon Reflexes are hypoactive or absent for the involved muscles.
Clinical Electromyographic Needle
Exam reveals electrical signs of denervation including abnormal potentials at
rest (fibrillation potentials, positive
sharp waves, and fasciculation potentials). Requests for volitional contraction shows
that Motor Unit Recruitment is
abnormal (reduced number of motor units that fire at a
rate than normal). Motor unit potentials that survive in incomplete lesions are abnormal (polyphasic and
giant potentials). The surviving motor unit axons sprout new axon
collaterals within the muscle to "rescue" denervated muscle fibers; this
results in an expanded motor unit territory for surviving motor units. Missing
in a true Anterior Horn Cell (LMN) Disease are the pathological reflexes typical of an Upper
Motor Neuron Disorder.
Abnormal synergies are absent, though the individual may substitute remaining muscles to
create actions to accomplish the task at hand. Weakness can be quantified
with a manual muscle test that isolates specific muscles/muscle groups. The
individual has no difficulty in isolating
specific muscle actions but those affected muscles will be weak (reduced force production). Diseases
of the spinal cord may show a combined
Upper Motor Neuron and Lower Motor Neuron Syndrome due to involvement of the descending tracts
and ventral horn gray, respectively. Amyotrophic Lateral Sclerosis (ALS) shows such a combined
UMN/LMN disease process.
ALS typically involves motor nuclei within the brainstem that may affect speech, swallowing,
tongue movements, and facial expressions.
The extraocular muscles are often sparred at least until late in the course of this fatal disease.