The peripheral nervous system (PNS) consists of those structures (including cranial nerves and spinal nerves) containing motor,
sensory and autonomic nerve fibers or axons that connect the central nervous system (CNS) with somatic and visceral end organs.
Lower motor neuron weakness refers to a lesion of the ventral spinal cord gray matter and its axon coursing to the muscle
through the spinal nerve roots, peripheral nerve and across the neuromuscular junction.
A motor unit is composed of a neuron cell body, its axon, the neuromuscular junction, and associated muscle fibers. A group
of myofibers innervated by one neuron is considered a motor unit. An abnormality in any portion of the motor unit can result
in clinical signs of neuromuscular disease – lower motor neuron. The functional component of the motor unit involves the reflex
arc. The arc consists of a sense organ, an afferent neuron (cell body in dorsal root ganglion), one synapse or more centrally,
an efferent neuron, and an effector organ. An all-or-none action potential is generated in the afferent nerve and modulated
centrally to be generated again as an all-or-none potential in the efferent nerve. The arrival of the action potential at
the axon terminal triggers release of acetylcholine, producing an end-plate potential which if enough causes the postsynaptic
membrane to depolarize.
Establishing an accurate diagnosis is based on following a logical sequence of diagnostic tests. History provides the signalment,
presenting clinical signs, background, and time of onset and temporal progression of clinical signs. Specifically, disorders
of the motor unit with acute onset include polyradiculoneuritis, tick paralysis, botulism and fulminant myasthenia gravis.
Myopathic and neuromuscular junction (i.e., myasthenia gravis) diseases often are episodic in onset. The term fatigability
describes when one or more muscles become weaker with repetitive but normal use and may imply neuromuscular junction disease.
History is especially important for determining exposure to toxins. Often the diagnosis of an acute neuromuscular disease
is determined by the history.
A physical examination is performed to localize the clinical signs and detect other systemic abnormalities. The neurologic
examination will establish existence of peripheral nervous system disease and further assist with determining disease symmetry
and distribution (focal, multifocal or diffuse). The neurologic examination should include sensory testing especially when
determining the extent of traumatic neuropathy. Clinical signs of lower motor neuron weakness manifest as paresis, paralysis,
hyporeflexia to areflexia and muscle atrophy that is severe and rapid-onset. Acute onset lower motor neuron diseases often
have muscle flaccidity. Acute peripheral neuropathy of cranial nerves can occur with focal, multifocal or diffuse (polyneuropathy)
disease. Mentation, gait, postural reactions and spinal reflexes are normal with focal neuropathy of a cranial nerve. Typically
only one cranial nerve is affected. If there is disease of the brainstem, abnormalities involve mentation, postural reactions,
gait and multiple cranial nerves. Neurologic deficits are more severe on the ipsilateral side. Myopathic and neuromuscular
junction diseases also can mimic neuropathy of cranial nerves, i.e. myasthenia gravis will manifest clinical signs of megaesophagus
(regurgitation). Proper neuroanatomic localization is crucial to the direction of the diagnostic approach.
A complete blood count, serum chemistry (including creatine kinase concentration and electrolytes) and a urinalysis serve
to establish a baseline health profile and further identify other systemic abnormalities. Thoracic radiography may show evidence
of concurrent megaesophagus and aspiration pneumonia which can be a sequela of peripheral neuropathy. Additionally, thoracic
radiography and abdominal ultrasonography are used to screen for underlying metastatic disease and evidence of paraneoplastic
neuropathy. CSF analysis will show abnormalities in cellularity and protein concentration with some peripheral neuropathies.
Serology is useful to evaluate for infectious and immune mediated diseases. Endocrine function testing, especially thyroid
hormone will further delineate underlying cause of the neuropathy.
Treatment protocols are guided by defining the underlying cause of the LMN signs.
Differential diagnosis of acute generalized lower motor neuron disorders