The flaccid dog (Proceedings)
Aug 01, 2009
CVC IN KANSAS CITY PROCEEDINGS
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.