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Acute peripheral neurologic lesions are a diverse group of disorders. By definition, they involve injury or disease in sensory and motor fibers outside of the central nervous system (CNS) extending to the neuromuscular junction. The peripheral nervous system (PNS) serves sensory, motor, and autonomic functions. Thus, the patient with a peripheral nerve lesion may have deficits in any combination of these functions. Exclude central processes, such as stroke or spinal cord injury, before considering an acute peripheral lesion.

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Use CNS and PNS neuroanatomy principles to distinguish lesions. Peripheral nerves contain varying amounts of motor, sensory, and autonomic fibers and follow well-described paths that make them prone to typical injuries. Thus, peripheral nerve lesions are more likely to be confined to one limb and to present with the involvement of multiple sensory modalities and motor symptoms. A typical example would be a nerve compression syndrome presenting with weakness, numbness, and tingling that developed after the arm was held in an unusual position for a prolonged period. However, weakness and numbness can be seen in both peripheral and central disorders. Hyporeflexia sometimes occurs with acute central lesions, but hyperreflexia and spasticity invariably develop with time. PNS disorders, like CNS diseases, can affect bulbar structures, resulting in diplopia, dysarthria, or dysphagia. Despite the overlap, CNS disorders have other features not seen in peripheral disease. For example, aphasia, apraxia, and vision loss are hallmarks of cortical disease. Most CNS lesions will result in upper motor neuron signs: hyperreflexia, hypertonia (spasticity), and extensor plantar (Babinski) reflexes. Perhaps the most important distinguishing component is the examination of deep tendon reflexes. Dorsiflexion of the great toe with fanning of remaining toes and flexion of the leg is a pathologic Babinski's sign, indicating a central disruption of the pyramidal tract. Although there can be many similarities between patients with CNS and PNS lesions, the distinctions are clear (Table 1). Lateralization of weakness, hyperreflexia, positive Babinski's sign, or any other CNS finding requires further investigation for a central rather than peripheral disorder.

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Table 1 Differentiating Central Nervous System from Peripheral Nervous System Disorders
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Patients with peripheral nerve disorders frequently require diagnostic evaluation unavailable during the initial ED encounter. A careful history and physical examination will exclude critical diagnoses and point toward the appropriate management.

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