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 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.
DISTINGUISHING CENTRAL AND PERIPHERAL LESIONS
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. Although there can be many similarities between patients with CNS and PNS lesions, the distinctions are clear (Table 172-1). Lateralization of weakness, hyperreflexia, positive Babinski’s sign, or any other CNS finding requires further investigation for a central rather than peripheral disorder.
TABLE 172-1Differentiating CNS from Peripheral Nervous System Disorders |Favorite Table|Download (.pdf) TABLE 172-1 Differentiating CNS from Peripheral Nervous System Disorders
| ||CNS ||Peripheral Nervous System |
|History || |
Weakness confined to one limb
Weakness with associated pain
Posture- or movement-dependent pain
Weakness after prolonged period in one position
|Physical Examination |
| Reflexes || |
Brisk reflexes (hyperreflexia)
| Motor || |
Asymmetric weakness of ipsilateral upper and lower extremity
|Symmetric proximal weakness |
| Sensory ||Asymmetric sensory loss in ipsilateral upper and lower extremity || |
Reproduction of symptoms with movement (compressive neuropathy)
All sensory modalities involved
| Coordination ||Discoordination without weakness ||Loss of proprioception |
LOCALIZING PERIPHERAL NERVE LESIONS
Patients with peripheral nerve disorders frequently require diagnostic evaluation unavailable during the initial ED encounter.
The elements required to localize the process will usually include the following: symmetry; proximal versus distal symptoms; sensory, motor, or autonomic involvement; and mono- versus polyneuropathy. Sensory symptoms may include numbness, tingling, dysesthesias, pain, ...