Highly symmetric involvement: A systemic cause (eg, a metabolic, toxic, drug-related, infectious, or postinfectious cause; vitamin deficiency) Clearly asymmetric involvement: A structural cause (eg, tumor, trauma, stroke, peripheral plexus or nerve compression, a focal or multifocal degenerative disorder) After location of the lesion, rate of onset, and degree of symmetry have been determined, the list of potential specific diagnoses is much smaller, so that focusing on clinical features that differentiate among them is practical (see table Some Causes of Numbness).