Anesthesia (anaesthesia) is a complete loss of sensation; Hypoesthesia (hypoaesthesia, hypesthesia) is a diminution of sensation. Hence in Jacksonian terms, these are negative sensory phenomena. Anesthesia may involve all sensory modalities (global anesthesia, as in general surgical anesthesia) or be selective (e.g., thermoanesthesia, analgesia). Regional patterns of anesthesia are described, e.g., "glove-and-stocking anesthesia" in peripheral neuropathies, "saddle anesthesia" involving S3-5 dermatomes resulting from a cauda equina syndrome.
Anesthesia is most often encountered after resection or lysis of a peripheral nerve segment, whereas paresthesia or dysesthesia (positive sensory phenomena) reflect damage to a nerve which is still in contact with the cell body.
Anesthesia dolorosa, or painful anesthesia, is a persistent unpleasant pain (i.e., a positive sensory phenomenon) which may be experienced in the distribution of a resected nerve, e.g., following neurolytic treatment for trigeminal neuralgia, usually with delayed onset. This deafferentation pain may respond to various medications, including tricyclic antidepressants, carbamazepine, gabapentin, pregabalin, and selective serotonin reuptake inhibitors.