Polyneuropathy is not a single disease, but rather refers to a group of syndromes arising from various underlying causes. Consequently, there are numerous different types. Polyneuropathies are generally classified according to their underlying causes, which can be metabolic, infectious, autoimmune, or genetic in origin. Furthermore, they are divided into two main types based on which part of the nerve is affected: 'demyelinating' polyneuropathies, where the myelin sheath surrounding the nerve is damaged, and 'axonal' polyneuropathies, where the main body of the nerve fiber (axon) is damaged. According to the rate of disease progression, polyneuropathies are categorized as acute (rapid onset) or chronic (slowly progressive). Finally, polyneuropathies can also be classified as 'acquired' or 'hereditary' (genetic) based on their etiology.