Interactions between the painful disorders and the autonomic nervous system

The autonomic nervous system (ANS) controls the heart rate, blood pressure, digestion, respiration, pupillary reactivity, sweating, urination, sexual arousal, and regulates the functions of internal organs. This system provides the homeostasis of the cells, tissues, and organs throughout the body and protects against the disturbances imposed by the external and internal stressors. The ANS has three main divisions: The sympathetic nervous system (SNS), the parasympathetic nervous system (PNS), and the enteric nervous system. In general, the SNS and PNS have opposing effects. Each region belonging to the "pain matrix" interacts with ANS. The descending system regulates pain and creates a regulatory effect by the contribution of aminergic neurotransmitters. Hypothalamus, amygdala, and periaqueductal gray are the main structures of this regulatory system. Dysfunction of the ANS is frequently observed in pain patients. The SNS induce, facilitate, or potentiate chronic pain. Increased responsiveness of injured sensory nerves to catecholamines, increased expression of alpha-1 adrenoreceptors on the primary afferent nociceptors and hyperalgesic skin, central sensitization rendering A beta mechanoreceptors, enhanced discharge and sympathetic sprouting in dorsal root ganglia, central sensitization, and dysfunction of the pain modulation is proposed mechanisms. In this review, the anatomical, physiological and pathological aspects of ANS and pain, and laboratory tests to evaluate autonomic functions will be discussed. Pathophysiological role of ANS in migraine, trigeminal autonomic cephalgias, trigeminal neuralgia, peripheral nerve injuries, small fiber neuropathies, myofascial pain syndrome, fibromyalgia, painful joint diseases, visceral pain, phantom limb pain, complex regional pain syndrome, and spinal cord injury will be discussed.