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The autonomic nervous system (ANS) is a powerful system that has important homeostatic functions. Disorders of this system can lead to many disease states and may contribute to some pain disorders such as complex regional pain syndrome (CRPS). Having a good working knowledge of the anatomy and physiology of the ANS is of paramount importance when considering sympathetic blocks. The higher neural centers in the brainstem, the hypothalamus, and the prefrontal cortex have nerve cells that tightly control the function and the output of the ANS. The ANS is composed of the sympathetic and parasympathetic nervous systems. The parasympathetic outflow has cranial (several cranial nerves), vagal (mainly via the vagus nerve, which innervates many intrathoracic and intraabdominal structures), and sacral outflow that controls the urinary and reproductive organs. The sympathetic nervous system is formed by neurons in the intermediolateral cell column of the thoracolumbar spinal cord. These are in turn modulated by descending autonomic projections from the hypothalamus, the suprachiasmatic nucleus, and the supraparaventricular nucleus.1 Axons from these neuron cells exit the spinal cord by the anterior spinal roots and white rami communicantes to the sympathetic chain ganglia located along the left and right anterolateral margins of the spinal column. Upon reaching these paravertebral ganglia, the preganglionic sympathetic axons may synapse, pass cephalad or caudad for variable distances within the sympathetic chain before synapsing, or continue uninterrupted to a more distant ganglion or plexus, such as the celiac or hypogastric plexus.

Nerves from the cervical and thoracic ganglia innervate the head and neck and control the blood supply to these structures. The nerves from the thoracic ganglia control the function of the heart and lungs. The nerves that bypass paravertebral sympathetic chain ganglia to more distant ganglia (celiac, hypogastric) are called splanchnic nerves. Splanchnic nerves are the primary sympathetic fibers that innervate visceral organs. Nerves from the lumbar ganglia innervate the legs and control the blood supply to the legs.

The A-δ and C fibers transmit pain and discomfort sensations from the visceral organs to the spinal cord dorsal horn lamina. These fibers develop from the dorsal root ganglion cells and travel along the blood vessels, ganglia, and mesenteric nerves. Hence, blockade of the sympathetic nerves not only causes sympatholysis but also sensory denervation of the abdominal organs.

Kappis originally used paravertebral sympathetic blocks2 as treatment for severe pain and visceral pain syndromes. Mandl3 first introduced percutaneous interruption of the sympathetic chain in the early 20th century, and for many years thereafter it was a mainstay therapy for vascular insufficiency of the lower extremities. Sympathetic nervous system blocks may be performed to diagnose sympathetically mediated pain (SMP), to diagnose visceral abdominal pain (by blocking splanchnic afferents), and for short-term benefit in conditions of pathologic vasoconstriction (e.g., Raynaud's disease)4 or when sympathectomy can provide therapeutic benefit (e.g., digit reimplantation, limb ischemia). Stellate ganglion block has also been successfully used ...

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