Denervation

Denervation is any loss of nerve supply regardless of the cause. If the nerves lost to denervation are part of neural communication to an organ system or for a specific tissue function, alterations to or compromise of physiological functioning can occur. Denervation can result from an injury or be a symptom of a disorder like amyotrophic lateral sclerosis (ALS), post-polio syndrome, or neuropathic postural orthostatic tachycardia syndrome (POTS). Intentional denervation is a valuable surgical technique for managing some medical conditions, such as renal denervation in the setting of uncontrolled hypertension. Pathological denervation, by contrast, is associated with serious health sequelae, including increased infection susceptibility and tissue dysfunction.

The loss of nerve supply can be caused by injury, disorders, or result from a surgical procedure.

Denervation can occur as a consequence of nerve injury. The three primary categories of nerve injury are neurapraxia, axonotmesis, and neurotmesis, each corresponding to varying degrees of damage and potential for recovery. In cases of nerve injury, the brain demonstrates an impressive ability to rewire or reorganize its neuronal circuitry. This plasticity enables the brain to compensate for the disruptions in neuronal communication that result from the injury.

Denervation processes are strongly associated with the symptoms experienced in post-polio syndrome. Individuals with post-polio syndrome undergo a continuous cycle of denervation and reinnervation that occurs after acute poliomyelitis. Over time, this cycle leads to an increase in the size of motor units in skeletal muscle fibers. Eventually, the motor unit areas grow to a point where reinnervation is no longer possible, resulting in uncompensated denervation of the motor units. This ultimately leads to muscle atrophy and myasthenia. Following an acute poliovirus infection, symptoms such as fatigue, asthenia, and pain are believed to be linked to muscle denervation.

Much like post-polio syndrome, ALS also has similar symptoms of motor neurodegeneration leading to general weakness and, in some cases, paralysis. The type of symptoms experienced can depend on which areas of the body experience the loss in nerve supply. This denervation process is different from post-polio syndrome in that it involves only upper and lower motor neuron degeneration and does not involve constant reinnervation and denervation.

In addition to the management of peripheral nerve injury, denervation is used as a medical procedure for various benefits resulting from eliminating nerve supply to a specific area of the body. Renal denervation involves using radio frequency or ultrasound to eliminate the sympathetic nerve supply to the kidney wall, aiming to lower blood pressure and treat chronic hypertension. Renal denervation has become less common in recent years due to new evidence indicating that the procedure does not significantly lower blood pressure. Additionally, there are recommendations against its use, as there has been insufficient proof demonstrating that renal denervation effectively reduces blood pressure.

Other prevalent surgical procedures involve intentionally reducing nerve supply to treat a variety of disorders. In a sympathectomy, a sympathetic ganglion is surgically removed to treat hyperhidrosis(excessive sweating). Surgical or radiologic ablation of the carotid sinus nerve is used to treat carotid sinus hypersensitivity. In a vagotomy, the vagus nerve is surgically removed to treat peptic ulcer disease by reducing stomach acid. In a rhizotomy, nerve fibers in the spinal cord are destroyed with the intent of eliminating chronic myalgia.

In regard to skeletal muscle denervation there are two distinct diagnoses: entrapment and compressive neuropathies or non-entrapment neuropathies. Entrapment and compressive neuropathy syndromes occur due to compression and/or constriction on a specific location for a segment of a single nerve or multiple nerve sites. This entrapment or compression can be diagnosed based on multiple factors including physical examination, electrodiagnostic test and clinical history.