Systemic Complications: Neurologic

Introduction

Nerves are complex in formation, surrounded by Schwann cells as the conductive myelin sheath with a supportive structure of extracellular matrix and vasculature. Smaller nerves are simpler, without such supportive structure, relying on diffusion through the interstitial space to supply their metabolic demands. Interestingly, the oxygen requirement of a mammalian nerve is less than that of other tissues, even with increased metabolism. In other words, nerves are inherently resistant to ischemia. The larger nerves of the extremities contain multiple networks of vessels supplied by branches of adjacent arteries, while the inner portions of the nerves contain an extensive pattern of epineural vessels. ^, Interruption of the blood supply to a segmental portion of the nerve usually does not result in ischemic nerve injury. ^, One notable exception is ischemic monomelic neuropathy (IMN), an infrequent ischemic nerve injury following acute limb malperfusion, which is characterized by multiple distal axonal infarctions and resultant motor and sensory mononeuropathies. ^,

There are two functional components of the peripheral nervous system. The somatic system is composed of motor and sensory fibers acting together to allow for muscle contraction and sensory function. Cranial and spinal nerves of the somatic system are composed of a ventral (motor) and dorsal (sensory) root. While the first two pairs of cranial nerves (the olfactory nerve and the optic nerve) originate from the cerebrum and are not a part of the peripheral nervous system, the other ten cranial nerves originate from the brain stem and enter the peripheral nervous system after leaving their associated nuclei. Along the spine, there are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 pair of coccygeal nerves.

The autonomic system is composed of sympathetic and parasympathetic fibers working without conscious control innervating organs. The sympathetic nervous system prepares the body for “fight or flight” response. The main neurotransmitters of the autonomic system released are acetylcholine and norepinephrine. The parasympathetic nervous system has digestive and elimination function. The main neurotransmitter released by the pathway is also acetylcholine.

Ischemic Neuropathy

The true incidence of ischemic neuropathy is difficult to determine. Symptoms of ischemic nerve injury may mimic rest pain associated with arterial insufficiency or decreased sensation due to diabetic neuropathy. Historically, the incidence of ischemic neuropathy associated with PAD has been estimated to be as high as 88%. This has been confirmed in contemporary studies reporting a similar incidence of nerve dysfunction detectable by electrophysiologic nerve conduction studies. Both acute and chronic ischemic conditions have the potential to cause nerve injury, however the mechanism of ischemic neuropathy specific to PAD has not been fully elucidated.

Traumatic Nerve Injuries

Peripheral nerves in proximity to major arteries and veins are at risk for iatrogenic injury during vascular interventions. Without a commanding knowledge of the anatomic relationships of structures, vascular surgeons may successfully restore flow to an extremity yet leave significant neurologic deficits because of surgical trauma. Fortunately, most peripheral nerve injuries that occur during operative exposure result in transient nerve dysfunction (neurapraxia) that recovers with time.

Traumatic injury occurs via several mechanisms. Division or ligation of a nerve renders the nerve permanently damaged. Reconstruction of divided nerves is possible with acceptable rates of functional recovery but the outcome is variable. Other mechanisms include stretch injury from improper retractor placement, thermal injury from electrocautery devices and compressive injury from hematoma formation or from improper patient positioning. Avoiding nerve injury requires constant vigilance for anatomic relationships and technical precision. Except in the case of severance, the peripheral nerves have remarkable capacity to regenerate and functional outcomes, in most instances, is favorable.

Other Causes of Nerve Injury

Some patients evaluated will have peripheral neuropathy not due to ischemia or trauma. Other causes of neuropathy should be included in the differential diagnosis ( Table 47.1 ). Diabetic neuropathy is the most commonly encountered alternative cause of peripheral neuropathy seen by vascular specialists. Risk factors include increasing age, length of diabetes diagnosis, poor glycemic control, poor lipid metabolism, inadequate blood pressure control, obesity, and metabolic syndrome. The etiology of the nerve damage is multifactorial and not completely understood. The metabolic derangements associated with chronically elevated blood glucose levels and abnormal insulin homeostasis are thought to be the major determinants. Vascular disease in the microcirculation is also believed to play a role in nerve damage, thus making the difference between diabetic neuropathy and ischemic neuropathy less distinct. Other causes of peripheral neuropathy include alcoholism, uremia, drug intoxication, medication side effects, vasculitis, autoimmune disorders, infectious causes, and inflammatory causes. Most often, these associated neuropathies are symmetric.

Diagnosing Nerve Injury