Chapter 15. Diseases of the peripheral nervous system

General issues of pathology of the peripheral nervous system

The PNS includes preganglionic nerve roots, ganglia of spinal and cranial nerves, cranial nerves, spinal nerves, plexuses and nerves of extremities, trunk and internal organs, that is, structures that provide communication of the CNS (brain and spinal cord) with other organs and tissues.

The proportion of PNS disorders is about 50% in the structure of neurological pathology and about 7% in the general morbidity of the population.

The pathophysiological basis of PNS diseases consists of functional and often structural disorders of motor and sensory neurons and their processes (Fig. 15.1). Neuron lesions are discussed in detail in the course of neurology. Neurosurgeons usually encounter damage to nerve fibers.

Fig. 15.1. Diagram of motor neuron: 1 — nucleus; 2 — nucleolus; 3 — nucleolus satellite; 4 — dendrite; 5 — endoplasmic reticulum and granules of ribonucleic acid (Nissl substance); 6 — synaptic terminal; 7 — granules deoxyribonucleic acid; 8 — Golgi apparatus; 9 — mitochondria; 10 — neurofibrils; 11 — axon; 12 — myelin sheath; 13 — node of Ranvier; 14 — Schwann cell nucleus; 15 — myofibril; 16 — muscle cell nucleus; 17 —muscular nucleus; 18 — axoplasm with the mitochondria; 19 — presynaptic Schwann cell; 20 — sarcoplasm with the mitochondria; 21 — actin filament; 22 — myosin filament

Etiology

The causes of damage to nerve fibers is diverse. More often, these are physical factors (injury), such as traumatic rupture or destruction of a nerve, neural acute or chronic compression, stretching, less often exposure to high or low temperature, electric current, vibration, ultrasound, electromagnetic radiation, penetrating radiation or a chemical substance (in particular, injected), acute or chronic ischemia (the role of ischemia is not as significant as previously thought), inflammatory process, tumor, hematoma.

Pathogenesis

Most nerves contain sensory fibers; therefore, with a moderate intensity of exposure to the etiological factor, paresthesia occurs in the area of nerve innervation and/or pain. With a higher energy of the damaging agent in the zone of its impact, the function of axons and their structure are disrupted.

Impaired integrity of the nerve and even persistent dysfunction with anatomical preservation of the nerve fiber (for example, with ischemia) leads to degeneration of the axon distal and a few centimeters proximal to the injury site. Such degeneration referred to as Wallerian by the name of author who described it begins approximately one day after the disruption of the movement of the axoplasm along the fiber and usually ends by 3 weeks. During this period, the electrical excitability of the axon is preserved, then it is lost (Fig. 15.2). After the completion of Wallerian degeneration, only outer shell of the axon group, the epineural tube, is intact.

Fig. 15.2. Wallerian degeneration: a — healthy cell with its axon; b — initial degenerative processes in the damaged axon; b — changes in the cell body and axon; с — degeneration of the distal portion of the axon and its myelin sheath, macrophage reaction; at the same time, small growths in the proximal part of the axon began to form — first visible signs of regeneration (from Lundborg G., 1988)

If the axon is damaged near the motor neuron body (for example, when the spinal root is torn off), the motor neuron dies. In other cases, repair processes are initiated in the body of the motor neuron, and a few weeks after the damage, regeneration of the axon begins (Fig. 15.3). The rate of regeneration of the nerve fiber in humans is approximately 1 mm per day.

Fig. 15.3. Beginning of the process of regeneration; the proliferation of Schwann cells in parts distal and proximal from the injury of the axon: 1, 2 — the proximal and distal end of the nerve at the site of interruption; 3 — sprouting regenerating nerve fibers; 4 — Schwann cell; 5 — macrophage; 6 — growing end; 7 — fibroblast (from Lundborg G., 1988)

A prerequisite for successful regeneration is the anatomical integrity of the epineural tube. With diastasis of its ends, no nerve recovery occurs, and regenerating axons, randomly intertwining, create a painful tumor-like formation referred to as a neuroma.

Clinically and morphologically, there are three forms of nerve damage.

• Neurapraxia is a dysfunction while preserving the anatomical integrity of both the membranes and nerve fibers. With neuropraxia, large myelin motor fibers suffer more, so sensitivity in the innervation zone may be partially preserved. The etiological factors of neuropraxia are usually stretching, compression and ischemia of the nerve, the most characteristic clinical forms are “Saturday night palsy”, “crutch palsy”. A local conduction block develops, leading to an impairment of nerve function. Recovery occurs within up to 3 weeks and is usually complete.
• Axonotmesis is an impairment of the integrity of nerve fibers, accompanied by loss of function, but with the preservation of the integrity of the neural membranes. Motor and sensory deficits are usually complete. Their causes are usually more intense compression, traction (including in the fracture zone), ischemia, compression with a tourniquet, intraneural injections, hypothermia. Nerve function is most often restored, but the extent of recovery depends on the distance that the regenerating fibers need to overcome: if it exceeds 50−55 cm, fibrosis of the distal parts of the neural tube developing in 1.5 years prevents full recovery.
• Neurotmesis is a complete impairment of the integrity of all nerve elements (fibers and membranes) due to various wounds, complex fractures of limb bones, iatrogenic injuries, nerve germination by malignant tumors. Spontaneous recovery is almost impossible; surgical intervention is necessary to restore the integrity of the damaged nerve.