Chapter 2: The Biomechanics of Peripheral Nerve Injury

Objectives

On completion of this chapter, the student/practitioner will be able to:

• Develop an evidenced-based evaluative algorithm for the assessment of suspected peripheral neuropathy.

• Develop sufficient knowledge of the cognitive components of the peripheral neuropathy evaluation to enable the development of the psychomotor components of the evaluation.

• Identify important signs and symptoms that may relate to peripheral neuropathy.

Key Terms

• Epineurium

• Lower motor neuron

• Mesoneurium

• Perineurium

• Vasa nervorum

Introduction

The peripheral nerve is a functional component of an intricate intrinsic conduction system that serves as a mediator for bidirectional transport between the central nervous system (CNS) and the peripheral nervous system (PNS). Peripheral nerves extend to distal tissues to aid in system regulation, homeostasis, repair, function, learning, posture, reproduction, mobility, and protection. For descriptive purposes, the peripheral nerves may be classified via segmental and peripheral nomenclature according to their function and site of CNS origin: The cranial nerves emerge from the base of the brain, the spinal nerves originate in the spinal cord, and the autonomic system is intimately associated with the cranial and spinal nerves but differs in function and in the details of structure and distribution.

Efferent, Afferent, and Autonomic Pathways

The bidirectional movement of action potentials along the peripheral nerve serves as afferent pathways, efferent pathways, and autonomic pathways. The afferent pathways are primarily sensory. A wide spectrum of sensory modalities exist: vision, hearing, smell, taste, rotational acceleration, linear acceleration, verticality, touch, pressure, warmth, cold, pain, proprioception, kinesthesia, muscle length, muscle tension, arterial blood pressure, central venous pressure, inflation of lung, temperature of blood in the head, osmotic pressure of plasma, and arteriovenous blood glucose difference. A loss or diminished function in any of these modalities may have devastating homeostatic and functional consequences. The signs and symptoms of loss are directly related to the specific sensory modality that is compromised. Positive signs of sensory nerve injury are typically described as sensations additive to normal perception, such as burning, formication, tingling, hyperalgesia, pain, or a feeling of temperature change. Negative signs are a reduction in sensory perception, such as numbness, ataxia, orthostasis, loss of visual acuity and tracking, deafness, movement degradation, and dyskinesia. In higher level animals, there is a redundancy of sensory modalities that allows for continued, albeit diminished, function with the loss of specific sensory modalities. An example of redundancy in humans is balance. The visual, vestibular, and sensorimotor systems all contribute to balance control. Functional performance with loss of one or more of these systems has been extensively studied.^1–6

The efferent pathways are primarily motoric. Motion ...