Chapter 11: Mechanisms of Pain and use of Therapeutic Modalities

INTRODUCTION

Pain is the most prevalent symptom and most common complaint among patients seeking rehabilitation services. The International Association for the Study of Pain (IASP) defines pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” (www.iasp-pain.org).¹ Perception of pain is influenced by a variety of factors such as cultural differences, motivation, emotional states, and past experiences with pain.² Although clinicians often treat patients with pain with the goal of reducing or eliminating pain, pain is an important protective sensation with significant biological meaning. In this manner, pain can serve as an indicator of pathology, physical stress, or injury. Therefore, when pain is present, treatment should not be simply aimed at eliminating the pain but rather eliminating the underlying cause of the pain.

TYPES OF PAIN

Pain is typically classified as acute, chronic, or referred. Pain can also be described according to the origin (e.g., neural or neuropathic pain) or the relationship between the symptoms and the underlying pathology (e.g., osteoarthritic pain or fibromyalgia). To best understand the mechanisms of electro-pain modulation and improve clinical management of pain, these three types of pain must be differentiated.

Acute Pain

Acute pain is a symptom resulting from injury and/or disease that causes, or can cause, tissue damage through infection, trauma, progression of a metabolic disorder, or a degenerative disease. Acute pain is generally, but not universally, described as pain lasting less than 12 weeks (i.e., 3 months). Acute pain is typically well located and defined, depending on the type of tissue involved. Superficial (e.g., skin) pain is typically sharp and easy to locate. On the other hand, acute deep-tissue pain from muscles, joints, or viscera can be diffuse and difficult to locate.¹ Acute pain serves to protect against further tissue damage, and when tissue injury is present, pain may be maintained to allow time for proper tissue healing. Therefore, the symptoms can reflect the underlying pathology.³ The clinical treatment of acute pain can be pharmacological or nonpharmacological, involve rehabilitation or surgery, or other procedures aimed at addressing the peripheral tissue damage.¹

The stimulus responsible for acute pain has a short latency and is associated with increased muscle tone, heart rate, blood pressure, skin impedance, and other manifestations related to the increase of activity of the sympathetic nervous system. Autonomic, psychological, and behavior responses persist while the stimulus is present.

Chronic Pain

Chronic pain is commonly defined as persistent or recurrent pain existing for 3 to 6 months or pain ...