Chapter 11: Mechanisms of Pain and Use of Therapeutic Modalities

INTRODUCTION

The International Association for the Study of Pain 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 Pain is the most common complaint and the most prevalent symptom that requires intervention among patients in rehabilitation programs. 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, it is important to note that pain is an important protective sensation. In this manner, pain can serve as an indicator or pathology, physical stress, or injury. Therefore, when pain is present, treatment should be targeted to solve the underlying cause.

TYPES OF PAIN

Pain can be described as acute, chronic, or referred. Pain can also be described according to the origin, or the relationship between the symptoms and the underlying pathology. To best understand the mechanisms of electro-pain modulation and improve clinical management of pain, the types of pain must be differentiated.

Acute Pain

Acute pain is a symptom that results from injury and/or disease that causes or can cause tissue damage through infection, trauma, the 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 in order 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 that persists beyond the normal time ...