Classification of Pain

Pain is a complicated physiological process that can be classified in terms of its duration, etiology, and physiology. Acute pain, which usually follows trauma to tissue, is limited in duration and is associated with temporal reductions in intensity. In contrast, chronic pain is of longer duration, often 3-6 months longer than expected. Chronic pain often has an unclear etiology and its prognosis is more unpredictable when compared to acute pain. Although acute pain and chronic pain have distinguishing characteristics, there is often overlap, making the diagnosis and management of pain challenging. Table 4.1 highlights some of these characteristics.

The etiologic classification of pain refers to the clinical context in which pain perception takes place. Thus, pain can be categorized as benign or adaptive, malignancy related, postsurgical, or degenerative. Identifying the etiology of pain is valuable in predicting prognosis and personalizing a patient's treatment strategy. For instance, a patient suffering from terminal pancreatic cancer may call for increasingly aggressive narcotic treatment with less concern for narcotic dependence and more concern for patient comfort.

N. Vadivelu et al. (eds.), Essentials of Pain Management,

DOI 10.1007/978-0-387-87579-8_4, © Springer Science+Business Media, LLC 2011

Table 4.1 Pain characteristics: Acute vs. Chronic

Acute pain

Chronic pain

1. Usually obvious tissue damage

1. Multiple causes (malignancy, benign)

2. Distinct onset

2. Gradual or distinct onset

3. Short, well-characterized duration

3. Persists after 3-6 months of healing

4. Resolves with healing

4. Can be a symptom or diagnosis

5. Serves a protective function

5. Serves no adaptive purpose

6. Effective therapy is available

6. May be refractory to treatment

Physiologic pain is defined as rapidly perceived non-traumatic discomfort of very short duration, alerting the individual of a dangerous stimulus. This is adaptive and initiates the withdrawal reflex that prevents and/or minimizes tissue injury.

Physiologic pain can be divided into neuropathic pain and nociceptive pain. Nociceptive pain can be further divided into somatic and visceral pain. Neuropathic pain results from irritation or damage to nerves. It is usually characterized as burning, electrical, and/or shooting in nature. However, a common characteristic of neuropathic pain is the paradoxical coexistence of sensory deficits in the setting of increased painful sensation.

Nociceptive pain is defined as noxious perception resulting from actual tissue damage following surgical, traumatic, or disease-related injuries. This pain is detected by specialized transducers called nociceptors, which are the peripheral endings of A-delta (AS) and C fibers. Nociceptive pain involves peripheral inflammation and the release of inflammatory mediators, which play a major role in its initiation and development.

Somatic nociceptive pain is well-localized sharp, crushing, or tearing pain that usually follows a dermatomal pattern and often occurs after mechanical trauma. In contrast, visceral nociceptive pain is poorly localized dull, cramping, or colicky pain generally associated with peritoneal irritation, dilation of smooth muscle, or tubular passages. Visceral pain radiating in a somatic dermatomal pattern is described as referred pain. Differences between the physiologic, neuropathic, and nociceptive pain are described in Table 4.2.

Table 4.2 Differences between the physiologic, nociceptive/inflammatory, neuropathic, and mixed pain.

Category

Cause

Symptoms

Examples

Physiologic

Brief exposure to a noxious

Rapid, yet brief pain perception

Touching a pin or hot object

stimulus

Nociceptive/inflammatory

Somatic or visceral tissue injury

Moderate to severe pain,

Surgical pain, traumatic

with mediators impacting on

described as crushing or

pain, sickle cell crisis

intact nervous tissue

stabbing; usually worsens after

the first 24 h

Neuropathic

Damage or dysfunction of

Severe lancinating, burning, or

Neuropathy, chronic regional

peripheral nerves or CNS

electrical shock-like pain

pain syndrome,

post-herpetic, neuralgia

Mixed

Combined somatic and nervous

Combinations of symptoms; soft

Low back pain, back surgery

tissue injury

tissue pain plus radicular pain

pain

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