By Jeannine Miesle MA, AAV

Part 1: History / Introduction

Part 2: Pain Perception and Signal Reception

Part 3: Pain Signal Transmission and Pain Pathways

Part 4: Types of Pain, Long-term effects, Referred Pain, and Pain Memory  (Please scroll down)

Part 5: Pain, Stress, and the Body’s Physiological Response to Them

Part 6: Pain in the Avian Species

Part 7: Anesthesia and Analgesia, Chronic Pain

Part 8: Quality-of-Life Issues

Part 9: Pain Assessment in Birds / Quality of Life

Part 10: Hospice and Palliative Care for Pets, Strategy for Comprehensive Care, & Conclusion

Part 4: Types of Pain, Long-term Effects, Referred Pain, and Pain Memory

There are different types of pain, as classified by physicians and neuroscientists.

These are general categories under which fall many sub-categories:

Acute Pain: caused by an injury to the body. It warns of potential damage that requires action by the brain, and it can develop slowly or quickly. It can last for a few minutes to six months and goes away when the injury heals. Examples: accidents, falls, surgery, cuts.

Chronic pain: persists long after the trauma has healed, and in some cases, it occurs in the absence of any trauma or tissue damage. Chronic pain does not warn the body to respond, and it usually lasts longer than six months. Examples: Arthritis, fibromyalgia, headaches, back pain.

Cancer (malignant) pain: associated with malignant tumors. Tumors invade Healthy tissues and exert pressure on nerves or blood vessels, producing pain. Cancer pain can also be associated with invasive procedures or treatments. Some physicians classify cancer pain with chronic pain.

The taxonomy of pain is also categorized based on the source. The purpose of each category is to suggest possible causes for pain, its severity and most appropriate therapy.  

Some examples include:

  • Disease: arthritis, pancreatitis, cancer

  • Anatomy: bladder, pancreatic, back, orthopedic

  • General location: superficial, visceral, deep

  • Duration: transient, acute, chronic

  • Intensity: mild, moderate, severe

  • Response to manipulation: palpation, response to commands, algesiometers (an instrument for determining the skin’s response to a painful stimulus; this is done by applying steady pressure with the instrument’s probe until you get a response from the patient.)

The longer the pain persists, the more intense it becomes. There are actual changes in sensory processing in the spinal cord. Central sensitization refers to activity in the entire central nervous system (CNS). When pain lasts a very long time, either from injury or chronic disease, it creates “windup,” or constant neuroexcitability, which means that the noxious stimuli produce prolonged, long-term sensitivity in the dorsal horn of the spinal cord. This sensitization of the dorsal horn (where pain signals are modulated and then sent on to the brain), is thought to be responsible for increases in the hyperalgesia (over-sensitivity to pain), and allodynia (pain resulting from activity that doesn’t usually cause pain, usually touch.) This lowers the threshold of pain on a permanent basis. Continuous noxious stimuli lasting longer than several hours actually change the genetic code and are responsible for long-term structural changes (neuroplasticity) to the CNS.

Visceral (internal organ) pain is not easily or quickly identified due to the lower number of nociceptors in the viscera compared to peripheral nociceptors. This means that it is difficult to pinpoint the exact location of pain since each nociceptor covers a large area. Also, pain may not even be perceived in the beginning of an assault by a noxious stimulus; however, generalized inflammation and tissue damage activate “silent” nociceptors in the intestines and bladder and increase sensitivity to otherwise innocuous mechanical stimuli. In other words, constant pain will cause the individual to feel pain from stimuli which ordinarily are not painful. They lower the threshold of pain to the point that ordinary touch is painful.

Referred Pain

Referred pain is pain that is felt in uninjured tissue a distance away from the causative injury or lesion. The “dermatome” is the area innervated by a nerve ganglion. Everything in this dermatome (muscles, bones, joints, and tendons) will feel the pain from the injury. It develops slowly, usually triggered by a deep somatic and visceral even rather than a superficial injury, and it referred to regions derived from the same dermatome.

Pain Memory

Pain perception and memory correlate strongly with the peak intensity of pain, but interestingly, not with its duration. The more the site is traumatized, the more severe the pain. This suggests that injury produces neurochemical changes in the CNS that affect nociceptive behavior. This is true for humans and animals.

If painful stimuli establish a memory of pain, then therapies that prevent central sensitization and neuroplastic changes should be beneficial in restoring normal pain sensitivity. This is the reason medical doctors and veterinarians are providing preemptive analgesia—pain medication given before the traumatic event occurs (such as surgery). Doctors reason that preventing the pain from occurring will prevent or reduce the subsequent pain, and the need for medication for additional pain will lessen. Once the intensity of the pain is raised to a high level (hyperexcitability), larger doses of analgesia are required, and often the treatment is ineffective. Therefore, all pain should be treated as early as possible and preemptively when possible.

Reference:

Muir W. Physiology and pathophysiology of Pain. In: Handbook of Veterinary Pain Management. Ed: James Gaynor, Wm Muir III. Mosby Inc, 2009