Afflictions linked with the somatosensory system such as neuropathic pain are common in people having neurologic and musculoskeletal ailments. But such conditions, remain a disorder in veterinary patients as it often overlooked. It is not easy to diagnose neuropathic pain in pets as they are not able to verbalize their pain, unlike humans who can report about their pain.
The progression of neuropathic pain is a complex phenomenon, and concepts associated with it are not generally considered in the conventional veterinary medical curriculum so as to make the veterinarians ignore it as a potential problem in their patients. The objective of this review is to examine fundamental concepts in the pathophysiology of neuropathic pain, offer definitions for common clinical terms associated with the condition, and discuss pharmacological therapy alternatives for dogs suffering from neuropathic pain.
Treatments For Reducing Neuropathic Pain
The advancement of neuropathic pain involves key mechanisms like ectopic afferent nerve activity, central sensitization, peripheral sensitization, defective inhibitory modulation, and pathologic stimulation of microglia (glial cells ) situated across the brain and the spinal cord.
Treatments directed towards reducing neuropathic pain involve one or more of these mechanisms. The drugs that are generally used to treat neuropathic pain in veterinary clinical set up are gabapentin, pregabalin, amitriptyline and amantadine. The said review discusses the mechanisms of action for each drug, and recognized pharmacokinetic profiles in dogs. There is strong evidence present in the human literature about the advantages of several of these treatments, but clinical specific literature is at present insufficient. Future research needs to concentrate on objective approaches to document neuropathic pain and control response to treatment in veterinary patients.
An Overview Of The Somatosensory System
The somatosensory system performs three primary functions. It permits perception and response to sensory stimuli arising within the body (interoceptive), reacts to stimuli arising outside the body (exteroceptive), and mediates proprioceptive tasks.
Interoception is an important sensory processing for perceiving intense emotional states like appetite, anger and sadness. Interoceptive sensations includes a much wider range of physiological sensations like muscular action, vasomotor sensations, pain and tickling, hunger, thirst, bladder distension, sensual touch and much more.
The general somatic afferent system (GSA) transmits information connected with thermal, chemical and mechanical stimuli from peripheral receptors towards the somatosensory cortex. The general proprioceptive (GP) system assists in detecting the motion and position of muscles and joints, covering both conscious and unconscious proprioceptive elements.
Pain And The General Somatic Afferent System
The source of pain from tissue injury results from four basic processes : transduction, modulation, transmission, and perception. Transduction causes the conversion of a harmful stimulus to a nociceptive signal at the nociceptor level. Transmission includes the process of transmission of nociceptive signals along nerve fibers from the region of original injury towards the CNS. Modulation involves the process by which nociceptive signals are changed within the CNS through either promotion or inhibition. Perception is an essential part of the pain experience which involves integration of cognitive and emotional reactions to the harmful stimulus.
Commonly, pain is classified as either nociceptive or neuropathic. Nociceptive pain results from harmful stimuli which are processed by an otherwise regularly working somatosensory system – like for instance pain linked with the generation of pro-inflammatory mediators following a broken bone. Nociceptive pain is beneficial at the beginning as it allows an animal to find out and react to a potentially injurious stimulus. It can be further ranked as somatic (arising from skin, joints, and muscles) or visceral ( starting from visceral organs).
Neuropathic pain is referred as the pain occurring from a disease or lesion, which causes damage or dysfunction of the somatosensory system. The term neuropathic and mixed pain are general but not well recognized phenomena in our veterinary patients with permanent neurologic, orthopedic or other ailments.
Neuropathic pain results from a wide range of conditions affecting any organ or tissue that contains nerve endings. It is a maladaptive phenomenon resulting from pathologic neuroplasticity and can turn into an ailment of the neurologic system in its own right by remaining beyond resolution of its provoking cause. Particularly conditions like peripheral neuropathy, spinal cord ailments, chronic musculoskeletal conditions, and brain lesions are generally mentioned. Symptoms of neuropathic pain involves both stimulus dependent hypersensitivity and involuntary pain.
In veterinary patients obvious symptoms of neuropathic pain may include changed response to touch, vocalization even when there is no overt painful impulse, excessive licking or self-injury, phantom scratching, and continuous lameness or reduced weight-bearing on a limb. Additional non-prominent symptoms may include reduced general activity level, unwillingness in climbing or descending stairs, decreased jumping behavior, problem in getting up from a seated position, difficulty in entering or coming out of the car, alterations in body posture and changed conduct or appetite.
Role Of Cannabinoids In Handling Neuropathic Pain
Currently, cannabinoids have evoked a lot of interest in managing neuropathic pain. Cannabinoids are a family of chemicals sourced from the cannabis plant. Several pharmacologically active components have been recognized but the two being evident for analgesic effects are tetrahydrocannabinol (THC), which also contains psychoactive effects, and cannabidiol (CBD). Cannabinoids exercise their effects in the body through the two well-researched receptors:CB1 and CB2. CB1 receptors are prevalent in high numbers within the brain and spinal cord and also the visceral organs and adipose tissue. Triggering of CB1 receptors causes inhibited release of dopamine, acetylcholine, and glutamate. CB1 also regulates opioid, MMDA, and GABA receptors. Most recognized side effects linked with cannabinoids are related to central CB1 receptor effects. CB2 receptors are situated in high concentration on hematopoietic and immune cells, including microglial cells. In regular health, CB2 receptors are expressed only at decreased levels within the CNS but are swiftly upregulated in both neurons and microglia following injury or swelling. Various peripherally restricted CB1 or CB2 receptor agonists have been generated to bypass the centrally meditated side effects of cannabinoids. These drugs have been examined for treating inflammation and neuropathic pain.
Future studies may support the application of novel restricted CB1 or CB2 receptor agonists in veterinary patients. Several non-pharmacologic processes for tackling neuropathic pain are indicated throughout the veterinary literature. For instance, acupuncture, cold or heat treatment, medical massage, shock wave therapy, platelet abundant plasma and dietary supplements are also mentioned.
Veterinary clinicians should know about neuropathic pain as a likely entity in patients and should weigh it while developing a multimodal approach to pain management.