Comparative analysis of chronic neuropathic pain and pain assessment in companion animals and humans

Abstract

Chronic neuropathic pain is underdiagnosed in companion animals. This paper will review the definition of pain and how classification and grading of neuropathic pain can be applied from human to veterinary medicine to increase the recognition of and the confidence in a neuropathic pain diagnosis. The mechanisms of nociception and the pathophysiology of the sensory systems that underlie the transition to chronic pain are described. Potential future methods for diagnosis and treatment of neuropathic pain in veterinary medicine are considered, utilizing the theoretical framework of pain behavior from humans and rodents. By discussing the current state of pain diagnosis in companion animals and increasing the recognition of chronic neuropathic pain, the goal is to increase understanding of chronic neuropathic pain in daily clinical practice and to aid the development of methods to diagnose and treat neuropathic pain.

Keywords: canine; chronic pain; neurology; neuropathic pain; translational medicine.

Figure 1

(A) Classification of neuropathic pain, including central and peripheral subtypes, with corresponding examples from humans and proposed corresponding neuropathic pain disorders in companion animals. (B) Grading system decision tree for confidence in neuropathic pain diagnosis. For patients presenting with pain, a history of relevant neurologic lesions and a matching distribution is consistent with possible neuropathic pain. The presence of either sensory signs (loss of sensation or increased/altered sensation) or lesion confirmation based on diagnostics results in the conclusion of probable or definite neuropathic pain. Created with BioRender.com.

Figure 2

Schematic diagram of pain pathways showing primary location of reflexive and ascending pain pathways. Mechanisms of chronic pain development are illustrated as well. (A) Encoding step, illustrating a free nerve ending that is activated due to acutely painful stimulus. (B) Transmission, the spinothalamic tract is pictured. Not pictured are other sensory pathways, including dorsal column postsynaptic pathway, the spinocervicothalamic pathway, the spinomesencephalic pathway and quintothalamic pathways. (C) Illustration of the regions of the brain that are associated with painful stimuli. These may have sensory or affective valence. In chronic pain, regions may either be differentially active, or there can be changes in brain volume. S1, primary sensory cortex; S2, secondary sensory cortex; Thal, thalamus; Ins, insula; PC, parietal cortex; CC, cingulate cortex; PfC, Prefrontal cortex; HP, hippocampus; Amg, amygdala; PaG, periacqueductal gray; Cb, Cerebellum; Med, Medulla. Created with BioRender.com.