Neuropathic Pain And How To Treat
Dr Holger Volk
 
Chiari-like malformation (CM) is nearly omnipresent in the Cavalier King Charles Spaniel (CKCS) population. CM results in a decreased space for the hindbrain. This condition is similar to human Chiari type I malformation, a common disease, the causes of which are unknown. The prevalence of Chiari malformation in humans is 0.5% of the population. As more severe this mismatch of skull and brain parenchyma is, as higher is the likelihood of syringomyelia (SM)1. CM/SM has been associated with the development of central neuropathic pain (NP) and progressive neurological deficits. Treatment can only ameliorate the clinical signs and a complete remission of signs is rarely achieved. The main reason for treatment failure is that both the aetiology and pathogenesis is unclear.

NP is pain arising as a direct consequence of a lesion or disease affecting the sensation processing system within the central nervous system. It cannot be explained by a single lesion or disease process. NP can present as continuous or episodic, spontaneously occurring abnormal sensations (dysaesthesias) or secondary to an external stimulus (allodynia). In human medicine common qualities include burning or coldness, “pins and needles” sensations, numbness and itching. Humans have difficulties describing this type of pain and it is even more difficult to quantify NP in dogs. Behavioural signs associated with NP in CKCS are for example phantom scratching (initiated by touch or spontaneously), dislike of touch to certain areas of skin (some dogs cannot wear a collar or a harness), vocalizations associated with change of posture2.

Although there is controversy as to how CM/SM results in pain. Recent studies suggest that the severity of SM is correlated with the severity of neuropathic pain signs3,4. Syrinx formation which are symmetrical or do not reach into the white matter tract have not been associated with the development of neuropathic pain. However, as in human medicine CM alone can be associated with headaches and facial skin hyperaesthesia, which might be secondary to a compression of pain-processing pathways in the brainstem5.

In human medicine, the Neuropathic Pain Special Interest Group of the International Association for the Study of Pain recently sponsored the development of evidence-based guidelines for the pharmacological treatment of neuropathic pain6. Tricyclic antidepressants (aminotryptilline), dual reuptake inhibitors of serotonin and norepinephrine and calcium channel alpha–2–delta ligands (ie, gabapentin and pregabalin). Opioid analgesics and tramadol were recommended as second–line treatments that can be considered for first-line use in certain clinical circumstances. In veterinary medicine we use similar treatments, but until now there has been no published study into the treatment of central NP in dogs.

References
  1. Relationship of brain parenchyma within the caudal cranial fossa and ventricle size to syringomyelia in Cavalier King Charles Spaniels.
    Driver CJ, Rusbridge C, Cross HR, et al. J Small Anim Pract 2010;51:382–386.
  2. Pathophysiology and treatment of neuropathic pain associated with syringomyelia.
    Rusbridge C, Jeffery ND. Vet J 2008;175:164–172.
  3. Morphology of the caudal fosaa in Cavalier King Charles Spaniels.
    Cerda–Gonzalez S, Olby NJ, Mccullough S, et al. Vet Radiol Ultrasound 2009;50:37–46.
  4. Syringomyelia in Cavalier King Charles Spaniels: the relationship between syrinx dimensions and pain.
    Rusbridge C, Carruthers H, Dube MP, et al. J Small Anim Pract 2007;48:432–436.
  5. Functional abnormalities of the cervical cord and lower medulla and their effect on pain: observations in chronic pain patients with incidental mild Chiari I malformation and moderate to severe cervical cord compression.
    Thimineur M, Kitaj M, Kravitz E, et al. Clin J Pain 2002;18:171–179.
  6. Recommendations for the pharmacological management of neuropathic pain: an overview and literature update.
    Dworkin RH, O’Connor AB, Audette J, et al. Mayo Clin Proc 85:S3-14.
 
 
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