Differential Efficacy of Ketamine in the Acute versus Chronic Stages of Complex Regional Pain Syndrome in Mice.
BACKGROUND
Complex regional pain syndrome (CRPS) is a painful, disabling, and often chronic condition, where many patients transition from an acute phase with prominent peripheral neurogenic inflammation to a chronic phase with evident central nervous system changes. Ketamine is a centrally acting agent believed to work through blockade of N-methyl-D- aspartate receptors and is being increasingly used for the treatment of refractory CRPS, although the basis for the drug's effects and efficacy at different stages of the syndrome remains unclear.


METHODS
The authors used a mouse model of CRPS (n = 8 to 12/group) involving tibia fracture/cast immobilization to test the efficacy of ketamine (2 mg kg day; 7 days) or vehicle infusion during acute (3 weeks after fracture) and chronic (7 weeks after fracture) stages.


RESULTS
Acute-phase fracture mice displayed increased limb temperature, edema, and nociceptive sensitization that were not reduced by ketamine. Fracture mice treated with ketamine during the chronic phase showed reduced nociceptive sensitization that persisted beyond completion of the infusion. During this chronic phase, ketamine also reduced latent nociceptive sensitization and improved motor function at 18 weeks after fracture. No side effects of the infusions were identified. These behavioral changes were associated with altered spinal astrocyte activation and expression of pain-related proteins including N-methyl-D-aspartate receptor 2b, Ca/calmodulin-dependent protein kinase II, and brain-derived neurotrophic factor.


CONCLUSIONS
Collectively, these results demonstrate that ketamine is efficacious in the chronic, but not acute, stage of CRPS, suggesting that the centrally acting drug is relatively ineffective in early CRPS when peripheral mechanisms are more critical for supporting nociceptive sensitization.
From the Veterans Affairs Palo Alto Health Care System, Palo Alto, California (M.T., J.D.C.); Department of Anesthesiology, Stanford University School of Medicine, Stanford, California (M.T., J.D.C.); Palo Alto Veterans Institute of Research and Education, Palo Alto, California (M.T., D.L., P.Y., T.T.H., J.D.C.); Geriatrics Research Education and Clinical Center, Veterans Affairs Palo Alto Health Care System, Palo Alto, California (T.T.H.); Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Stanford, California (D.L., P.Y., T.T.H.); and Physical Medicine and Rehabilitation Service, Veterans Affairs Palo Alto Health Care System, Palo Alto, California (W.S.K.).
Maral
M
Tajerian
David
D
Leu
Phillip
P
Yang
Ting Ting
TT
Huang
Wade S
WS
Kingery
J David
JD
Clark
D000208:Acute Disease; D000700:Analgesics / Q000494:pharmacology*; D000818:Animals; D002908:Chronic Disease; D020918:Complex Regional Pain Syndromes / Q000188:drug therapy* / Q000209:etiology*; D004195:Disease Models, Animal; D005500:Follow-Up Studies; D007649:Ketamine / Q000494:pharmacology*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D013978:Tibial Fractures / Q000150:complications*; D016896:Treatment Outcome
D000700:Analgesics; D007649:Ketamine