Postoperative pain is a common complication of surgery that is associated with delirium and cognitive dysfunction, particularly in the elderly. To better understand the connection between postoperative pain and cognitive dysfunction, researchers studied pain-associated behavior and nerve injury biomarker expression in mice undergoing orthopedic surgery to repair fractured bones.

Mice exhibited increased cold sensitivity, characteristic of neuropathic pain, following surgery, along with increased expression of brain-derived neurotrophic factor (BDNF) in sensory neurons. BDNF expression in sensory neurons also increased following transection of the sciatic nerve, suggesting that BDNF is a marker of nerve injury.

However, elevated BDNF levels persisted for at least 2 weeks following surgery, whereas following nerve transection, BDNF returned to baseline levels or less after 2 weeks. BDNF levels also increased in mossy fibers in the hippocampus following surgery, even though the corresponding mRNA levels did not change and hippocampal neurogenesis, which BDNF stimulates, decreased.

The results suggest that bone fracture and surgery led to impaired BDNF signaling in the hippocampus, possibly by inhibiting BDNF release from mossy fibers. This impaired BDNF signaling might contribute to postoperative cognitive dysfunction, according to the authors.

http://www.pnas.org/content/early/2016/10/06/1614017113