Toothaches: Why head and facial pain cause particular suffering
DURHAM, N.C., U.S.: People consistently rate pain affecting the head, face, eyeballs, ears and teeth as more disruptive and emotionally draining than pain elsewhere in the body. Now, scientists have discovered why this may be so in the case of head and facial pain.
A research team from Duke University in the U.S. has found that sensory neurons from the head and face are wired directly to one of the brain’s principal emotional signaling hubs, while sensory neurons from the body are connected only indirectly. Pain signals from the head versus those from the body are carried to the brain through two different groups of sensory neurons, and it is thus possible that neurons from the head are simply more sensitive to pain than neurons from the body are.
However, differences in sensitivity would not explain the greater fear and emotional suffering that patients experience in response to head and facial pain compared with body pain, explained senior author of the study Prof. Fan Wang, from the university’s Department of Neurobiology. “Usually doctors focus on treating the sensation of pain, but this shows that we really need to treat the emotional aspects of pain as well,” she added.
Personal accounts of greater fear and suffering are supported by functional magnetic resonance imaging, which has shown greater activity in the amygdala—a region of the brain involved in emotional experiences—in response to head pain than in response to body pain.
“There has been this observation in human studies that pain in the head and face seems to activate the emotional system more extensively,” Wang said, “but the underlying mechanisms remained unclear.”
To examine the neural circuitry underlying the two types of pain, Wang and her team tracked brain activity in mice after irritating either a paw or the face. It was found that irritating the face led to higher activity in the brain’s parabrachial nucleus (PBL), a region that is directly wired to the brain’s instinctive and emotional centers. They then used methods based on a novel technology recently pioneered by Wang’s group to pinpoint the sources of neurons that caused this elevated activity.
“It was a eureka moment because the body neurons only have this indirect pathway to the PBL, whereas the head and face neurons, in addition to this indirect pathway, also have a direct input,” Wang said. “This could explain why you have stronger activation in the amygdala and the brain’s emotional centers from head and face pain.”
Further experiments showed that activating this pathway prompted facial pain, while silencing the pathway reduced it.
“We have the first biological explanation for why this type of pain can be so much more emotionally taxing than others,” said co-author Prof. Wolfgang Liedtke, who operates at the Duke University Medical Center and treats patients with head and facial pain. “This will open the door toward not only a more profound understanding of chronic head and face pain, but also toward translating this insight into treatments that will benefit people.”
The results may pave the way toward more effective treatments for pain mediated by the craniofacial nerve, such as chronic headaches, neuropathic facial pain and the discomfort after dental surgery.
The study, titled “A craniofacial-specific monosynaptic circuit enables heightened affective pain,” was published online on Nov. 13 in the Nature Neuroscience journal.