Why a rotten tooth is hard to find

When it comes to a toothache, the brain doesn’t discriminate. A new imaging study shows that to the brain, a painful upper tooth feels a lot like a painful lower tooth. The results, which will be published in the journal Pain, help explain why patients are notoriously bad at pinpointing a toothache.

For the most part, humans are exquisitely tuned to pain. The brain can immediately distinguish between a splinter in the index finger and a paper cut on the thumb, even though the digits are next-door neighbors. But in the mouth this can be more difficult, depending where and how intense the ache is.

“We don’t know much about tooth pain,” comments dentist and neuroscientist Alexandre DaSilva of the University of Michigan in Ann Arbor, who was not part of the new research. The new study is one of the first to address the puzzle of toothache localization, he says.

In the study, researchers led by Clemens Forster of the University of Erlangen-Nuremberg in Germany analyzed brain activity in healthy — and brave — volunteers as they experienced tooth pain. The researchers delivered short electrical pulses to either the upper left canine tooth (the pointy one) or the lower left canine tooth in the subjects. These bursts of electrical stimulation produced a painful sensation similar to that felt when biting into an ice cube, Forster says, and were tuned such that the subject always rated the pain to be about 60 percent, with 100 percent being the worst pain imaginable.

To see how the brain responds to pain emanating from different teeth, the researchers used fMRI to monitor changes in activity when the upper tooth or the lower tooth was zapped. “At the beginning, we expected a good difference, but that was not the case,” Forster says. 

Many brain regions responded to top and bottom tooth pain — carried by signals from two distinct branches of a fiber called the trigeminal nerve — in the same way. The V2 branch carries pain signals from the upper jaw, and the V3 branch carries pain signals from the lower jaw.

In particular, the researchers found that regions in the cerebral cortex, including the somatosensory cortex, the insular cortex and the cingulate cortex, all behaved similarly for both toothaches. These brain regions are known to play important roles in the pain projection system, yet none showed major differences between the two toothaches. “The activation was more or less the same,” Forster says, although he adds that their experiments might have missed subtle differences that could account for why some tooth pain can be localized.

Because the same regions were active in both toothaches, the brain — and the person — couldn’t tell where the pain was coming from. “Dentists should be aware that patients aren’t always able to locate the pain,” Forster says. “There are physiological and anatomical reasons for that.”

DaSilva agrees that the brain’s inability to tell top-tooth pain from bottom-tooth pain “pairs really well with what we see in the clinic.”

Understanding the pathway from tooth to brain may help researchers devise better treatments for acute tooth pain, such as cavities or infections, and more chronic conditions, DaSilva says. One such condition is phantom pain that persists in the mouth after a tooth has been removed.