A study published in the November 23 edition of the Journal of Neuroscience, as quoted from LiveScience  claims to have observed that “the brains of people with chronic backaches were up to 11 percent smaller than those of non-sufferers” 
The reader will notice, in this study that: “No attempt was made to correlate brain size to brain function.” 
According to “study leader A. Vania Apkarian of Northwestern University”  “people afflicted with other long-term pain and stress might face similar brain shrinkage.” 
Without defining pain’s neural function it is not surprising the deductions made involved size and speculation of emotions.
“The results suggest those with constant pain lose gray matter equal to an oversized pea for each year of pain. Gray matter is an outer layer of the brain rich in nerve cells and crucial to information and memory processing.” 
“It is possible it’s just the stress of having to live with the condition,” Apkarian told LiveScience. “The neurons become overactive or tired of the activity.” 
Sorry Dr. Apkarian, neurons do not become ‘tired’ or ‘overactive’. Neurons just process data. It is what causes a neuron to be made, increasing the depth of processing that is involved in reducing the number of neurons, and their relational glials.
“Another possibility is that people born with smaller numbers of neurons are predisposed to suffering chronic pain. But some of the differences measured ‘must be directly related to the condition,’ Apkarian said.” 
Without defining pain in the brain it is dangerous to speculate on things that are causes.
“The research involved a one-time brain scan of 26 people who’d had unrelenting back pain for at least a year (and in one case for up to 35 years), along with a pain-free control group. Pain sufferers had lost 5 to 11 percent of gray matter over and above what normal aging would take away. ‘People who have had pain for longer times have had more brain atrophy,’ Apkarian said.” 
So there would be something involved in brain function that would account for such an observation.
“No attempt was made to correlate brain size to brain function. It is possible that some of the shrinkage involves relatively noncrucial tissue — other than neurons — and that some of the effects are reversible if the pain is eliminated.” 
The loss of neurons and support cells due to pain has nothing to do with a reversible issue. It has to do with the function of the brain.
Pain is the level of base signal amplitude, exceeded. Neurons process data that is represented as being negative to the positive of the electrical signal of charge states and pulses of neuron discharges. As long as data values remain below ‘0’ the neuron processes the data and discharges the data to the next neuron process. The more intense the pain the lower the negative value of the signal and the closer to ‘0’ the signal becomes. When a neuron receives a signal of ‘0’ is has nothing to becomes charged with and therefore does not discharge. A neuron not charged is a neuron not needed and they slowly collapse. What this study has found is that a long history of pain does indeed result in less processing taking place in the brain and therefore less need for neurons and support cells to process it with.
“‘Suffering of pain is fundamentally an emotional condition,’ Apkarian said. ‘Different types of pain will have different types of emotional parameters, which will probably result in different types of atrophy — different amounts and in different brain regions.'” 
There is a major difference between the suffering of pain and actual pain. In Chapter Two of The Brain Is A Wonderful Thing the analogy is used of remembering pain. One can remember the emotional condition a severe pain resulted in but it is completely impossible to recall, ie: relive or retrieve, pain itself. That is because, pain is the value of ‘null’ in brain function. There is no record of it. There is a record of the results of that pain and many people, Apkarian included misconstrue the result of pain with pain itself.
The brain function of pain (the ‘null’ value of signal) causes less need for further processing and results in the decay or atrophy of cellular structure. It is nice to see a study showing support for the explanation of pain provided in the books offered through the Enticy Institute.