Everyone experiences pain. In fact, it’s a healthy physical response that keeps us alert to danger and helps avoid injury. But what if an ache started one day and didn’t go away? Worse yet, what if no one—including your doctor—knew what was going on, or how to make it stop?
It’s hard to imagine, but this is a reality for many suffering from chronic neuropathic pain. Fortunately, researchers at the University of Toronto’s Centre for the Study of Pain are making in roads into the treatment of this complex condition, gaining profound insights into our brain and how it works.
Neuropathic pain is caused by damage to the nervous system, which is made up of the central nervous system (the brain and the spinal cord), and the peripheral nervous system (the sensory and motor neurons and nerves in the rest of the body). It can be brought on by a number of different conditions, such as cancer, multiple sclerosis, diabetes, viral infections like AIDS, and even routine surgery. Yet, its trigger remains a mystery.
Though it is often described as a burning, numbing, or stabbing sensation, neuropathic pain can differ greatly depending on an individual’s experience. It can happen spontaneously, or in response to stimuli under which normal circumstances would cause no pain at all. For those suffering from this condition, even the lightest touch can be excruciating. Unlike nociceptive pain, which is felt in response to tissue damage and dissipates once the injury has healed, chronic neuropathic pain often occurs completely out of the blue. Like its name suggests, the sensation can persist for months, years, or even a lifetime.
Dr. Thuan Dao, an Associate Professor with the University of Toronto’s Faculty of Dentistry, is a researcher attempting to gain a better understanding of neuropathic pain. She is interested in orofacial neuropathic pain, which affects the face and inside the mouth. Currently testing a medication called pregabalin, Dao and her team believe it could offer relief to sufferers of orofacial neuropathic pain.
It is thought that orofacial neuropathic pain is triggered by injury to the nerves during routine dental procedures like root canal treatment, surgery, or filling a deep cavity. Dr. Dao is quick to point out that this doesn’t necessarily mean a dentist is doing their job incorrectly; it may simply be that a person’s anatomy or genetic background makes them more prone to this type of injury or pain.
Neuropathic pain is prone to misdiagnosis, as it can appear long after treatment in locations far away from the initial procedure. Health professionals often intervene without understanding the neuropathic nature of their patient’s discomfort. This can have unfortunate consequences: one patient’s misdiagnosis resulted in 32 root canals, after which he still had a toothache.
When asked why the pain continues after the damaged nerve heals, Dr. Dao exclaimed, “That is the million dollar question!” The key to understanding nerve pain is in the continuous remodeling of the peripheral and central nervous system, known as neuroplasticity. After an injury changes the periphery of the nervous system, it can lead to further effects all the way up into the spinal cord and the brain. These changes occur down to gene level, resulting in a continuous reverberation of the pain signal long after the initial stimulus has stopped.
Once patients have received a diagnosis, they are usually treated with a cocktail of antidepressants, anticonvulsants, opioids, and canabinoids. These drugs interfere with the ability of nervous system cells to communicate with each another, effectively blocking the pain signal from reaching or being commanded by the brain. Ideally, Dr. Dao says, the neurons that are misfiring eventually “go to sleep,” as the patient is weaned off the medication.
An effective drug cocktail is found for only half to a third of all patients, and some can have long-term side effects. A small percentage of patients are forced to seek invasive surgical intervention such as deep brain stimulation. In this therapy, electrodes that deliver electrical impulses are inserted deep into the brain. How this treatment works is still unclear, though it appears to interfere with the neural activity at the insertion site, disrupting the pain signal. Hopefully, research done at the Centre for the Study of Pain will someday bring lasting relief to sufferers of neuropathic pain, making these treatments a distant memory.
New research hopes to gain a better understanding of neuroplasticity and why some patients respond better to certain therapies than others. This will allow for better screening and more effective, individualized treatments. New drugs and vaccines are also being developed to treat diseases that trigger neuropathic pain.
