MANAGING PAIN: An Introduction to Drug Therapies for Pain Management

By David A. N. Siegel, MD

Severe pain has long been known to be one of the most devastating consequences of spinal cord injuries (SCIs). Different studies have reported the prevalence of pain after SCI to be up to 94%, with the majority of the estimates ranging from 40% to 75% of patients. Pain is reported to be moderate to severe in up to 60% of people with SCI, with up to 70% reporting their pain as chronic and up to 37% stating that their pain is so severe as to be disabling. One study illustrated the gravity of the pain problem by showing that up to 37% of people with pain after SCI would trade pain relief for loss of bladder, bowel, or sexual function.

The International Association for the Study of Pain (IASP) has defined central pain as: “pain caused by a lesion or dysfunction in the central nervous system.” It can occur in a multitude of neurological disorders and is often severe and difficult to treat. Affected patients include those with SCI, stroke, traumatic brain injury, multiple sclerosis, epilepsy, and Parkinson’s disease. Estimates of the prevalence of central pain are difficult to obtain. There is reason to believe, however, that the numbers are considerable.

There are numerous theories on what causes pain after an injury to the central nervous system (CNS). Some people believe that it is caused by cross connections between nerves as they try to regenerate. Others think that it is caused by an increased excitability of the nerves that have been damaged. Still others attribute it to a loss of the normal control of the balance between the different regulatory systems in the CNS. All of these theories have something to be said for them, but are incomplete. This means that treatment must still be largely based on trial and error.

The treatment of central neuropathic pain is often challenging since it usually does not respond to standard pain medications like morphine. It often requires the use of adjuvant analgesics, which are medications that were developed for problems other than pain but which possess activity on the central nervous system. Most of these medications are from the fields of psychiatry and neurology. These include antidepressants, anesthetics, antispasmodics, and anticonvulsants.

The exception to the rule about “morphine-like” drugs is methadone. Unlike morphine, methadone also works on receptors in the central nervous system called NMDA. It works by blocking a neurotransmitter called glutamate from exciting the pain pathways in the spinal cord and brain. Other drugs that work by blocking NMDA include an anesthetic called ketamine, a cough medicine called dextromethorphan and a new medication for Parkinson’s called memantine.

Certain anti-depressants work by increasing the neurotransmitters serotonin and norepinephrine in the CNS. These neurotransmitters are important because they help to suppress pain signals as they travel up the spinal cord. By increasing them you can reduce the levels of pain signals that reach the brain. Examples include amitriptyline and nortriptyline, which are in a class called tricyclic antidepressants or newer medications called serotonin/norepinephrine re-uptake inhibitors like venlafaxine and duloxetine.

Anti-convulsants are medications that were originally used to treat epilepsy. They work by decreasing the excitability of nerves, thereby reducing their spontaneous transmission. This has the effect of reducing the number and strength of pain signals that are sent to the brain. Different anti-convulsants do this in slightly different ways. This may explain why different people respond to some and not others.

In general, no one medication can be expected to completely control a person’s central pain and often a combination is required. There is a high degree of variability of pain relief to different classes of medications and different medications within each class. In complex and challenging cases like central pain after SCI, patients need to anticipate a period of trial and error in their treatment before an optimal therapeutic treatment combination can be found.

This column is the first in an ongoing series of columns on pain management after SCI. In the future we will be going into more depth about the subjects touched upon in this article, as well many new ones, including the most recent research on pain and how it relates to SCI. I invite you to e-mail me with your questions at action@unitedspinal.org.

Additionally, if you are a veteran in the New York City area with SCI and pain, please contact the Pain Management Clinic at the James J. Peters Veterans Affairs Medical Center for more information and potential treatment. You can find out how to make an appointment by calling the clinic at 718-584-9000, ext. 5080 or 6581.

David A. N. Siegel, MD, is director of Pain Medicine at the James J. Peters Veterans Affairs Medical Center in the Bronx, New York.