– Q and A with Dr. Benjamin Greenberg, MD, MHS from the University of Texas at Southwestern in Dallas
Is pain a typical symptom in diseases like Transverse Myelitis (TM)?
One of the most common issues that patients afflicted with transverse myelitis experience is pain. It can come in many forms, but the most common is a burning or stabbing pain that occurs in an arm, leg or around the trunk. It is often described as a burning, aching or stabbing pain. When the pain occurs in the chest or abdomen it is often described as a squeezing sensation. Frequently the pain worsens with exertion, stress, heat or in the evening when trying to go to sleep. It is also frequently experienced in an area that had previous sensory changes. This type of pain is often not present at the onset of TM, but develops in the weeks or months after TM.
What are the different types of pain?
Medically, there are many types of pain that affect human beings. These include nociceptive pain, phantom pain and neuropathic pain. Nociceptive pain includes pain that occurs in the setting of tissue injury, such as a cut, burn or broken bone. Phantom pain occurs in the setting of a lost limb and is a perceived pain when the brain no longer receives signals from a limb. Neuropathic pain occurs when there is damage to a part of the nervous system and after that event and normal sensation is replaced with uncomfortable sensations.
Why is neuropathic pain experienced in neuro-immunologic conditions?
As you might expect, neuropathic pain has a different cause, biology and treatment than nociceptive pain. When you place your hand on a hot stove, it hurts. It is supposed to hurt. A signal moves from your hand to your brain and is interpreted as pain. The wound is painful even during the healing stages. This process is there to protect animals from tissue injury. We are supposed to learn that placing hands on hot stoves is dangerous! The medications used to treat this pain include opiates (e.g. narcotics) because the brain’s pain centers express large numbers of opiate receptors. When opiate medications bind to these receptors it dampens down the perceived pain. As the medication wears off, the pain returns. In reality, the pain signals are always there – transmitted from the wound to the brain, but are ignored by the brain when opiates are present.
Neuropathic pain is different. Very different! While a person’s foot may burn at night, there is no flame near the skin! So why does the brain perceive pain? The answer has to do with the pain pathways to the brain and pain centers in the brain. When sensation fibers in the spinal cord are damaged by transverse myelitis there is often a loss of normal sensory input to the brain. As a result, the sensation networks in the spinal cord and sensory centers in the brain are left with incomplete input of signals. The brain is used to receiving billions of signals every second from our bodies. Temperature, vibration, pressure, movement, light touch and pain inputs bombard our brain constantly. Every square inch of skin includes thousands of nerve endings responsible for a multitude of signal types. If the pathways responsible for vibration are damaged in the spinal cord, then the brain receives an incomplete “sensory picture” about what is happening to the feet. The spinal cord is left to manage incomplete sensory inputs. As a result of these changes the spinal cord can lead to amplification of some sensations (in an unpleasant fashion) and the brain can “fill in the gap” of missing sensation with unpleasant sensations (burning, squeezing, stabbing pains).
Why does neuropathic pain get worse at night?
Many patients indicate that their pain is worse in the evening when trying to go to sleep. You may wonder why this occurs! If the theory of neuropathic pain is correct, specifically, that the brain “fills in gaps” you might expect for distraction to lessen this phenomena. Thus, while at work or busy, people may not experience the pain, but when less distracted, their brain may be free to ‘make things up’! Just as a person tries to relax, their brain kicks into gear and the pain intensifies. Neuropathic pain is not supposed to be there – no damage to the affected area exists. As such, the treatment for this type of pain would be expected to be different than the treatment for nociceptive pain (broken bones, wounds, etc.).
What are common treatments for neuropathic pain?
Treatment of neuropathic pain usually does not involve opiates. Often patients with neuropathic pain will indicate that the use of opiates ‘took the edge off’, but did not rid them of pain. As such, we usually use antidepressant or antiepileptic medications to treat neuropathic pain. Are patients depressed or seizing? NO! These classes of medications act on cells in the brain and spinal cord to dampen down the ‘made up’ signals that are interpreted as pain and as such are perfect for neuropathic pain. Examples include amitriptyline, pregabilin, gabapentin and carbamezapine. There are many other options that have been used in patients. Beyond medication, many patients will find benefit from topical anesthetics to reduce all sensory signaling, acupuncture and/or avoidance of pain triggers. A careful discussion with your physician is needed to discuss your pain, what it feels like, what triggers it, what has helped in the past and what medications might be indicated. Pain needs to be aggressively treated as it can worsen mood and energy levels. Often multiple agents need to be attempted so that an appropriate one can be found.
UT Southwestern, Dallas, TX