Expert in Back Pain Clears Up Misunderstanding About Pain Types
Low back pain with or without leg pain comes in many flavors. There could be just back pain or back pain that radiates (spreads) into the buttock region. Back pain accompanied by shooting pain down the leg can be from two separate problems. The shooting pain down the leg is referred to as sciatica or radicular pain.
Then there's the patient who experiences allodynia. That's a medical term for pain caused by a stimulus that normally doesn't cause pain. For example, gentle pressure or a light touch results in moderate to severe pain. Mild hot or cold temperatures in contact with the skin can cause allodynia. Pain felt with gentle brushing the skin can also be referred to as allodynia.
All of these painful experiences really fall under one of four separate types of pain: nociceptive, somatic referred, radicular, and radiculopathy. And identifying which type or types of pain are present is key to choosing the right treatment. Defining and understanding each of these terms is essential for the physician diagnosing and treating low back pain.
That's why Nikolai Bogduk, a well-known back pain expert from Australia was asked to write this review defining and describing each of these pain types. Most of the time, a patient with just one of these problems is easy to evaluate and diagnose. But sometimes clinicians confuse one type of problem for another. Then a delay in diagnosis or even a mistake in diagnosis can occur. To help avoid such a situation, physicians must be able to distinguish one type of pain from another. The real challenge comes when patients have more than one type of pain at the same time. Any combination of nociceptive, somatic referred, radicular, and/or radiculopathy can throw a monkey wrench in the diagnostic process.
Dr. Bogduk offers a complete summary of each type of pain based on findings from years of animal and human studies performed and confirmed around the world. For example, nociceptive back pain occurs when an anatomical structure in the spine is stimulated by mechanical or chemical means. Studies of nociceptive pain have used normal subjects and targeted spinal ligaments, spinal joints, discs, sacroiliac joints, and back muscles. By stimulating each of these areas individually, scientists have been able to identify responses used by patients to describe nociceptive pain as dull and/or aching. There are no shooting pains, no numbness, and no allodynia with nociceptive pain.
What about somatic referred pain? What does that look like? Well, let's understand the term soma (or somatic) first. The soma is actually the physical body as it is made up of structures such as skin, muscle, joint, tendons, ligaments, connective or myofascial tissue, and even bone. Referred means the pain originates in one place but is felt in a separate location. There are no spinal nerve roots involved in somatic referred pain. The patient with just somatic referred pain does not have any neurologic signs or symptoms. On examination, there is no numbness, no muscle weakness, and no change in deep tendon reflexes (all signs of neurologic or nerve involvement).
Instead, nerve endings called neurons are affected. These tiny nerve structures are located within the somatic structures just described. For a better understanding of somatic referred pain, it's helpful to know that there are three basic types of neurons that send messages from the soma (body) to the brain and from the brain back to the soma. Sensory neurons respond to touch, sound, vibration, light, and other stimuli. The sensory neuron sends signals to the spinal cord and up to the brain so that the individual experiences each of these sensations. Motor neurons receive signals sent down from the brain through the spinal cord and cause muscles to contract. Interneurons connect neurons to other neurons within the same region of the brain or spinal cord.
Patients describe pain of a somatic referred source as dull, aching, and sometimes like an expanding pressure. They often can't put a single finger on the exact spot that hurts. The pain is more diffuse and spread out over a larger area. A closer look at the map of the pain will point to the underlying cause because the pain distribution will match the nerve innervation of the affected area. For example, a commonly recognized somatic referred pain pattern is pain in the middle of the buttocks on one side. The pain may also go down into the upper portion of the thigh. Mechanical pressure on (or misalignment of) the spinal joints in the lumbar spine (low back) can result in this type of pain report.
Then there is radicular pain. The spinal cord or spinal nerve root is injured or affected in some way. It could be a herniated disc pressing against either or both of these two neural structures. Or sometimes, the damaged or degenerated disc sends out chemicals that irritate the spinal nerve root with the same end result. Technically speaking, there really isn't pain. The patient is more likely to report a sharp, burning or stabbing sensation that goes down the full pathway of the nerve. The sensation may be alternately described as an electric shock. These patients report the pain goes down past the knee to the foot. You may have heard the term sciatica to describe this type of sensation. But a better understanding of how the nerve pathway is involved has led us away from the use of the word sciatica. Instead, the term radicular pain is the correct description.
The fourth and final type of low back pain is radiculopathy. Any time a word ends in the suffix -opathy you know there's something wrong. In this case, there is enough damage to the nerve that it can no longer transmit messages clearly and correctly. The block can occur at the sensory or motor portion of the nerve. In either case, there are neurologic signs and symptoms present. The patient may report numbness and weakness. The examiner may see altered reflexes (e.g., decreased knee jerk response when the patellar tendon is tapped with a reflex hammer). There may not be any pain at all. Only when radicular pain is present along with radiculopathy does the patient experience pain along with the neurologic signs and symptoms.
Putting this all together, here's what it can look like in the clinic. A patient comes in with shooting pain down the leg. No position is comfortable or reduces the pain. There is weakness of the leg muscles and numbness in the skin. There is a nerve involved here. It's either being pinched or pressed with an inflammatory response. The patient would be diagnosed with radicular pain and radiculopathy. On the other hand, a patient with back pain that spreads to the buttock but without shooting pain with full leg strength has no nerve involvement. That's simply a case of nociceptive (back) and somatic referred (leg) pain mixed together.
Dr. Bogduk acknowledges that there's a lot of confusion about the cause of back and/or leg pain. Unnecessary medical and surgical procedures may be done unless physicians clearly understand the differences between the four types of pain reviewed in this article. Anyone examining and diagnosing patients with low back pain would benefit from a careful reading of this detailed review of the physiology of back pain.
Nikolai Bogduk. On the Definitions and Physiology of Back Pain, Referred Pain, and Radicular Pain. In Pain. December 2009. Vol. 147. No 1-3. Pp. 17-19.