• Race to a Cure Authors

Referred Pain: your pain could be more serious than you think

Cliquez ici pour accéder l’article en Français/Click here to view this article in French


According to Healthline, pain in the jaw or teeth can often be one of the first symptoms of a heart attack. Pain felt in the shoulder blades could specifically indicate a ruptured or injured spleen. This is a phenomenon called referred pain, where the pain in one part of the body is actually caused by injury or pain in a different part of the body.


While some may have thought that pain is imagined due to the lack of injury in that specific organ, a research study conducted by the University of Sydney suggests that pain referral is thought to have a neural basis, which means that there are specific pathways and connections in the brain that scientists believe are responsible for pain referral. One of those neural phenomena that play an important role is called convergence.


Image is courtesy of cnx.org

Convergence


Information about the world is conveyed to the brain through action potentials along sensory nerve fibers with the help of sensory receptors in areas such as the skin, muscles, joints, etc. Specifically, the human body has sensory nerve fibres called nociceptive nerve fibres or nociceptors for pain. These receptors convey action potentials into the brainstem, where two significant actions take place. The first is that nociceptors from the orofacial area can end on the same group of second-order neurons, which means that some nociceptors from the skin, tooth, and jaw can converge onto one-second order neuron. The second one is that both nociceptors and other non-pain-related receptors (ex. mechanoreceptors) can converge onto one second-order neuron. This phenomenon does not have an apparent biological reason, but it seems to be the reason for referred pain, the study states.


Second-order neurons


Second-order neurons are part of the neural pathway that conveys sensory information to higher education centers for integration and interpretation. Since there is a great deal of convergence between different sensory data originating from other places in the body onto the same second-order neurons, they may provide ambiguous information regarding the exact location of the painful stimulus. This mechanism is believed to be one way that the higher centres of the brain can be confused and not know the precise location of the stimulus.


Unmasking latent synaptic connections

This phenomenon could also help explain what may occur with the activation of nociceptors. Once the afferent nociceptive nerve fibers enter the brainstem, they branch to terminate on different second-order neurons. Some of the connections are ineffective or latent, meaning that the action potentials do not activate the second-order neurons at the synaptic connections in the afferent pathway. Sometimes, when there is a tedious, painful stimulation, such as muscle trauma or repeated clenching of the same body part, the latent synapses may become effective. When this is the case, action potentials could be transmitted along pathways to convey information from the orofacial area, which is unrelated to the source of a painful stimulus. In this case, the brain can become confused and not realize the primary source of the noxious stimulus.


Diagnostic test of tooth pain vs. pain referral to a tooth


To distinguish between pain caused by a tooth versus a pain referral to a tooth, clinicians have to test by administering a local diagnostic anesthetic. This causes inactivation of the nerves at the site of the pain, in this case, a tooth. If the pain is arising from the tooth, the pain should stop. If it is a referred pain, the pain should persist. This could be a symptom of an underlying problem that the patient is not aware of.


Example: Kehr's Sign

A Rice University study explains that this is a nervous system disorder, with a pain in the left shoulder, chest, and neck following rupture of the spleen. While the spleen is present in the upper-left side of the abdomen, the pain is present in the left shoulder, chest, and neck. This is due to the sympathetic fibers in the spleen that originate from the celiac ganglion, in the mid to lower thoracic region, compared to the parasympathetic fibers found in the vagus nerve, which connects to the medulla. In addition, the neck and the shoulder are connected to the spinal cord. These connections do not match the predicted correspondence of visceral and somatosensory fibers joining in the spinal cord at the same level.


The visceral fibers originate from the diaphragm; the phrenic nerve is attached to the spinal cord at C3 to C5. This nerve's motor fibers are in charge of the muscular contractions that regulate ventilation. These fibers have exited the spinal cord and entered the phrenic nerve, indicating that spinal cord injury below the mid-cervical level does not result in death by preventing breathing. As a result, the diaphragm's visceral fibers enter the spinal cord at the same level as the somatosensory fibers originating from the shoulders and the neck.


The diaphragm plays a vital role in Kehr's sign. This is due to the placement of the spleen, which is directly under the diaphragm. Thus, when the spleen ruptures, the blood spills into this general region, applying pressure on the diaphragm. The referred pain is in the region that corresponds to the diaphragm rather than the spleen.



Article Author: Celine Guirguis

Article Editors: Maria Giroux, Clara Han