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Human body is not perfect, there are many imperfection in almost every organ system but that is what makes us and other living beings unique as well as interesting . Today we will be discussing one of such imperfections , "The Referred pain" which I have called a Glitch in the Nervous system and you will know why as you read this post.And we will also be discussing its Medical implications.
What is a Referred Pain?
" A pain which is felt at the distant site from the organ diseased but not felt at the diseased organ itself is referred pain."
Didn't get it? ..
When a part of body organ is damaged or diseased you will not feel pain there, instead you will feel it on some other distant organ. That is basically the referred pain.
That's strange, isn't it?..How & Why would a person feel pain on the distant site and not on the involved site? Is it an abnormal thing? ... we will find that out!
To know about how referred pain occurs we should first learn about Basics of pain pathway
Fig- Basic Pain Pathway(image source)
The picture above shows the neuronal circuit of pain , its similar to neuronal circuits of other sensation like touch, pressure, cold/warmth etc. and consists of following components.-
1.Receptor organ
2.Sensory nerve
3.Central Nervous system (integration Centre)
4.Motor nerve
5.Effector organ
Lets take an example of the picture above; due to arthritic inflammation the receptors present in the joint of wrist and thumb are stimulated. This causes elctrophysiological change in sensory nerve terminals which then pass the information to spinal cord. From there group of nerve fibre known as Spinothalamic tract which are present in the spinal cord itself pass the info further to higher centres. The spinothalamic tract ends in thalamus which then finally gives the information to different areas of Cerebral cortex.
Stimulation of somatosensory cortex helps localizing the site of pain(like the GPS ), it finds out from which part of the body the information has came. Stimulation of insula causes the actual peception of pain,.i.e pain sensation is due to this. And another part of cortex is also stimulated which is cingulate gyrus- this produces various emotions associated with pain (fear, avoidance).
Important things to know here are - Sensory nerves which carry information of pain from the affected part and somatosensory cortex which localizes the pain.
Fig- Dermatomes (Image source)
Dermatomes are the areas/segments of skin supplied by particular segment of spinal cord(see the picture above). When a particular dermatome is injured the information is passed via the sensory nerve to the corresponding segment of spinal cord, and from there to the higher centres. Thats how brain can precisely localize your pain. If you prick palmer aspecct of your thumb - sensory nerve will send information to 6th cervical segment of spinal cord and then brain. Same is applied for the muscles and other internal visceras.
Then, What happens in the referred pain?
By now , some of you must have got the mystery behind referred pain.
Some of internal visceras and the Dermatomes are supplied by the same segment of the spinal cord. This is the cause of origin of referred pain. when some internal viscrea is diseased/injured, sensory nerve from it goes to particular segment of spainal cord but some other sensory nerve supplying another part of the body (area of skin) goes to the same segment of spinal cord.
When this happens, Our brain becomes confused about where the information came from, a dermatome or the Internal viscera?.. Literally, the cerabral cortex is fooled here.
Fig-Convergence-Projection theory of referred pain(image source)
out of the two groups, the somatic sensory nerves stimulate the higher neurons(2nd order neurons ) reaching the thalamus instead of visceral sensory nerves from injured viscera. Brain interprets that the information has come from somatic area(skin), so instead of feeling pain in the injured internal organ we feel pain in the distant somatic area.
This was Convergence-projection theory for mechanism of referred pain.
Some Examples of Referred pain and Their Medical Implications :
Fig- Site of referred pain for different organs(Image source)
Heart - In case of coronary artery disease like Angina pectoris or Myocardial infacrion, the injury to Myocardiium(Heart muscles) elicits pain in the inner aspect of left shoulder as both part are supplied by same segment of spinal cord i.e Thoracic- 1st to 4th
Liver and Gallbladder - Irritation of diaphragm in case of biliary colic & acute cholecystitis causes pain in the tip of shoulder and inferior angle of scapula because of common sensory supply i.e from Thoracic 7th to 9th
Stomach, Small intestine -In diseases of these organ Pain is felt in the epigastrium(just below the xiphisternum) . Here the sensory supply is from Thoracic segment 5th to 8th
Appendix - In acute appendicitis , pain is felt around umbilicus, the segments of spinal cord involved here are Thoracic 9th to 11th
colon - Pain is felt around hypogastrium(below umbilicus) , spinal segment 11th thoracic to 2nd lumbar
There are many other examples which I will not mention here.
So, what medical importance does this referred pain bear?
In medical practice, referred pain is a crucial indicator of early stage diseases . we have seen from the above examples that the site of referred pain are organ specific(Inner aspect of left shoulder for heart, inferior angle of scapula for gall bladder, epigastrium for foregut etc). They usually occur in initial stages of disease where insults to organ has just started, later on pain will be felt on organ itself which suggest progression of disease to more serious stage.
When knowledge of this is combined with proper history and other examination we can reach to correct diagnosis of the disease.
Referred pain is different from Radiating pain and Migrating pain !
It is important to differentiate Referred pain from Radiating and Migrating pain.
- Migrating pain
Here, at first pain is felt at one site, later on pain disappears in that site and occurs at another site.Perfect example for thsi is Acute appendicitis where pain is first felt around umbilicus for few hours and then pain shifts to right iliac fossa(Right lower abdomen) but original pain around umbilicus disappears. When this is present in a patient, its almost sure sign of acute appendicitis.
Fig- Migrating pain is Acute appendicitis(Image source)
Here the first pain is referred pain but 2nd on is due to irritation of parietal peritonium(i.e somatic pain).
- Radiating pain
Here, pain occurs at one site at first and then pain occurs to another site but pain persists at the original site. For example, in gastric ulcer pain is usually felt at epigastrium but when it perforates pain will be also felt at back. The first pain here is referred pain but radiated pain is due to peritoneal involvement.
Thats all for today my friends . In this post we have learned some basics of pain, mechanism and medical importance of Referred pain.We learned that its a glitch in the nervous system but a a very useful one. And we have also learned some different types of pain.
If you have any queries regarding this topic or some extra information feel free to write it in the comment, if you liked this post please upvote and resteem as well so it reaches to more people. And follow 
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References:
1.A Manual On Clinical Surgery, S.Das, 11th Edition
2.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4327510/
3.https://www.slideshare.net/KemUnited/referred-pain-final
4.http://positivemed.com/2015/10/18/9-types-of-referred-pain-you-should-know-to-save-your-life/
5.http://www.med.umich.edu/lrc/coursepages/m1/anatomy2010/html/cardiovascular_system/heart_case.html
Images
Sources of all the images are cited along with them, no copyright infingement is intended.If there is any issue with the content as well as use of images in this post, you can contact me .
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