Referred pain is pain which is felt in a part of the body away from the cause/stimulus of the pain.
Here are a few examples,
Myocardial infarction (heart attack) which is caused by a lack of oxygenated blood supplying the heart muscle, can bring about referred pain into the left shoulder arm and hand instead of pain in the chest.
Conditions affecting the liver, gall bladder, spleen and lungs can also produce referred pain to the shoulder and arm, disease of the stomach may produce pain between the shoulder blades.
The site where the pain is felt is not necessarily painful on palpation but may feel a little sensitive.
There are a few theories as to how the phenomenon of referred pain exists. Perhaps the most widely accepted is as follows.
We have 31 levels in our spine and a pair of nerves emerge from right and left at each level. One job of these nerves is to supply the brain with sensory information from your limbs and torso such as pain, hot and cold, the position of your joints in space, pins and needles, stretch etc. Our organs also have sensory input to the brain however it is much weaker that those from the musculoskeletal structures and skin.
Most of our organs are tucked under the diaphragm which is supplied by the right and left phrenic nerves which leave the spinal cord at C3,C4 and C5 which also happen to supply the shoulder and upper arm. Any sickness in the organs may irritate the diaphragm which in turns sends pain signals up the phrenic nerves to C3 4 and 5 which the brain (not being accustomed to receiving regular mail from the organs) interprets as shoulder pain.
Another example. Disorder in the stomach can produce pain between the shoulder blades because of overflow at their shared level in the spinal cord at T5 and T6.
Another form of referred pain can be explained using the Dermatome Map. A dermatome is an area of skin supplied by a level in the spinal column. For example, your thumb is supplied by the 6th cervical vertebral level – C6
When a nerve root is pinched where it exits the spine such as by a herniated disc or bony irritation, the pain is not always where the cause of the problem exists in the spinal column.
A nerve root irritation at L5 S1 can cause pain in the buttock or down the back of the thigh and into the foot. This is the dermatome distribution for the sciatic nerve.
Likewise a problem with the C5, C6 level can cause pain on the outside of the elbow. This is a fairly common problem and often confused with tennis elbow.
Differential diagnosis is most important as referred pain can mimic other conditions or hide more serious ones.