How do you explain the persistent pain I’m having in my shoulder? My sister (who is a physical therapist) thinks I have something called impingement. But when she tested my arm, everything hurt from the shoulder down to the hand. How can my hand hurt if it’s my shoulder that’s the problem?

You may be experiencing one of two things: referred pain or centralized hypersensitivity. Or you could be having both things at the same time. Referred pain means the problem is in the shoulder (for example, something is getting impinged or “pinched”) but the pain pattern causes symptoms at a site distant from the source. You still have what’s called a peripheral pain problem (the arm).

In the case of centralized pain, the nervous system itself has become heightened in its responses. So even a light touch that would normally not hardly register creates a painful response. Pain is felt with lower mechanical pressure than would normally cause pain. And this doesn’t just occur in the arm or leg that’s hurt. The heightened pain response is perceived throughout the body. That’s what we mean by “central” hypersensitivity.

Pain can be a good thing. For one thing, it is protective. It is a normal response of the body when there is an injury. The body uses pain to protect itself while it is healing. But in time, the healing processes are completed and the pain goes away. That is the normal healing response.

But it’s not good when a persistent pain response gets turned on in the central nervous system and it doesn’t get turned off. The result is chronic pain from this phenomenon we call central hypersensitivity.

People with central hypersensitivity clearly have reduced pain thresholds (it takes less pressure than normal to cause pain). The reduced pain thresholds are observed at the site of the problem (shoulder for you) and in other parts of the arm unaffected by the injury.

People with centralized hypersensitivity have much lower pain thresholds compared with normals. For example, people with centralized hypersensitivity feel a painful response when a rubber tip is touched to the skin sooner and with less pressure compared with normal adults. And the sensation causes pain in the centralized hypersensitive group when it was just felt as pressure to the control group.

There are other possible causes of persistent pain that should be ruled out before going any further. You may need more advanced imaging studies to examine the bone and soft tissues to fully identify what’s going on in the shoulder.