Subacromial impingement syndrome is a common cause of shoulder pain that has multiple causes (subacromial bursitis to rotator cuff tendinopathy and full-thickness rotator cuff tears). Unfortunately, for almost half of people afflicted with this syndrome, medical treatment is not successful and they will continue to have shoulder pain 2 years later [1]. Some of these patients, in spite of apparently normal anatomy, continue to suffer from shoulder pain [2,3]. It is not known why some people have shoulder pain long past the time that healing should take. Recent research may have an explanation for this continued shoulder pain called hypersensitivity.
Hypersensitivity, a revving up of the body’s pain detection system, is a useful tool that the body uses to alert you to an injury [4]. For example, when you cut your finger while cooking dinner, your finger becomes immediately painful because your body is warning you about the potential or actual tissue damage. This is hypersensitivity in action; your finger is now hypersensitive to painful and non-painful stimulation like touching your throbbing finger [5-7]. After the cut on your finger has healed, this hypersensitivity eventually goes away. However, we now think that people with chronic pain, such as having shoulder pain for longer than 6 months, are experiencing unresolved hypersensitivity [8]. People who are experiencing central hypersensitivity may continue to perceive pain even after adequate treatment for their shoulder and time to heal, which may be due to hypersensitivity not shutting off as it should in the normal situation.
What is interesting about hypersensitivity in patients with chronic shoulder pain is that the hypersensitivity is not just present at the injured shoulder but is found all over the body. This type of hypersensitivity is called central hypersensitivity [5-7]. One way to measure central hypersensitivity is through pressure-pain thresholds, which sounds scary but really is just a scientist applying slight pressure until the subject starts to feel the tiniest amount of pain. We measure this point of pressure and call it a pressure pain threshold. We can test if a group with chronic shoulder pain has central hypersensitivity by measuring their pressure-pain thresholds and comparing them to the pressure-pain thresholds of a pain-free group. If the pressure-pain thresholds are low at the injured shoulder and at other sites such as their leg or the opposite shoulder, then it provides evidence that central hypersensitivity is present [8]. In our study, we found that patients with chronic shoulder pain had lower pressure pain thresholds all over their body when compared to people with no pain [9].
Other studies have found central hypersensitivity in many other chronic conditions such as whiplash injury, fibromyalgia, low back pain, osteoarthritis and hemiplegic shoulder pain [10-11]. The next step is to find a treatment that targets central hypersensitivity and once this is accomplished, we may be able to treat chronic pain successfully.
About Tracy Paul
Tracy obtained her B.S. in Chemistry at Emory University and she is currently a third-year medical student at Case Western Reserve University School of Medicine. She worked under the guidance of Dr. Richard Wilson at the Department of Physical Medicine & Rehabilitation, MetroHealth Rehabilitation Institute of Ohio. She is interested in researching the etiology and management of chronic pain syndromes.
References
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[2] Dorrestijn, O., Stevens, M., Winters, J., van der Meer, K., & Diercks, R. (2009). Conservative or surgical treatment for subacromial impingement syndrome? A systematic review J Shoulder Elbow Surg, 18 (4), 652-660 DOI: 10.1016/j.jse.2009.01.010
[3] Ketola S, Lehtinen J, Arnala I, Nissinen M, Westenius H, Sintonen H, Aronen P, Konttinen YT, Malmivaara A, & Rousi T (2009). Does arthroscopic acromioplasty provide any additional value in the treatment of shoulder impingement syndrome?: a two-year randomised controlled trial. J Bone Joint Surg Br, 91 (10), 1326-34 PMID: 19794168
[4] Greene CS. (2009). Neuroplasticity and sensitization. JADA 140: 676-8.
[5] Petersen-Felix S, Curatolo M. (2002). Neuroplasticity – an important factor in acute and chronic pain. Swiss Med Wkly 132: 273-8.
[6] Curatolo, M., Arendt-Nielsen, L., & Petersen-Felix, S. (2006). Central Hypersensitivity in Chronic Pain: Mechanisms and Clinical Implications Phys Med Rehabil Clin North Am, 17 (2), 287-302 DOI: 10.1016/j.pmr.2005.12.010
[7] Latremoliere A, & Woolf CJ (2009). Central sensitization: a generator of pain hypersensitivity by central neural plasticity. J Pain, 10 (9), 895-926 PMID: 19712899
[8] Hidalgo-Lozano A, Fernández-de-las-Peñas C, Alonso-Blanco C, Ge HY, Arendt-Nielsen L, & Arroyo-Morales M (2010). Muscle trigger points and pressure pain hyperalgesia in the shoulder muscles in patients with unilateral shoulder impingement: a blinded, controlled study. Exp Brain Res, 202 (4), 915-25 PMID: 20186400
[9] Paul, T., Soo Hoo, J., Chae, J., & Wilson, R. (2012). Central Hypersensitivity in Patients With Subacromial Impingement Syndrome Arch Phys Med Rehabil, 93 (12), 2206-2209 DOI: 10.1016/j.apmr.2012.06.026
[10] Fernández-de-las-Peñas C, de la Llave-Rincón AI, Fernández-Carnero J, Cuadrado ML, Arendt-Nielsen L, & Pareja JA (2009). Bilateral widespread mechanical pain sensitivity in carpal tunnel syndrome: evidence of central processing in unilateral neuropathy. Brain, 132 (Pt 6), 1472-9 PMID: 19336461
[11] Soo Hoo J, Paul T, Chae J, & Wilson RD (2013). Central hypersensitivity in chronic hemiplegic shoulder pain. Am J Phys Med Rehabil, 92 (1) PMID: 23255268