SSS may play a key role in managing chronic neuromusculoskeletal pain
Understanding the mechanisms behind spinal segmental sensitization (SSS) may offer valuable insight into, and more effective management of, chronic neuromusculoskeletal pain, according to Dr Reynaldo Rey-Matias of the Philippine Academy of Rehabilitation Medicine and Philippine Board of Pain Medicine.
In a presentation at the 7th Association of South-East Asian Pain Societies Congress (ASEAPS 2017) held in Yangon, Myanmar, Dr Rey-Matias discussed the role of SSS in chronic musculoskeletal pain, as well as its clinical manifestations, evaluation and available treatment modalities.
A novel concept proposed by the late Professor Andrew Fischer from New York University, SSS is a hyperactive state of the dorsal horn caused by bombardment of nociceptive impulses from sensitized and/or damaged tissues which, under normal circumstances, should be modulated by the descending pathways from the cortex or brainstem. Previous studies have shown SSS to be consistently associated with chronic musculoskeletal pain states.
Common pain symptoms in SSS include hyperalgesia, allodynia and hyperpathia. Hyperalgesia is defined as an increased response to a stimulus normally perceived as painful, whereas allodynia is a painful response to a stimulus that does not normally provoke pain. Hyperpathia refers to increased magnitude of pain.
“Hyperalgesia of central origin is so prevalent… It is responsible for about 61 percent of patients suffering from arthroses,” said Dr Rey-Matias, citing findings from studies supporting SSS.
In one study, Imamura and colleagues examined changes in pain pressure threshold (PPT) in 40 patients with chronic nonspecific low back pain and found that these patients had lower PPT values and significant hyperalgesia. In another study, they found that nervous system hyperalgesia causes significant disability and impacts the quality of life in patients with osteoarthritis of the knee. Meanwhile, Ratmansky, et al, used neuromyotherapy for stroke patients and found it effective in decreasing shoulder pain. [Spine (Phila Pa 1976) 2013;38:2098-107; Arthritis Rheum 2008;59:1424-31; J Rehabil Med 2012;44:830-6]
Mechanisms of SSS
According to Dr Rey-Matias, sensitization occurs when the central nervous system becomes hyperactive and hyperexcitable.
“These reactions are spread further down into the motor system of the spinal segments, generating irritative foci marked by tender or trigger points, which follows the rule of myotomal distribution of spinal segments,” he said, adding that other mechanisms involving wide dynamic range neurons and emergence of new spinal terminals may also contribute to sensitization.
Dr Rey-Matias differentiated anterior horn sensitization, which causes rigidity, muscle spasm and myofascial trigger points, from posterior horn sensitization, which induces dermatomal hyperalgesia and pressure pain sensitivity of the sclerotome supplied by the same sensitized spinal segment.
In spinal facilitation, the dorsal root reflex activates the dorsal root ganglia, which increases the production and release of central and peripheral vasoactive neuropeptides.
“The ventral horn outflow would result in local tissue tenderness and mechanical hyperalgesia; also, adjacent spinal segments may be progressively sensitized,” he said.
Dr Rey-Matias explained that the foundation of SSS is in understanding the embryological origins of tissues and the nerves that supply them. In the developing embryo, each dermatome, myotome and sclerotome receives its innervation from the spinal nerves that originate from the same segment.
“When the muscle hurts, the periosteum, skin [and] subcutaneous tissue of the same segment would hurt because they have the same innervation,” he said.
According to Dr Rey-Matias, another underappreciated fact is the trifurcation of primary afferent nociceptive fibres as they enter the dorsal horn; that is, one branch enters the dorsal horn at that segmental level while the other two branches may respectively ascend and descend along the dorsal margins of the dorsal horn.
Thus, “when we have some involvement of the thoracic spinal levels (ie, T1 to T12), this facilitates abdominal pain and somatovisceral symptoms which can mimic a lot of gastrointestinal conditions,” said Dr Rey-Matias.
Diagnosis and treatment of SSS
The specific dermatomal, myotomal and sclerotomal distribution patterns are considered hallmarks of SSS. Thus, diagnosis and management of SSS in the clinic would require identification of the sensitized spinal segment using several clinical and diagnostic examination techniques which, according to Dr Rey-Matias, can lead to precise diagnosis and optimal pain treatment.
Dermatomal hyperalgesia can be elicited by making use of a pin or clip to scratch over the different spinal segmental levels then asking the patient where the pain is located and where it will actually increase or decrease (ie, by sharpening or dulling of the pain) during the procedure.
For subcutaneous allodynia, a pinch and roll test, using the thumb and forefingers to roll the tissue underneath (which is supposed to be a non-noxious stimulus), may be done. If it elicits pain over a particular area, the test is positive.
“[This test] is both diagnostic and therapeutic; it can be used to desensitize a particular area as much as it is used to determine subcutaneous allodynia,” said Dr Rey-Matias.
Segmental myofascial pain may be elicited by palpating segmentally related tendons, enthuses or bursae on that area. Applying a pressure algometer may also be used to elicit tenderness. However, Dr Rey-Matias said that it has to be significant in that it is “at least 2 kg/cm2 lower than that of the normal sensitive control point.”
These methods of palpation and pressure algometry may also be used to identify vertebral paravertebral allodynia.
Other tests include musculoskeletal ultrasound (MSU), where a trigger point may appear hypoechoic or a localized stiffer region identified by a focal decrease of color variance. MSU can also be used to determine active and latent trigger points.
An active trigger point can be seen in Doppler studies as “reversal of blood flow reversal in diastole due to the high resistance of the vascular bed, leading to compression of the blood vessel by local muscle contraction and biochemically related vasoconstriction,” said Dr Rey-Matias.
Lastly, Dr Rey-Matias enumerated several treatment strategies for SSS. These include paraspinous injection, which involves injecting the paraspinal muscles adjacent to the spinous process to reach the deep layers after the vertebral lamina, and dry needling especially of the peripheral trigger points and taut bands.
“These are the same innervations embryologically so any peripheral trigger point can also be desensitized by desensitization of the paraspinal area,” said Dr Rey-Matias.Skin stretching (ie, pinch and roll test), connective tissue massage, therapeutic exercises, osteopathic techniques and other physical rehabilitation procedures are also commonly employed treatment modalities for SSS.