J Manipulative Physiol Ther. 2012 (Oct); 35 (8): 608–613 ~ FULL TEXT
Sonsoles Muñoz-Muñoz, PT, María T. Muñoz-García, PT,
Francisco Alburquerque-Sendín, PT, PhD,
Manuel Arroyo-Morales, PT, MD, PhD,
César Fernández-de-las-Peñas, PT, PhD
Mutua de Accidentes de Trabajo y Enfermedades
Profesionales de la Seguridad Social,
OBJECTIVE: The purpose of this study was to investigate the presence of active myofascial trigger points (MTrPs) in a greater number of muscles than previous studies and the relation between the presence of MTrPs, the intensity of pain, disability, and sleep quality in mechanical neck pain.
METHODS: Fifteen patients with mechanical neck pain (80% women) and 12 comparable controls participated. Myofascial trigger points were bilaterally explored in the upper trapezius, splenius capitis, semispinalis capitis, sternocleidomastoid, levator scapulae, and scalene muscles in a blinded design. Myofascial trigger points were considered active if the subject recognized the elicited referred pain as a familiar symptom. Myofascial trigger points were considered latent if the elicited referred pain was not recognized as a symptom. Pain was collected with a numerical pain rate scale (0-10); disability was assessed with Neck Disability Index; and sleep quality, with the Pittsburgh Sleep Quality Index.
RESULTS: Patients exhibited a greater disability and worse sleep quality than controls (P < .001). The Pittsburgh Sleep Quality Index score was associated with the worst intensity of pain (r = 0.589; P = .021) and disability (r = 0.552; P = .033). Patients showed a greater (P = .002) number of active MTrPs (mean, 2 ± 2) and similar number (P = .505) of latent MTrPs (1.6 ± 1.4) than controls (latent MTrPs, 1.3 ± 1.4). No significant association between the number of latent or active MTrPs and pain, disability, or sleep quality was found.
CONCLUSIONS: The referred pain elicited by active MTrPs in the neck and shoulder muscles contributed to symptoms in mechanical neck pain. Patients exhibited higher disability and worse sleep quality than controls. Sleep quality was associated with pain intensity and disability. No association between active MTrPs and the intensity of pain, disability, or sleep quality was found.
Key Indexing Terms: Neck Pain, Trigger Points, Myofascial Pain Syndromes, Disability, Sleep
From the Full-Text Article:
Neck pain can have an insidious (mechanical) or traumatic (whiplash-associated neck pain) onset. Mechanical neck pain is defined as pain in the cervical spine and/or shoulder area with symptoms provoked by neck postures, neck movement, or palpation of the cervical muscles. Neck pain constitutes a significant health care problem affecting 45% to 54% of the general population.  A systematic review reported 1-year prevalence for neck pain ranging from 16.7% to 75.1% (mean, 37.2%).  In addition, the economic burden associated with the management of neck pain is second only to low back pain in annual workers' compensation costs in the United States. 
It has been proposed that myofascial trigger points (MTrPs) can be involved in pain processes in patients with mechanical neck pain; however, few studies had included MTrPs therapy for the management of these patients. [4, 5] Simons et al  define an MTrP as a hyperirritable spot in a taut band of a skeletal muscle that is painful on contraction, stretching, or stimulation and elicits a referred pain distant from the point. Myofascial trigger points can be clinically classified as active or latent. Active MTrPs are those causing spontaneous pain symptoms, which elicited referred pain reproduces the patient's symptoms and is recognized as a familiar phenomenon for the patient. Latent MTrPs are those not responsible of symptoms of the patient. Clinical distinction between active and latent MTrPs has been substantiated by histochemical findings because higher levels of some algogenic substances and chemical mediators (ie, bradykinin, serotonin, or substance P) have been found in active MTrPs as compared with latent MTrPs and non-MTrPs.  The presence of algogenic substances and chemical mediators within active MTrPs may be involved in sensitization processes found in patients with mechanical neck pain. 
Fernández-de-las-Peñas et al  found that the referred pain elicited by MTrPs in the upper trapezius, levator scapulae, and sternocleidomastoid muscles reproduced the symptoms in patients with mechanical neck pain. However, this study only included the exploration of these muscles and did not assess other outcomes such as disability or sleep quality. In addition, there is some evidence demonstrating that treatment of active MTrPs is effective for reducing symptoms in patients with mechanical neck pain. [10-12] These studies suggest that MTrPs can be involved in the genesis of mechanical neck pain.
In addition to muscle impairments, sleep disturbances are considered an essential element in patients with chronic pain. [13-16] It has been proposed that addressing the ongoing cycle of pain and sleep disturbances is essential for treatment for patients with mechanical neck pain. We do not know if the presence of active MTrPs is related to functional impairments such as sleep quality or disability in this population.
Therefore, the aims of this study were
(1) to investigate the presence of active MTrPs in a greater number of muscles in individuals
with mechanical neck pain and
(2) to determine the relationship between the presence of active MTrPs, the intensity of pain,
disability, and sleep quality in individuals with mechanical neck pain.
The current study found that the referred pain elicited by active MTrPs in neck and shoulder muscles contributes to pain symptoms in individuals with mechanical neck pain. When the assessor applied pressure to active MTrPs, patients reported: “Yes, this is exactly the pain that I usually feel.” In addition, individuals with mechanical neck pain exhibited higher disability and worse sleep quality than healthy controls. Sleep quality showed different associations with the intensity of neck pain and disability, but not an association between the presence of active MTrPs and pain, disability, or sleep quality was found.
The presence of active MTrPs in patients with mechanical neck pain has been previously reported by Fernández-de-las-Peñas et al.  The current study found that the upper trapezius and levator scapulae muscles were the most affected by MTrPs in mechanical neck pain, which agrees with previous findings.  Nevertheless, our study is the first one investigating the presence of MTrPs in some cervical musculature such as the splenius capitis, semispinalis capitis, or scalene muscles in mechanical neck pain, which has not been previously reported for this population. However, the presence of active MTrPs in these muscles was lower than in the upper trapezius or levator scapulae. This can be related to the fact that active MTrPs in the splenius capitis have been found more frequently in individuals with chronic whiplash.  It is possible that active MTrPs in patients with mechanical neck pain, who experience an insidious onset of their symptoms, would be present in different muscles compared with those patients with a whiplash injury, who experience a traumatic onset of their symptoms. Future studies should investigate this hypothesis.
An interesting finding was that patients with mechanical neck pain exhibited similar number of latent MTrPs than healthy controls, which has been previously found in our previous study,  but contrary to previous studies conducted in individuals with tension type headache, [29, 30] lateral epicondylalgia, [31, 32] or shoulder impingement  where healthy controls had lower number of latent MTrPs than the symptomatic populations. The presence of some latent MTrPs in healthy asymptomatic people should be expected; however, it may be that the prevalence of latent MTrPs in healthy people would be greater in the cervical region than in other areas of the body, although future studies should clarify this situation. The presence of latent, but not active, MTrPs in our control group supports that we were able to find an appropriate comparison group to accomplish a properly blinded study. It is important to note that the clinical relevance of latent MTrPs is increasing with recent studies  demonstrating that latent MTrPs disturb normal pattern of motor recruitment and movement efficiency  and induce sensitization mechanisms.  In addition, several studies have studied the response of different therapeutic interventions over latent MTrPs. [37, 38]
We also observed that our small group of patients exhibited moderate disability (NDI, 25) and that 68% (n = 10) patients reported poor sleep quality (PSQI >8). These data suggest similarities to those previously reported by Valenza et al  for individuals with mechanical and traumatic neck pain. In addition, we reported an association of sleep quality with the intensity of pain and disability suggesting an interaction between these factors. In agreement with current results, a relationship between pain intensity and sleep quality has been also reported in chronic low back pain. [38, 39] However, we should recognize that our sample size was smaller than previous studies.
On the contrary, the presence of active MTrPs was not directly associated with sleep quality or disability. Our results suggest that sleep disorders and musculoskeletal impairments (active MTrPs) can contribute by different mechanisms in the symptoms of mechanical neck pain. Whether cause or consequence, sleep disorders must be taken into account in the overall management of these patients, in the same way as active MTrPs, pain, and disability. It seems essential to address these different aspects as an integral part of the evaluation and treatment of individuals with mechanical neck pain.
First, the sample size was small, which may explain the lack of significance in some outcomes. It is possible that some correlational analyses were underpowered due to the small sample size. Second, the cross-sectional nature of the study does not permit us to establish a cause and effect relationship between pain, MTrPs, disability, and sleep quality. The small sample size did not permit us to conduct a regression analysis to determine potential interactions among these outcomes. Therefore, current results should be considered as preliminary. Third, we did not collect data on other factors associated with pain, such as catastrophizing, anxiety, fear, uncomfortable sleep position, medication side effects, or workplace features. Future longitudinal studies with larger sample sizes and including these related outcomes should be considered to further confirm or refute a relationship between active MTrPs, neck pain, and sleep quality.
This study suggests that the referred pain elicited by active MTrPs in the neck and shoulder muscles contributes to pain symptoms in individuals with mechanical neck pain. Patients with mechanical neck pain seem to exhibit higher disability and worse sleep quality than controls. Sleep quality was negatively associated with the intensity of neck pain and disability. No association between the presence of active MTrPs and the intensity of pain, disability, or sleep quality was found. Our results suggest that different pain contributors are related to symptoms in individuals with mechanical neck pain.
This study suggest that the referred pain elicited by active MTrPs in the neck
and shoulder musculature contributes to pain in mechanical neck pain.
Patients with mechanical neck pain seem to have higher disability and
worse sleep quality than controls.
No association between the presence of active MTrPs and the intensity of pain,
disability, or sleep quality was found.
These results suggest that different pain contributors are related to
symptoms in mechanical neck pain.