Whiplash neck injury, which commonly occurs in motor vehicle accidents, causes acute and chronic whiplash-associated disorders (WAD) [
1‐
3]. Its cumulative annual incidence is reported to be more than 300/100,000 people [
4]. The Quebec Task Force defined whiplash neck injury as an injury caused by an acceleration-deceleration mechanism of energy transfer to the neck [
5]. The energy may impact and cause injury to various components of the neck including bones, intervertebral discs, facet joints, spinal cord, nerve roots, and cervical muscles [
5‐
7]. Mostly, however, the mechanical stress on the neck from motor vehicle accidents is not so strong or severe enough to injure bones, intervertebral discs, facet joints, spinal cord, or nerve roots, although it may cause functional disorders like cramps or spasms of cervical muscles [
4,
8]. Hence, pain or stiffness of the neck or shoulder, the local and main symptom caused by nociceptive mechanical stimulus of the muscles, usually recovers within two to three weeks following the injury by conventional outpatient care, such as medication or neck rest [
1,
8]. Alternatively, up to 30–50% of patients with WAD report variable and indefinite symptoms involving the whole body, despite there being no evidence of direct injury to organs other than the neck [
1‐
4,
9,
10]. The reported whole-body indefinite symptoms include headache, vertigo or dizziness, palpitation, chest tightness, vision loss, dazzling, dry eyes, dry mouth, nausea or appetite loss, gastrointestinal symptoms (stomachache, diarrhea, and constipation), hyperhidrosis, cold sensation or poor circulation, unstable blood pressure, unknown fever, sleeping disorder, general malaise or fatigue, depression, distraction or obsession, irritability, and lack of endurance.
Treatment for patients with WAD remains controversial. A review reports that some active treatments, although not specified, have a tendency to be more effective than only resting of the neck [
11]. Several reports have shown that pharmacological interventions such as paracetamol, nonsteroidal anti-inflammatory drugs (NSAIDs), and opioids have slight or moderate effects [
12,
13]. However, these studies are limited to the management of local pain in the neck or shoulder, performed as outpatient care for acute WAD. The management of other whole-body indefinite symptoms or intensive treatment by hospitalization has not been extensively examined.
We have previously reported that cervical muscle disorders may possibly be associated with these indefinite and variable symptoms and propose a new medical concept, namely cervical neuro-muscular syndrome [
14]. On the basis of our clinical experience, we believe that functional disorders in cervical muscles, such as cramps or spasms after injury, are responsible not only for neck and shoulder symptoms but also for other whole-body symptoms of WAD. On palpation of 34 points of the neck, which is the original diagnostic method used at our institutions (Additional file
2: Figure S1), we observed that patients with WAD exhibit tenderness and hardening at specific sites of the posterior and lateral cervical muscles, such as trapezius, semispinalis capitis, splenius capitis, and sternocleidomastoid. These muscle lesions cannot be diagnosed using images such as plain radiographs or magnetic resonance imaging (MRI). For the treatment, we attempted prescribing medication or neck rest with a cervical collar for functional disorders of the cervical muscles as outpatient care. Although these were partly effective for local symptoms of the neck or shoulder, they were hardly effective for other symptoms in the whole body.
Among the many interventions for local modulation of the cervical muscles, low-frequency electric stimulation has recently been reported to be effective for recovery of muscle tone of the erector spinae [
15,
16]. In addition, a randomized, double-blind, placebo-controlled pilot study has shown that far-infrared irradiation significantly decreased the stiffness of cervical muscles [
17]. We also believe that a combination of the two physical therapies can decrease tenderness and hardening of the cervical muscles much more effectively than conventional treatments, through the aforementioned palpation of the neck. The present study therefore examined the effect of a combination of the two physical therapies at the cervical muscles of inpatients with WAD who were resistant to outpatient care and showed variable symptoms in the whole body.