4.1. Prevalence of NRP Tenderness
To our knowledge, this is the first report on the epidemiology of neck reflex points (NRPs). NRPs are six areas on each side of the cervical neck which may become tender, and which can be detected upon physical palpation [
1].
NRP tenderness examination shows a high inter-rater reliability, independent from the examiner’s experience [
5]. NRPs seem to be a clinical entity, different from muscular trigger points (MTrPs). They show none of the typical signs of MTrPs [
7]. There is some empirical evidence that NRP tenderness can serve as an indicator for remote chronic disorders of the trigeminal region, i.e., in the visceral cranium [
1]. They are to become tender as a reaction to a chronic irritation of the trigeminal area. NRP tenderness can be significantly reduced by therapeutic interventions: injections of local anesthetics to the pharyngeal region specifically reduce NRP tenderness [
4]. This reduction may indicate a reduction in the chronic inflammation of the respective area in the visceral cranium.
Knowing a symptom’s prevalence is an important prerequisite to assessing the strength of a clinical test. We examined two independent populations for the prevalence of NRP tenderness: chronically ill patients, and students as a cross-sectional study group of young individuals.
No influence of age. Tender NRPs are thought to be signs of chronic inflammation in the trigeminal area, such as chronic sinusitis and pharyngitis. The incidence of chronic inflammations increases with age. We therefore postulated that the number of tender NRPs increases with age as well. We expected that patients “accumulate” tenderness of the NRPs during their lifetime. However, no age dependence of the NRPs was found, even when the data were adjusted to the factors “group” (patient vs. student) and “gender”. NRP tenderness seems to occur independently of the age of the individual. In contrast, a correlation was found in patients: they showed a significantly higher prevalence of tender NRPs than students (odds ratio, 3.4). We postulate that NRP examination discriminates against individuals with a low incidence of diseases from ill individuals seeking medical advice.
No influence of body weight. We expected a lower prevalence of tender NRPs in individuals with a higher body weight, assuming that body fat of the cervical neck may cover tenderness of NRPs. However, there was no correlation. We conclude that NRP testing can be applied independently of the BMI of the respective individual.
Gender difference. A higher prevalence of tender NRPs was found in females. The reasons are unclear. The difference may be based on differences of the female soft and connective tissue of their musculoskeletal system. It is also possible that the defined palpation pressure used in this survey, as well as in previous studies [
19], is too low to examine males, possibly due to their higher muscular mass in the neck. But the gender difference may just as well reflect physiological gender differences in the NRP prevalence. Future studies investigating the clinical correlation of NRP tenderness will answer this question.
Clinical correlation of NRP tenderness. We do not know the clinical implications of tender NRPs. Except for the intervention study mentioned above [
4], there are no data on NRP correlation with clinical findings of the visceral cranium. The Spanish dentist E. Adler, who first observed the NRP phenomenon [
2], postulated a level-specific correlation of tender NRPs with diseases of the trigeminal forehead region. He ascribed NRP-C0–C1 to be correlated to chronic sinusitis, and NRP-C2–NRP-C3 to a dental chronic inflammation, such as an apical osteitis. Later, NRP-C4 and NRP-C7 were ascribed to chronic pharyngitis [
1,
3]. This assumption was supported by Uehleke et al. [
8], who observed a high clinical correlation between pharyngitis and tenderness of NRP-C7. We did not find a correlation of NRP tenderness with pre-existing diseases. In fact, NRPs are intended to detect
inapparent (otherwise unknown), pre-existing silent inflammations of the trigeminal area, but not
apparent diseases. Therefore, this result of no correlation with apparent diseases is congruent with our hypothesis.
Independence of contralateral NRPs. Assuming the postulate of Adler that each NRP level indicates a specific and localized segmental disorder of the trigeminal region [
2], we postulated that NRPs become tender independently from each other. We therefore examined the interdependence of NRPs with their contralateral counterparts. We did not find a correlation in NRP-C0 through NRP-C4 (kappa < 0.4). Only NRP-C7 showed a moderate correlation between left and right NRPs. NRP-C7 is claimed to be an area corresponding to the pharyngeal region. Chronic pharyngitis usually is a bilateral disease. This may reflect the agreement at the NRP-C7 level. In contrast, there was no left–right correlation between NRPs at all other levels, NRP-C0-C4. These NRPs are claimed to reflect chronic sinusitis and chronic dental osteitis. Both conditions may occur unilaterally, which corresponds with the findings of the independence of left and right NRPs.
Independence of the neck levels. We found no significant agreement between a certain NRP with its immediate, or with the next or other levels (kappa < 0.3). This is congruent with the clinical experience that local trigeminal irritations induced by a chronic inflammation, such as dental osteitis, occur independently from another inflammation site, e.g., frontal sinusitis. The findings of independence support the proposed specific relation of tender NRPs to a segment-specific disorder of the trigeminal region in the forehead.
4.2. Limitations and Strength
Selection bias in the patient group. Due to the location of the pain clinic within an OB/GYN unit, we examined mostly women in the patient group. In contrast, our student group was recruited from young individuals from the Medical School of Heidelberg University, with ~60% of the students being females. The average age and the gender distribution differed significantly between the two groups, meaning the two groups studied are not assimilable: we examined two different populations. As none of the students were patients in the OB/GYN pain clinic, there was no overlap between the groups, and all data could be calculated as independent samples. We found significant differences in the NRP prevalence between the patient and the student group. We conclude that patients are characterized by a higher prevalence of NRPs compared to controls, independent from age and gender.
Limited information about the health status of the student group. In the student group, no further factors (confounders) were evaluated; that is, we did not know anything about the true health status of the students. Based on the young age of the students, severe diseases may be less likely in this group, and, if few were present, their influence may have been diluted due to the magnitude of this cohort. In order to exactly describe the disease prevalence in a defined healthy population, cohorts with known health status should be evaluated in further studies. Nevertheless, the prevalence in a normal healthy population needs to be investigated in further studies. Based on the results of the current study, future case-number calculations can be performed.
Reliability and objectivity of the NRP test. The examinations were not double-checked by two independent examiners, so an examiner bias cannot be excluded. However, in a previous study, we could demonstrate a high inter-rater reliability even between an experienced and an inexperienced examiner [
5]. We therefore assume that the individual examiner bias was low also in this investigation. It is noteworthy to mention that the base of the results is the pain expression by the patient only. Other investigators using the palpation findings of the examiner only did not find any reproducibility [
6].
Left-side predominance. We found a higher prevalence of NRP tenderness on the left side in some of the NRP levels. The reasons for this are unclear. The difference may have been caused by the examination mode, in which the physician examines the individual with one hand standing laterally to the person. Other examination modes (standing in front of the patient) were described in the literature [
1]. We decided to use this method because it was used in previous studies on NRPs [
4,
5]. Although examiners were trained to apply a defined pressure of 4 kp on each side, a palpation difference between the thumb and middle finger cannot be excluded. However, the predominance of the left-side tenderness, especially in female students, at the NRP-C2 level may be based on a currently unknown physiological correlation.
Measurement scale. The three-level scale (PI = 0, 1, 2) used in this survey may be discussed. Other scales such as the Nominal Analogue Scale (NAS) 0–10 may be used as well. The three-digit scale was introduced by Andersen and coworkers [
15] for the examination of trigger points of the cervical neck. Here, as well as in previous NRP studies [
4,
5], this scale has revealed to be an easy-to perform, reliable, and easy-to-understand measure for both, patient, and examiner. From the patient’s point of view, it is much easier to answer than a 10-digit scale. From the physician’s view, a three-level distinction is completely sufficient when searching for a sign of chronic (silent) inflammation on a certain level. We therefore recommend maintaining this easy-to-use scale in further studies.
Nomenclature. The denomination of the NRP from “NRP-C0” to “NRP-C7” is historic and has been discussed in detail [
5]. They are not necessarily correlated with the transverse processes of the respective cervical spine vertebrae. As in previous publications, we decided not to change these historic denominations, as this is of no relevance to the meaning of this clinical phenomenon. Apart from these nomenclature issues, the clinical relevance may be caused by a segment-based interrelation to the trigeminal region.
Cutoff for positive findings. Which result is clinically relevant for finding sites of chronic inflammation, moderate (PI = 1) or marked tenderness (PI = 2)? Using marked tenderness results in a lower number of positive results and allows for a better distinction between patients and students. As long as there are no correlation analyses with clinical findings, we suggest using the results of marked tenderness (PI = 2) as “positive” findings.