Topic Area: Prognosis
Article: Salo P. Neck Muscle Strength and Mobility of the Cervical Spine as Predictors of Neck Pain: A prospective 6-year study. Spine 2012;37(12):1036-1040.
- It is well documented that individuals with neck pain have significant loss of neck muscle strength and ROM …but the question is, chicken or the egg? Is it possible that those with weaker and stiffer necks are simply more likely to end up with neck pain?
The purpose of this 6-year study was to determine whether neck muscle strength or passive mobility of the cervical spine among initially pain-free working-age women could be used to predict future neck pain.
The researchers screened 220 pain-free, healthy female volunteers where isometric neck muscle strength and passive ROM of the cervical spine were measured.
After 6 years of following these patients 19% admitted to having neck pain for at least 7 days during the past year.
However, isometric neck muscle strength or passive mobility could not predict future occurrences of neck pain in healthy pain-free women. We can safely conclude that the neck weakness and loss of passive ROM comes AFTER the onset of neck pain, and not before …so the chicken came first!
- Was the defined representative sample of patients assembled at a common (usually early) point in the course of their disease?
The sample was indeed assembled at a common point in time. All 220 subjects were deemed to be pain-free after answering a screening questionnaire on health status, occupation, time used for leisure time physical activity, and level of physical workload. The sample itself was not representative of the general population as it only consisted of females that were employed at the largest companies in the city of Jyvaskyla, Finland.
- Was patient follow-up sufficiently long and complete?
Yes, follow-up was sufficiently long and complete. A questionnaire package was sent to each subject 6 years after their baseline measurement. Questions included onset of neck pain in the last year and also during the past 6 years, amount and length of neck pain, medication, health care utilization, and accidents or illnesses associated with their neck pain. One hundred and ninety-two subjects returned the questionnaire. In terms of length of time for follow-up, the authors admitted that 6 years was too long suggesting the results were less reliable when participants had to recall pain symptoms during such a long period. They also indicated that the results of the study may have been stronger if the sample had been screened regularly for neck pain during the whole 6-year period.
- Were outcome criteria either objective or applied in a blind fashion?
While the authors did not specifically state any “blinding” methods for the researchers, the design of this study may be that “blinding” was not required since there was no treatment provided nor were measurements taken a second time. A single tester performed the initial range and strength measurements with reliable and valid instruments used in clinical practice. Participants provided the remainder of outcome data subjectively via postal questionnaire.
- If subgroups with different prognosis are identified, did adjustment for important prognostic factors take place?
No, as the authors were looking for isometric neck muscle strength and/or passive range of motion as predictive ability between the two groups, they used bias-corrected bootstrap CI’s to determine cut-off points for strength and mobility. Ninety-five percent CI values were used to calculate sensitivity, specificity, positive predictive value and likelihood ratios. Differences between the groups in the area under the curve (receiver operator characteristic curves) were also evaluated.
Additionally, the t test, x2 test, Mann-Whitney U test or Fisher Exact test (alpha level at 0.05) were used to provide statistical comparisons of baseline characteristics between those who did and did not return the survey. Statistical comparison was also made of baseline characteristics between those who did not (NPO) and did (NP) experience pain in the past year or 6 year interval from the time of initial intake.
Baseline characteristics of the subjects who did not return the questionnaire (n=28) did not differ from those who returned the questionnaire. In both groups (pain or no pain) baseline characteristics were similar except for the physical workload. Those without neck pain tended to participate in more medium work activities more so than those reporting neck pain at 6 months.
- How likely are the outcomes over time?
Of the 220 healthy female volunteers, 192 (87%) responded at 6 year follow-up via a postal survey. This is only a 13% attrition rate. Thirty-seven (19%) reported having neck pain for 7 days or greater in the past 6 years, while 155 (81%) reported neck pain for less than 7 days (or none at all) in the previous 6 years.
- How precise are the prognostic estimated?
A 95% CI was established for isometric neck strength: .52-.56 (.41-.66), and cervical mobility: .54-.56 (44-.76). A positive predictive value for all items assessed remained between .21-.29 for all variables (strength and mobility assessment). When the area under the curve (AUC) is under .70 the test is described as a poor predictor, and if the AUC is < .6 then the test is a very poor predictor, per the University of Nebraska Medical Center1. Therefore, according to the study, both cervical mobility assessment and baseline cervical strength in healthy women are poor predictors of future neck pain.
- Can I apply this evidence about prognosis to my patient?
The participants in the study were all healthy and were excluded if they answered positively for any neck or shoulder pain in last 6 months, had previous or current neck injuries or other disorders of the neck or shoulder area, arthritis, fibromyalgia, severe depression or mental disorder, or an active competitive sports career. This is not representative of the typical patient seen in clinic where pain, immobility and/or loss of function often triggers a referral for physical therapy management. Rarely do patients seek physical therapy for prophylactic treatment alone.
Likewise, all participants were women between the ages of 20-59 which is not representative of the total population typically seen in clinic. Furthermore, participants in the study were volunteers that were employed at the largest companies in the city of Jyvaskyla, Finland which may have also potentially biased the outcomes of the study.
The predictive indicators (isometric neck muscle strength and passive mobility) and the outcome measures (isometric neck measurement system and cervical measurement system) chosen by the authors to determine the likelihood for future neck pain was a little surprising considering the evidence linking weakness of the segmental deep neck flexors with ‘chronic’ neck pain and passive range of motion versus segmental mobility (PIVM’s). Nevertheless, what we can take away from this study is that isometric neck muscle strength and passive ROM are not predictive of future occurrences of neck pain in pain-free working age women and perhaps other indicators need to be examined.
- Tape, TG. “The Area Under An ROC Curve.” University of Nebraska Medical Center. 29 July 2012. < http://gim.unmc.edu/dxtests/roc3.htm>