Background/Purpose: To reevaluate the usefulness of the nociceptive flexion reflex (NFR) in distinguishing between FM patients and healthy controls, compared to computerized algometry and a summated measure of pain in 24 anatomical sites. NFR is a reflex contraction of the biceps femoris muscle elicited by progressive electrical stimulation of the sural nerve. It is measured as the least current (mA) required to stimulate the biceps contraction. Previous studies have reported that FM patients required less current to elicit biceps contraction than healthy controls (HC) (Arthritis Rheum. 2003 48(5):1420-9, Pain. 2004 Jan;107(1):7-15.)

Methods: The NFR threshold (NFRz) was evaluated by using a single ascending series of stimulations of 4 mA increments, and the biceps femoris contraction (EMG) was timed in relation to the onset of the stimulus to define the RIII reflex. The evaluation of the NFRz used a computerized NFR Interval z score, defined as NFR Interval Mean-baseline divided by the baseline SD, as previously described (Pain. 2009 Sep;145(1-2):211-8). B. Computerized algometry was performed at 4 paired sites (volar forearm, mid trapezius, mid gluteal area and mid-point of anterior thigh) with the average of 3 readings at each site, using an AlgoMed Pressure Algometer (Medoc, Durham, NC. ) All subjects estimated their current pain level (VAS 0 -10) at 24 locations representative of widespread pain (Arthritis Res Ther. 2009;11(4); the VASs at the 24 sites were summated into a single figure (0 – 240).

Results: The study was approved by the OHSU IRB. All subjects were female and FM diagnosis was based on the 1990 ACR criteria. The ages of the FM and HC subjects were similar, but the FM had a higher BMI. In contradistinction to 2 previously reported studies, comparing FM to HC, the NFRz was slightly higher (NS 0.18) in the FM subjects (table 1). As was expected the FM subjects had higher pain ratings on activation of the reflex, but endured more electrical stimulations (NS). Both the mean algometer scores at 4 paired sites and the summated pain at 24 locations provided a very significant discrimination between FM and HC.

	FM N=30	HC N=30	P value
Age	53.2	46.1	0.16
BMI	32.58	28.44	0.039
Current activating biceps reflex NFRz (mA)	24.94	19.10	0.180
Pain VAS at NFRz	68.06	39.66	0.004
Max Current Reached during testing (mA)	33.00	24.48	0.170
Max VAS Rating Given	87.50	66.44	0.002
Number of electrical stimulations	20.57	18.60	0.361
Mean algometer scores at  4 paired sited (kPa)	113.8	465.5	≤0.0001
Summated pain in 24 areas	111	12	≤0.0001

Conclusion: In this study, we were unable to confirm that the nociceptive flexion reflex is decreased in FM patients compared to HC. This difference maybe due to the employment, in the current study, of a computerized evaluation of the NFRz which used a strict definition of the NFRz. On the other hand, 2 less technically demanding methodologies,  namely pressure pain at 4 paired sites and a summation of pain VAS at 24 widespread locations, provided a highly significant discrimination between FM and HC. These latter 2 methods need to be studied in a broad variety of rheumatic disorders to evaluate their utility in diagnosing FM in the clinical setting.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/a-comparison-of-the-nociceptive-flexion-reflex-pressure-algometry-and-summated-widespread-pain-in-the-diagnosis-of-fibromyalgia/