There exits some similarity between symptoms of carpal tunnel syndrome (CTS) and myofascial pain related to trigger points (TPs) in the infraspinatus muscle.
The aim was to examine what proportion of patients referred with a clinical suspicion of CTS had myofascial pain in the infraspinatus muscles and how their occurrence was related to the outcome of nerve conduction studies (NCS).
We examined the occurrence of CTS and of TPs in infraspinatus muscles in a cohort of patients suspected for CTS and referred to the Department of Clinical Neurophysiology, Aarhus University
Hospital from October 2003 to February 2004. Patients underwent electrodiagnostic studies of the median and ulnar nerves. Additional tests were performed if necessary. Patients were examined for TPs by a blinded examiner immediately after NCS.
We included 335 patients (202 females; 133 males, mean age 46). Two hundred one patients (60%) had abnormal NCS indicating CTS. Fifteen patients (4%) had other electrodiagnostic
abnormalities. One hundred nineteen patients (36%) had normal NCS. Forty nine percent (58 out of 119) of the subjects with normal NCS had TPs in the infraspinatus muscles whereas only 26% (53 out of 201) of the patients with abnormal NCS indicating CTS had TPs. Likewise, 26% (4 out of 15) of the patients with other electrodiagnostic abnormalities had TPs.
Approximately one third of patients referred with a clinical suspicion of CTS had TPs in the infraspinatus muscles. This occurrence was higher in the group with normal NCS than in the group with
abnormal NCS indicating CTS. When CTS is suspected clinically, physicians must be aware of TPs in the infraspinatus muscles as a possible cause of the symptoms; thus, NCS are important in these patients.
Carpal tunnel syndrome (CTS) is a common peripheral entrapment mononeuropathy, caused by median nerve compression at the wrist. The symptoms of CTS often appear as a painful tingling in the hand during the night. Furthermore swelling, weakness or clumsiness of the hand are reported together with sensory deficits in the median innervated region of the hand (Padua et al., 2001a,b,1999; Practice parameter for electrodiagnostic studies in carpal tunnel syndrome, 1993). Parasthesia and pain can spread to the arm and shoulder (Kummel and Zazanis, 1973; Cherington, 1974). Atypical presentation of CTS commonly occur and present a diagnostic challenge (Hoffman, 1975. Sometimes shoulder pain is reported as the presenting complaint in CTS (Kummel and Zazanis, 1973).
Myofascial pain symptoms arising from muscle trigger points (TPs) are quite common musculoskeletal complaints (Travell and Simons, 1999) (Bennett, 2007). Pain and tenderness from TPs can
be referred to regions beyond their immediate vicinity (Simons, 1993). Referred pain is an important characteristic of TPs and differentiates TPs from tender points, which is associated with pain
at the site of palpation only (Alvarez and Rockwell, 2002; Hopwood and Abram, 1994) and is commonly described in fibromyalgia (Wolfe et al., 1990). TPs in the infraspinatus muscles are reported to be a major source of shoulder pain (Travell and Simons, 1999). The pain usually projects to the back of the neck, between scapula and spine and down to the anterolateral aspect of the arm, to lateral forearm, to radial part of the hand or to the radial fingers (Travell and Simons, 1999). Experimental studies with hypertonic saline in this muscle reported pain projected to the antero-posterolateral side of the upper arm (Kellgren, 1938).
The aim of the present study was to determine the proportion of patients referred with a clinical suspicion of CTS that had myofascial pain in the infraspinatus muscle and whether this occurrence
was related to the outcome of the nerve conduction studies (NCS). We hypothesized that a group of patients referred with a clinical suspicion of CTS, instead of CTS have myofascial pain related to TPs in the infraspinatus muscle. We further hypothesized that patients with NCS indicating CTS have a lesser occurrence of infraspinatus muscle TPs than those with normal NCS.
All patients that were referred to the Department of Clinical Neurophysiology at Aarhus University Hospital from October 2003 to February 2004 with a clinical suspicion of CTS, were considered for participation. These patients were referred from general practitioners, orthopaedics, neurologists and rheumatologists. According to the routine procedure of the department, the referrals in our study were evaluated beforehand by a skilled neurophysiologist who either redirected patients to more extended electrophysiological studies if the symptoms from the referrals suggested other disorders or returned the referrals that did not indicate a CTS problem, often with a suggestion of obtaining a more precise clinical examination.
Of 345 patients referred, 202 females and 133 males of mean age 46 (range 17–89) consented to participate and were included. Data about occupation, symptoms and symptoms’ localisation and
duration were collected from the referrals and from the patients’ reports at the time of the study. Information about paraesthesia and pain, and hypoesthesia in hands, arm and shoulder were obtained during the examination. The amount of information obtained on other symptoms and signs depended on the referring physicians and varied. Possible causes of TPs in the infraspinatus muscles such as fibromyalgia, rheumatoid arthritis, tenosinovitis of the scapulahumeral joint, or Pancoast syndrome were not explored. A neurophysiology technician informed all patients about the project immediately after the NCS. The patients that consented to participate, were all examined for TPs in the infraspinatus muscles by the same examiner (EQ).
The NCS were performed by experienced neurophysiology technicians, using a Keypoint EMG system (previously Medtronic Functional Diagnostics, now Alpine, Skovlunde, Denmark). The NCS were performed with surface electrodes and included: (1) antidrom sensory studies (conduction velocity and amplitude) of median and ulnar nerve from the wrist to the respective fingers and (2) motor studies (distal latency, amplitude and conduction velocity) of median nerve from the wrist to abductor pollicis brevis muscle and of ulnar nerve from the wrist and the forearm to abductor digiti minimi muscle. A subgroup underwent further electrophysiological examinations e.g. EMG of the brachial brachii muscles and ENG of other nerves if polyneuropathy, radiculopathy or brachial plexus disorders were suspected at the time of the study. Reference values of the department were used for the studies. The severity of abnormalities of the sensory and motor median NCS were classified using z-scores e.g. standard deviations (SD) from the mean of controls (Tankisi et al., 2005, 2002). The following categories were used: (1) minimal CTS (palmar sensory slowing only), (2) mild CTS (sensory conduction velocity from wrist to digits decreased more than 2 SD and less than 4.5 SD), (3) moderate CTS (sensory conduction velocity decreased more than 4.5 SD and less than 5.5 SD) and (4) severe CTS (sensory conduction velocity decreased more than 5.5 SD). In the cases, in which motor conduction studies revealed amplitude reduction of more than 5 SD or distal latency increase of more than 6 SD the category was increased by one.
The examiner (EQ), who was blinded to the results of NCS, asked the patients to give a short description of their symptoms in order to determine whether the palpation of the TPs provoked the same symptoms, so that the recognition criteria (see below) for TPs could be fulfilled. Thereafter, the examiner palpated the infraspinatus muscles on both sides by applying pressure on different places of the muscles. The examiner was aware that clinical assessment could influence the trigger point testing, and patients were reminded not to report the results of the NCS.
The pressure applied was approximately 150–200 kPa. The presence of TPs in infraspinatus muscles was confirmed by following criteria: (1) a taut muscle band palpable in the muscle, (2) a painful spot in the taut band, (3) recognition of pain and sensory complaints by pressure, and (4) local twitch response evoked by snapping palpation (Travell and Simons, 1999).
The study was approved by the Regional Ethics Committee (Aarhus, j.no. 20010244), the Danish Medicines Agency (j.no. 2612–2276) and the Danish Data protection Agency.
Data are presented as mean (SD) if not otherwise stated. The Fishers’ exact test was used for analysis of 2 2 tables. The independent samples t-test was used and non-parametric tests such as Mann–Whitney or Wilcoxon rank sum test were used for non-normal distributed data. p-values < 0.05 were considered to be significant.
4.1. Paraesthesia, pain, and hypoesthesia
Three hundred twenty four patients (96%) reported parasthesia and pain in at least one digit innervated by the median nerve. One hundred and sixty four patients (49%) reported additional parasthesia and pain in digit IV or V. Twentyfour patients (7%) reported parasthesia and pain in the whole arm. Most frequently reported was paraesthesia in the T1 dermatom in the forearm (180 patients (53%).
Two hundred fifty six patients (76%) reported hypoesthesia in at least one digit innervated by the median nerve. One hundred twenty patients reported additional hypoesthesia in digit IV or V. No patients reported hypoesthesia in the whole arm. Most frequently reported was hypoesthesia in the T1 dermatom (10 patients (3%).
The distribution of parasthesia, and pain, and hypoesthesia in all patients is shown in Table 1.
4.2. Duration of symptoms
Three hundred and three patients reported data about duration of symptoms. The mean duration of symptoms in this group was 57 weeks (range 1–500). For 32 patients the duration of symptoms
was not registered.
4.3. Occupational data
Occupational data were possible to obtain from 231 patients: manual work (70 patients), office work or students (50 patients), work within the nursery and care system (37 patients), pensioners or unemployed (64 patients), other (19).
4.4. Nerve conduction studies
Two hundred and one patients (60%) had abnormal NCS in the median nerve indicating carpal tunnel syndrome and normal ulnar nerve. One hundred seven patients (52%) had CTS in the right hand, 38 patients (19%) in the left hand and 56 patients (28%) had bilateral CTS. Eleven patients had palmar slowing of the median sensory nerve conduction, 90 had mild, 29 had moderate and 71 patients had severe electrodiagnostic signs of CTS according to the above mentioned severity scale. In one hundred nineteen patients (36%) NCS of median and ulnar nerves were normal.
Fifteen patients (4%) had abnormal ulnar nerves that prompted additional tests e.g. EMGof the brachial brachii muscles, sensory and motor studies of ulnar nerve over the elbow, orNCSof the lower extremities. These tests revealed the following electrodiagnostic abnormalities (ulnar neuropathy at the cubital level (1), cervical radiculopathy/myelopathy (4), brachial plexus neuropathy (1), polyneuropathy (8).
4.5. Trigger points
TPs in the infraspinatus muscle were found in 49% of patients (58 out of 119) with normal NCS while only 26% of the patients (53 out of 201) with abnormal NCS had TPs (Fisher’s Exact test p = 0.000037).
Likewise, 26% (4 out of 15) of the patients with other electrodiagnostic abnormalities had TPs, all four had polyneuropathy.
The distribution of severity of CTS in patients with and without TPs is shown in the Fig. 1. In the group with abnormal NCS indicating CTS and TPs in the infraspinatus muscle, the patients with mild CTS (including palmar slowing) had a significant higher occurrence of TPs than the patients with moderate and severe CTS (33% vs. 20%) (Fisher’s Exact test p = 0.017).
4.6. Parasthesia, pain, and hypoesthesia related to results of NCS and the presence of TPs
There was no difference in the distribution of paraesthesia and pain, and hypoesthesia in all fingers between patients with abnormal NCS indicating CTS and with TPs, and patients with abnormal
NCS indicating CTS and without TPs. There was a significant difference in the distribution of paraesthesia and pain in finger 3 and finger 5 between patients with abnormal NCS indicating CTS and without TPs and patients with normal NCS and with TPs (finger 3: t = 2.49, p = 0.014; finger 5: t = 5.61, p = 0.000007). There was a significant difference in the distribution of hypoesthesia in all fingers between patients with abnormal NCS indicating CTS and without TPs, and patients with normal NCS and with TPs (finger 1: t = 1.01, p = 0.004; finger 2: t = 2.43, p = 0.01; finger 3: t = 2.61, p = 0.01; finger 4: t = 2.12, p = 0.03; finger 5: t = 4.34, p = 0.00004) (Table 2).
This single-blinded study investigated the occurrence of myofascial pain from the TPs in the infraspinatus muscle in a patient population (335 patients) referred for CTS. About half (49%) of the patients with normal electrophysiological findings had myofascial pain from TPs in the infraspinatus muscle associated with parasthesia and pain referred to the arm and the fingers. In these patients TPs possibly explain the patients’ symptoms. This is in line with another study (Lo et al., 2002) that found up to 55% of the same group of patients had myofascial pain. The TPs in the infraspinatus muscle of the symptomatic side(s) were more likely to be found in patients with normal NCS than in those with electrophysiological signs of CTS (49% vs. 26%). In addition, we found that patients with mild electrophysiological signs of CTS had a significantly higher occurrence of TPs in the infraspinatus muscle of the symptomatic arm compared with patients with moderate to severe electrophysiological CTS (33% vs. 20%). TPs in the infraspinatus muscle may thus be a confounding factor in patients with mild CTS.
We found that 36% of the referred patients had normal NCS. Previous studies have reported from 5% (Padua et al., 1999) to 51% (Lo et al., 2002) of patients referred for CTS to have normal NCS. Review of the literature has indicated that clinical diagnosis of carpal tunnel syndrome in different settings is often made according to highly variable diagnostic criteria (Graham et al., 2006; Katz et al., 1990; MacDermid and Wessel, 2004; Padua et al., 2001b, 1999; Priganc and Henry, 2003).
We did not systematically collect information about disorders such as fibromyalgia, rheumatic disorders, tenosynovitis of the scapulahumeral joint, or Pancoast syndrome that can be associated with TPs in the shoulder girdle muscles. We cannot rule out that such disorders can explain the existence of the TPs and the symptoms in some of the patients with normal NCS. Nevertheless, the possibility of the myofascial pain from TPs should be kept in mind when considering patients referred for CTS as the myofascial pain is quite common in the general population and even more common in patients with pain complaints in the arms (Bennett, 2007).
The severity scale used in our study was developed according to Z scores (Tankisi et al., 2005) and uses standard deviations from the mean of controls of the sensory conduction velocity as the primary classification tool. This scale differs slightly from a previously reported classification (Padua et al., 1997).
Whether surgery in CTS is based on NCS’ findings or not depends on the educational background of the surgeon and may vary depending on medical school, hospital and country. (Storm et al., 2005; Jordan et al., 2002) The use of electrophysiological studies in predicting the surgical outcome has been challenged (Glowacki et al., 1996) and some surgeons perform CTS surgery without having performed NCS (Storm et al., 2005). Our study shows that almost half of the patients referred for CTS and with no or only mild electrophysiological signs has TPs that could explain some of the symptoms. Thus, care is warranted when patients are referred to surgery without performing NCS. We suggest that in patients referred for CTS and where the NCS are normal, a check for TPs should be made.
As a conclusion, myofascial pain from TPs in infraspinatus muscle gives rise to referred pain and parasthesia in shoulder, arm and hands and these symptoms may mimic the carpal tunnel symptoms. When CTS is suspected clinically, physicians should be aware of myofascial pain in the infraspinatus muscle, especially in patients with normal nerve conduction studies.
Conflict of interest
The authors thank the Danish Insurance and Pension Fund for Grant support, Lotte Faarbk, Bente Allen and Anja Holm who performed NCS, Holger Killerich for technical assistance, and Paul Cumming, PhD for manuscript revision.
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