Diaz, James H. MD, DrPH
In addition to cardiologists, dentists, and dental hygienists, nurse anesthetists (NAs) may represent another high-risk occupation for carpal tunnel syndrome (CTS) in the health care industry (1). Unlike other health industry workers, NAs are exposed to unique mechanical and environmental stresses in the operating room (OR) workplace. To investigate a small cluster of nondominant, left-hand CTS in female NAs in one work site, a second confirmatory cross-sectional study was designed to compare the risks of CTS and nondominant hand CTS in female NAs with a control population of OR nurses (ORNs) with similar physical characteristics and environmental exposures working in 11 metropolitan-area hospitals.
Research questions addressed by this regional prevalence study included 1) Are there significant sentinel clusters of CTS in NAs as compared with other OR workers, specifically ORNs? 2) Is nurse anesthesia a high-risk health care occupation for upper extremity cumulative trauma disorders, and, if so, are the disorders lateralized or affected by hand dominance?
This study was conducted in an anonymous manner with IRB approval for the work sites studied as part of a citywide (New Orleans, LA) investigation. The IRB of the Louisiana State University Health Sciences Center serves as the official Committee for Human Research for all human investigations conducted by state health science academic investigators. The investigator was blinded as to the participants in this study and to the participating institutions. A female research assistant (senior medical student in the combined MD-MPH degree program), assigned and trained by the investigator and by a faculty hand surgeon, distributed and collected all study questionnaires at the participating work sites, confirmed historical, physical, and diagnostic findings, and reviewed charts as required by the study protocol. Study participants were provided with a written statement describing the investigation and participated in the study with fully informed consent by voluntarily completing questionnaires of baseline characteristics, medical history, diagnostic tests, and outcomes of treatment anonymously. Prior permission to distribute and to collect survey questionnaires at work sites was obtained from the nursing administration at each participating hospital. There was no prior announcement as to the nature of the survey, nor was any attempt made to contact nonrespondents on vacation, on sick leave, or off duty. There was no attempt to correlate CTS with past or current compensation claims or with sick leaves related to CTS.
The study population included 63 female, certified registered NAs and 181 female, registered ORNs from 11 work sites in the New Orleans metropolitan area. There was no attempt made to match NAs with ORNs on the basis of physiologic characteristics or length of service, because such a design would have promoted misclassification biases from case-control similarities. In addition, a ratio of more controls (ORNs) to cases (NAs) was determined a priori to not exceed 4:1 to add study subjects, increase sample size, adjust for an anticipated small number of cases of CTS, and detect small, but significant, differences in risk ratios (2). The sample site study populations included approximately 50% of the NA staff and 60% of the ORN staff in the metropolitan region, as a result of vacations, sick leaves, and call assignments. The larger female population from which the study site subjects were drawn included approximately 125 NAs and 253 ORNs from the 11 participating hospitals. The 11 hospital work sites included all general acute care hospitals in the New Orleans metropolitan area and excluded outpatient surgical facilities and obstetric delivery facilities.
Study exclusion criteria included male sex (because of inadequate numbers of male NAs and ORNs at participating hospitals), pregnancy, puerperium (up to 12 wk postpartum), and obesity (body mass index more than 30 kg/m2). Musicians were not intentionally excluded from the study because of synergistic, nonoccupational risk factors for CTS. Nevertheless, the study included no musicians or patients with rheumatic, connective tissue, or endocrine disorders, specifically hypothyroidism and diabetes mellitus.
The case definition of CTS included either a history of surgical median nerve decompression or a combination of four positive historical (nocturnal hand pain and hand pain diagram) and physical findings (positive Tinel’s sign and Phalen’s test). The precise electrophysiologic diagnosis of CTS had been previously confirmed in three patients (two NAs and one ORN), who also met the clinical case definition for CTS by the following abnormalities in median nerve conduction velocity from palm to wrist: 1) median nerve sensory latency of 3.5 ms or more and 2) median nerve motor latency of 4.5 ms or more. Nerve conduction testing was not offered in this cross-sectional study but was used to confirm the clinical case definition of CTS in 3 of 20 patients with CTS. All subjects worked in OR suites at ambient OR temperatures of 19.4°C–20.6°C. All waste anesthetic gases in the ORs were scavenged with vacuum systems.
The continuous baseline characteristics of the study subjects divided by occupational risk group (NAs = 63, ORNs = 181), including age, weight, and length of service in the same occupation (in years), were expressed as means (sd) and analyzed for significant mean differences by using unpaired, two-tailed t-tests, with a preestablished significance (α) level of P < 0.05. The dichotomous, categorical variables of unilateral or bilateral CTS present or absent were analyzed for exposure odds ratios (with 95% confidence intervals [CIs]) and χ2 values by 2 × 2 table contingency analysis. An a priori power and sample size of proportion analysis directed sample sizes of at least 59 subjects for each sample to achieve 80% statistical power at the significance level of 0.05.
Odds ratio contingency table cells with no observations were completed with surrogate frequencies of 0.5 to calculate odds ratios by the cross-multiplication technique: Odds ratio = ad/bc. Crude odds ratios and χ2 values included all clinically defined (and in three cases, electrophysiologically confirmed) cases of CTS, irrespective of side, bilaterality, or hand dominance. Odds ratios and χ2 values were also adjusted for hand dominance and calculated in the same manner as the crude, unadjusted contingency ratios. The 95% CI for each odds ratio was calculated by using the Taylor series expansion formula (3) :
The prevalence (exposure) odds ratio was considered the most precise estimate of the relative risk of disease (CTS) in a cross-sectional investigation. CIs for odds ratios that did not include and exceeded the no-effect level of 1.00 were considered clinically significant and permitted the rejection of the null hypothesis of no occupational exposure effect. Odds ratios larger than 2.5 were considered a priori to represent statistical significance in this prevalence analysis with few anticipated cases of CTS and reflected an acceptable approximation of the relative risk of disease in a longitudinal case-cohort investigation with a larger sample size.
In addition to the odds ratios for CTS, Yates-corrected χ2 values were also calculated for the same crude and adjusted dichotomous, categorical conditions of unilateral or bilateral CTS and CTS by hand dominance. For the χ2 values, significant differences were determined by a preestablished significance (α) level of P < 0.05.
The baseline characteristics of the subjects (n = 244) are shown in Table 1. NAs (n = 63) were older (41.98 ± 9.28 yr) and weighed more (62.10 ± 5.11 kg) than ORNs (n = 181, 38.41 ± 7.48 yr, 56.47 ± 7.02 kg) (Table 1). Notably, both NAs and ORNs had similar mean lengths of service in the same occupation at the same work site of longer than 10 yr (Table 1). The ratio of controls (ORNs) to cases (NAs) was 2.87:1.00, in keeping with the recommended design of no more than four controls for each case for epidemiologic studies restricted by sample size (n = 244) and the number of CTS cases (n = 20) (2).
There were 10 cases of CTS among 63 NAs and 10 cases of CTS among 181 ORNs. Two NAs and two ORNs reported prior surgical decompression of the median nerve on one side. The influence of hand dominance on the distribution pattern of CTS was noted and reported in Table 2. The crude and dominant-hand adjusted odds ratios with 95% CIs and the Yates-corrected χ2 values with exact P values were analyzed and reported in Table 3.
The crude odds ratio for CTS in NAs as compared with ORNs was 3.23 (CI, 1.27–8.17). This odds ratio was supported by a χ2 of 5.35 (P = 0.021) at a power of the χ2 of 75%.
Right-handed NAs were more likely to have left-hand and bilateral CTS than ORNs with an odds ratio of 3.85 (CI, 1.05–12.16) and a supporting χ2 of 5.08 (P = 0.024) at a power of the χ2 of 72%. Left-handed NAs did not seem to have the same risks for developing right-hand, left-hand, and bilateral CTS that right-handed NAs did as compared with similarly handed ORNs. Although not offering any protection from CTS as indicated by a negative odds ratio, left-hand dominance among NAs was associated with an insignificant risk of left unilateral and bilateral CTS with an OR of 1.44 (CI, 0.03–74.2) and a χ2 of 0.133 (P = 0.715).
Cumulative trauma disorders of the upper extremity are one of the fastest-growing occupational disorders in US industry (4). More than half (57%) of the occupational upper-extremity disorders reported in the federal work force were diagnosed as CTS (5). Product industries, especially meat packing and poultry processing, pose greater occupation-related risks of CTS in workers than service industries, such as health care (6). Some hobbies and avocations, such as knitting and string-instrument playing, also predispose individuals to CTS and may magnify and compound coexisting occupational risk factors (6).
Women between 31 and 50 years of age have now been identified as a high-risk population for CTS, predisposed to both occupation- and nonoccupation-related CTS (5). A recent shift to service industry work and more video display unit use, has, however, not correlated as well with increasing upper extremity cumulative trauma disorders in women as have increased numbers of women in the workforce and heightened public awareness of CTS (7). Despite increasing proportions of women in the US workforce, the distribution of Liberty Mutual Group Workers’ Compensation Insurance claims by sex has remained constant over the years, with women accounting for 65% of all upper-extremity cumulative trauma disorders, but only 30% of all claims (7). A compounding risks model has been proposed to illustrate the synergistic effects of work and home exposures on cumulative trauma disorders and can be easily adapted for CTS in women (8) (Fig. 1).
Several health care and allied health professions have been associated with unilateral CTS of the dominant hand and include interventional cardiology, dentistry, dental hygiene, and sign language communication (1). Repetitive pinching and advancement of small-bore vascular catheters with surgical gloves on may predispose interventional cardiologists to CTS (1). Vibrating dental buffers and drills may predispose dental hygienists and dentists, respectively, to CTS (1). Forced finger flexion during signing may predispose sign language interpreters to CTS (1). In all such cases among health industry workers, CTS is usually unilateral and confined to the dominant, preferred hand (1).
NAs have a number of unique, potentially neurodestructive, environmental and mechanical exposures in their professional workplace, the OR. The cumulative exposure of the upper extremity to these workplace exposures may predispose NAs to CTS. OR workers are habitually exposed to cold, ambient OR temperatures and waste anesthetics, especially nitrous oxide. Both cold and nitrous oxide exposures can reduce nerve conduction, and moderate nitrous oxide exposure can cause permanent symmetric, distal axonopathy (9). These environmental exposures can, however, be effectively mitigated by (1) wearing undergarments and layering OR garb (2), increasing OR temperatures, and (3) scavenging waste anesthetic gases.
Mechanical exposures unique to nurse anesthesia in this work site investigation included 1) rigid laryngoscopy for endotracheal intubation by using laryngoscopes and blades traditionally designed for left-hand use only and 2) wearing disposable, loose fitting gloves during laryngoscopy and other anesthesia procedures contaminated with body fluids. Rigid laryngoscopy performed with the left hand, in fact, exposed NAs to five of the six occupation-related risk factors for CTS as described by Silverstein et al. (10), with the single exception being use of vibrating, hand-held instruments, as in dentistry. These exposure risks during rigid laryngoscopy performed by NAs with the left hand included 1) repetitive left wrist deviation during laryngoscopy, 2) exaggerated left-hand grip during laryngoscopy, 3) left midpalmar compression by the laryngoscope handle during laryngoscopy, 4) extreme left-wrist flexion and extension during laryngoscopy, and 5) wearing poorly fitting gloves during laryngoscopy, which further exaggerated hand grip (10).
According to the classification of Silverstein et al. of mechanical exposures associated with work-related upper extremity cumulative trauma disorders, the task of rigid laryngoscopy may be classified as a high-force, low-repetition task (10,11). High force was defined as a force of 6 kg or more as estimated by surface electromyography (11). Low repetition was defined as a task lasting longer than 30 seconds or consisting of redundant tasks performed for 50% or more of the work-action cycle (11). Successful laryngoscopy for tracheal intubation often takes longer than 30 seconds, but less than one or two minutes, and requires a force that can extend the head (9% or more of body weight) off a flat surface and rigidly align to approximately 180 degrees the three axes of the airway (oral, laryngeal, and tracheal). The use of loose, bulky, or otherwise poorly fitted gloves, such as disposable, one-size-fits-all examination gloves, contributed to fingertip sensory feedback loss and resulted in hypercontraction of carpal tunnel tendons and increased grip forces in the model of Silverstein et al. (11).
In this investigation, NAs demonstrated a fivefold increased risk for CTS compared with ORNs. When risk adjustment was made for left-hand CTS, NAs continued to demonstrate an increased risk for nondominant, left-hand CTS and were nearly four times more likely to have been diagnosed clinically, and in some cases, electrophysiologically, with left-hand CTS than ORNs. The task of rigid laryngoscopy, which can be performed only with the left hand by using conventional laryngoscopes, and the wearing of loose-fitting, disposable gloves during rigid laryngoscopy contributed five of six known mechanical exposure forces associated with CTS in manual workers (10). Successful mitigation of these mechanical exposures might include laryngoscope redesign; replacing conventional, rigid laryngoscopy with flexible, fiberoptic laryngoscopy for tracheal intubation; and wearing fitted gloves for airway procedures (12).
The strengths of this investigation included 1) a double-blinded design at the time of data collection and analysis; 2) the same workplace occupational stresses, cold-temperatures, and scavenged waste-gas environmental exposures for all study subjects; and 3) the most sensitive and specific combination of two historical and two physical findings to confirm a clinical diagnosis of CTS with or without electrophysiologic diagnosis (13).
In making a clinical diagnosis of CTS, a positive Tinel’s sign (sensitivity 44%–63%, specificity 55%–94%) or a positive Phalen’s test (sensitivity 25%–71%, 47%–80%) do not offer the dependability that a combination of clinical tests and examinations can offer (6). A hand pain diagram as a diagnostic tool for CTS is an even weaker and more subjective indicator of CTS than provocative physical tests with a sensitivity of 61% and a specificity of 71%(6). When combined, however, as in this investigation, a Tinel’s sign and a hand pain diagram can offer a specificity of 89% and a positive predictive value of 71%(6). A combination of a positive Phalen’s test and a hand pain diagram can offer a specificity of 83% and a positive predictive value of 83%(6). This study was not designed to compare the sensitivity and specificity of combined nocturnal hand pain history, hand pain diagram, positive Tinel’s sign, and positive Phalen’s sign with the “gold standard” electrophysiologic diagnosis of CTS. The predetermined sensitivities and specificities of combined clinical tests as compared with nerve conduction tests were, however, assumed to be additive.
Weaknesses of this investigation included 1) the small sample size (n = 244) with no male nurses included; 2) a cross-sectional study design comparing prevalent cases, which could estimate only relative risk for incident cases of CTS; 3) no physiologic mea-surements of force and force duration (force × time) during rigid laryngoscopy, such as surface electromyography, grip force, force-posture interactions, and carpal tunnel compartment pressures; and 4) an inability to confirm the clinical diagnosis of CTS in 17 of 20 cases with electrodiagnostic testing. In addition, the temporal, synergistic risk factors for CTS could not be measured in this study, including shift lengths, repetitive laryngoscopies per day, work-rest ratios, and off-duty or sick-leave times (Fig. 1). The exact temporal responses and the repetitive stress exposures per day could not even be estimated because of rotating duty rosters, vacation and call schedules, and sick leaves, which institutions were not required to reveal for purposes of subject confidentiality and anonymity. Finally, the investigators were not permitted to contact off-duty nonresponders on call, vacation, or sick leave, creating two types of information bias (nonresponse and volunteer biases) that could not be controlled for in a prevalent study.
In adapting Kerk’s (8) model of compounding risk factors for musculoskeletal injuries in the workplace, a personal risk factor model for CTS in NAs should also have included a number of observed physical risk factors (solid boxes, Fig. 1) compounded by many unknown occupational, nonoccupational, and psychosocial risk factors (open boxes, Fig. 1) (8). Such a model cannot be replicated in a cross-sectional, bivariate investigation and will require prospective studies with multivariate analysis (Fig. 1) (8). This cross-sectional investigation at 11 work sites will require further confirmation of its findings in larger, both-sex, historical cohorts of OR workers from multiple professions in many work sites before generalizing its findings on CTS, left-hand CTS, and bilateral CTS to the universe of all NAs performing rigid laryngoscopy with the left hand.
On the basis of our data analysis, however, other female anesthesiology workers, such as female anesthesiologists and female anesthesiology physician assistants, will have the same increased exposure odds ratios for left-hand and bilateral CTS as female NAs. Although CTS occurred with increased frequency in both sexes in other health professions, such as interventional cardiology and dentistry; it was always a unilateral disorder of the dominant hand in these professions and not uniquely lateralized to the nondominant left hand (1). Male anesthesiology workers, such as male NAs, anesthesiologists, and physician assistants, will remain at decreased risk of CTS compared with female anesthesiology workers because of sex and increased upper extremity strength, but they could be predisposed to CTS by coexisting conditions, such as arthritis and endocrinopathy.
In conclusion, the answers to the research questions posed by this regional prevalence study included the following: 1) There were significant sentinel clusters of CTS in female NAs as compared with other OR workers, specifically female ORNs in 11 regional work sites. 2) Nurse anesthesia may represent another high-risk health care occupation for upper-extremity cumulative trauma disorders. Nondominant left-hand CTS and bilateral CTS were significantly more common among female NAs than female ORNs. As compared with other health care workers at increased risk of unilateral and dominant hand CTS, the cases of CTS in NAs observed in this study were bilateral or lateralized to the left hand and affected by hand dominance (1). Rigid laryngoscopy performed with the left hand exposed NAs to five of the six occupation-related risk factors for CTS as identified by Silverstein et al. (10,11). Despite precise inclusion criteria, the physical risk factors for CTS in all study subjects may have been magnified by sex, musculoskeletal strength, endocrine activity, and other physiologic variables, such as menses and premenstrual edema (1,8). Nevertheless, any possible confounding from risk magnification in women was evenly represented in same-sex cases and controls. On the basis of our data analysis, nondominant left-hand CTS and bilateral CTS were significantly more prevalent in NAs than ORNs.
The author acknowledges and appreciates the statistical advice and support of Professor and Chair Miguel A. Guzman, PhD, and Assistant Professor Donald E. Mercante, PhD, of the Department of Biometry and Genetics, Louisiana State University School of Medicine.