The largest contemporary comprehensive study of neurological complications after CNB was published by Moen et al. (51) in 2004. The large, albeit approximate, number of CNBs (1,260,000 spinal and 450,000 epidural anesthetics) captured reflects the long study period (1990–99) as well as the authors' efforts to accumulate data from multiple sources, including a postal survey to anesthesiologists, the national Swedish database for mandatory reporting of adverse events, and that country's predominant manufacturer of neuraxial local anesthetics. The next largest study was conducted by Aromaa et al. (46) in 1997. These authors collected all claims of neurological complications associated with CNB that were reported by patients to Finland's legislated no-fault patient compensation insurance program between 1987 and 1993, and retrospectively estimated the total number of CNBs administered in that country (550,000 spinal and 170,000 epidural anesthetics) over the same time period. Scott and Tunstall (50) performed a prospective survey that captured 14,856 obstetrical spinal anesthetics and 108,133 obstetrical epidural anesthetics performed between 1990 and 1991 in 79 obstetrical units across the United Kingdom. The next two largest comprehensive studies of neurological complications after CNB were performed by Auroy et al. (45,47) These two widely cited studies prospectively surveyed hundreds of practicing anesthesiologists in France to determine the frequency of major complications associated with all RA techniques. In addition to gathering the most extensive data on complications after PNB, Auroy et al. included 40,640 spinal and 30,413 epidural anesthetics performed in 1994 (47) and 41,079 spinal and 35,293 epidural anesthetics performed in 1998–1999 (45).
There are a limited number of contemporary prospective studies in the literature examining the risk of neurological injury after PNB. Most of the available data involves upper extremity, rather than lower extremity, PNB, which reflects the preference for brachial plexus blockade in contemporary RA practice (71). In the two large prospective studies performed by Auroy et al., eight cases of neurological injury were identified among 21,278 PNBs (3.8:10,000) in 1997 (47) and 12 cases among 43,946 PNBs (2.7:10,000) in 1998–1999 (45). In the latter study, neurological symptoms were still present 6 mo after the PNB in 7 of the 12 cases of reported peripheral neuropathy (45). Unfortunately, however, neither of these two studies provides sufficient detail to determine the overall frequency of permanent neurological deficit. For all PNB studies, the present review suggests that interscalene block carries the highest risk of transient neurological deficit, specifically, 2.84:100 (95% CI: 1.33–5.98:100) (Table 4). Among the 16 studies in which complications were sought 12 mo after PNB, only one case of permanent neuropathy was reported (69) (Table 2).
Although there is a limited number of contemporary large scale studies examining neurological complications after PNB available for review, there are even fewer available for historical comparison with our present findings. In 1985, Winchell and Wolfe (74) prospectively followed 854 consecutive patients who underwent brachial plexus blockade for upper extremity surgery and found a 0.4% incidence of postoperative neuropathy and no cases of permanent neurological deficit. Weeks et al. (75) followed 834 patients who underwent axillary brachial plexus blockade and found that four patients (0.5%) suffered persistent pain unrelated to the surgical site when assessed at 2 yr postoperatively. Finally, in an observational study examining 242 consecutive axillary and 266 consecutive interscalene brachial plexus blocks for upper extremity surgery, Urban and Urquhart (76) determined the incidence of neurological deficit to be 5% and 3%, respectively, at 2 wk postoperatively, with only one patient in each group (0.4%) experiencing persistent deficit beyond 4 wk.
The heterogeneity and quality of the available source studies included in an article such as this calls for caution when interpreting the validity of our risk estimates. Differences in sample size, patient populations, comorbidities, and surgical procedures undermine faithful comparisons of neurological complications reported in each study. Moreover, the presentation, investigation, and diagnosis of anesthesia-related nerve injury is complex (77,78) and inconsistent among studies, likely resulting in under-reporting in some studies and over-reporting in others. For example, identification of neurological complications likely varied depending on direct anesthesia follow-up (24,25,53,54,56,57,59,60,63,69), surgeon referral (49), voluntary reporting by anesthesiologists (45,47,50,51,55,68), retrospective chart review (48,52,58,62,64,70,73), or patient self-reporting (46,58,59), the latter associated with a relatively higher rate of neurological symptoms after nerve blockade.
The time at which assessment or follow-up occurred surely affected the incidence of complications as neurological symptoms after CNB and PNB diminish with time. In some studies, one or more anesthesiologists (24,45,47,49,53,55–57,59,67), neurologists (47,49,54,55), or surgeons (24,49,55,56) undertook diagnosis, whereas in most other studies it is unclear who, if anyone, was charged with diagnosing the etiology of nerve injury. Finally, none of the studies presently reviewed were of prospective controlled design. Rather, the largest of the source studies reviewed relied, in all or in part, on self-reporting from anesthesia providers (45–47,50,51). The significant potential for under-reporting of anesthesia-related complications is the predominant limitation when self-reporting is sought from anesthesiologists (79). Although the tendency for under-reporting may be greater in voluntary self-reporting systems [e.g., Auroy et al. (45,47)] (80), mandatory self-reporting [e.g., Moen et al. (51)] does not guarantee that all adverse events will be reported either (79). Nonetheless, voluntary or mandatory self-reporting is one of the only practical means to capture rare (approximate incidence 1:10,000–1:100,000) occurrences (79). A more reliable and valid method to capture the true incidence of rare neurological complications would be an international, multicenter, prospective, standardized trial (79), the logistics of which can be highly impractical. For extremely rare events (approximate incidence 1:1,000,000), such as paraplegia after CNB, preemptive risk modeling would be ideal, but this strategy is still premature in our specialty (79). At present, collating and adjusting the reported rates of neurological complications and calculating CI (81) are likely our best means to quantify and estimate the incidence of such rare occurrences.
In summary, our review suggests that the rate of neurological complications after CNB is <4:10,000, or 0.04%. The rate of neuropathy after PNB is <3:100, or 3%. However, permanent neurological injury after RA is rare in contemporary anesthetic practice. The rate of neurological complications presented in this article may be under-estimated, because much of the source data relied on self-reporting from anesthesia providers rather than prospective controlled trials.
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