Anesthesiologists are often alarmed by studies documenting the frequency of neurological symptoms occurring after regional block: up to 20% of patients will report vague symptoms in the first few days after surgery. On the surface, this might seem disturbingly high; however, if one evaluates these studies more critically, the situation is far from concerning.
First, in studies reporting relatively high “complication” rates, interrogation for symptoms typically occurred during the first 2 weeks after surgery. The natural history of these symptoms is for rapid resolution within a few weeks. Second, most symptoms are minor: they may be reported when patients are prompted during interrogation, but they may not be bothersome enough to be volunteered otherwise. When assessing the significance of these minor symptoms, it is important to note that many patients undergoing regional block are undergoing surgical procedures that require months of rehabilitation. A minor symptom (e.g. patchy numbness) resolving within a couple of weeks may therefore be inconsequential in the context of the surgical rehabilitation. Third, minor symptoms may be unrelated to the block. Many large prospective series have been conducted documenting the incidence of neurological complications following regional anesthesia and surgery. However, few studies have been conducted to document the frequency of postoperative neurological symptoms occurring following surgery without regional block. We therefore do not know the relative impact of the block versus other causes. In fact, studies have shown that most postoperative neurological symptoms have causes unrelated to the block. 2,3
Caution should also be exercised comparing postoperative neurological complication rates between large prospective studies. The rarity of prolonged neurological complications reported in previous studies renders the 95% confidence intervals so wide that one needs to be cautious making any sort of comparison between studies. Furthermore, there are other major limitations to making between study comparisons. First, the study protocols of large prospective studies differ in their definition of what constitutes a complication (or symptom): the threshold criteria for significant sensory symptoms ranges from simply “altered sensation”, 1, 2 to “non-resolving paresthesia”, 3 to “sensory deficit with clinical or electrophysiological evidence of a peripheral site of injury” 4 to the extreme of the continuum, “neurological deterioration, allodynia or severe pain”. 5 Second, there is the inescapable reality of reporting bias. 6 A “neurological complication” in the eyes of one investigator can be insignificant to another investigator. In fact, there have been at least two previous prospective studies in the last couple of years involving hundreds of patients, both claiming a neurological rate of zero despite direct patient interrogation in the first two weeks after surgery. Third, large prospective studies vary in their level of follow up. Very large, multicenter studies inevitably have lower follow up rates. Single institution and/or single operator studies, on the other hand, typically have high (>95%) follow up rates, and it is an unlikely coincidence that these high follow up rates are associated with high neurological complication rates.
The aforementioned limitations are reflected in the marked disparity in the short-term neurological complication rates between studies. In the very early postoperative period (< 2 weeks), neurological complications have been reported to occur with a frequency of 0.02%, 4 0.2%, 5 0.5%, 3 8% 1, 2 and 11%: rates that are statistically very different (p < 0.0001). Are these differences indicative of the risks associated with different techniques and/or different operators? Clearly not.
Such limitations make the literal comparison of one series against another imprecise at best. When these limitations are combined with the issue of very low historical baseline neurological rates and very wide confidence intervals, all that can be concluded is that the neurological complication rate from a particular series is not markedly higher than similar previous studies.
1. Borgeat A, Dullenkopf A, Ekatodramis G, et al. Evaluation of the lateral modified approach for continuous interscalene block after shoulder surgery. Anesthesiology 2003;99: 436-42.
2. Fredrickson MJ, Kilfoyle DH. Neurological complication analysis of 1000 ultrasound guided peripheral nerve blocks for elective orthopaedic surgery: a prospective study. Anaesthesia 2009;64: 836-44.
3. Barrington MJ, Watts SA, Gledhill SR, et al. Preliminary results of the Australasian Regional Anaesthesia Collaboration: a prospective audit of more than 7000 peripheral nerve and plexus blocks for neurologic and other complications. Reg Anesth Pain Med 2009;34: 534-41.
4. Auroy Y, Benhamou D, Bargues L, et al. Major complications of regional anesthesia in France: The SOS Regional Anesthesia Hotline Service. Anesthesiology 2002;97: 1274-80.
5. Capdevila X, Pirat P, Bringuier S, et al. Continuous peripheral nerve blocks in hospital wards after orthopedic surgery: a multicenter prospective analysis of the quality of postoperative analgesia and complications in 1,416 patients. Anesthesiology 2005;103: 1035-45.
6. Brull R, McCartney CJ, Chan VW, et al. Neurological complications after regional anesthesia: contemporary estimates of risk. Anesth Analg 2007;104: 965-74.