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Correspondence 6. Rennotte MT, Baele P, Aubert G, Rodenstein DO: Nasal con- tinuous positive airway pressure in the perioperative man- agement of patients with obstructive sleep apnea submitted to surgery. Chest 1995; 107:367–74 7. Wieczorek PM, Carli F: Obstructive sleep apnea uncovered after high spinal anesthesia: A case report. Can J Anaesth (Accepted for publication June 3, 2014.) Single-injection and Continuous Femoral Nerve Blocks Are Associated with Different Risks of Falling To the Editor: I read with interest the superb retrospective database study of Memtsoudis et al. titled, “Inpatients falls after total knee arthro- plasty: The role of anesthesia type and peripheral nerve blocks.” 1 Of more than 190,000 patients who underwent total knee arthroplasty (TKA), there was a fall incidence of 1.6%. Twelve percent of all patients had a “peripheral nerve block” (PNB), yet, as stated in the abstract, “Contrary to common concerns, no association was found between PNB and IF [inpatient falls].” However, within the limitations section, the authors explain that, “The database used contains limited clinical infor- mation and thus some important factors cannot be taken into account… With regard to information concerning PNB, spe- cific details on the exact type of block, if it was a continuous or single-shot application… are not readily discernible.” In other words, it remains unknown how many—or even if any—of the nearly 23,000 patients with “PNB” had a continuous PNB. This is a critical piece of (missing) information because the available data from previous studies that were able to differ- entiate between single-injection and continuous PNBs sug- gest a strong association of the latter with an increased risk of falls. In a retrospective database study, Wasserstein et al. found that—like Memtsoudis et al.—patients who underwent TKA with a single-injection femoral nerve block had the same risk of falling as patients without any type of PNB. 2 However, the presence of a continuous femoral nerve block increased the odds ratio of falling to 4.4 (p = 0.04). In a meta-analysis of three randomized, placebo-controlled trials involving femoral and posterior lumbar plexus catheters for TKA and total hip arthroplasty, respectively, Ilfeld et al. found that no subjects receiving perineural saline (n = 86) fell (0%) while there were seven falls (7%) in patients receiving perineural ropivacaine (n = 85; p = 0.01), strongly suggesting a causal relationship between the continuous blocks and falling. 3 Since there were no falls in the placebo group, an odds ratio cannot be calcu- lated; but, if even just a single fall occurred in this group, the odds ratio would be 5.5 (therefore, the actual odds ratio is at least 5.5, but possibly higher). Finally, an additional meta- analysis including 4 randomized, controlled trials and one retrospective cohort study, Johnson et al. calculated an odds ratio of 3.9 (p < 0.01) of falling for subjects with a continu- ous femoral or posterior lumbar plexus block of greater than 12 h (incidence = 2.2%) compared with subjects with either no block, a single-injection block, or a perineural infusion of less than 12 h (incidence = 0.5%). 4 To my knowledge, there are no data contradicting these findings when single-injection and continuous PNBs are differentiated. Why this apparent difference in the risk of falling exists between single-injection and continuous PNB remains unknown. Nevertheless, one may speculate that the reason single-injection blocks do not increase the risk of falling is simply because patients with flaccid quadriceps are not permitted out of bed—and do not attempt ambulation— without a good deal of caution and assistance. In contrast, patients with continuous PNB are not only permitted to get out of bed, but ambulate repeatedly in the early hours/ days following surgery. Given that falls with 4-day continu- ous PNB occur not only in the two days following surgery, but postoperative days 3 and 4 as well—after patients have successfully ambulated multiple times during physical ther- apy—there is a high probability that patients become more confident and do not continue to take the same precautions as during early ambulation attempts. 3 To support this sup- position, the majority of falls in patients with continuous PNB occur when patients are unaccompanied/unassisted and going to the restroom, often in the middle of the night. 5 Therefore, while I agree with the authors’ statement that their, “data should provide encouragement to not shy away from the use of PNB;” it should also not lull healthcare pro- viders into a false sense of security regarding the risks of con- tinuous PNB. Research involving the etiology of patient falls and their association with various regional analgesic inter- ventions must not decrease due to the important findings reported in the recent study by Memtsoudis et al. In addition, this letter should not be construed as criticizing the study by Memtsoudis et al.—the authors accurately and responsibly identified the limitation of their analysis within their discus- sion section—but, rather, a caution to readers of their article. Practitioners should at least be aware of the data specific to continuous PNB, as decreased cognizance or even denial of the issue may only increase the potential risk to our patients. Competing Interests The author declares no competing interests. Brian M. Ilfeld, M.D., M.S., University of California San Diego, San Diego, California. bilfeld@ucsd.edu References 1. Memtsoudis SG, Danninger T, Rasul R, Poeran J, Gerner P, Stundner O, Mariano ER, Mazumdar M: Inpatient falls after total knee arthroplasty: The role of anesthesia type and peripheral nerve blocks. A nesthesiology 2014; 120:551–63 2. Wasserstein D, Farlinger C, Brull R, Mahomed N, Gandhi R: Advanced age, obesity and continuous femoral nerve Anesthesiology 2014; 121:662-9 668 Correspondence