The escalating burden of type 2 diabetes mellitus and its accompanying morbidities is a significant concern in healthcare. According to the International Diabetes Federation, over 530 million adults live with diabetes worldwide. The incidence is expected to rise above 750 million adults by 2,045, accounting for an estimated annual expenditure of almost 1 trillion US dollars (1). Accordingly, diabetes is present in an escalating number of patients presenting for surgical procedures, prompting a critical need to optimize perioperative care for this growing population (2–4). While patients with diabetes are known to have increased complications in the immediate post-operative period compared to non-diabetic patients, long-term complications, such as worsening of peripheral neuropathy, have not been studied (5). Approximately 50% of patients with diabetes will be affected by peripheral neuropathy in their lifetime (6). The sensorimotor impairments of diabetic peripheral neuropathy are progressive and can lead to debilitating chronic pain, poor functional status, and impaired quality of life (6). Diabetes-related peripheral neuropathy places patients at increased risk of developing foot injuries and infections, which can ultimately result in lower extremity amputation (6, 7). Due to the gravity of such debilitating outcomes, measures to safeguard against the worsening of peripheral neuropathy should be taken in the perioperative period.

Perioperative pain control is a critical component of patient care. Adequate pain control after surgery contributes to decreased morbidity, increased quality of life, and patient satisfaction (8, 9). By using local anesthetics for nerve block procedures, regional anesthesia improves perioperative pain management and has become an essential component for enhanced recovery after surgery (10). In addition, the use of perioperative nerve blocks can reduce the need for general anesthesia and decrease the use of systemic opioids (8, 9). While infrequent, adverse neurologic outcomes have been linked to local anesthetic use (8, 11, 12). Clinical dogma has purported that patients with pre-existing nerve dysfunction, including diabetic peripheral neuropathy, are more susceptible to toxicity from local anesthetics and subsequent risk for worsening neuropathy after a peripheral nerve block (13–17). Bupivacaine is one of the most used local anesthetics for peripheral nerve blocks. However, studies using bupivacaine have reported cellular toxicity (18–22). Additionally, we have previously reported that bupivacaine, but not liposomal bupivacaine, exacerbates motoneuron death and delays functional recovery in a peripheral nerve injury model, suggesting that pre-existing nerve injury increases the risk of bupivacaine toxicity (23, 24). Others have described delayed motor and sensory recovery and increased nerve damage after a sciatic nerve block using bupivacaine in diabetic patients and/or animals (16, 25–29). Recently, novel local anesthetic formulations, such as liposomal bupivacaine, are being developed and may also be less toxic to nerves. Several studies report that liposomal bupivacaine causes less cellular toxicity than standard bupivacaine and postulate the slow release and longer time to peak concentration may account for this difference (30, 31). The long-term consequences of perineural local anesthetics in patients with diabetic peripheral neuropathy are unknown (32), and there is no expert consensus on the safest practice regarding peripheral nerve blocks in patients with diabetes. Therefore, a better understanding of the effects of commonly used nerve block anesthetics should be prioritized to inform the development of safe peripheral nerve block strategies and preserve long-term functional independence in patients with or at risk for diabetic peripheral neuropathy.

We employed the high-fat diet/low-dose streptozotocin rodent model of diabetes to study the functional consequences of repeated sciatic nerve block with bupivacaine and liposomal bupivacaine. Outcome measures included mechanical allodynia, thermal hyperalgesia, exploratory and locomotor behaviors, and nerve conduction velocity. We hypothesized that perineural bupivacaine, but not liposomal bupivacaine, would exacerbate the severity of sensorimotor dysfunction in an animal model of diabetic peripheral neuropathy.

Given the poor clinical sequelae related to progressive diabetic peripheral neuropathy, practitioners should consider the long-term consequences of peripheral nerve blocks using known toxic agents. Our goals are to identify neurotoxic risks associated with perineural local anesthetics and understand the mechanisms of local anesthetic toxicity to better inform the development of optimized dosing or agents that can reduce long-term dysfunction in this ever-increasing patient population with diabetes. Here, we tested the hypothesis that perineural bupivacaine would exacerbate the severity of sensorimotor dysfunction in high-fat diet/low-dose streptozotocin-treated animals. We first established that the high-fat diet/low-dose streptozotocin model of diabetes resulted in persistent hyperglycemia for the extended study period. Next, we showed peripheral neuropathy as evidenced by tactile allodynia and thermal hyperalgesia in diabetic animals. Finally, we performed unilateral sciatic nerve blocks using saline, bupivacaine, or liposomal bupivacaine on control and diabetic animals and evaluated evoked sensorimotor functions and general exploratory and locomotor behavior twelve weeks later. Our data demonstrate the negative effects of perineural bupivacaine on all sensorimotor functions tested in both control and diabetic animals. Furthermore, bupivacaine treatment in diabetic animals with pre-existing neuropathy caused continued worsening of sensorimotor function in some cases. These findings address concerns consistently raised in the literature regarding the safety of local anesthetic nerve blocks in patients with at-risk nerves and provide further evidence that bupivacaine has negative effects in both control and diabetic animals (5, 13–15, 22, 29, 30, 39–44). We describe long-term sensory and locomotor deficits in both control and diabetic animals after a sciatic nerve block using bupivacaine as compared to saline and liposomal bupivacaine.

The high-fat diet plus low-dose streptozotocin rodent model is a more recent model used to mimic the clinical condition of type 2 diabetes (45–47). Although no animal model is perfect, the high-fat diet/low-dose streptozotocin model of type 2 diabetes is now the most commonly used and best characterized, lending its value to testing functional outcomes of local anesthetic toxicity in regional anesthesia (39, 45–50). In line with others, our data show that animals fed a high-fat diet for 10 weeks initially increased their body weight compared to the control-fed animals. After treatment with low-dose streptozotocin, diabetic animals slowly decrease their body weight over time, presumably from the development of insulin resistance, as previously reported by others (51, 52). Few studies have evaluated this diabetic model at prolonged time points. While one study followed their model for 56 weeks, they repeated the streptozotocin injection every six weeks to maintain the hyperglycemic phenotype (53). To ensure the stability of this model for our experiments, we measured fasting blood glucose levels throughout the study period. Our data demonstrate that a single low-dose injection of streptozotocin is sufficient to induce sustained hyperglycemia at least up to 18 weeks after injection. Finally, our data confirmed the presence of both tactile and thermal allodynia in diabetic animals six and 18 weeks after streptozotocin treatment, supporting previously reported development of peripheral neuropathy in this rodent model (39, 47–49).

After confirming our experimental group had a phenotype consistent with diabetes, we tested the long-term effects of performing sciatic nerve blocks with bupivacaine and liposomal bupivacaine on tactile allodynia, thermal hyperalgesia, nerve conduction velocity, and spontaneous locomotor activity. Bupivacaine concentration (0.5%) was chosen based on common clinical practice and used an equal volume of liposomal bupivacaine. Importantly, our data reveal that even in control animals, treatment with bupivacaine resulted in long-term tactile allodynia, thermal hyperalgesia, and slowed nerve conduction velocity, while liposomal bupivacaine did not. Additionally, control animals treated with bupivacaine were less mobile, moved slower, and reared less when compared to saline and liposomal bupivacaine treatment. Our data also show that diabetes led to behavioral and neurophysiological changes consistent with worsening function for all local anesthetic treatment groups. Less clear are the exacerbating effects of bupivacaine on diabetic nerves. In diabetic animals, bupivacaine treatment led to worse tactile allodynia and slower NCV when compared to diabetic animals treated with saline or liposomal bupivacaine. However, no evidence of exacerbated nerve injury was found in other outcomes tested. Given the significant negative effect of diabetes alone, our study is likely underpowered to detect additional exacerbation by bupivacaine.

The safety of peripheral nerve blocks in patients with diabetic peripheral neuropathy has been discussed repeatedly in the clinical literature (5, 13, 14, 25, 32, 54–56). Studies have demonstrated increased sensitivity and prolonged block duration in animals and patients with diabetic peripheral neuropathy (28), and pre-clinical models have demonstrated increased local anesthetic toxicity in rodent models of diabetes (25, 57, 58). There is a growing number of reports demonstrating the neurotoxic effects of bupivacaine (22, 25, 39, 59). Specifically, bupivacaine has been identified as an independent risk factor for paresthesia/dysesthesia after use (22). While the overall reported incidence of acute nerve dysfunction after a peripheral nerve block is low, there is no long-term clinical data regarding the onset and progression of peripheral neuropathy in diabetic patients exposed to regional anesthesia. This is likely because following long-term outcomes is difficult in a clinical setting, given slow disease progression, lack of long-term follow-up, and evolving clinical practices. To our knowledge, this is the first study to describe the long-term effects of bupivacaine and liposomal bupivacaine in an experimentally induced model of diabetes. Given the increasing number of patients with diabetes who are presenting for surgical procedures, future research should continue to optimize our approach toward improved short- and long-term outcomes.

The mechanism of bupivacaine toxicity is not completely understood. It is known that the toxic effects appear to be time- and concentration-dependent. To perform a peripheral nerve block, relatively large amounts of local anesthetics are deposited perineurally to generate a concentration gradient of local anesthetic into the nerve. Furthermore, some patients undergo repeated nerve blocks, sustained nerve blocks with nerve catheters, or are exposed to adjuvants to increase the duration of analgesia. For these reasons, we performed repeated nerve blocks using both a short- and long-acting formulation of bupivacaine. Interestingly, strong clinical suspicion for increased nerve injury risk has led The American Society for Regional Anesthesia to suggest that clinicians use lower concentrations, avoid prolonged exposure, or use alternative methods in “at-risk” patients (56). Liposomal bupivacaine is a currently approved formulation marketed to provide ease of use in a single-shot nerve block with prolonged analgesic effectiveness (60). Interestingly, efficacy studies comparing bupivacaine and liposomal bupivacaine have revealed that liposomal bupivacaine may be less toxic than standard bupivacaine (30). Previously, we have demonstrated that liposomal bupivacaine did not exacerbate motoneuron death or delay functional recovery after a peripheral nerve injury (23). In the present study, we similarly found that liposomal bupivacaine caused less evidence of peripheral neuropathy in both control and diabetic animals. However, these findings only suggest that the liposomal formulation of bupivacaine should be further studied. Perhaps the slow release of bupivacaine from a liposomal formulation may expose nerves to lower concentrations over a given time or result in lower systemic exposure. Future studies should continue to focus on understanding the mechanism(s) of local anesthetic neurotoxicity and optimizing agents used for peripheral nerve blocks.

Nonetheless, the present results must be interpreted cautiously, and several limitations must be addressed. First, the differences between rodents and humans in terms of diabetes and neuropathy are unknown. Although the injection of streptozotocin is a common model of diabetes in rats, one should be cautious when extrapolating these results to humans. We used the von Frey and Hargreaves methods as surrogates for pain behavior since they are currently the most widely used and published methods for quantifying the severity of allodynia and hyperalgesia. However, we recognize that these are reflexive pain behaviors, and better assessments are needed that can measure in a more ethological manner. To overcome this limitation, we used the open-field test on a small cohort of animals in the present study to observe locomotor behavior. While others have also used this outcome measure for pain-like behavior, its validity is unclear (61). Cho et al. (62) reported reduced rears and distance traveled in a high-dose STZ model of diabetic neuropathic pain in mice. Future studies will utilize additional assessments that may reflect functional and affective pain-related features such as grimace scales and/or burrowing behaviors.

We believe that peripheral nerve blocks should continue to be a therapeutic option for patients, including those with diabetic peripheral neuropathy; however, a better understanding of the long-term risks of functional impairments is needed. Despite decades of clinical practice, research has not kept pace with the science needed to establish long-term safety. There is abundant space for further research to better understand the safety of peripheral nerve blocks in patients with diabetic peripheral neuropathy. Future studies can help elucidate the mechanism of neurotoxicity from local anesthetics, impact of tissue injury at the time of nerve block (surgical procedure for which acute pain is being treated), role of sex differences and age, and test alternative therapies.

This study revealed that bupivacaine has long-term negative effects on the development of tactile allodynia and thermal hyperalgesia and results in the slowing of nerve conduction velocity in both control and diabetic animals. It is premature to extrapolate these findings to clinical guidance; however, given the escalating growth of the diabetic patient population who are already vulnerable to perioperative complications and progressive debilitation, research addressing long-term consequences, mechanisms of toxicity, and search for less toxic peripheral nerve block techniques should continue to be prioritized.