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The investigators will conduct a retrospective cohort study of patients who have undergone i.v. lidocaine infusions in the previous 2 years for suspected neuropathic pain of heterogeneous origin. The investigators hypothesize that the analgesic response to intravenous (i.v.) lidocaine will be bimodal with clear responders and clear non-responders. The investigators also hypothesize that more refractory patients, who have failed previous multimodal analgesic therapy, will be less likely to respond to i.v. lidocaine. The investigators goals are to report what percentage of patients will achieve relief, the degree of relief that can be expected, and identify the type of patients who will most likely to benefit from systemic lidocaine. The investigators secondary objective is to report the correlation between infusion rates and patterns of infusion rate adjustments with regard to efficacy and adverse effects.

HYPOTHESIS On the basis of investigators' clinical experience, The investigators hypothesize that the analgesic response to intravenous (i.v.) lidocaine will be bimodal with clear responders and clear non-responders. The investigators also hypothesize that more refractory patients, specifically patients with pain of longer duration and patients who have failed previous multimodal analgesic therapy, will be less likely to respond to i.v. lidocaine. To test the hypotheses, we will identify the distribution of pain relief in the enrolled cohort. The investigators will define responders as patients achieving Numerical Rating Score (NRS) reductions of 30% or greater based on literature defined criteria for meaningful reductions in pain scores.The investigators goals are to report what proportion of patients will achieve relief, the degree of relief that can be expected, and identify the type of patients who will most likely to benefit from systemic lidocaine. The investigators also hypothesize that degree of pain relief in a responder to i.v. lidocaine will be translated in the form of concomitant decrease in quantitative measures of PHQ-9 scales and Oswestry index scales. The investigators secondary objective is to report the correlation between infusion rates and patterns of infusion rate adjustments with regard to efficacy and adverse effects.

DESIGN AND SETTING The investigators will conduct a retrospective cohort study of patients who have undergone IV lidocaine infusions for suspected neuropathic pain. All patients will be recruited from the Pain Management Clinic at Henry Ford Health System.

STUDY PARTICIPANTS The investigators will secure approval from the Institutional Review Board and ethics committee of the institution. The investigators will review patients' information and their treatment data through the electronic medical record (EPIC) of the pain management clinic. The investigators will screen sequential charts of previous two years to identify patients who have undergone i.v. lidocaine infusions for Neuropathic Pain of heterogeneous origin. Inclusion criteria will include: neuropathic pain suspected by the board certified pain specialist assessing the patient based on the presence of allodynia, hyperalgesia, hyperpathia and hyperesthesia or hypesthesia. The exclusion criteria will include patients that refused subsequent i.v. lidocaine infusions or were lost to follow up.

DATA COLLECTION AND INDEPENDENT VARIABLES Independent variables examined will include the patient's age, sex, body mass index, indication, duration of pain problem, dates of treatment, dose, infusion time, adverse events, action taken as a result of an adverse event, duration of treatment, reason for discontinuation, pre and post infusion NRSs of pain, vital signs that were taken pre-infusion, during infusion and post-infusion, previous trials of alternative pain medication and current or previous substance abuse.

STATISTICAL ANALYSIS Initial analysis of patient's age, sex, pain severity, pain duration, number alternative medication tried, with comparison between lidocaine responders and non-responders will be done using Student t test or χ2 test as appropriate. All statistical tests will be 2-sided. A p value less than 0.05 will be considered statistically significant. Normality of data will be assessed with Kolmogorov-Smirnov test. Odds ratios with 95% confidence intervals (CIs) will be calculated. SAS version 9.4 (SAS Institute, Cary, NC) will be used for all analysis.

Inclusion Criteria:

• patient with neuropathic pain
• patient enrolled for lidocaine infusion

Exclusion Criteria:

• refusal to consent
• lost to follow-up

Patients' information and their treatment data will be identified through the electronic medical record (EPIC) of the pain management clinic. The investigators will screen sequential charts of previous two years to identify patients who have undergone i.v. lidocaine infusions for Neuropathic Pain of heterogeneous origin (e.g., post-herpetic neuralgia, post surgical nerve injury, lumbar radiculopathy).

Inclusion criteria will include: neuropathic pain suspected by the board certified pain specialist assessing the patient based on the presence of allodynia, hyperalgesia, hyperpathia and hyperesthesia or hypesthesia.

The exclusion criteria will include patients that refused subsequent i.v. lidocaine infusions or were lost to follow up.