Enhanced recovery after surgery (ERAS) is a recognized, evidence-based and patient-centered clinical pathway that has an array of benefits. Minimally invasive techniques, a cyclopedic pain management plan and precise administration of anesthetics, which will render patients a rapid and comfortable recovery if executed correctly, followed by early mobilization and discharge.

Pain management practice is traveling through a paradigm shift as opioid crises arise in the western countries. Opioid-based pain control is being disarmed and replaced by multimodal analgesia (MMA) and becoming the mainstay strategy. Opioids are increasingly being reserved as rescue medications. MMA target different parts of the nociceptive pathway, preventing its wind-up during surgery. Decreased firing of the nociceptive neurons may be linked to lower postoperative pain scores or even the suppressing chronic pain incidence. In our ERAS pathway, we implement erector spinae plane block (ESPB) as the main analgesic firepower.

As postoperative pain decreases, an observed rising complaint is "sng", or soreness, in native Taiwanese in our ERAS spine patients. It is very different from the nociceptive "pain" we are familiar with. Patients avoid movements if it causes pain, but they tend to stretch or adjust postures if sngception dominates. The term "sngception" has been proposed in 2018 by Taiwanese scholars. It is believed to be a sense of acidosis, possibly within the muscles. Acidosis and associated pain are well documented, such as in muscle aches from exercise, cancer or diabetic ketoacidosis. The underlying mechanism is yet to be established but does not entirely overlap with nociception. There are numerous similarities of sngception in our patients and sngception:

1. a sensation different from nociception,
2. usual painkillers are ineffective, suggesting an alternative route of transmission,
3. relieved by movement,
4. inflammation and acidosis in the vicinity of surgical wound. In this study, we intended to characterize sngception by observing various perioperative factors, as well as the short- and long- term outcomes they bring. This will be done through a detailed sngception and pain trajectory analysis. Only when we know the main causative factor(s), we can design treatment plans toward guarding against sngception. This further improves the quality of postoperative recovery and safety as less opioids may be required as rescue medications.

Inclusion Criteria:

1. Patients undergoing routine ERAS protocol for spine surgeries
2. American Society of Anesthesiologists Physical Status (ASA-PS) I~III

Exclusion Criteria:

1. Communication issues that preclude proper preoperative counselling for ERAS and study design.
2. Active alcohol or drug addiction.
3. Pregnancy
4. Allergy to main anesthetics that preclude the use of routine ERAS anesthesia management.
5. Patient refusal to participate in study.
6. Emergent operations.

Patients receiving routine ERAS management for spine surgeries. All patients will be screened for eligibility.