Postoperative pain (POP) following abdominal surgery can vary from a few hours to several days. This acute unrelieved pain can become chronic, requiring patients to take analgesics on an almost daily basis for comfort [1] . Postoperatively, more than two-thirds of patients report moderate to severe pain, but less than half of them report adequate pain relief. Many patients are even very dissatisfied with their pain management. This pain is correlated with increased morbidity and mortality, longer length of stay, delayed ambulation and regaining of autonomy, and a risk of pain becoming chronic [2] .

The French Society of Anaesthesia and Critical Care (SFAR) recommends prescribing a strong opioid (morphine or oxycodone) systemically for severe postoperative pain, or if weaker analgesics are not strong enough to relieve patients [3] . However, the use of systemic morphine has many disadvantages (respiratory distress, paralytic ileus, pruritus, etc.) but also carries the risk of developing hyperalgesia and chronic post-surgical pain [3] [4] .

Enhanced recovery after surgery (ERAS) programs have made morphine sparing one of their keystones, with the aim of limiting the use of systemic opioids. The aim is to limit the use of systemic opioids, through the use of co-analgesics and/or regional analgesia techniques (multimodal analgesia) [5] [6] . Morphine sparing reduces the length of hospital stay and helps prevent serious complications, particularly respiratory and digestive, such as paralytic ileus [7] , as well as urinary retention, somnolence and postoperative delirium in the elderly [8] .

Pain after major laparoscopic abdominal surgery is intense, but relatively short-lived compared with open surgery. It is a major component of the stress response, and if not adequately treated, can have adverse consequences, hence the importance of appropriate analgesia.

Numerous multimodal analgesia techniques have been developed, such as intravenous lidocaine [9] [10] , abdominal trunk blocks [11] (ultrasound-guided or under direct laparoscopic guidance) or intraperitoneal injection of local anesthetic [12] . These methods provide adequate postoperative analgesia, but often require systemic administration of morphine.

Epidural analgesia is recommended for laparotomy colonic surgery [5] . However, its use in laparoscopic surgery is associated with prolonged hospital stay due to delayed mobilization [13] [14] .

Intrathecal morphine injection (ITM) appears to have a significant analgesic effect [15] and could be an interesting method in this type of surgery. Indeed, intrathecal morphine has limited systemic absorption due to its hydrophilic properties, and therefore a minor effect on intestinal motility, unlike intravenous morphine. In addition, the doses of morphine required for analgesia are lower with this technique.

Reported disadvantages of this technique include the risks associated with intrathecal injection, pruritus and delayed respiratory depression [16] [17] . However, when a low dose of morphine is used, there does not appear to be any more respiratory depression than with systemic opioids [18] .

ITM therefore appears to be an attractive strategy for patients undergoing laparoscopic colonic resection. There are currently few studies evaluating ITM in major laparoscopic surgery. The aim of this study was to compare the efficacy of intrathecal morphine on postoperative pain after laparoscopic colectomy versus conventional analgesia at the Hôpital Nord in Marseille.

This study was conducted to compare the analgesic efficacy of intrathecal morphine (ITM group) versus conventional analgesia using systemic morphine (control group). We observed a decrease in pain intensity in the ITM group compared with the control group (p < 0.0001*), a decrease in morphine consumption postoperatively for cumulative doses and on day zero (p < 0.0001*), more side effects in the control group than in the ITM group (p < 0.0001*) and a slight trend towards shorter length of stay in the ITM group than in the control group, with no significant difference (p = 0.054).

Our cohort size (193 patients) falls within the range of studies in Koning’s meta-analysis [18] , which was between 80 and 150 patients. Chronic inflammatory bowel disease (CIBD) and neoplastic pathologies were the predominant underlying conditions characterized by chronic pain and therefore at high risk of chronic postoperative pain [19] , underlining the importance of better pain management, notably with intrathecal morphine. The dose of intrathecal morphine used (100 - 300 µg, average 212.5 µg ± 73) is recommended to avoid side effects [16] [20] . Ketamine for the prevention of postoperative hyperalgesia was predominantly administered in the ITM group, in accordance with the SFAR guidelines [21] , but also represents a bias in this study. Postoperative pain management in this cohort complies with the SFAR and the French Society of Digestive Surgery recommendations, giving a Grade 1+ (Strong) to the prescription of a multimodal analgesia technique favouring non-morphine analgesic agents and/or a regional anaesthesia technique [5] . Studies on the intraoperative use of intravenous lidocaine by electric syringe [9] [10] , and TAP block [11] [12] show that these techniques provide adequate postoperative analgesia, but often require systemic morphine administration.

On the primary endpoint of postoperative pain intensity (numerical scale) at 0, 30, 60, 90, and 120 minutes from awakening, and the maximum pain scale from D₀ to D₅, the ITM protocol showed significant advantages (NS < 3, p < 0.001*), as reported by other authors [22] . This effective analgesia over the first 24 hours was noted in Meylan’s meta-analysis [17] . However, this analgesic efficacy became less significant with each passing day, until it was almost non-existent at D₅. This is in line with the study by Wongyingsinn et al. [23] which, in addition to effective analgesia over the first 24 hours, also describes a low pain threshold up to 72 hours in the ITM group, albeit with an increased need for analgesics in the latter.

In the ITM group, there was a significant reduction in total morphine consumption at D0 compared with the control group. These results corroborate those of Wongyingsinn [23] , who found lower morphine consumption in the ITM group, i.e. between 3 - 15 mg in the ITM group vs. 23 - 47 mg in the PCA group for laparoscopic colonic resection. Koning et al. [18] , in their meta-analysis of 40 studies of ITM in abdominal surgery, found a low rate of morphine consumption in patients receiving ITM. In our cohort, there was no uniformity of intrathecal sufentanil and/or clonidine doses, due to the absence of a well-defined protocol, even though this practice is recommended [20] .

The cumulative dose of morphine consumed from D₀-D₅ was significantly lower in the ITM group, i.e. 6 mg (±12) vs. 31 mg (±24) in the control group, in favour of the improved rehabilitation after surgery program in abdominal surgery. Morphine sparing is the keystone of improved rehabilitation after surgery [5] [6] [24] . The length of hospital stay was not significantly different between the two groups, unlike in Koning’s meta-analysis [18] , due to the power of the study. Complications of ITM are dose-dependent [20] .

In our cohort, side effects were more frequent in the control group due to the high dose of morphine. Respiratory distress was more frequent in the control group (35%) than in the ITM group (6%). Controversy remains over the correlation between the dose of morphine administered intrathecally and the risk of occurrence of delayed respiratory distress. This distress is due to the cephalic migration of morphine, which occurs after 6^ième hours and persists until 12^ième hours on average [25] . The meta-analysis by Meylan et al. [17] demonstrated the efficacy of ITM in major and abdominal surgery within the first 24 hours postoperatively, despite a higher rate of respiratory complications in the ITM group, probably due to the wide variability in morphine doses used in the studies (100 - 4000 µg). This was not the case in our cohort, due to the low dose of intrathecal morphine used (100 to 300 µg). In a recent study of over 600 patients, Merchea et al. [26] reported the efficacy and safety of ITM in colorectal surgery for low intrathecal morphine doses (mostly below 200 µg). Their rate of respiratory distress was only 0.2. In a meta-analysis published in 2020, Koning et al. [18] confirmed the efficacy of ITM in reducing intra- and postoperative morphine consumption, and that the risk of respiratory distress appeared at doses >1000 µg. Guidelines [20] recommends intrathecal morphine doses of between 100 and 300 µg for effective analgesia, and not to exceed 300 µg to prevent the risk of respiratory distress. This 300 µg threshold is corroborated by the work of Gehling et al. [16] and our own. The occurrence of PONV was 18% in the control group versus 3% in the ITM group, in contradiction with the meta-analysis by Koning et al. [18] and the study by Wongyingsinn et al. [23] , both of which found no significant difference. The explanation lies in the low doses of morphine used in our study, as recommended by experts [20] . There was no significant difference between the two groups for the occurrence of ileus and urinary retention as described in the literature [18] [23] . In our cohort, we recorded a spinal analgesia failure rate of 3.22%, in line with the 1% to 6% reported in the literature [26] [27] .

The results of our study suggest that intrathecal morphine in laparoscopic colectomy provides effective analgesia for acute postoperative pain, reduced postoperative morphine consumption and fewer morphine-related side effects than conventional analgesia.

In view of the above-mentioned advantages, it would be interesting to carry out additional prospective and more robust studies in resource-limited settings to confirm the benefits, and the impact on reducing the cost of analgesia, and to establish evidence-based protocols.

The present study confirms the analgesic efficacy of morphine spinal analgesia in abdominal surgery as described in the literature [28] - [30] , as well as its contribution to improved rehabilitation after surgery program in abdominal surgery, the key to which is to minimize systemic morphine consumption [5] [6] . It also confirms the absence of major side effects in this simple technique, which has a high success rate.

The monocentricity and retrospective nature of this study may have led to a selection bias, even though the basic characteristics of the patients were comparable. Patients in the ITM group were therefore operated on more recently than those in the control group. This confounding bias is limited by the fact that over the three years of data collection, anaesthetic management (in addition to ITM) and surgical management of these patients remained identical. As the practice of ITM in our team was not protocolized, morphine dosage and the addition of Sufentanil or Clonidine were left to the free will of the anaesthetist. We limited, but did not eliminate, the information bias associated with missing data by using precise markers to diagnose postoperative complications (prescription of an antiemetic or oxygen, evacuating bladder catheterization), but the retrospective nature of our study means that we cannot guarantee the completeness of the collection of these complications.

There was also a co-intervention bias due to the higher proportion of Ketamine use in the ITM group, linked to the retrospective nature of the study. However, this bias was negligible, insofar as the proportion of patients in the control group was not negligible, but this did not prevent the occurrence of postoperative pain.

We would like to thank the entire team of anaesthetic of Marseille’s Nord Hospital.

Lionel Diyamona: Study design, drafting of the manuscript and data collection;

John Nsiala, Bruno Pastene, Manon Colin, Marc Leone: study design and correction of the manuscript;

Wilfrid Mbombo: Study design and drafting of the manuscript;

All other authors: Reading and correction of the manuscript.