VRS is commonly performed under different techniques of RA; PBB without sedation provides excellent postoperative analgesia depending on the anaesthetic technique and local anaesthetic solutions used [35, 36]. Nevertheless, in selected subgroups of patients, intolerable pain perception during surgery negatively influenced patient satisfaction from the surgery in some reports [35; 37].
Therefore, in some elderly patients who are unable to co-operate during VRS under RA, those refuse to consent for RA alone, and those with contraindications to RA , GA is performed to ensure adequate immobilisation on the operating table for the comfort of the surgeon. This is in spite of an additional 20% cost incurred with GA compared to RA with monitored anaesthesia care (MAC) . Because intraoperative use of OA during GA carries a risk of PONV , different RA techniques are added to GA to reduce intraoperative OA requirement, and reduce the incidence of PIPP.
Surgical Pleth Index (SPI)
Addition of RA to GA was reported to reduce the necessity of rescue OA administration, but did not eliminate it completely. Therefore, the aim of the current study was to assess the utility of SPI to guide OA administration in case of intraoperative afferent nociceptive stimulation due to incomplete effect of preventive RA.
Volatile anaesthetics administered during GA have been shown to blunt the haemodynamic response to nociceptive stimulation . During surgical manipulation, patients receiving volatile anaesthetics may experience afferent nociceptive stimulation without change in haemodynamic parameters. The anaesthesiologist may not administer rescue OA in the absence of changes in blood pressure and heart rate; this may lead to PIPP due to central sensitization. Struys et al.  have shown that SPI values change in response to afferent nociceptive stimulation; SPI monitoring constitutes a better measure of nociception/anti-nociception balance than haemodynamic parameters including heart rate and blood pressure variations in optimising intraoperative OA titration . SPI value is derived by finger plethysmography and displayed on the screen; hence, it is simple to use and does not require any preoperative preparation. Bergmann et al.  reported that SPI guidance during GA resulted in rational titration of rescue doses of OA and a reduction in the cumulative dose of OA administered during GA. Ledowski et al.  showed that the change in SPI value after a bolus of rescue FNT enabled monitoring of intraoperative titration. Hence, SPI guidance of rescue OA administration also helps to monitor the effectiveness of rescue bolus doses of FNT. Based on these studies, we hypothesised that the use of SPI in conjunction with RA techniques could produce lead to improved perioperative outcomes with a lower incidence of OCR, haemodynamic instability, PIPP and PONV.
Upton et al.  utilized Anti-Nociception Index (ANI) guidance to administer FNT intraoperatively during GA with sevoflurane for lumbar discectomy and laminectomy. They observed that a more objective, ANI-guided intraoperative FNT administration resulted in decreased perception of pain intensity in the immediate postoperative period compared to a standard practice of FNT administration based on observation of haemodynamic fluctuations and anaesthesiologist’s judgement. On the contrary, Wennervirta et al.  showed that the addition of RA to GA was more efficient in providing perioperative analgesia than SPI-guided OA titration in patients who underwent GA combined with brachial plexus block (BPB).
Preventive regional analgesia
Different types of preventive regional analgesia techniques were utilised to ensure smooth postoperative recovery. Analgesia initiated before a nociceptive afferent surgical stimulation is considered to be more effective than analgesia commenced afterwards, as it suppresses the afferent nociceptive barrage perioperatively; this is the concept of pre-emptive analgesia . The action of local anaesthetics (LAs) is a result of a reversible block of sodium channels that prevents the propagation of painful afferent nerve impulses from the cornea, conjunctiva and sclera .
Although addition of PBB to GA was reported to diminish the requirement for intraoperative rescue OA, techniques of RA are not free from potential complications. During the course of the current study, no adverse events were observed; nevertheless, PBB was reported to result in transient vision impairment, which may be an unwelcome, distressing experience for patients postoperatively [40, 41], whereas TA may cause local allergic reactions. After PBB, systemic LA toxicity was reported to be likely to induce cardiac arrhythmias, an increase in mean arterial blood pressure and heart rate  or a severe decrease in systolic blood pressure . Perioperative haemodynamic fluctuations constitute a subsequent risk factor of destabilization of atherosclerotic plaques that may result in life-threatening cardiac and cerebrovascular events . Central retinal vein occlusion , brainstem anaesthesia , transient complete visual loss and a partial third nerve palsy , pulmonary oedema , ocular explosion  and generalized tonic-clonic seizures  have been reported following PBB due to LA toxicity.
Postoperative Intolerable Pain Perception (PIPP)
VRS may cause PIPP. In a study by Fekrat S et al. , 56% of patients complained of some eye pain after VRS, whereas 48% requested an analgesic within 5 hours after surgery, and 27% percent of patients required OA. Schönfeld CL et al.  found GA with PBB using 0.75% ropivacaine with 75 IU of hyaluronidase in a volume of 5 ml superior for the prevention of PIPP compared to a volume of 1 or 3 ml. Sixty percent of patients receiving 5 ml experienced no PIPP after one hour of VRS in this study. Ghali et al.  observed reduced PIPP in patients receiving PBB combined with GA compared to GA alone in patients undergoing VRS with scleral buckling. In the PBB group, 7% of patients reported acute postoperative pain perception defined as a score of >7 on the Visual Analogue Scale (VAS) compared to 30% of patients in GA group. Rescue doses of tramadol and total diclofenac consumption administered for moderate pain perception (VAS >4), was also significantly higher in the GA group compared to the group that had GA combined with PBB.
In our study, among 90 patients included in the final analysis, 14 patients (15.7%) complained of moderate pain and one patient (1.1%) reported PIPP in the immediate postoperative period in the recovery room. Interestingly, performance of neither PBB nor TA resulted in a significant decrease in PIPP expressed by NRS values; PIPP was reported by four patients in PBB group (13.3%), six in TA group (20%) and five in the GA group (16.7%).
Jaichandran et al.  observed the efficacy of PBB, expressed as adequate globe akinesia, to be 20–60%, depending on the LA mixture used; hence, on average, half of PBBs may theoretically provide insufficient analgesia. This is because estimation of sensory block by abolition of the corneal reflex may not always imply that there is no sensory perception of surgical manipulations. Therefore, supplementation of PBB with intraoperative intravenous OA during GA under SPI guidance, may be necessary in case of partly or completely failed block [35, 37]. This may play a key role in the utility of SPI guidance for supplemental FNT administration. Introduction of ultrasound-guided, perineural stimulation-directed (dual guidance) interscalene BPB increased its efficacy from 41.46% with the perineural stimulation technique to 80.43% with the dual guidance technique . Hopefully, similar to dual guidance BPB, ultrasound-guided PBB combined with GA in patients undergoing VRS may in the future , improve the efficacy of PBB by more precise needle placement and observation of LA deposition at the target destination. This may reduce the necessity of intraoperative rescue OA administration using SPI guidance, as in the current study the demand for rescue FNT was the lowest in the PBB group compared to other groups, although this was not statistically significant.
Overall, in our study, we did not observe marked improvement in perioperative outcomes in patients receiving preventive PBB or TA, compared to GA alone, although there was no statistically significant difference in the dose of intraoperative FNT administered.
In the current analysis, with the use of SPI-guided FNT administration during GA, afferent nociceptive stimulus was reflected as an intraoperative delta SPI >15 compared to the baseline value. Inadequate RA detected by SPI monitoring resulted in more efficient suppression of central sensitisation. As a result, the use of preventive RA using either PBB or TA did not influence the incidence of PIPP; with a similar incidence in all three groups.
We believe that preventive RA, with the added risk of rare, but serious complications, seems no longer justified in combination with GA to reduce the incidence of PIPP after VRS.
A similar result was observed by Bayerl et al.  with preoperative RBB using bupivacaine 0.5% and mepivacaine 1% in combination with GA in patients undergoing VRS. They induced GA with fentanyl and propofol and maintained anaesthesia using propofol and remifentanil administered by observing haemodynamic parameters and anaesthesiologist judgement. They observed no advantage by combining RBB with GA as compared to GA alone with postoperative analgesia in the early postoperative period; however, in the GA group, cyclooxygenase inhibitors or non-steroidal anti-inflammatory drugs were infused before emergence, which may have influenced the incidence of PIPP.
Haemodynamic instability during GA constitutes a serious risk factor of the development of cardiac and cerebrovascular events . During VRS (stage 3) significantly lower mean values of SAP, MAP and DAP were observed in PBB group compared to GA and TA group, whereas maximum values of SAP, MAP and DAP were observed to be lower in patients allocated to PBB group. Although such differences seem to bear little clinical significance (on average, not more than 10 mmHg and 10 beats/min), and no complications were observed, PBB combined with GA with SPI-guided FNT administration resulted in the most stable conduct of GA. Nevertheless, the use of SPI-guided intraoperative analgesia with FNT in all groups could have resulted in stable haemodynamics during VRS, with a tendency for the most stable heart rate and blood pressure in the PBB group.
Every episode of PONV leads to a rapid increase in intraocular pressure, which may even lead to wound dehiscence; besides, it impairs patient satisfaction in the recovery period. In recent literature, the incidence of PONV after intraocular surgery is 18–56% [51, 52]. According to Nitahara et al. , who analysed risk factors of PONV after VRS, the overall incidence of nausea and vomiting was 15% and 23% respectively. Fekrat et al.  reported PONV in 16% of patients, most likely occurring in those receiving OA. Shende et al.  observed the incidence of PONV in 12 out of 30 patients receiving GA with PBB for retinal detachment surgery. In many previous reports, preventive analgesia techniques were utilised to reduce intraoperative OA administration which reduced the rate of PONV. Ghali et al.  observed PONV in 10 out of 30 patients receiving GA only (33%) and in 3 patients out of 30 (10%) in whom additional PBB was performed. In all abovementioned studies, intraoperative FNT administration was performed based on observation of haemodynamic fluctuations and the anaesthesiologist’s judgement. In our study, only 9 patients out of 90 (9%) developed of PONV. The incidence of PONV was lowest in the PBB group compared to the TA and GA groups [1/30 (3.3%) vs. 4/30 (13.3%) vs. 4/30 (13.3%), respectively; p = 0.38], although this was not statistically significant. We believe that appropriate administration of intraoperative rescue FNT under SPI guidance probably prevented an excessive dose which resulted in a lower incidence of PONV compared to abovementioned data from literature.
Oculocardiac reflex is a trigeminal-vagal reﬂex triggered by intraoperative pressure on the eyeball, traction of extra-ocular muscles and is defined as rapid decrease in heart rate by over 20%  that may lead to serious cardiac arrhythmias with haemodynamic instability (bradycardia, ectopic beats, nodal rhythm, ventricular ﬁbrillation or asystole) and therefore constitutes an intraoperative complication of a great concern.
Ghali et al.  observed OCR in 8 out of 30 patients (27%) receiving GA alone and in 2 patients out of 30 (7%) in whom additional PBB was performed.
Shende et al.  compared patients undergoing elective retinal detachment repair under GA alone and in combination with PBB and observed an incidence of OCR in 9 patients out of 30 in PBB group (30%) and in 21 out of 30 patients in GA-only group (70%).
Contrary to literature data, in our study the use of SPI-guided FNT administration resulted in OCR 7 out of 90 patients included in the final analysis (7.78%). The incidence of OCR was observed to be similar in all groups [PBB: 2/30 (6.7%) vs. TA: 2/30 (6.7%) vs. GA: 3/30 (10%); p = 0.38].
In our view, adequate administration of intraoperative rescue OA using SPI-guided FNT administration probably led to stable intraoperative analgesia and therefore resulted in a low incidence of PONV compared to abovementioned data from literature. SPI-guided FNT administration during GA reduced afferent nociceptive stimulation evoked by intraoperative muscle traction, which could have also prevented central hyperexcitability , thus reducing the incidence of OCR.