Postoperative analgesia by intra-articular neostigmine in patients undergoing knee arthroscopy

Background: Recently, the spinal administration of neostigmine was shown to produce a dose-dependent analgesia. However, this analgesia is limited by adverse effects. The purpose of this study was to examine the analgesic action of peripheral muscarinic receptors by administering intra-articular neostigmine after operation in patients undergoing knee arthroscopy. Methods: Sixty patients (classified as American Society of Anesthesiologists status I or II) having arthroscopic meniscus repair during general anesthesia were randomized to receive, in a double-blind manner, after operation 125, 250, or 500 μg intra-articular neostigmine; 2 mg intra-articular morphine; or as control groups intra-articular saline or 500 μg neostigmine given subcutaneously (SC). Visual analog pain scores (VAS), duration of analgesia as defined by first demand for patient-controlled analgesia by morphine, and subsequent 48-h consumption of morphine were evaluated. Results: Intra- articular (500 μg) neostigmine resulted in significant VAS reduction 1 h after injection compared with patients given intra-articular saline and with those given intra-articular morphine. Analgesia lasted longer after 500 μg intra-articular neostigmine (350 ± 126 min) compared with intra-articular morphine (196 ± 138 min; P < 0,05) or with the control groups (intra- articular saline, 51 ± 11 min; SC neostigmine, 46 ± 8 min; P < 0,05). The need for supplementary analgesia was significantly higher in control groups than for patients given intra-articular morphine or 500 μg intra-articular neostigmine. No significant analgesic effects were observed for the two lower doses of intra-articular neostigmine. Among all study groups, no adverse effects were observed. Conclusions: Intra-articular injection of the acetylcholinesterase inhibitor neostigmine produced a moderate but significant analgesic effect. Several mechanisms such as the hyperpolarization of neurons, reduction in the release of pronociceptive neurotransmitters, or activation of the nitric oxide-cyclic guanosine monophosphate pathway might mediate this peripheral cholinergic antinociception by elevating endogenous acetytcholine.