When used as an anaesthetic opioids may caue muscle rigidity. - Opioid induced rigidity is characterized by increased muscle tone progressing at times to severe stiffness. - When the rigidity increases and involves thoracic and abdominal muscle, it may impair ventilation. - Rigidity of the abdominal and/or thoracic muscles was thought to underlie opioid induced impairment of spontaneous or controlled ventilation. The precise mechanism by which opioids can cause muscular rigidity is not clearly understood. Muscle rigidity is not due to direct action on muscle fibres because it can be decreased or prevented by pretreatment with muscle relaxants. In addition, opioid induced muscle rigidity is not associated with increase in creatinine kinase a .finding suggesting that little or no muscle damage occurs during this period. - Opioids do not have any significant effect on neuromuscular conduction and result in only minimal depression of monosynaptic reflexes associated with muscle stretch receptors. - Some investigators have suggested that opioids produce rigidity by altering dopamine concentrations within the striatum.s of the brain. Rigidity may occur due to the result of stimulation of GABA ergic interneurons which can be blocked by lesion in the striatum. Continuous research and data seems to indicate that the main mechanism responsible for opioid induced muscle rigidity is stimulation of it receptors. - Opioids appear to produce their physiological effects by.binding to at least three types of opioid receptors the mu (p), delta (8) and Kappa (x) receptors. - Muscle rigidity occurs after administration of supra analgesic doses of potent p preferring opioids. - The present data demonstrates that whereas systemic opiate induced muscle rigidity is primarily due to the activation of central mu receptors, supraspinal delta and kapp receptors may attenuate this effect. Anaesthesia review by Michelle Bowman Howard states (p134) -All opioids given in high doses may cause muscular rigidity. It is most likely related to I.i receptors in the caudate nucleus. It is more commonly seen with Fentanyl.