Opioids can increase muscle tone and may cause muscle rigidity. The incidence of rigidity noted with opioid anesthetic techniques varies significantly because of differences in dose and speed of opioid administration, the concomitant use of N2O, the presence or absence of muscle relaxants, and the patient's age. Opioid-induced rigidity is characterized by increased muscle tone that sometimes progresses to severe stiffness with the potential for serious problems. Clinically significant opioid-induced rigidity usually begins just as or after a patient loses consciousness. Mild manifestations of rigidity, such as hoarseness, can occur in conscious patients. Vocal cord closure is primarily responsible for the difficult ventilation with a bag and mask that follows the administration of opioids. The precise mechanism by which opioids cause muscle rigidity is not clearly understood. Muscle rigidity is not the result of a direct action on muscle fibers because it can be decreased or prevented by pretreatment with muscle relaxants. Mechanisms of opioid-induced muscle rigidity involving the CNS have been postulated. Pharmacologic investigation using selective agonists and antagonists suggest that systemic opioid-induced muscle rigidity is primarily caused by activation of central m receptors, whereas supraspinal d1 and k1 receptors may attenuate this effect. Some aspects of opioid-induced catatonia and rigidity (increased incidence with age, muscle movements resembling extrapyramidal side effects) are similar to Parkinson's disease and suggest similarities in neurochemical mechanisms. Patients with Parkinson's disease, paicularly if they are inadequately treated, may experience reactions such as dystonia following opioid administration. Pretreatment with or concomitant use of nondepolarizing muscle relaxants can decrease the incidence and severity of rigidity. Induction doses of sodium thiopental and subanesthetic doses of diazepam and midazolam can prevent, attenuate, or successfully treat rigidity. Rapid administration of larger doses of opioids (paicularly fentanyl, sufentanil, remifentanil, and alfentanil) can induce chest wall rigidity severe enough to make ventilation with bag and mask nearly impossible. Ref: Miller's anesthesia 8th edition Ref: Morgan & Mikhail's clinical anesthesiology 6e.