Myoclonus and urinary retention following subarachnoid morphine injection in a dog

A 5-year-old German Shepherd dog which presented for total hip replacement developed myoclonus and urinary retention after the subarachnoid injection of preservative-free morphine. Myoclonus was resistant to treatment, except pentobarbital anesthesia. Urinary retention was treated with bethanechol and subsided within a few days. Involuntary muscular activity can result from the epidural, subarachnoid or systemic injection of various opioid drugs, or as a result of the toxic or irritant effect of preservatives or autologous blood. The latter were not causative factors in this case. Opioid agonist inhibition of central inhibitory neurotransmitter action may have explained the myoclonus. Postoperative urinary retention was attributed to the spinal action of morphine inhibiting efferent parasympathetic nervous activity.