Unintentional subdural catheter placement during labor analgesia shows typical radiological pattern but atypical response to the Tsui test.

Subdural injection may be associated with abnormally extensive or limited spread of local anesthetics during neuraxial anesthesia. This complication is difficult to diagnose clinically. Radiological imaging is the gold standard for confirming the location of subdural catheter, but electrical stimulation of the catheter has also been described as a useful diagnostic tool. We present the case of an obstetric patient with unintentional subdural catheter placement that presented as a failed epidural block associated with severe upper back and scapular pain on catheter injection. Electrical stimulation of the catheter did not elicit muscle contractions until a current of 4 mAmp was attained, which is the response pattern of epidural placement. Subdural location of the catheter was subsequently confirmed by contrast radiography. This case adds to the evidence that subdural catheters are difficult to identify clinically, and that electrical stimulation may not differentiate them from epidural catheters.