Sensory blockade of S3 dermatome prevents pain during bladder catheterization

We often encounter patients who do not complain of pain on undergoing invasive urogenital or rectal procedures, despite incomplete epidural blockade of sacral cutaneous sensation. To clarify whether or not urethral pain is blocked faster than sacral cutaneous sensation during lumbar epidural anesthesia, we investigated the correlation between occurrence of urethral pain and loss of cold sensation in the S1-3 dermatomes.

In 46 gynecological patients, Group A (n=22) received 15 ml of 2% mepivacaine via an epidural catheter inserted cephaladly. Group B (n=24) received 5 ml of 2% mepivacaine directly in the epidural needle directed caudally and 10 ml of 2% mepivacaine via the epidural catheter inserted cephaladly. A Foley catheter was inserted into the urethra 30 min after the injection.

Urethral pain, which was defined as a pained facial expression and/or complaint of pain, was observed in seven patients in Group A, and none in Group B. The caudad level of epidural blockade was significantly lower in patients without urethral pain (S3, median) than with urethral pain (L4) (P <0.05). In 39 patients without urethral pain, 19 (49%) experienced loss of cold sensation in the S1 dermatome, 27 (69%) in the S2 and 38 (97%) in the S3 25 min after the injection.

Blockade of urethral visceral pain often occurs before complete sacral somatosensory blockade, and S3 somatosensory blockade is the important sacral level as an indicator of successful urethral sensory blockade.