Sorry I haven't gotten back to you. I've been in Europe where internet access is very expensive. I posted an away message in the announcements thread, but it isn't that prominent, so it's easy to miss. The most serious risk of intrathecal narcotics is, on rare occasions, profound depression of maternal breathing, as in: if somebody doesn't ventilate the woman, she's going to die. There is also a trial in which women were assigned to have intrathecal narcotic followed by epidural in one group (combined spinal-epidural) and systemic narcotic followed by epidural in the other group in which babies were more likely to experience alarming fetal heart rate changes in the combined spinal-epidural group compared with the systemic narcotic-epidural group.
Wong CA, Scavone BM, Peaceman AM, et al. The risk of cesarean delivery with neuraxial analgesia given early versus late in labor. N Engl J Med 2005;352(7):655-65.
Intrathecal narcotic is generally used as a pain relief method for early labor, advancing to an epidural once labor gets active, so for most women interested in pain relief medication, it isn't intrathecal narcotic or epidural, it's intrathecal narcotic followed by epidural. To learn more about the potential harms of epidurals, here's a link to Labor Pain: Epidural and Spinal on the Childbirth Connection website.