Epidural analgesia is now the commonest method to provide pain relief during labor in many institutions. The use of epidural analgesia has become so widespread in recent years that many women are now requesting to repeat epidural analgesia for their second or subsequent labor [1].

Repeated epidural block due to repeated delivery or any other operation during which epidural block was used may be associated with the formation of adhesions and fibrous septa in the epidural space which causes misdistribution of the local anesthetic in the epidural space and plays a major role in the occurrence of unilateral epidural block and failure to control labor pain [2].

The reported incidence of the unilateral epidural block in obstetric patients varies from 5-21%. This complication is annoying for both the patient and the anesthesiologist and may require a redo of the epidural block with subsequent psychological and financial burdens [3].

The effect of previous epidural injection on performance and outcome of subsequent epidurals has not received much attention. It was our impression that there was a higher incidence of unilateral blockade among women receiving their second epidural.

The purpose of the study is to examine performance and outcome in women receiving their first versus repeated epidural.

One hundred and forty women were studied, 70 were primipara had received their first epidural and 70 had received epidural analgesia or anesthesia during a previous pregnancy. The two groups were comparable with respect to weight and height (Table 1). The multiparas were, predictably, older than the primipara and had greater cervical dilatation at epidural insertion. The time between epidural insertion and delivery was also shorter in the multipara. There were 22 caesarean sections in the primipara compared with two in the multipara. The number of repeated injections of bupivacaine till delivery showed no significant difference between the two groups (Table 2).

There is no statistical difference between the two groups as regards visual analogue scale assessment before the epidural block but there was a significant reduction of the VAS in both groups after the block indicating more relief of labor pain after the epidural block (Table 2 and Fig. 1).

Five primipara fulfilled the criteria for a unilateral block (7.1%) as did 15 multipara (21.4%) (P < 0.02). This difference was statistically significant between the two groups (Table 2 and Fig. 2). There were no differences between the groups with respect to paraesthesia, the difficulty of catheter insertion or blood appearing in the catheter. There were no dural taps during the study.

Number of fentanyl usage (50 microgram diluted in 5 mL of normal saline was given in the lateral position) due to unilateral block or inadequate block was increased in group B than in group A but fentanyl usage did not improve the unilateral or the inadequate block (Table 2).

Fig. (1). Visual Analogue Scale (VAS) in the two groups. Bars represent Mean ± SD.

Fig. (2). Incidence of unilateral epidural block.

Unilateral epidural analgesia is a common and persistent issue after epidural block. The phenomenon of unilateral epidural analgesia was first described in 1967 through radiographic analysis and examples of unilateral epidural analgesia continued to exist a half-century later [1].

The unilateral epidural blockade has an incidence between 5% and 21% of all epidural blocks. Persistent unilateral blockade is reported in 0.5% - 2% [2].

We evaluated the effectiveness of epidural analgesia between first and multiple epidural blockades in normal vaginal delivery and the incidence of unilateral epidural blockade. The major finding of our study was that the incidence of the unilateral epidural blockade was significantly higher in patients who received multiple epidurals than those who received it for the first time.

Consistent to our result, Narchi et al. [7] reported a unilateral block rate of 16% in repeated epidural analgesia against 7% in women receiving first epidurals. The point of difference was that the assessment was retrospective by means of a questionnaire administered in the post-partum period and the incidence of the persistent unilateral block was not recorded.

However, Bray and Carrie found no difference between women receiving their first or subsequent epidurals for labour when compared the efficacy of first with second epidurals [8].

Narchi et al. described a 13% incidence of the persistent unilateral block in women who had previous lumbar and caudal epidurals and 7% in women receiving their first epidurals [7].

Some studies reported the failure of epidural analgesia and its relation to a repeated epidural procedure. Camann reported an increased proportion of failed blocks as the number of previous epidurals increased [1].

Many hypotheses assumed to be the cause of unilateral epidural block. One of these causes is the presence of median connective tissue band, which may act as a barrier to the bilateral spread of local anesthetic. However, in autopsy specimens, resin injection studies have disagreed as to the consistent presence of a dorso-median fold [9].

Another method of examination was performed in cadavers; Epiduroscopy that had demonstrated a dorso-median fold in all the 48 cases studied [10].

Epidurography with computed tomography was also done to investigate the presence of a dorso-median fold in living patients. It has resulted in confirmation of its presence in 100% of examined people with 76% of them having sufficient midline tissue to be a potential impediment to catheter passage. This result is obviously grossly out of proportion to the clinical problem, suggesting that local anesthetic must diffuse through the connective tissue bands and the cause of unilateral block is not likely due to the dorso-median connective tissue band [11]. As barriers to the spread of local anesthetics are thought to be congenital, it may be also acquired as well as being congenital. It has been supposed formation of fibrous adhesions in the epidural space after the passage of an epidural catheter, mostly due to local irritation. There was one case of blood and granulation tissue in the epidural space in an autopsy after morphine infusion for a long time via an epidural catheter [12].

Localized adhesion bands and fibrosis after epidural blood patch are expected to be another cause as to affect subsequent local anesthetic spread as Selwyn Crawford that reported an incidence of two cases of an incomplete block (one unilateral block and one missed segment) in 17 women when received epidural analgesia after previous blood patch [13].

The sensory level could not be extended above T10 in Cesarean section in another case. A similar case has been reported with the failure of the block to spread above L2 following a blood patch performed at L1/2 three years previously [14].

The distance between skin and epidural space was also studied as a risk factor for the unilateral blockade. It has been suggested that the unilateral blockade is resulted from lateral catheter placement because of the deviation of the epidural Tuohy needle tip from the midline. The conclusion of a retrospective study of 3011 epidurals supported this theory as it found a strong positive correlation between the occurrence of unilateral blockade and distance between the skin and the epidural space. The incidence of the unsatisfactory block was more than 40% when the distance between skin and epidural space was greater than 6 cm [15].

Although the distance between the skin and the epidural space was not measured in our study, this factor is undoubtedly not responsible for the difference in the unilateral blockade in our study since there was no difference between the two groups in relation to weight (which has been shown to be directly related to the skin with the space of the epidural) [16].

The main conclusion of this study is that there is an increase in the incidence of unilateral block amongst women receiving repeated epidurals for labour than those receiving their first epidural block. Further investigation of this problem could be achieved by the study of a group of multigravida receiving their first epidural analgesia, a group difficult to find in our practice where the epidural rate is very high in primigravida. The role of the epidural catheter could be evaluated by injecting the initial dose through the needle prior to catheter insertion.