During the second stage of labour, pain largely results from distension of the lower vagina, vulva and perineum. In addition, the musculature of the anterior compartment of the pelvis must be negotiated by the descending presenting part. These structures obtain most of their sensory and motor innervation from the second to fourth sacral nerve roots via the pudendal nerve. Pudendal nerve blocks are of value in providing analgesia for the second stage when contraindications to neuraxial blockade exist and for the provision of pelvic floor relaxation when forceps delivery is required. They may also be used to provide anaesthesia of the perineum in order to create or repair an episiotomy. Pudendal nerve blocks have been utilized in the past as an alternative to epidurals in assisted twin and breech deliveries (Figure 14.2). Increasing evidence supporting the safety of epidurals in these situations has contributed to a diminished use of pudendal nerve blocks (1).

The anatomy of the pudendal nerve (Figure 21.1) (2) should be considered when performing the block. The block is performed with the patient in the lithotomy position. Two approaches have been described: transvaginal and transcutaneous or perineal. The basis of both approaches is to block the nerve before it gives off its terminal branches. Its most reliable landmarks are exploited - the ischial spine in the transvaginal, and the ischial tuberosity, in the perineal approach.

Transvaginal approach:
In the transvaginal approach, the ischial spine can be palpated either transvaginally or per rectum. It is important to use a needle with a guide in order to limit the depth of submucosal penetration ("Huber" security point ). When a left sided block is performed, the ischial spine is palpated (Figure 21.2) with the index finger of the left hand, the syringe is held in the right hand and the needle is guided between the index and middle fingers of the left hand towards the ischial spine. The sacrospinous ligament lies 1 cm medial and posterior to the spine. The needle is passed through the ligament for a distance of 1 cm until a loss of resistance is appreciated. The tip now lies in the area of the pudendal nerve. The pudendal vessels are closely associated. After aspiration, 10 mls of local anaesthetic solution are injected. The block is repeated on the other side.

Perineal approach:
The perineal approach is considered valuable when the engaged head makes vaginal palpation difficult. The ischial tuberosity is located by palpation (Figure 21.2). The needle is introduced slightly medial to this point, for a distance of 2.5 cm. The nerve is usually encountered without eliciting paraesthesia. Up to 8 mls of solution is infiltrated at this point. The needle is then withdrawn and directed into the deep and superficial tissue of the vulva along its anterior margin in order to block the the ilioinguinal and genitofemoral component (Figure 21.1). The block is repeated on the other side.

The limitations of this block include:
- Failure to provide adequate analgesia. It has been reported that up to half of all bilateral pudendal blocks are ineffective on one or both sides (1). A pudendal nerve block will not abolish sensation to the anterior part of the perineum because this region is supplied by branches of the ilioinguinal and genitofemoral nerves. Subcutaneous infiltration anteriorly along the vulva is therefore described as a component of this technique. Failure to wait sufficient time is a commonly cited reason for failure of a pudendal block. Pudendal nerve blocks do not abolish the pain of uterine contraction and cervical dilation which is transmitted via sympathetic nerve fibres derived from spinal levels T10 - L2 (Figure 46.1). It does not cause relaxation of the uterus.

- Intravascular injection with subsequent local anaesthetic toxicity is a risk imposed by the close proximity of the pudendal vessels (Chapter 36).

- Fetal complications are uncommon. Exposure of the fetus to a high plasma level of local anaesthetic is a possibility that should be considered. Delivery usually occurs within a short period of time which may not allow for clearance of local anaesthetic from fetus via the placenta. 1% lignocaine is the most commonly used agent. Peak concentrations occur within 10 - 20 minutes. The maternal plasma concentrations which occur after this block are much lower than after either epidural or paracervical blocks.

This block has been reported to prolong the second stage of labour as a result of loss of the bearing-down reflex. The addition of adrenaline may further prolong the second stage of labour (3).

References:
1 Scudamore JH, Yates MJ: Pudendal block - a misnomer?

2 Ellis H and Feldman S ' Anatomy for Anaesthetists '

3 Norris MC Obstetric Anesthesia Norris MC (ed)