Mueller
published the first description of pudendal nerve block in 1908. Several
variations of this description followed, until Klink in 1953 demonstrated the
transperineal approach. The transvaginal approach was used by several
individuals, including Kobak, whose name is now associated with the specially
guarded needle he designed. However, the first article to describe the
transvaginal approach was written by Kohl in 1954. This approach can be
recommended because it is technically simple, carries little risk of hemorrhage
or infection and causes little discomfort.
The pudendal
nerve is the largest branch of the pudendal plexus and is composed of
somatosensory, somatomotor and autonomic elements derived from the anterior
primary divisions of the second, third, and fourth sacral nerves. The
nerve trunk is found about 1 cm proximal to the spine of the ischium and passes
anterior and inferior, out of the pelvic cavity, before returning through the
lesser sciatic foramen, at which point it passes anterior through Alcock's
canal. The pudendal nerve divides into three main branches; the inferior
rectal nerve, the perineal nerve, and the dorsal nerve of the penis or
clitoris.
The trunk of
the pudendal nerve passes posterior to the junction of the ischial spine and
the sacrospinous ligament. Here, the nerve is medial to the internal
pudendal vessels, and it is at this point that the nerve is
blocked. In close proximity, but posterior and lateral, is the posterior
femoral cutaneous nerve, which is often blocked at the same time. The
inferior gluteal vessels are also near the pudendal nerve, and their proximity
poses the risk of intravascular injection of local anesthetic. Like the
pudendal vessels, they can also suffer injury.
Pudendal
block is used primarily to relieve pain in the second and third stages of labor
for both normal and instrumental (outlet) delivery. Obviously, the
transvaginal approach is precluded once the presenting part occupies the
vagina. Pudendal block is also indicated for perineal surgery.
While lumbar epidural analgesia will continue to replace pudendal block as the
technique of choice, there are and will always remain situations where
anesthesia service is unavailable. In this event, pudendal block provides
a suitable alternative means of relieving pain during parturition. In
fact, when carried out with a full knowledge of the anatomy, physiology and
associated pharmacology, it is a safe procedure with few complications.
Pudendal
nerve block provides satisfactory perineal anesthesia for normal delivery, low
forceps manipulation and episiotomy. The advantage of this block over a
major conduction block is the lack of circulatory and respiratory
disturbance. Furthermore, the course and duration of labor are
practically unaffected, even though there may be impairment for the
perineal-diaphragmatic-abdominal reflex.
Care must be
taken on both sides during the course of injection to ensure that the needle is
not inserted intravascularly and that the dose and volume used will preclude
any toxic effects from the local anesthetic and the vasoconstrictor.
When this
block is combined with paracervical blocks (which utilize the same needle), the
painful uterine contractions of the late first stage of labor are also
relieved.
Generally, a
short-acting agent, such as lidocaine 1% or prilocaine 1%, produces very
satisfactory analgesia for 1.5-2.5 h. When analgesia of very rapid onset
is required, 2-chloroprocaine 2% can be used. The duration of this agent
will, however, be no more than 1.5 h. A volume of 20 ml should not be
exceeded for a block of both nerves.
Pudendal
nerve block can result in complications such as maternal hematoma, systemic
toxic reaction, trauma to the sciatic nerve and puncture of the rectum.
1.
Transperineal Approach
The patient
is placed in the lithotomy position. After appropriate skin preparation,
the point of needle insertion is identified. This point, located by palpating
the tuberosity of the ischium, lies 2.5 cm posteromedial to the latter
structure.
A skin wheal
is made with a 25-gauge needle, after which a 20-cm 20-gauge needle is
introduced at right angle to the skin on all planes. The syringe is not
attached until the needle has reached its final position.
The left
index finger is inserted into the rectum or vagina to guide the needle, palpate
the ischial spine and prevent the needle from passing through the rectum.
The needle should at all times remain within the ischiorectal fossa. As
the needle approaches the spine of the ischium, it is possible to move the
needle back and forth as if the point were not engaged. This generally
signifies its correct placement. The syringe is not connected, and an
aspiration rest is performed while the needle is rotated 180E. If this
test is negative, 3-4 ml local anesthetic is injected. The needle is then
advanced another cm, a further attempt to aspirate is made and another 4 ml
local anesthetic is injected.
As with the
transvaginal approach, this maneuver should ensure block of the inferior rectal
nerve. The same procedure is carried out on the other side. Whether
the same guiding finger is used or not will depend on the preference of the
operator. Obviously, if the operator changes hands to guide the needle on
the other side, it will necessitate changing gloves.
In order to
avoid possible contamination, some individuals prefer to use the vaginal route
for the guiding finger, although this may well be precluded by the descent of
the presenting part, which is really the primary indication for the
transperineal approach.
2.
Transvaginal Approach
The patient
should be placed in the lithotomy position, and after appropriate preparation
the status of the presenting part and the location of the ischial spines should
be determined by vaginal examination.
It is
preferable to use a special needle, such as the one designed by Kobak or the
Iowa "trumpet," where the needle point is guarded until the ischial
spine is palpated with the tip of the third finger. When using an
unguarded needle, it should be held so as to lie in the groove formed between
the second and third fingers.
The fingers
with the needle and attached syringe are advanced into the vagina until the tip
of the ischial spine is felt. If a Kobak needle is used, it is rotated so
that the stud slides down the bayonet slot, allowing the needle to protrude
beyond its guard tube. At this point, the needle is carefully advanced
through the mucosa until the tip is embedded in the sacrospinous
ligament. While maintaining constant pressure on the plunger of the
syringe, the needle is slowly advanced until the sudden loss of resistance
emerged from the other side of the ligament and is now lying in the same tissue
plane as the pudendal nerve and vessels.
After attempted
aspiration through a 180† rotation to ensure that the bevel is not in a blood
vessel, 3-5 ml local anesthetic solution is injected. The needle is then
advanced 1 cm further and another 3-5 ml solution is injected. The left
hand is used as the dominant hand for the left pudendal nerve and the right
hand must be used for the right side.
It should be noted that when the needle passes through the sacrospinous ligament some anesthetic will escape, thereby anesthetizing this sensitive structure. The ability to inject local anesthetic solution into this ligament is in sharp contrast to the complete inability to infiltrate the ligamentum flavum during the loss of resistance test for identification of the epidural space. This maneuver also achieves block of the inferior rectal nerve when the latter pierces the sacrospinous ligament.