28. CAUDAL EPIDURAL ANAESTHESIA

Caudal anaesthesia has been used since 1901 until Page described the lumbar approach in 1921. It is suitable for anaesthesia and analgesia below the umbilicus in adult and paediatric patients, obstetric analgesia and chonic pain problems. In adults caudal anaesthesia may be used alone. In children, caudal anaesthesia is usually combined with sedation or general anaesthesia. In labour, as the pain of the first stage of labour is transmitted by T10 to L1, caudal anaesthesia is unlikely to be useful as a sole technique of analgesia. However, it is excellent for the second stage or instrumental deliveries. Care must be taken that the foetal head does not lie close to the site of injection, as there have been at least four case reports of direct injection of local anaesthetic into the foetus.

Contraindications

Caudal anaesthesia should not be performed if there is infection near the site of injection, coagulopathy or congenital abnormalities of the lower spine or meninges, or if the patient refuses the technique.

Anatomy

The sacrum is a triangular bone that consists of the five fused sacral vertebrae (S1 to S5). It joins above with the fifth lumbar vertebra and below with the coccyx. The back (dorsal) surface is convex and irregular with important prominences representing fused elements of the sacral vertebrae. The sacral hiatus is a defect at the lower end on the posterior wall from the failure of fusion of S5 and/or S4. The thick fibrous posterior sacroccygeal ligament covers it. Unfortunately there is considerable variation in the anatomy of the sacrum. Frequently bony landmarks are obscured to a degree by asymmetric bony growth and by overlying fibrous or fatty tissues. Distorted anatomy is less common in the younger patients and rare in children. The sacral canal is a continuation of the lumbar spinal canal, which terminates at the sacral hiatus.

The sacral canal has an average volume of 30 to 34 ml in the adult. It contains (1) the terminal part of the dural sac, ending between S1 and S3, (2) the filum terminale which exits though the sacral hiatus and attaches to the back of the coccyx, (3) epidural fat which is variable in nature and sacral epidural veins which generally end at S4 and (4) the five sacral nerves and coccygeal nerves making up the cauda equina.

The sacral nerves give rise to the posterior cutaneous nerve of thigh, perforating cutaneous nerve, pudendal nerve, anococcygeal nerve, pelvic splanchnic nerves and muscular branches. They provide total sensory input from the vagina, anorectal region, floor of the perineum, anal and bladder sphincters, uretha, scrotal skin, vulva (except the far most anterior margin) and penis (except the base) along with a narrow band of skin extending from the posterior aspect of the gluteal region to the plantar and lateral surface of the foot.

Caudal Anaesthesia

The patient should be fasted and all appropriate equipment and drugs for treating complications of epidural anaesthesia (e.g. intravascular injection, total spinal) available. The anaesthetist must be prepared to ventilate the patient, and treat fitting and hypotension. An intravenous cannula must always be inserted before performing caudal anaesthesia. The procedure must be performed with a strict aseptic technique. The skin should be cleaned with an antiseptic and the anaesthetist must wear gloves. Caudal anaesthesia may be performed with the patient lying face down or on their side.

Usually the patient is placed in the Simms position (on their side with the upper leg fully flexed and lower leg partially flexed). This helps to part the buttocks. Finding the bony landmarks is the key to success. The sacral hiatus may be identified by feeling the tip of the coccyx and the moving the finger towards the head about 4 to 5 cm in the adult. It is important to keep the finger in the midline. Sagging of the buttocks may cause confusion in confirming the midline. It may be helpful to have an assistant hold the upper buttock up. Once over the sacral hiatus, the prominent sacral cornua can be felt for on each side by rocking the palpating finger.

Once identified, a needle is inserted at about 45 degrees to the skin though the sacrococcygeal ligament, often with a distinct pop. After perforating the sacrococcygeal ligament the needle should be depressed towards the skin to align the needle approximately with the long axis of the canal and inserted a further 1 cm. The needle should not be inserted more than 2 cm into the caudal space. If the needle is inserted further than 2 cm it may enter a blood vessel or the spinal space. Intravascular injection may cause local anaesthetic toxicity, and intraspinal injection may cause a total spinal. The needle should be aspirated looking for CSF or blood. It may be useful to turn the needle 90 degrees and aspirate again. A negative aspiration does not always exclude the needle being in a vessel or in the spinal space. The anaesthetist must always be aware that the needle may be in the wrong place and give a test dose and never give the full dose more quickly than 10 ml/30 seconds. There should be no resistance to injection.

A small amount (4 ml) of local anaesthetic should be injected (test dose). The anaesthetist must look for signs of intravascular injection (arrhythmias, tingling around the mouth, hypotension). The test dose should not produce a lump beneath the skin. This would show that the needle is not in the caudal space but was beneath the skin. If the test dose is normal then the whole dose may be given slowly.

Suggested Local Anaesthetic Dosage for Caudal Anaesthesia

Both lignocaine 1% and bupivacaine 0.25% (or ropivacaine 0.75%) are commonly used for caudal anaesthesia. The anaesthetist must not give more than the maximum amount allowed of 2 mg/kg bupivacaine or 4 mg/kg lignocaine.

There are various factors that are known (age, weight, height and speed of injection) and unknown (size of caudal space 12 to 65 ml in adult, size and patency of anterior sacral foramina, amount of bony distortion, presence of septa and amount and nature of soft tissues), which may explain the various dosage regimes that have been suggested.

In children

Lignocaine 1% at 0.1 ml/segment/year + 0.1 ml/segment or

Bupivacaine 0.25% at 0.5 ml/kg for lumbosacral block, 1 ml/kg for thoracolumbar block and 1.25 ml/kg for a mid thoracic block produce reliable blocks.

In adults

20 ml of 2% lignocaine with adrenaline or 0.5% bupivacaine with adrenaline (5 micrograms per ml) will spread approximately 9 segments (T9 to L5)

10 ml of 2% lignocaine with adrenaline or 0.5% bupivacaine with adrenaline will spread approximately 7 segments (T11 to L5)

5 ml of 2% lignocaine with adrenaline or 0.5% bupivacaine with adrenaline will spread approximately 4 segments (L1 to L5).

Complications of Caudal Anaesthesia

Failure

Intravenous injection – the needle should not be inserted more than 1 cm and sacral epidural vein puncture excluded by negative aspiration. Intravascular injection can cause fitting and/or cardio-respiratory arrest.

Dural puncture – should be excluded by negative aspiration for CSF. Injection into the CSF may cause a total spinal. The anaesthetist must be skilled at paediatric airway management. Dural puncture may occur in 1:2000 to 1:3000 cases.

Foetal injection

Urinary retention – occurs occasionally in the postoperative period. The incidence is only increased if opioids are administered into the caudal space.

Leg weakness

Neurological complication – very rare.

Infection – superficial and deep abscesses may rarely occur.