Anaesthesia UK : Educational points for nerve injury resulting from a labour epidural

Introduction

Epidural anaesthesia is a central neuraxial block technique with many applications. The epidural space was first described by Corning in 1901, and Fidel Pages first used epidural anaesthesia in humans in 1921. In 1945, Tuohy introduced the needle which is still most commonly used for epidural anaesthesia. Improvements in equipment, drugs and technique have made it a popular and versatile anaesthetic technique, with applications in surgery, obstetrics and pain control. Both single injection and catheter techniques can be used. Its versatility means that it can be used as an anaesthetic, an analgesic adjuvant to general anaesthesia and for postoperative analgesia in procedures involving the lower limbs, perineum, pelvis, abdomen and thorax. In obstetrics, continuous lumbar techniques were first used in 1946. Most units now provide a 24 hour service. Successful epidural analgesia provides blockade of the nerve supply of:

- the uterus: sympathetic pathways T11-12
- the cervix, possibly due to blockade of S2-4
- the birth canal (pudendal nerves S2-4), genitofemoral ilioinguinal nerves and sacral nerves.

Contraindications

Absolute

• Patient refusal.
• Coagulopathy. Insertion of an epidural needle or catheter into the epidural space may cause traumatic bleeding into the epidural space. Clotting abnormalities may lead to the development of a large haematoma, leading to spinal cord compression.
• Therapeutic anticoagulation. As above.
• Skin infection at injection site. Insertion of the epidural needle through an area of skin infection may introduce pathogenic bacteria into the epidural space, leading to serious complications such as meningitis or epidural abscess.
• Raised intracranial pressure (ICP). Accidental dural puncture in a patient with raised ICP may lead to brainstem herniation (coning).
• Hypovolaemia. The sympathetic blockade produced by epidurals, in combination with uncorrected hypovolaemia, may cause profound circulatory collapse.

Relative

• Uncooperative patients may be impossible to position correctly, and be unable to remain still enough to safely insert an epidural.
• Pre-existing neurological disorders, such as multiple sclerosis, may be a contraindication, because any new neurological symptoms may be ascribed to the epidural. It is important to document existing neurological symptoms prior to instituting regional anaesthesia if choosing to do so.
• Fixed cardiac output states. Probably relative rather than absolute. This includes aortic stenosis, hypertrophic obstructive cardiomyopathy (HOCM), mitral stenosis and complete heart block. Patients with these cardiovascular abnormalities are unable to increase their cardiac output in response to the peripheral vasodilatation caused by epidural blockade, and may develop profound circulatory collapse, which is very difficult to treat.
• Anatomical abnormalities of the vertebral column may make the placement of an epidural technically impossible.

Loss of resistance technique: midline approach

After checking the availability of resuscitation drugs and equipment, appropriate monitors are placed and the patient is positioned.

The patient may be sitting or in the lateral position, according to the anaesthetist’s preference. The parturient may find it more comfortable in the lateral position. The intercristal line and midline are identified. The point of needle insertion is marked with a skin marker or with a cruciform mark made by thumbnail pressure applied in the vertical and horizontal planes.

The skin is widely prepared and the field draped.

A skin wheal of 1% lidocaine is made with a short 32 gauge needle.

A 25 gauge 1.5 inch standard bevel needle is used to infiltrate 1% lidocaine in the supraspinous and interspinous ligaments

Portex epidural kit

An 18 gauge Tuohy needle with a stylet is inserted perpendicular to the skin, with the bevel facing cephalad. The depth of the needle in the supraspinous ligament is limited to 2 cm before the stylet is removed. A 5 or 10 ml saline-filled loss of resistance syringe is then attached to the needle hub. Some anaesthetists prefer to use loss of resistance to air.

Bromage describes the technique as follows:

"The needle is gripped with the thumb on top and the proximal and distal phalanges of the crooked forefinger below. The hand is supinated and the wrist partially flexed and the back of the carpus braced against the patient's back. Forward motion is imparted on the needle by a gradual extension of the wrist, and the carpus and metacarpus roll in toward the back like an eccentric cam driving a piston".

The non-dominant hand rests against the patient's back and stabilises the needle to prevent any sudden forward motion. Constant unremitting pressure is placed on the plunger of the saline-filled syringe with the thumb.

Ligamentum flavum (click for larger image)

A sudden loss of resistance is felt when the bevel pierces the ligamentum flavum. Injection, which was previously obstructed, should suddenly become easy. The forward motion of the needle should be stopped immediately. The jet of saline pushes the dura away from the advancing needle. The average depth of the epidural space measures 5 cm, with a range between 2 - 8.5 cm. No cerebrospinal fluid (CSF) or blood should flow from the needle after the syringe is detached from the hub.

Rotation of the needle during needle advancement and after entering the epidural space may increase the likelihood of lacerating epidural veins as well as the dura. Subdural or subarachnoid placement of the needle bevel and epidural catheter may result.

Some anaesthetists recommend injection of a volume of saline or part of the initial dose of local anaesthetic through the needle prior to threading the catheter. This may open up the epidural space, facilitate passage of the catheter, decrease the incidence of paresthesias and intravenous cannulation and shorten the onset of the initial dose. The incidence of intravenous cannulation is decreased significantly (1% vs 14%) when 10 ml of saline are injected through the needle, but is no different when volumes of less than 10 ml are injected prior to catheter insertion.

A 20 gauge epidural catheter is threaded through the needle, with attention paid to the depth markings on the catheter. Moderate pressure might be required to pass the catheter tip beyond the orifice of the needle, but only light and delicate pressure should be needed to advance it further. Catheter advancement frequently produces a mild paresthesia, described as a poorly localised burning sensation radiating to the hip or leg. The optimal distance in the space probably ranges between 2-6 cm.

The potential for shearing the catheter with the needle exists once the tip of the catheter has passed the bevel of the needle. The catheter should not be removed with the needle left in place.

The catheter is grasped at its entry into the skin between the thumb and index finger as the needle is removed. The depth of the catheter at the skin is noted.

Spontaneous flow of CSF or blood from the catheter should be absent when the end of the catheter is held in a dependent position. An aspiration test for CSF or blood is attempted with a 3 ml syringe. Following a negative aspiration, an appropriate test dose is administered.

The catheter is secured to the skin using a transparent dressing. A loop in the catheter as it exits the skin may prevent outward migration of the catheter by absorbing sudden tension placed on the proximal end of the catheter. Some anaesthetists recommend suture fixation or subcutaneous tunnelling of the catheter to prevent catheter dislodgement when prolonged catheterisation is anticipated, although studies have questioned the effectiveness of subcutaneous tunnelling. The catheter is taped lateral to the spinous processes, without crossing the midline.

Following a negative test dose, local anaesthetic is administered in incremental doses.

Paramedian approach

The paramedian approach may offer several advantages over the midline approach to the epidural space. Studies demonstrate that identification of the epidural space on the first attempt is improved and repeat attempts at needle insertion are decreased with the paramedian approach. Decreased incidence of failure to thread the catheter, resistance to injection through the catheter and paresthesia with catheter positioning have also been demonstrated. Rigid epiduroscopy during catheter placement has demonstrated that catheters placed with a paramedian approach take a straight cephalad course more often, and with less tenting of the dura with the tip of the catheter during catheter advancement compared with placement with the midline approach.

The paramedian approach may be the only possible approach in patients who are unable to flex the lumbar spine, in patients with severely calcified supraspinous and interspinous ligaments, and when performing epidural anaesthesia in the midthoracic region (T5-T9), where the steep angulation of the spinous processes and overlap with the laminae precludes use of the midline approach.

Paramedian approach: technique

A mark is made 1.5 cm lateral to the inferior border of the spinous process. After a skin wheal of local anaesthetic is made, a 1.5 inch 25 gauge needle is used to infiltrate local anaesthetic in the paraspinous muscles in a path directed ventrally and slightly medially. The needle is used to contact and identify lamina. The needle is then withdrawn to the skin, redirected slightly more cephalad and advanced until either the lamina or the superior edge of the lamina is contacted. In this manner, the needle is "walked" in fine increments off the superior edge of the lamina. Longer (spinal) needles may be required to identify the lamina in patients with generous subcutaneous tissue and paraspinous muscles. Once the superior edge of the lamina has been identified, the needle is removed and an 18 gauge Tuohy needle is inserted through the skin and subcutaneous tissue with the bevel facing cephalad. The stylet is removed and the needle advanced to contact the lamina. The needle is then "walked" off the superior edge of the lamina until the ligamentum flavum is contacted. The needle is advanced using the loss of resistance technique.

Technical problems

Headache - post-dural puncture headache

Approximately 1% of women will experience a moderately severe to very severe headache following epidural analgesia. A similar percentage will follow spinal analgesia or combined-spinal-epidural (CSE) analgesia; however, these are possibly of a lesser severity. Not enough data are available as yet to clarify this.

The epidural space is relatively empty and hence the distance across it is very small. If the epidural needle is pushed a little too far, it may pierce the membrane (the dura) which forms the spinal sac. If there is a hole in the dura, the fluid can leak out into the epidural space. This fluid leak can cause a ‘low pressure’ headache. Although CSF is formed continually (the volume of CSF is about 150 ml, but 600 ml are formed each day, so it is continually being absorbed), with a hole in the dura, the loss will be greater than usual and the pressure around the brain and spinal cord will be lower than usual. It is this low pressure which causes the headache associated with a dural puncture. This also explains the fact that the headache is better when the patient is lying down (there is less ‘drag’ pulling the brain down and stimulating pain sensitive structures).

The headache is typically felt in the front or back of the head and neck and gets worse upon sitting or standing up. It usually develops within 18 hours of the epidural and lasts for 4-5 days. Uncommonly, it lasts longer. Resolution indicates that the body has created a seal over the puncture and that the CSF is no longer leaking. The severity varies. A dural puncture headache may be mild and go away by itself. Not uncommonly, it is severe enough to significantly impair the mother’s ability to nurse and care for the baby.

In itself, an accidental dural puncture is not a serious complication. It is, in fact, performed deliberately in a spinal anaesthetics or a CSE technique. The calibre of spinal needles is significantly smaller than epidural needles, which results in a smaller puncture and possibly an overall reduced severity of headache.

Dural puncture headaches can be treated. Things which can help to improve the headache include bed rest, drinking plenty of fluid, caffeine and simple analgesics, such as paracetamol. If these fail, then the most effective treatment is a ‘blood patch’. This is a simple procedure which involves another epidural, with a few ml of the patient’s own blood injected through the needle into the epidural space. This completely and permanently resolves the headache in over 80% of women within a few hours. In the remainder, the headache is either not relieved, or returns some time later. In these cases, a second blood patch will resolve 95% of headaches.

Back pain

Back pain is very common after pregnancy and labour. There may be some local tenderness where the needle was put in, which may last up to a week. 10% of women will develop lower back pain either during their pregnancy or around the time of the delivery. The likelihood of having continuing back pain after delivery are the same whether or not epidural analgesia was employed during labour.

Injury to nerves

About 1 in 3000 women having a baby will have some temporary damage to peripheral nerves, that is, individual nerves outside of the spinal cord. This usually occurs during delivery, as a result of traction or direct pressure from the foetal head. Direct injury from instruments like forceps and epidurals is possible; however, the incidence is the same, whether or not an epidural has been employed.

Signs suggesting nerve injury include an area of numbness, weakness or pain. It is usually experienced in the legs, although loss of control of the bladder and anal sphincters is another manifestation. Over 99% of these will recover spontaneously. One large study revealed that out of 50 cases reported, all had either completely or significantly resolved by 12 weeks.

Permanent injury to the spinal cord

Permanent paralysis resulting from epidural analgesia during labour is so rare that clear figures on its incidence are not available. A recent review of 500,000 cases performed in the United Kingdom did not reveal a single case. One case has been reported in Australia. In this case, the patient had a rare malformation of blood vessels around the spinal cord. This was unknown to the patient and resulted in a blood clot which caused compression injury to the spinal cord.

Anaesthetists consider this to be an extremely serious potential, but very rare, complication of epidural analgesia. Experience from the general surgical population reveals that this risk is increased in patients on anticoagulants or who suffer from disorders of blood clotting. This may occur in association with severe hypertension during pregnancy. Anaesthetists employ conservative guidelines when advising labouring women about this risk.

Permanent injury to the spinal cord could also occur as a result of infection. For this reason, a sterile technique similar to surgical operations is enforced.

Problems regarding the dose of local anaesthetic

The local anaesthetic which is injected into the epidural space has the potential to block all of the nerves it contacts.

Hypotension

Hypotension invariably occurs after receiving an epidural. There are two main reasons for this. First, the blood pressure is almost always raised before the epidural is inserted - for the simple reason that the patient is usually in pain. Once the pain is relieved, the blood pressure starts to fall. The second reason for a fall in blood pressure is that the epidural relaxes the muscles in the walls of blood vessels. This means that vessels which were previously constricted (secondary to pain) now become dilated. Consequently, the blood has to circulate through more blood vessels.

These circulatory effects are of little importance - provided that the parturient is not lying on her back. In this event, ‘supine hypotension’ may occur. Since hypotension is an expected effect of epidural analgesia, anaesthetists insert a cannula and commence intravenous therapy prior to the establishment of analgesia. Following this, they ensure that the parturient is positioned on her side. As a result of this preventative management, it is unusual to experience low blood pressure when the epidural is used for labour analgesia. Nonetheless, the blood pressure is closely monitored and additional fluid or medication is readily available to keep it normal.

If hypotension does not respond to change in posture and intravenous fluids, and is a cause for concern, then it is very easily treated by adding to the infusion a vasopressor. The drug which is usually used (ephedrine) acts within a couple of minutes and is perfectly safe.

Bladder distension

Muscle weakness associated with epidurals has the potential to cause the bladder to become over-full. This may lead to problems in the future with passing urine. This is compounded by loss of sensation of the discomfort caused by bladder distension.

The problem is reduced by the use of dilute solutions of local anaesthetic which preserve bladder sensation. In addition, bladder function is part of the normal care provided by midwives during labour. It may require a temporary catheter to ensure that the bladder drains.

Effects of epidurals on the newborn

The effect of regional (epidural, spinal, CSA) analgesia on the newborn has been extensively investigated. It carries both potential benefits and potential risks to the foetus. The overall effect is to neither harm nor help the unborn child.

Potential benefits

Severe pain, when experienced by a woman in labour, may result in a stress response. Despite there being pronounced differences in the philosophy and technical application of the various methods of pain relief in childbirth, many have made a similar observation - that stress can interfere with the normal progress of labour and can increase the perception of pain. The effects of stress in the mother may, in addition, be transferred to the foetus.

Any technique used during labour which decreases the mother’s stress should be of benefit to the child. Epidural analgesia has been demonstrated to have this effect.

Catecholamines released during labour as a result of pain or anxiety will be of little benefit to the mother or foetus under normal conditions. Circulating adrenaline can diminish the strength and synchrony of the uterine contractions. ‘Dyscoordinate’ labour may ensue because the cervix dilates much more slowly. Much research has addressed the effect of epidurals on the progress of labour. The paradoxical ‘speeding-up’ of a poorly progressing labour which has been observed has been attributed to a reduction in the production of these catecholamines by the mother when effective pain relief is achieved.

Epidural analgesia has been demonstrated to be of benefit in some labours because it normalises the levels of catecholamines and the mother’s breathing pattern. The mother relaxes, uterine contractions regain their effectiveness and the foetus shows fewer signs of stress. These effects, however, are by no means predictable.

The concentration of oxygen in the mother’s blood may decrease if she is heavily sedated. This may occur if the level of the epidural block rises high enough to weaken her respiratory muscles. This is a potential problem with anaesthesia for caesarean section, where higher concentrations of local anaesthetic are needed. The dilute solutions used for labour make this a rare (and treatable) complication in this setting. Finally, an inadvertent injection of a large amount of local anaesthetic may cause the mother to fit. This would also threaten delivery of oxygen to the foetus. This is also a rare complication.

A severe (and untreated) fall in the mother’s blood pressure may decrease blood flow to the placenta, such that the delivery of oxygen to the foetus is inadequate. Supine hypotension may occur when an epidural is in progress, and this is why it is important for the mother to either lay on her side or with one hip ‘wedged’ up on a pillow.
ArticleDate:20040406
SiteSection: Article