Epidural space

The epidural space (sometimes called the extradural space) is a part of the human spine inside the spinal canal separated from the spinal cord and its surrounding cerebrospinal fluid by the dura mater. The term epidural is often short for epidural anesthesia, a form of regional anesthesia involving injection of drugs through a catheter placed into the epidural space. The injection can cause both a loss of sensation and a loss of pain (analgesia).

The epidural space is also of clinical interest since its cranial part contains vessels (e.g. the middle meningeal artery) susceptible to lesions after head traumas, causing a potentially lethal (if not treated immediately) collection of blood called an extradural or epidural hematoma.

Anatomical points
The epidural space is the space outside the tough membrane called the dura mater (sometimes called the "dura"), and within the spinal canal, which is formed by the surrounding vertebrae. Adherent to the inside of the dura is a much thinner and more fragile membrane, the arachnoid mater. Inside the arachnoid is the subarachnoid space, which contains the cerebrospinal fluid, and the spinal cord.

In the spine, the epidural space contains loose fatty tissue, and a network of large, thin-walled blood vessels called the epidural venous plexus.

The upper limit of the epidural space is the foramen magnum, which is the point where the spinal cord enters the base of the skull. The lower limit is at the tip of the sacrum, at the sacrococcygeal membrane.

In the head, the dura is continuous with the periosteum, the tough fibrous lining of the inside of the skull. This means that, in the head, the epidural space is known as a potential space, which means that normally it does not exist. In rare circumstances, a torn artery may cause bleeding which is sufficient to separate both the dura and periosteum from the bone; this is an epidural hematoma.

The space between the dura and the arachnoid (in both head and spine), the subdural space, is also a potential space. Bleeding may also occur here.

Epidural anesthesia
Most commonly, the anesthesiologist conducting an epidural places the catheter in the lumbar, or lower back region of the spine, although sometimes a catheter is placed in the thoracic (chest) or cervical (neck) region. In adults, the spinal cord terminates at the first lumbar vertebra, below which lies a bundle of nerves known as the cauda equina ("horse's tail"). Hence lumbar epidurals carry virtually no risk of injuring the spinal cord.

A patient getting a modern epidural for pain relief generally receives a combination of local anesthetics and opioids. Common local anesthetics include lidocaine, bupivicaine, ropivicaine, and chloroprocaine. Common opioids are morphine, fentanyl, sufentanil, and pethidine (known as meperidine in the U.S.). These are then injected in relatively small doses. Occasionally other agents may be used, such as clonidine or ketamine.

An epidural for pain relief (e.g. in childbirth) is unlikely to cause loss of muscle power, but is not usually sufficient for surgery.

In epidural anesthesia, to allow surgical procedures, larger doses are given in order to remove all feeling (sensory block) and to relax the muscles (motor block) in a large region of the body. Motor block is experienced as short-term weakness or immobility of the affected region, not to be confused with the disease process paralysis.

The pain relief from the epidural can be maintained for some days, if necessary. Sometimes the anesthesiologist will place an epidural in a patient, then go on to use a general anaesthetic for the operation, and then use the epidural for pain relief afterwards.

Technique
Using a strict aseptic technique a small volume of local anaesthetic, such as 1% lignocaine (lidocaine in the U.S.), is injected into the skin and interspinous ligament. A 16, 17, or 18 gauge Tuohy needle is then inserted into the interspinous ligament and a "loss of resistance" technique is used to identify the epidural space.

Traditionally anaesthetists have used either air or saline for identifying the epidural space, depending on personal preference. However, evidence is accumulating that saline may result in more rapid and satisfactory quality of analgesia.

After placement of the tip of the Tuohy needle into the epidural space the catheter is threaded through the needle. The needle is then removed. Generally the catheter is then withdrawn slightly so that 4-6 cm remains in the epidural space. The catheter is a fine plastic tube, down which anaesthetics may be given into the epidural space.

In some unusual instances, it may not be required to insert a catheter into the epidural space, e.g. for steroid injections; see below. The anesthesiologist may inject medication into the epidural space through the needle, then remove the needle.

Caudal epidurals
The epidural space may be entered through the sacrococcygeal membrane, using a standard 21G needle. Injecting a volume of local anaesthetic here provides good analgesia of the perineum and genital areas. This is typically a single-injection technique and a catheter is not normally placed. This is known as a caudal epidural or "caudal".

The caudal epidural is an effective and safe analgesic technique in children undergoing pelvic or perineal surgery. It is usually combined with general anaesthesia.

Side effects

 * Confinement to bed
 * Loss of ability to move around actively during labor
 * Loss of sense of needing to urinate requiring placement of a urinary catheter
 * Pain in the area of placement is not uncommon for up to a year after an epidural
 * Increase in fetal malpositions due to confinement in bed
 * Sudden drop in blood pressure

Complications
These include:
 * Accidental dural puncture. The epidural space in the adult lumbar spine is only 3-5mm deep, which means it is comparatively easy to cross it and accidentally puncture the dura (and arachnoid) with the needle. This may cause cerebrospinal fluid (CSF) to leak out into the epidural space, which may in turn cause the dural puncture headache. This happens about 1 in 100 epidurals, and can be severe and last several days and in some cases weeks or months. It is caused by a reduction in CSF pressure and is characterised by exacerbation when the patient raises their head above the lying position. If severe it may be successfully treated with a "blood patch" (a small amount of the patient's own blood given via another epidural needle). Most cases resolve spontaneously with time.
 * Bloody tap (about 1 in 30-50). It is easy to injure an epidural vein with the needle. In patients who have normal blood clotting, it is extremely rare (e.g. 1 in 100,000) for problems to develop. However, in a patient who has a coagulopathy, the patient may be at risk of epidural hematoma. If blood comes back down the needle, the anesthesiologist will normally site the epidural at another level.
 * Block failure (about 1 in 20). Partial failure may still give satisfactory pain relief. However, if pain relief is inadequate, another epidural may have to be performed.
 * Hypotension which may briefly affect baby.
 * Significant damage to a single nerve (rare, less than 1:10,000).
 * Paraplegia (extremely rare, less than 1:100,000).
 * Death (extremely, extremely rare, less than 1:100,000).

Contraindications

 * Patient refusal
 * Bleeding disorder (coagulopathy)
 * Infection overlying area spine to be injected (or infection in the blood which may "seed" onto the catheter)
 * Anatomical anomalies, such as spina bifida or previous spinal surgery
 * Large lower back tattoos (unless sufficient ink-free space is available)
 * Certain heart-valve problems (such as aortic stenosis) may make epidurals more risky

Epidural analgesia
Epidural drug infusion can change the perception of pain and sensation. Epidural analgesia is similar to epidural anesthesia but uses lower concentrations of local anesthetic drugs to remove most, but not necessarily all, pain. Therefore, epidural analgesia causes less muscle weakness, or paralysis, than epidural anesthesia. It is possible to continue epidural analgesia for several weeks, although there is an increasing risk of infection if the catheter is left in place for more than four or five days.

A common solution for epidural infusion in childbirth or for post-operative analgesia is 0.2 percent ropivicaine and 2 &mu;g/mL of fentanyl. This solution is infused at a rate between 4 and 14 mL/hour, following a loading dose to initiate the nerve block.

Epidural in childbirth
Epidural analgesia is a relatively safe and effective method of relieving pain in labor. It provides immediate pain relief. Epidural analgesia is associated with longer labor. Some claim that it is correlated with an increased chance of operational intervention. The clinical research data on this topic is conflicting. For example, a recent study in Australia (Roberts, Tracy, Peat, 2000) demonstrated that having an epidural reduced the woman's chances of having a vaginal birth, without further interventions (such as episiotomy, forceps, ventouse or caesarean section) from 71.4% to 37.8%. Conversely, a 2001 study by researchers at the National Institute of Child Health and Human Development and a 2002 study by researchers at Cornell University and the University of Ontario demonstrated that epidurals do not increase the likelihood of a caesarean section. In 2005, a meta-analysis of 21 studies also showed that epidurals do not increase the likelihood of caesarean section, but they do increase the chance of a forceps or ventouse delivery by a factor of about 1.4 (Anim-Somuah, Cochrane Review, 2005).

What explains these differing outcomes? There is some data that demonstrates that the likelihood of increased intervention is directly related to the quality of the institution or practitioner providing the care: epidurals administered at top-rated institutions do not generally result in a clinically significant increase in caesarean rates, whereas the risk of caesarean delivery at the worst-ranked practitioners seems to increase with the use of epidural An alternative explanation is that women having difficult labours are more likely to request epidurals, and are also less likely to have an unassisted vaginal birth.

It is important that expectant mothers receive accurate information about the benefits and risks of the procedure, as well as about their other pain-relief options, in order that they may make an informed decision. Less common in labor is spinal anaesthesia in which a much smaller needle (26G or 27G) is advanced slightly further to penetrate the dura (and arachnoid) and allow a rapid achievement of analgesia or anaesthesia depending on the dose given.

Epidural steroid injection
An epidural injection, or epidural steroid injection, is used to help reduce pain caused by a herniated disc, degenerative disc disease, or spinal stenosis. These spinal disorders often affect the cervical (neck) and lumbar (low back) levels of the spine. Pain may be accompanied by numbness or tingling that radiates into the arms or legs. An epidural steroid injection (ESI) may be part of a patient’s multidisciplinary treatment plan that includes physical therapy. The effects of an epidural steroid injection may be temporary or long-term. The injection works by reducing the inflammation and/or swelling of nerves in the spine’s epidural space. The epidural space surrounds the spinal cord and nerves that branch off from the cord.

Epidural steroid injections are administered in a sterile setting such as an outpatient facility or hospital. The medicine used in the injection is a combination of a local anesthetic (such as lidocaine) and a steroid. The procedure involves numbing the skin by injection of a local anesthetic, allowing time for the anesthetic to work, and then inserting a needle into the epidural space. The procedure is performed using fluoroscopy (a live x-ray) which enables the physician to view the placement of the needle. When the needle is properly positioned, the steroid is injected into the epidural space.

After the procedure, the patient is returned to the recovery area and monitored for a period of time before being released home. Patients may be asked to keep a pain diary to help them discuss their pain progress during a follow-up appointment. Some patients who have some residual pain after the first injection may receive a second or third epidural steroid injection. Patients who do not receive any relief from the first injection usually do not receive a second injection.

It is important that patients scheduled for an epidural steroid injection follow the pre-procedure instructions provided. Instructions include stopping certain medications such as blood thinning agents (e.g. aspirin, warfarin, clopidogrel) which can increase the risk of bleeding and hence epidural hematoma formation. An epidural steroid injection, like other medical procedures is not risk-free. There is a possibility of side effects and complications from the needle puncture and medications used.

Other reading

 * Roberts C, Tracy S, Peat B,Rates for obstetric intervention among private and public patients in Australia: population based descriptive study, British Medical Journal (BMJ), v321:p137, 15 July 2000


 * Jun Zhang, Michael K. Yancey, Mark A. Klebanoff, Jenifer Schwarz and Dina Schweitzer, Does epidural analgesia prolong labor and increase risk of cesarean delivery? A natural experiment, American Journal of Obstetrics and Gynecology, Volume 185, Issue 1, July 2001, Pages 128-134.


 * Barbara L. Leighton and Stephen H. Halpern, The effects of epidural analgesia on labor, maternal, and neonatal outcomes: A systematic review, American Journal of Obstetrics and Gynecology, Volume 186, Issue 5, Part 2, May 2002, Pages S69-S77. Also available online.


 * Boqing Chen and Patrick M. Foye, UMDNJ: New Jersey Medical School, Epidural Steroid Injections: Non-surgical Treatment of Spine Pain, eMedicine: Physical Medicine and Rehabilitation (PM&R), August 2005. Also available online.