Spinal & Epidural Anaesthesia

INTRODUCTION

The first use of local anaesthetic in the form of cocaine solution trickled under the eyelids was by Sigmund Freud and Karl Koller in 1884. They described how the cocaine removed all sensation from the conjunctiva of their eyes. This experiment was followed with James Leonard Corning experimenting with animals and instilling cocaine close to the spinal cord and observing how it induced anaesthesia in the lower half of the body. He proceeded to try this on a human and described his work in 1886. It was not until later work by Bier published in 1898 that an appreciation of the difference between spinal and epidural anaesthesia was understood. Since then, enormous advances in spinal and epidural anaesthesia have been made and a current summary will be presented in this section.

The principal uses of spinal and epidural anaesthesia include

(1) The relief of pain during surgical operations - on its own or in conjunction with a general anaesthetic

(2) The relief of pain during labour

(3) Relief of pain after an operation

ANATOMY

The vertebral canal contains the spinal cord and the nerves that branch from it and is formed by the vertebrae of the neck, back and lumbar region. The spinal cord extends from the brain and ends at the top of the lumbar region (the lower part of the back). Below this the nerves roots supplying the lower half of the body continue in the spinal canal until they exit to supply the abdomen and legs. The spinal cord is covered by a layer of three membranes and is cushioned within this space by spinal fluid.

A spinal anaesthetic involves injecting a small volume of local anaesthetic (1-3.5ml) into the spinal space below the level at which the spinal cord ends. An epidural anaesthetic involves injecting a larger volume of local anaesthetic into the space that surrounds the spinal canal (outside the membranes). Due to the fact that it is outside the spinal canal, an epidural may be performed in any region of the back. This will be explained in greater detail.

SPINAL ANAESTHESIA

Spinal anaesthesia is used for surgical procedures in the lower abdomen and legs. The local anaesthetic agents that are used provide surgical anaesthesia for between 1 - 3 hours. For this reason, a spinal anaesthetic can only be used when the operation will take less than time.

The procedure is performed with the patient in the sitting position. After insertion of an intravenous cannula (drip), the lower part of the back is cleaned with a surgical cleaning solution and a small dose of local anaesthetic my be inserted under the skin to decrease any pain from the spinal itself. An extremely fine needle is used to reach the spinal space. The space is identified by the fact that clear spinal fluid will leak back through the needle. The local anaesthetic is then injected. This procedure (including positioning and cleaning) usually takes between 5 - 10 minutes to perform.

After the injection of the anaesthetic, the patient lies back on the operating table (with the help of the medical staff) and waits for the local anaesthetic to take effect. This usually takes 3 - 15 minutes again depending on the agents that were used. Onset of the spinal block is marked by a feeling of warmth and tingling in the legs. Testing the block may be performed using a bock of ice - the patient will be unable to feel the cold. Further evidence of the block is in the inability of the patient to move or raise the legs. These changes are temporary and will wear off over a period of 3 - 8 hours. Once the block is working adequately the surgical staff will proceed with the operation.

The operations that are commonly performed using a spinal anaesthetic include caesarian section, urological procedures in men and orthopaedic procedures in the legs.

The complications will be discussed along with those for an epidural anaesthetic as they are similar.

EPIDURAL ANAESTHESIA

An epidural is similar to but nonetheless quite different to a spinal anaesthetic. It is performed in a similar manner however a larger needle with a different shape. This allows for the anaesthetist to be able to identify the epidural space. Once the space is identified, a small plastic catheter is inserted into the space through the hollow epidural needle. The needle is removed and the catheter remains in the space. The local anaesthetic is injected through the catheter. Due to the fact that the catheter is lying outside of the spinal canal (in the epidural space) a larger volume of local anaesthetic is needed to achieve the same effect as a spinal anaesthetic.

The main differences between a spinal and epidural anaesthetic are as follows:-

(1) The onset of the block is slightly slower as the anaesthetic has to be injected over a longer period of time.

(2) The quality of the block can be varied by using local anaesthetics of different concentrations

(3) The duration of the block can be extended by injecting further doses of local anaesthetic

(4) The epidural catheter can be left in place for a period of up to 72 hours and used for control of pain in the postoperative period

(5) Epidural blocks can be inserted in the back region and this allows for the anaesthetic to extend to the upper abdomen and even the chest.

(6) Epidural anaesthetics can be used in conjunction with a general anaesthetic for major surgery in the upper abdomen and the chest

EPIDURAL PROCEDURE

The following pictures demonstrate the performance of a thoracic epidural that will be used for post-operative pain control. The performance of a spinal is similar except a different needle is used, there is no catheter threaded and it is only ever performed in the lumbar region (lower back).

An epidural needle - larger than spinal needle with a special bevel on the end

Patient's back has been cleaned, draped and local anaesthetic is being injected under the skin to lessen the discomfort/pain from the insertion of the epidural

The epidural needle being inserted. An air filled syringe is attached and this is used to locate the epidural space.

The epidural needle is in the epidural space and a thin, plastic catheter is being fed through the needle into the space

The catheter is taped to the patient and the local anaesthetic drugs are then injected. The catheter can be left in place for up to 3 days and an infusion of local anaesthetic drugs used to continue the effect of the epidural.

ADVANTAGES

Spinal and epidural anaesthesia has been proven to be a very safe and effective procedure for appropriate surgical procedures. During child birth, should a mother require caesarian section, a spinal/epidural anaesthetic allows for painless delivery of the child while allowing the mother to be fully conscious and participate in the special occasion.

Epidural anaesthesia has specific application for the control of post operative pain following major abdominal or chest surgery. The pain control thus achieved is probably superior to that achieved with other methods.

COMPLICATIONS

The complications from spinal and epidural anaesthesia are fortunately uncommon to exceedingly rare and they are as follows:-

(1) Headache - following a spinal anaesthetic or an epidural that is inserted too far, there may be a leak of spinal fluid into the surrounding tissues and this can cause a "low pressure" headache. This headache may respond to conservative treatment such as strict bed rest for a few days. However if this fails it may be necessary to perform another epidural procedure where the patients own blood is injected into the epidural space (blood patch) in an attempt to seal the hole a stop the leak.

(2) Fall in blood pressure - this occurs within the first few minutes after a block and may require intravenous fluids and blood pressure agents to correct the problem. The anaesthetist will be paying close attention and this problem can be corrected rapidly.

(3) Urinary retention - this problem can be treated with a urinary catheter

(4) Nausea and vomitting

(5) Infection (meningitis) - a rare complication following spinal or epidural anaesthesia.

(6) Epidural blood clot or abscess - this is fortunately a very rare complication with an incidence of approximately 1 in 120 000. However if the diagnosis of the problem is delayed, it can result in parpalysis.

(7) Other nerve damage - other nerve roots may be damaged if the needle is inserted incorrectly. Fortunately this is also a very rare event.