Spinal anaesthesia


Spinal anaesthesia, also called spinal block, subarachnoid block, intradural block and intrathecal block, is a form of neuraxial regional anaesthesia involving the injection of a local anaesthetic or opioid into the subarachnoid space, generally through a fine needle, usually long. It is a safe and effective form of anesthesia performed by anesthesiologists and nurse anesthetists which can be used as an alternative to general anesthesia commonly in surgeries involving the lower extremities and surgeries below the umbilicus. The local anesthetic or opioid injected into the cerebrospinal fluid provides anesthesia, analgesia, and motor and sensory blockade. The tip of the spinal needle has a point or small bevel. Recently, pencil point needles have been made available.

Indications

Spinal anaesthesia is a commonly used technique, either on its own or in combination with sedation or general anaesthesia. It is most commonly used for surgeries below the umbilicus, however recently its uses have extended to some surgeries above the umbilicus as well as for postoperative analgesia. Procedures which use spinal anesthesia include:
Spinal anaesthesia is the technique of choice for Caesarean section as it avoids a general anaesthetic and the risk of failed intubation. It also means the mother is conscious and the partner is able to be present at the birth of the child. The post operative analgesia from intrathecal opioids in addition to non-steroidal anti-inflammatory drugs is also good.
Spinal anesthesia is a favorable alternative, when the surgical site is amenable to spinal blockade, for patients with severe respiratory disease such as COPD as it avoids potential respiratory consequences of intubation and ventilation. It may also be useful, when the surgical site is amenable to spinal blockade, in patients where anatomical abnormalities may make tracheal intubation very difficult.
In pediatric patients, spinal anesthesia is particularly useful in children with difficult airways and those who have are poor candidates for endotracheal anesthesia such as increased respiratory risks or presence of full stomach.
This can also be used to effectively treat and prevent pain following surgery, particularly thoracic, abdominal pelvic, and lower extremity orthopedic procedures.

Contraindications

Prior to receiving spinal anesthesia, it is important to provide a thorough medical evaluation to ensure there are no absolute contraindications and to minimize risks and complications. Although contraindications are rare, below are some of them:
Complications of spinal anesthesia can result from the physiologic effects on the nervous system and can also be related to placement technique. Most of the common side effects are minor and are self-resolving or easily treatable while major complications can result in more serious and permanent neurological damage and rarely death. These symptoms can occur immediately after administration of the anesthetic or arise up to 48 hours after surgery.
Common and minor complications include:
Serious and permanent complications are rare but are usually related to physiologic effects on the cardiovascular system and neurological system or when the injection has been unintentionally at the wrong site. The following are some major complications:
Regardless of the anaesthetic agent used, the desired effect is to block the transmission of afferent nerve signals from peripheral nociceptors. Sensory signals from the site are blocked, thereby eliminating pain. The degree of neuronal blockade depends on the amount and concentration of local anaesthetic used and the properties of the axon. Thin unmyelinated C-fibres associated with pain are blocked first, while thick, heavily myelinated A-alpha motor neurons are blocked moderately. Heavily myelinated, small preganglionic sympathetic fibers are blocked last. The desired result is total numbness of the area. A pressure sensation is permissible and often occurs due to incomplete blockade of the thicker A-beta mechanoreceptors. This allows surgical procedures to be performed with no painful sensation to the person undergoing the procedure.
Some sedation is sometimes provided to help the patient relax and pass the time during the procedure, but with a successful spinal anaesthetic the surgery can be performed with the patient wide awake.

Anatomy

In spinal anesthesia, the needle is placed past the dura mater in subarachnoid space and between lumbar vertebrae. In order to reach this space, the needle must pierce through several layers of tissue and ligaments which include the supraspinous ligament, interspinous ligament, and ligamentum flavum. Because the spinal cord is typically at the L1 or L2 level of the spine, the needle should be inserted below this between L3 and L4 space or L4 and L5 space in order to avoid injury to the spinal cord.

Positioning

Patient positioning is essential to the success of the procedure and can affect how the anesthetic spreads following administration. There are 3 different positions which are used: sitting, lateral decubitus, and prone. The sitting and lateral decubitus positions are the most common.
Sitting- The patient sits upright at the edge of the exam table with their back facing the provider and their legs hanging off the end of the table and feet resting on a stool. Patients should roll their shoulders and upper back forward.
Lateral decubitus- In this position, the patient lays on their side with their back at the edge of the bed and facing the provider. The patient should curl their shoulder and legs and arch out their lower back.
Prone- The patient is positioned face down and their back facing upwards in a jackknife position.

Limitations

Spinal anaesthetics are typically limited to procedures involving most structures below the upper abdomen. To administer a spinal anaesthetic to higher levels may affect the ability to breathe by paralysing the intercostal respiratory muscles, or even the diaphragm in extreme cases, as well as the body's ability to control the heart rate via the cardiac accelerator fibres. Also, injection of spinal anaesthesia higher than the level of L1 can cause damage to the spinal cord, and is therefore usually not done.

Difference from epidural anesthesia

is a technique whereby a local anesthetic drug is injected through a catheter placed into the epidural space. This technique is similar to spinal anesthesia as both are neuraxial, and the two techniques may be easily confused with each other. Differences include:
is the local anaesthetic most commonly used, although lidocaine, tetracaine, procaine, ropivacaine, levobupivicaine, prilocaine, or cinchocaine may also be used. Commonly opioids are added to improve the block and provide post-operative pain relief, examples include morphine, fentanyl, diamorphine, and buprenorphine. Non-opioids like clonidine or epinephrine may also be added to prolong the duration of analgesia. In the United Kingdom, since 2004 the National Institute for Health and Care Excellence recommends that spinal anaesthesia for Caesarean section is supplemented with intrathecal diamorphine and this combination is now the modal form of anaesthesia for this indication in that country. In the United States Morphine is used for cesareans for the same purpose since diamorphine is not used in clinical practice in the US.
Baricity refers to the density of a substance compared to the density of human cerebrospinal fluid. Baricity is used in anaesthesia to determine the manner in which a particular drug will spread in the intrathecal space. Usually, the hyperbaric, is chosen, as its spread can be effectively and predictably controlled by the Anaesthesiologist, by tilting the patient. Hyperbaric solutions are made more dense by adding glucose to the mixture.
Baricity is one factor that determines the spread of a spinal anaesthetic but the effect of adding a solute to a solvent, i.e. solvation or dissolution, also has an effect on the spread of the spinal anaesthetic. In tetracaine spinal anaesthesia, it was discovered that the rate of onset of analgesia was faster and the maximum level of analgesia was higher with a 10% glucose solution than with a 5% glucose spinal anaesthetic solution. Also, the amount of ephedrine required was less in the patients who received the 5% glucose solution. In another study this time with 0.5% bupivacaine the mean maximum extent of sensory block was significantly higher with 8% glucose than with 0.83% glucose or 0.33% glucose. Also the rate of onset of sensory block to T12 was fastest with solutions containing 8% glucose.

History

The first spinal analgesia was administered in 1885 by James Leonard Corning, a neurologist in New York. He was experimenting with cocaine on the spinal nerves of a dog when he accidentally pierced the dura mater.
The first planned spinal anaesthesia for surgery in man was administered by August Bier on 16 August 1898, in Kiel, when he injected 3 ml of 0.5% cocaine solution into a 34-year-old labourer. After using it on 6 patients, he and his assistant each injected cocaine into the other's spine. They recommended it for surgeries of legs, but gave it up due to the toxicity of cocaine.