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 with or without an opioid into the subarachnoid space. Usually a single-shot dose is administrered through a fine needle, alternatively continuous spinal anaesthesia through a intrathecal catheter can be performed. It is a safe and effective form of anesthesia usually performed by anesthesiologists that can be used as an alternative to general anesthesia commonly in surgeries involving the lower extremities and surgeries below the umbilicus. The local anesthetic with or without an opioid injected into the cerebrospinal fluid provides locoregional anaesthesia: true anaesthesia, motor, sensory and autonomic blockade.
Administering analgesics in the cerebrospinal fluid without a local anaesthetic produces locoregional analgesia: markedly reduced pain sensation, some autonomic blockade, but no sensory or motor block.
Locoregional analgesia, due to mainly the absence of motor and sympathetic block may be preferred over locoregional anaesthesia in some postoperative care settings.
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 may be favored when the surgical site is amenable to spinal blockade for patients with severe respiratory disease such as COPD as it avoids the potential respiratory consequences of intubation and ventilation. It may also be useful in patients where anatomical abnormalities may make tracheal intubation relatively difficult.
In pediatric patients, spinal anesthesia is particularly useful in children with difficult airways and those who 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:
  • Patient refusal
  • Local infection or sepsis at the site of injection
  • Bleeding disorders, thrombocytopaenia, or systemic anticoagulation
  • Severe aortic stenosis
  • Increased intracranial pressure
  • Space occupying lesions of the brain
  • Anatomical disorders of the spine such as scoliosis
  • Hypovolaemia e.g. following massive haemorrhage, including in obstetric patients
  • Allergy
Relative Contraindication
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 be delayed.
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 three 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 lies 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.

Differences with epidural anaesthesia

is a technique whereby a local anaesthetic drug is injected through a catheter placed into the epidural space. This technique is similar to spinal anaesthesia as both are neuraxial, and the two techniques may be easily confused with each other. Differences include:
  • A spinal anaesthetic delivers drug to the subarachnoid space and into the cerebrospinal fluid, allowing it to act on the spinal cord directly. An epidural delivers drugs outside the dura, and has its main effect on nerve roots leaving the dura at the level of the epidural, rather than on the spinal cord itself.
  • A spinal gives profound block of all motor and sensory function below the level of injection, whereas an epidural blocks a 'band' of nerve roots around the site of injection, with normal function above, and close-to-normal function below the levels blocked.
  • The injected dose for an epidural is larger, being about 10–20 mL compared to 1.5–3.5 mL in a spinal.
  • In an epidural, an indwelling catheter may be placed that allows for redosing injections, while a spinal is almost always a one-shot only. Therefore, spinal anaesthesia is more often used for shorter procedures relative to procedures which require epidural anaesthesia.
  • The onset of analgesia is approximately 25–30 minutes in an epidural, while it is approximately 5 minutes in a spinal.
  • An epidural often does not cause as significant a neuromuscular block as a spinal, unless specific local anaesthetics are also used which block motor fibres as readily as sensory nerve fibres.
  • An epidural may be given at a cervical, thoracic, or lumbar site, while a spinal must be injected below L2 to avoid piercing the spinal cord.