Spinal Anesthesia: Indications, Technique, and Complications

Spinal, or subarachnoid anesthesia (SA), is a method of neuraxial anesthesia in which a local anesthetic (with or without adjuvants) is injected into the lumbar cerebrospinal fluid to achieve a predictable sensory and motor block by acting on the spinal nerve roots and spinal structures stemming from the spinal cord. Spinal anesthesia is used for achieving anesthesia and/or analgesia during various interventions on the spine, lower abdomen, pelvis, perineum, and lower extremities.

Indications

Spinal anesthesia can be used as a sole method or in conjunction with general anesthesia (GA) for procedures on the lower half of the body:

• Surgery on the lower extremities, including orthopedic and traumatological interventions, and vascular interventions distal to the inguinal ligament;
• Surgery on the lower abdomen, including urological surgeries (transurethral interventions, bladder surgeries), proctological (rectal) surgeries, and general surgery interventions (inguinal hernia repair);
• Obstetric and gynecological surgeries, including cesarean section, surgeries on the uterus and adnexa, as well as vaginal surgery;
• As intra- and postoperative analgesia component to reduce the exposure to opioid pain relievers.

Contraindications

Absolute contraindications

• Impossibility to obtain the patient’s consent;
• Infection at the intervention site (risk of meningitis development);
• Increased intracranial pressure (ICP) in the event of an intracranial mass (risk of herniation);
• Allergy.

Relative contraindications

• Coagulopathy (previously thought to be an absolute contraindication) may now be considered depending on the degree of dysfunction;
• Systemic infection or sepsis;
• Preload dependence in cardiac disorders: aortic stenosis, hypertrophic cardiomyopathy, severe hypovolemia (risk of significant reduction in cardiac output and hypotension as a result of sympathetic block accompanying SA);
• Significant vertebral column deformities, making SA technically challenging or impossible and associated with a high risk of neurological complications;
• Progressive neurological disease;
• Hydrocephalus with a shunt (theoretically poses a risk of shunt infection).

Anatomical and physiological aspects of spinal anesthesia

Spinal anesthesia (SA) is not only a “puncture technique” but primarily a managed intervention into the autonomic and conductive body pathways. Understanding the functional anatomy of the spine, the meninges, and the subarachnoid space, as well as the physiological effects of sympathetic, sensory, and motor blocks, allows for prediction of block spread and complication prevention. Operator’s anatomical and physiological literacy is directly related to the patient safety and predictability of their outcomes.

Anatomical factors

Performing spinal anesthesia requires the operator to have knowledge of the functional anatomy of the spinal block, which is impossible without deep knowledge of the structure of the spine, spinal cord, and spinal nerves.

• The vertebral column consists of 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal segments).
• The vertebral column normally forms three curves. The cervical and lumbar curves are convex anteriorly, while the thoracic curve is convex posteriorly. The curves of the vertebral column, along with gravity, the baricity of the local anesthetic, and patient positioning, affect the spread of local anesthetics within the subarachnoid space.

• Five ligaments hold the vertebral column together. The supraspinal ligaments connect the tips (apices) of the spinous processes of the vertebrae, from the seventh cervical vertebra (C7) to the sacrum. The interspinal ligaments directly connect the spinous processes. The yellow ligament connects the laminae of the vertebrae above and below. The posterior and anterior longitudinal ligaments bind the vertebral bodies together.
• The spinal cord is protected by three meninges: dura mater, arachnoid mater, and pia mater. The dura mater is the outermost layer. The space between the arachnoid mater and pia mater is known as the subarachnoid space, where spinal nerves run and cerebrospinal fluid circulates.
• As previously mentioned, SA is performed only in the lumbar region, at a lower level of the lumbar spine to avoid spinal cord injury and prevent the effect of intrathecally administered medications on the upper thoracic and cervical regions. The lower end of the spinal cord (cauda equina) is usually located at the lower border of the first or second (L1-L2) lumbar vertebra (in children, the cauda equina is situated lower, usually ending at the L3 level). For these reasons, the special needle for spinal anesthesia is typically inserted at the L3-4 or L4-5 level.

• Spinal anesthesia is performed by inserting a needle between the lumbar vertebrae through the dura mater to introduce an anesthetic agent.
• The line between the iliac crests is used as an approximate landmark for inserting the spinal needle. In most patients, this line intersects the L4 vertebral body. Since the landmark does not allow for precise prediction of the intervertebral space, the spinal needle should be inserted at or below the intercristal line.
• Midline and paramedian puncture techniques are available. The paramedian approach is often used in patients who cannot flex their spine or when the midline approach is ineffective.
• When performing SA using midline access, the spinal needle passes through the following layers:

1. Skin;

2. Subcutaneous adipose tissue;

3. Supraspinal ligament;

4. Interspinal ligament;

5. Ligamenta flava;

6. Dura mater;

7. Subdural space;

8. Arachnoid mater;

9. Subarachnoid space.

• In the case of the paramedian approach, the spinal needle passes through the following layers:

1. Skin;

2. Subcutaneous adipose tissue;

3. Paraspinal muscles;

4. Ligamenta flava;

5. Dura mater;

6. Subdural space;

7. Arachnoid mater;

8. Subarachnoid space.

• Performing SA is impossible without understanding the dermatomes concept, the importance of which is crucial for successfully conducting the procedure. A dermatome is an area of skin innervated by sensory fibers from a single spinal nerve root. In addition to covering the area of the skin incision and manipulation, the block level should account for visceral pain, which requires higher coverage than superficial cutaneous innervation. A dermatome map is necessary to assess the sensory block: adequacy of anesthesia, symmetry of block, and risk of excessive block of thoracic segments.

• For instance, the tenth thoracic (Th10) dermatome corresponds to the umbilicus and is suitable for hip surgery and vaginal deliveries, the sixth thoracic (Th6) to the xiphoid process, thereby ensuring an adequate block for urological and surgical procedures, and the fourth thoracic (Th4) to the nipples; for cesarean section, the target sensory level is Th4 (considering peritoneal irritation and visceral manipulations).

Physiological aspects of spinal anesthesia

SA induces a block of sympathetic, sensory, and motor fibers, compensatory reflexes, and vagotonia. From a clinical point of view, it is important that the level of sympathetic block usually exceeds the level of sensory one; therefore, hemodynamic changes may occur earlier and be more significant than expected based on the level of skin sensitivity.

Cardiovascular effects.

Hypotension and bradycardia are the most common and significant physiological effects of neuraxial anesthesia, resulting from sympathetic block.

1. Mechanism of hypotension

Sympathetic nerve block causes hypotension by affecting preload, afterload, contractility, and heart rate. In other words, a decrease in systemic vascular resistance and reduced venous return lead to hypotension.

Additional risk factors for the development of hypotension include pregnancy, pre-existing hypovolemia and blood loss, age over 40, obesity, chronic alcohol consumption, and hypertension.

2. Changes in heart rate

The heart rate due to SA can increase (as a result of hypotension) or decrease (due to sympathetic fiber block, decreased venous return, the so-called Bainbridge reflex, and also as a result of the Bezold-Jarisch reflex, that is, the stimulation of left ventricular mechanoreceptors).

Risk factors for bradycardia include younger age, a heart rate less than 60 bpm, ASA class I, beta-blocker therapy, and a prolonged PR interval on the ECG.

Respiratory effects

In patients with normal lung function, spinal anesthesia affects it to the minimal extent. Minute ventilation (MV), dead space volume, shunt fraction, and arterial blood gases change insignificantly.

With a high spinal block, exhalation may be difficult due to paralysis of the intercostal and chest muscles (maximum MV and expiratory reserve volume are reduced). Therefore, patients with obstructive lung diseases, who require auxiliary muscles for adequate ventilation, should be monitored more closely after spinal block. Patients with normal lung function may complain of dyspnea in the event of a high spinal block (most often due to the inability to feel chest movement during breathing; describing the situation to the patient helps to ease their symptoms).

Since high SA does not usually affect the cervical spine, the function of the phrenic nerve and diaphragm is preserved.

Arterial blood gases do not change during high SA in patients breathing spontaneously at room temperature.

Effects on the gastrointestinal tract, liver, and kidneys.

• Increased vagal activity following sympathetic nerve block intensifies gastrointestinal tract peristalsis, which can causenausea. Nausea can also result from intestinal ischemia caused by hypotension.
• Liver blood flow correlates with arterial blood flow, so maintaining blood pressure at the proper level will not change liver perfusion. There are no limitations on the use of SA in patients with liver diseases.
• SA does not physiologically affect the kidneys as long as adequate hydration and blood pressure are maintained. In the presence of hypotension, the volume of diuresis may decrease, but it normalizes once blood pressure is restored.

Thermoregulation

Vasodilation and decreased systemic vascular resistance lead to increased heat loss and shivering, which require active methods of warming the patient (external warming and the use of heated intravenous fluids), as during general anesthesia.

Procedure technique

The practical sequence of the procedure includes the following steps:

1. Assessment and preparation of equipment

• A thoroughly taken medical history (use of anticoagulants, beta-blockers), obtaining consent, and physical examination of the patient;
• Provision of baseline monitoring of the patient’s vital functions before the procedure (non-invasive BP measurement, ECG, pulse oximetry), as well as additional methods of monitoring if necessary (invasive blood pressure measurement);
• Determination of the number of venous access sites depends on the patient’s health status and the extent of surgical intervention;
• Checking for the availability of the airway management devices and resuscitation equipment;
• Preparation of all medications (including vasopressors [phenylephrine, ephedrine] and anticholinergics [atropine and glycopyrrolate]) before positioning the patient.

2. Positioning

Sitting and side-lying positions with maximum lumbar flexion are used, the choice of which depends on the physician’s preference, the planned position of the patient during the surgical procedure, the patient’s constitution, and their comfort. The physician may perform the procedure while sitting or standing. The prone jack-knife position (lying prone on the abdomen with bent legs) is used in rare cases during proctological or perineal surgeries. For these procedures, it is preferable to use a hypobaric or isobaric solution of local anesthetic.

3. Aseptic technique

The skin is treated with an antiseptic solution (e.g., 0.5% chlorhexidine). The solution must dry completely before continuing with the procedure.

4. Puncture

A small amount of local anesthetic (1% lidocaine solution) is injected into the target puncture site to infiltrate the skin and subcutaneous tissue.

Preoperative ultrasound examination of the patient is recommended if there are difficulties palpating anatomical landmarks.

Using small-diameter needles (24-27 G) and needles with rounded, non-cutting bevels reduces the incidence of post-puncture headache.

With midline access, the guide needle is inserted at a slight cranial angle until denser tissue is felt, indicating that the tip of the needle is in the interspinal ligament.

Next, the spinal needle is inserted into the guide-needle. The needle passes through the ligamentum flavum, then the epidural space, and finally the dura mater. As the needle passes through each of these layers, a change in resistance is felt. A “pop” is often felt when piercing the dura mater. In patients with normal constitution, the depth of needle penetration from skin to dura mater is 5.0±1.0 cm.

5. Confirmation of position and injection of the anesthetic

After the “pop” or loss of resistance, the stylet should be removed, and if the maneuver is correctly performed, cerebrospinal fluid (CSF) will begin to flow through the needle tip. Outflow through smaller-diameter needles may be very slow, especially if the patient is in a side-lying position.

If there is no outflow, as the needle may be blocked by a nerve root, rotate the needle 90°.

Once free CSF outflow is ensured, slowly inject the local anesthetic at a rate of no more than 0.5 ml/sec. After completing the injection, the introducer and spinal needle are removed from the patient’s back.

Spinal anesthesia is usually administered as a single injection. Continuous spinal anesthesia via a catheter placed in the subarachnoid space is no longer used due to the frequency of neurological complications (cauda equina syndrome) and the high risk of post-puncture headache.

6. Assessment of block level and further patient management

This includes assessment of sensory and motor blocks, as well as readiness to manage potential side effects such as hypotension and bradycardia.

If necessary, sedatives should be used to achieve the optimal effect in order to reduce patient anxiety, considering the direct sedative effect resulting from SA.

Spinal anesthesia agents

In SA, local anesthetics and adjuvants should be chosen to achieve the required level of spinal anesthesia and provide the necessary duration of anesthesia for the duration of the surgical procedure.

The most important factors determining the degree of sensory block are the dose and baricity (the ratio of the density of the solution to the density of cerebrospinal fluid; anesthetics are divided into hyperbaric, isobaric, and hypobaric) of the anesthetic solution.

Baricity affects the distribution of the anesthetic solution in the subarachnoid space: hyperbaric solutions tend to “sink” within the CSF relative to the injection site, while hypobaric solutions rise above the injection site.

Hyperbaric solutions result in a faster onset of action, greater sensory block, and shorter duration of action compared to isobaric solutions. It’s important to note that local anesthetics can be neurotoxic at higher concentrations.

Local anesthetics

• Bupivacaine is a long-acting amide anesthetic that is the most commonly used for SA. It’s available as both hyperbaric and hypobaric solutions. The dose of bupivacaine may vary from 6 to 15 mg. The duration of anesthesia used for surgery is from 1.5 to 2.5 hours.
• Lidocaine is a short-acting amide local anesthetic. Although lidocaine administered intrathecally has become less popular due to the high frequency of transient neurological symptoms, it is occasionally used for short procedures. Its dose may vary from 40 to 100 mg. The duration of anesthesia used for surgery is from 45 to 75 minutes.
• Ropivacaine is a pure L-enantiomer of an amide local anesthetic, developed as a less toxic alternative to bupivacaine. It has lower cardiotoxicity. It is less effective than bupivacaine and may provide a lower motor-to-sensory block ratio compared to bupivacaine. The dose is from 15 to 20 mg. The duration of anesthesia used for surgery is from 75 to 120 minutes.
• 2-chloroprocaine is a short-acting ester anesthetic used for short procedures. The dose is from 20 to 60 mg. The duration of anesthesia used for surgery is from 30 to 50 minutes.

Adjuvants

Intrathecal opioids specified below may be added to improve intraoperative and postoperative analgesia.

• Morphine and hydromorphone (common side effects of both opioids include nausea, vomiting, and itching, as well as delayed respiratory depression requiring observation in the intensive care unit);
• Fentanyl and sufentanil provide minimal postoperative analgesia due to short duration of action and are not associated with delayed respiratory depression; itching is a dose-dependent common side effect.

α2-agonists enhance sensory and motor blocks, prolong duration of action, and promote postoperative analgesia by acting on alpha-adrenergic receptors in the spinal cord. Please see some examples below:

• Clonidine, dexmedetomidine (extend block duration but increase hypotension and sedation);
• Epinephrine (direct alpha-adrenergic agonistic action on central nervous system receptors causes vasoconstriction, reducing the absorption of local anesthetic and increasing duration of action).

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Complications of spinal anesthesia

Expected complications include hypotension and bradycardia due to sympathetic block, nausea (hence prophylaxis with antiemetics is advised), patient discomfort due to motor and sensory block, as well as shivering.

Potentially severe complications of SA

• High spinal block is the most severe complication because of respiratory failure requiring airway management, severe hypotension, and bradycardia;
• Post-puncture headache is less likely when using needles of smaller diameter;
• Epidural and/or spinal hematomas are rare but extremely serious complications that can lead to compression of the spinal cord or nerve roots and progressive neurological deficits: back pain, sensory disturbances, weakness or numbness in the legs, and pelvic organ dysfunction;
• Infectious complications include arachnoiditis and abscess at the puncture site;
• Transient neurological symptoms (TNS) are more commonly associated with lidocaine and manifest as pain or burning in the lower back, buttocks, or posterior thighs after the regression of spinal block and resolve within 2-5 days;
• Root injury is a rare complication manifested by sharp pain or paresthesia during puncture and subsequent symptoms thereafter.