Residency · Residency · Anesthesiology
Neuraxial Anesthesia: Spinal and Epidural Techniques
Spinal Cord and Vertebral Anatomy
Vertebral Column
The vertebral column consists of 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal vertebrae. The spinal cord terminates at the L1-L2 level in adults (the conus medullaris), though it extends to L3 in neonates. The thecal sac (dural sac) continues inferiorly and ends at S2. Lumbar puncture is performed at or below the L3-L4 interspace to avoid injury to the spinal cord.
Layers Traversed (Midline Approach)
A midline neuraxial needle passes through nine layers in sequence: skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum (the key landmark for epidural placement, identified by loss of resistance), the epidural space (which contains fat, veins, and nerve roots), dura mater, arachnoid mater, and finally the subarachnoid space (containing CSF and the spinal cord or cauda equina).
Surface Landmarks
Tuffier's line (the intercristal line) connects the iliac crests and crosses the L4 spinous process or the L4-L5 interspace. However, its reliability is limited in obese patients, and ultrasound improves accuracy of level identification. Other useful landmarks include the C7 prominent spinous process and T7 at the inferior tip of the scapula.
<image>Sagittal cross-section of the lumbar spine showing all tissue layers from skin to subarachnoid space. Each layer is labeled with its depth and characteristics. The epidural space is highlighted between the ligamentum flavum and dura, and the subarachnoid space containing CSF and cauda equina is shown. The conus medullaris is depicted ending at L1-L2, with a dotted line at Tuffier's line crossing L4.</image>
Spinal (Subarachnoid) Anesthesia
Indications
Spinal anesthesia is indicated for lower abdominal, pelvic, perineal, and lower extremity surgery, cesarean delivery, urologic procedures such as TURP and cystoscopy, and hip and knee arthroplasty.
Technique
The patient is positioned sitting or in lateral decubitus. A midline or paramedian approach is used at the L3-L4 or L4-L5 interspace. A 25G or 27G pencil-point needle (Whitacre or Sprotte) is preferred because it produces a significantly lower rate of post-dural puncture headache compared to cutting-tip needles. Free flow of CSF is confirmed before injection, and the drug is injected slowly over 10-15 seconds.
Intrathecal Drug Pharmacology
Bupivacaine (Most Common)
Bupivacaine is available in a hyperbaric formulation (0.75% in 8.25% dextrose) at doses of 7.5-15 mg and an isobaric formulation (0.5%) at doses of 10-15 mg. The baricity of the solution determines how it spreads within the subarachnoid space: hyperbaric solutions settle to the most dependent areas, and positioning the patient controls the distribution of the block.
Lidocaine
Intrathecal lidocaine was historically common but has been largely abandoned because of its association with transient neurologic symptoms (TNS), a burning pain in the buttocks and legs that resolves over days but occurs in 15-30% of patients.
Other Agents
Mepivacaine provides intermediate duration with a lower TNS risk than lidocaine. Preservative-free chloroprocaine offers a short duration of 40-90 minutes and is ideal for ambulatory cases. 2-Chloroprocaine is gaining popularity as a short-acting alternative for outpatient procedures.
| Intrathecal Agent | Dose | Onset | Duration | Baricity | Key Feature |
|---|---|---|---|---|---|
| Bupivacaine (hyperbaric) | 7.5–15 mg | 3–5 min | 90–150 min | Hyperbaric | Position-dependent spread; most commonly used |
| Bupivacaine (isobaric) | 10–15 mg | 3–5 min | 90–150 min | Isobaric | Less position-dependent |
| Lidocaine | 40–100 mg | 1–3 min | 60–90 min | Variable | TNS risk 15–30%; largely abandoned |
| Mepivacaine | 40–80 mg | 2–4 min | 90–120 min | Isobaric | Lower TNS risk than lidocaine |
| 2-Chloroprocaine (PF) | 30–60 mg | 2–3 min | 40–90 min | Isobaric | Shortest duration; ideal for ambulatory |
Intrathecal Adjuvants
Fentanyl (10-25 mcg) improves block quality and slightly extends duration, with dose-dependent pruritus as the main side effect. Morphine (0.1-0.2 mg) provides prolonged postoperative analgesia lasting 12-24 hours but carries a risk of delayed respiratory depression that mandates respiratory monitoring for 24 hours. Clonidine (15-75 mcg) prolongs the block but causes hypotension and sedation. Epinephrine (0.1-0.2 mg) prolongs block duration by reducing vascular absorption but is less commonly used in current practice.
Factors Affecting Spinal Block Height
The most important controllable factor affecting spinal block height is the baricity of the solution. Other factors include patient position during and immediately after injection, the total dose of local anesthetic (the mass of drug, not the volume or concentration), and the speed of injection (faster injection produces higher spread). Patient height has a minimal clinical effect. Pregnancy and obesity reduce CSF volume, resulting in a higher block for a given dose.
Epidural Anesthesia and Analgesia
Indications
The most common indication for epidural anesthesia is labor analgesia. Other indications include cesarean delivery (surgical epidural), thoracic and upper abdominal surgery (using a thoracic epidural), postoperative analgesia for major surgery, and chronic pain management.
Technique
The epidural space is identified using the loss of resistance (LOR) technique with either saline or air. The epidural is typically placed at the vertebral level corresponding to the surgical dermatome: T6-T8 for upper abdominal surgery, T8-T10 for lower abdominal surgery, and L3-L4 for lower extremity and obstetric applications. A 17G or 18G Tuohy needle is used, and the catheter is threaded 3-5 cm into the epidural space. Placement is confirmed with a test dose.
Epidural Test Dose
The classic test dose consists of 3 mL of 1.5% lidocaine with 1:200,000 epinephrine (45 mg lidocaine plus 15 mcg epinephrine). Intrathecal injection is signaled by a rapid, dense motor block within 3-5 minutes. Intravascular injection is signaled by a heart rate increase of 20 bpm or more, or a systolic blood pressure increase of 15 mmHg or more, within 60 seconds. Limitations include the fact that beta-blockers blunt the tachycardia response and that laboring patients have baseline heart rate variability that can confound interpretation.
Epidural Drug Pharmacology
For labor analgesia, bupivacaine 0.0625-0.125% is standard, while 0.5% is used for surgical anesthesia. Ropivacaine 0.1-0.2% provides analgesia and may produce less motor block. Lidocaine 2% is used for surgical epidural anesthesia with a faster onset. 2-Chloroprocaine 3% provides the most rapid surgical anesthesia, making it ideal for emergency cesarean delivery top-up. Epidural opioids such as fentanyl (1-2 mcg/mL) or hydromorphone are commonly added to the local anesthetic infusion.
Epidural Maintenance
Programmed intermittent epidural bolus (PIEB) is now the preferred maintenance strategy over traditional continuous epidural infusion (CEI). PIEB delivers automated intermittent boluses that generate higher injection pressures, resulting in better circumferential spread of local anesthetic within the epidural space. Compared to CEI, PIEB provides more uniform analgesia, less motor block, less breakthrough pain, and lower total local anesthetic consumption. Patient-controlled epidural analgesia (PCEA) is added for breakthrough pain management.
<image>Step-by-step illustration of the epidural technique showing: (1) patient in sitting position with landmarks identified, (2) Tuohy needle advancing through tissue layers with loss-of-resistance syringe attached, (3) cross-sectional view at the moment of loss of resistance as the needle enters the epidural space, and (4) catheter threading through the needle into the epidural space with the tip position 3-5 cm beyond the needle. Each step includes key technical points annotated.</image>
Combined Spinal-Epidural (CSE)
Technique
The needle-through-needle technique is most common: a Tuohy needle is placed in the epidural space, then a long spinal needle is passed through the Tuohy needle into the subarachnoid space. The spinal dose is administered, the spinal needle is removed, and the epidural catheter is threaded. An alternative is the separate interspace technique, where the spinal and epidural are placed at different vertebral levels.
Advantages
The CSE combines the rapid onset of spinal anesthesia with the ability to extend duration through the epidural catheter. A lower spinal dose can be used because the epidural catheter provides backup. This technique is popular for labor analgesia (the low-dose spinal "walking epidural") and cesarean delivery.
Concerns
The epidural catheter remains untested until it is actually needed, meaning its function is not confirmed until an attempt is made to use it. Epidural drugs may potentially migrate intrathecally through the dural puncture site. The PDPH rate is higher than with epidural alone because of the intentional dural puncture with the spinal needle.
Complications of Neuraxial Anesthesia
Hypotension
Hypotension is the most common side effect of spinal anesthesia. It results from sympathetic blockade, which causes vasodilation, decreased preload, and decreased SVR. The effect is more pronounced with higher block levels, in the setting of hypovolemia, and in obstetric patients where aortocaval compression compounds the problem. Prevention strategies include IV fluid co-loading (crystalloid administered at the time of the spinal) and vasopressor prophylaxis (phenylephrine or norepinephrine infusion for cesarean delivery).
Post-Dural Puncture Headache (PDPH)
PDPH occurs in 1-2% of cases with 25-27G pencil-point needles but up to 70% with 16-18G cutting needles. The pathophysiology involves CSF leaking through the dural puncture, causing intracranial hypotension with brain sagging that stretches the meninges and bridging veins. It presents as a positional headache that worsens in the upright position and improves when supine, typically beginning 24-48 hours after the puncture. Associated symptoms include neck stiffness, nausea, photophobia, hearing changes, and cranial nerve palsies (the sixth cranial nerve being most commonly affected). Treatment for mild cases is conservative (hydration, caffeine, analgesics). The definitive treatment is an epidural blood patch, using 15-20 mL of autologous blood, which has a success rate exceeding 85%.
Epidural Hematoma
Epidural hematoma is rare but devastating, with an incidence of approximately 1:150,000 for epidural and 1:220,000 for spinal anesthesia. Risk factors include anticoagulation, coagulopathy, thrombocytopenia, and difficult or traumatic needle placement. It presents with severe back pain, progressive motor weakness, and bowel or bladder dysfunction. Emergency MRI and neurosurgical decompression within 8-12 hours are critical for the best neurologic outcome. Prevention depends on strict adherence to ASRA anticoagulation guidelines.
High/Total Spinal
High or total spinal results from excessive cephalad spread of intrathecal local anesthetic. Symptoms progress from upper extremity weakness to dyspnea (from intercostal and phrenic nerve blockade), hypotension, bradycardia, loss of consciousness, and respiratory arrest. Management is supportive: secure the airway (intubate if needed), administer vasopressors (epinephrine for bradycardia and hypotension), provide IV fluids, and maintain supportive care until the block regresses.
Neurologic Injury
Permanent neurologic injury from neuraxial anesthesia is rare but includes cauda equina syndrome, adhesive arachnoiditis, and anterior spinal artery syndrome. Risk factors include multiple attempts, paresthesia during placement, and injection into neural tissue. Use of pencil-point needles and avoiding injection when pain or paresthesia is elicited help reduce this risk.
<image>Comparison diagram showing spinal versus epidural anesthesia side by side. Left panel: spinal needle (pencil-point) entering the subarachnoid space with CSF return, injection site below conus medullaris. Right panel: Tuohy needle in the epidural space with catheter threaded, loss-of-resistance technique depicted. Below each, a table compares onset time, duration, typical drugs, controllability, and common complications of each technique.</image>
Clinical Pearls
The conus medullaris ends at L1-L2 in adults; a spinal should never be performed above L2-L3 to avoid cord injury. Pencil-point spinal needles (Whitacre, Sprotte) dramatically reduce the incidence of PDPH compared to cutting-tip (Quincke) needles. For cesarean delivery, hyperbaric bupivacaine is preferred because positioning controls spread; a standard dose of 10-12 mg with intrathecal fentanyl and morphine is the norm. PIEB is now the preferred epidural maintenance strategy over CEI for labor analgesia because it provides better distribution and less motor block. If an epidural blood patch fails on first attempt, a second patch has a success rate exceeding 95%. After an unintentional dural puncture with an epidural needle, threading the catheter intrathecally and using it as a continuous spinal is a valid management option. Vasopressors should always be drawn and immediately available before performing spinal anesthesia.
References
- Hadzic A, ed. Hadzic's Textbook of Regional Anesthesia and Acute Pain Management. 2nd ed. McGraw-Hill; 2017.
- Arendt K, Segal S. Why epidurals do not always work. Reviews in Obstetric Gynecology. 2008;1(2):49-55.
- Van de Velde M, Carvalho B. Remifentanil for labor analgesia: an evidence-based narrative review. International Journal of Obstetric Anesthesia. 2016;25:66-74.
- Choi PT, Galinski SE, Takeuchi L, et al. PDPH is a common complication of neuraxial blockade in parturients: a meta-analysis. Canadian Journal of Anesthesia. 2003;50(5):460-469.
- George RB, Allen TK, Habib AS. Intermittent epidural bolus compared with continuous epidural infusions for labor analgesia: a systematic review and meta-analysis. Anesthesia & Analgesia. 2013;116(1):133-144.
