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Spinal Anesthesia - A Complete Overview
1. What Is Spinal Anesthesia?
Spinal anesthesia (also called subarachnoid block or intrathecal block) is the injection of local anesthetic - with or without adjuvants - directly into the cerebrospinal fluid (CSF) in the lumbar subarachnoid space. It produces dense, rapidly-onset sensory AND motor block of the lower body.
Key anatomical fact: In most adults, the spinal cord ends at L1-L2. Below that, the lumbar and sacral nerve roots float freely in a generous volume of CSF (the cauda equina). This is exactly why lumbar spinal injections are safe - there is no cord to puncture, just floating roots bathed in CSF.
- Goodman & Gilman's Pharmacological Basis of Therapeutics, p. 519
2. Anatomy of the Injection Site
The needle passes through these layers, in order:
- Skin and subcutaneous fat
- Supraspinous ligament
- Interspinous ligament
- Ligamentum flavum
- Epidural space (not the target)
- Dura mater
- Subdural space (narrow, not the target)
- Arachnoid mater
- Subarachnoid space - target reached when free CSF flows back
The classic approach is midline at L3-L4 or L4-L5 (below the conus medullaris). A paramedian approach is used when spinal stenosis, obesity, or degenerative disease makes midline access difficult.
3. Mechanism of Action
Local anesthetics block voltage-gated sodium channels in nerve axon membranes, preventing depolarization and impulse conduction. In the CSF, all nerve roots passing through the subarachnoid space are exposed. This produces a differential block - fiber sensitivity from most to least:
| Fiber Type | Function | Blocked First? |
|---|---|---|
| B fibers (preganglionic sympathetic) | Vasomotion | Yes - most sensitive |
| C fibers (pain, temperature, slow) | Nociception | Yes |
| A-delta fibers | Sharp pain, temperature | Yes |
| A-beta fibers | Touch, pressure | Intermediate |
| A-alpha fibers (motor) | Skeletal muscle | Last - require highest conc. |
This explains why sympathetic block typically extends 2 dermatomal levels higher than the sensory block, which in turn extends higher than the motor block.
- Goodman & Gilman's Pharmacological Basis of Therapeutics, p. 519
4. Physiological Effects (Know These Cold)
Cardiovascular
The most important and most dangerous physiological consequence is sympathetic blockade.
- Sympathetic fibers exit the cord from T1-L2. Spinal anesthesia blocks these preganglionic fibers.
- Venous dilation >> arterial dilation: blood pools in capacitance vessels, reducing venous return, reducing cardiac output.
- Cardiac accelerator fibers (T1-T4): if the block reaches this level, bradycardia occurs (Bezold-Jarisch reflex can cause sudden asystole).
- Result: hypotension - the most common complication. Managed with IV fluids (crystalloid co-loading), vasopressors (ephedrine, phenylephrine), and modest head-down tilt.
At high block levels, the parasympathetic nervous system becomes relatively dominant because its sympathetic counterpart is blocked.
- Goodman & Gilman's Pharmacological Basis of Therapeutics, p. 519; Fuster and Hurst's The Heart, 15th Edition
Respiratory
- At low-to-mid thoracic block levels, respiratory function is largely preserved (diaphragm = C3-C5, largely spared).
- Intercostal muscle paralysis reduces accessory muscle use but rarely matters in healthy patients.
- At total spinal levels (cervical spread), apnea occurs - a life-threatening emergency requiring immediate airway control.
Gastrointestinal
- Beneficial: Sympathetic fibers from T5-L1 inhibit peristalsis. Their blockade produces a contracted, quiescent bowel - ideal for abdominal and pelvic surgery. Parasympathetic tone is relatively unopposed, which improves gut motility.
5. Indications
Spinal anesthesia is ideal for:
- Lower limb surgery (hip arthroplasty, knee surgery, fracture fixation)
- Urologic procedures (TURP - awake patient detects hyponatremia/bladder perforation)
- Anorectal and perineal procedures
- Cesarean section (preferred over general anesthesia - lower maternal morbidity and mortality)
- Lower abdominal surgery (hernia repair, appendectomy)
- Obstetric "saddle block" for operative vaginal delivery
- Sabiston Textbook of Surgery, p. 348; Morgan and Mikhail's Clinical Anesthesiology, 7e
6. Factors Controlling Block Height
This is critical for the clinician. Block height determines what surgery is possible AND what complications may occur.
A. Drug Factors
Baricity (density of solution relative to CSF density = 1.00059 g/mL):
| Baricity | Behavior | Positioning Used |
|---|---|---|
| Hyperbaric (+ dextrose) | Sinks to dependent regions | Sitting = saddle block; lateral = one-sided block |
| Isobaric | Stays near injection site | Position-independent, predictable spread |
| Hypobaric (+ sterile water) | Rises to non-dependent regions | Prone position - perianal surgery |
Hyperbaric solutions have the most predictable spread with least interpatient variability.
Dose: Higher dose = greater cephalad spread and longer duration.
- Miller's Anesthesia, 10e, p. 6044-6045
B. Patient Factors
Higher block than expected with:
- Obesity
- Pregnancy (engorged epidural veins compress subarachnoid space)
- Increased intraabdominal pressure
- Elderly patients (more sensitive to intrathecal agents)
- Abnormal spinal curvature or previous spine surgery
C. Injection Technique
- Rapid injection → turbulent flow → unpredictable spread (inject slowly)
- Patient position during and immediately after injection critically determines block level with hyperbaric solutions
7. Local Anesthetics Used for Spinal Anesthesia
| Drug | Concentration | Total Dose | Baricity | Duration |
|---|---|---|---|---|
| Bupivacaine | 0.5% (isobaric) or 0.75% (hyperbaric) | 7.5-22.5 mg | Both available | 75-200 min |
| Ropivacaine | 0.5-1% | 12-25 mg | Isobaric | 80-210 min |
| Lidocaine | 1.5-5% | 30-100 mg | Hyperbaric | 30-60 min |
| Chloroprocaine | 3% | 30-60 mg | Isobaric | 40-80 min |
| Mepivacaine | 1.5% | 30-80 mg | Isobaric | 120-180 min |
Bupivacaine is the most widely used drug for spinal anesthesia. Lidocaine use has declined due to transient neurologic symptoms (TNS).
- Sabiston Textbook of Surgery, Table 20.8, p. 348
8. Adjuvants (Added to Prolong/Enhance the Block)
Opioids
| Drug | Dose | Onset | Duration | Notes |
|---|---|---|---|---|
| Fentanyl | 10-30 mcg | 10-20 min | 4-6 hrs | Good for ambulatory surgery |
| Sufentanil | 5-7.5 mcg | Fast | 4-6 hrs | Commonly used in obstetrics |
| Morphine | 0.1-0.3 mg | Slow (hydrophilic) | 12-24 hrs | Best for post-op analgesia |
| Diamorphine (UK) | 0.3-0.4 mg | Intermediate | ~12 hrs | Common in UK for C-section |
Morphine's hydrophilicity means slow onset but prolonged analgesia - and also delayed respiratory depression (monitor for up to 24 hours).
Vasoconstrictors
- Epinephrine (0.1-0.6 mg) and phenylephrine (2-5 mg) prolong block duration by α1-mediated vasoconstriction reducing systemic uptake.
- Epinephrine may also act via α2 receptors for direct analgesia enhancement.
- Miller's Anesthesia, 10e, p. 6061-6062
9. Needle Types
| Needle | Design | PDPH Risk |
|---|---|---|
| Quincke (cutting) | Beveled cutting tip | Higher |
| Whitacre (pencil-point) | Solid tip, side port | Lower |
| Sprotte (pencil-point) | Solid tip, side port | Lower |
| Gertie Marx | Pencil-point variant | Lower |
Pencil-point needles separate rather than cut dural fibers, resulting in a smaller effective dural opening and lower incidence of post-dural puncture headache (PDPH). Smaller gauge also reduces PDPH - 25-27 gauge preferred.
10. Complications
A. Hypotension
- Most common complication (~30% of patients)
- Mechanism: sympathectomy → vasodilation → decreased venous return
- Treatment: IV fluid co-loading (crystalloid equally efficacious as colloid), vasopressors (phenylephrine or ephedrine), left uterine displacement in pregnancy
B. Post-Dural Puncture Headache (PDPH)
- Mechanism: CSF leaks through dural hole → traction on meninges → positional headache (worse sitting/standing, better lying flat)
- Risk factors: young age, female sex, larger needle, cutting needle tip
- Treatment: bed rest, caffeine, hydration; definitive: epidural blood patch (autologous blood 15-20 mL injected epidurally to tamponade the leak)
C. Bradycardia and Cardiac Arrest
- High block blocks T1-T4 cardiac accelerators
- Bezold-Jarisch reflex: stretch receptors in an empty ventricle → vagal bradycardia → asystole
- Treatment: atropine, epinephrine, CPR if needed
D. Total Spinal Anesthesia
- Excessive cephalad spread reaches cervical levels
- Apnea, loss of consciousness, cardiovascular collapse
- Management: immediate airway control, vasopressors, CPR
E. Transient Neurologic Symptoms (TNS)
- Back pain and radicular symptoms (buttocks/legs) within 24 hours of resolution, lasting 2-7 days
- Especially associated with lidocaine (5% hyperbaric) and lithotomy position
- Usually self-limiting, no permanent deficit
F. Post-Dural Puncture Backache
- Common but usually resolves within days
G. Urinary Retention
- Sacral block → bladder atony → retention; monitor and catheterize as needed
H. Neurologic Injury (Rare)
- Paraplegia: ~0.1 per 10,000; multifactorial
- Cauda equina syndrome: associated with microcatheters and hyperbaric lidocaine (continuous spinal - now largely abandoned)
- Epidural/spinal hematoma: rare but catastrophic; risk elevated with anticoagulants (follow ASRA guidelines)
- Meningitis / arachnoiditis: rare infectious/chemical complication
- Sabiston Textbook of Surgery, p. 349; Miller's Anesthesia, 10e; Goodman & Gilman's, p. 519
11. Contraindications
| Absolute | Relative |
|---|---|
| Patient refusal | Coagulopathy / anticoagulant use (follow ASRA guidelines) |
| Infection at injection site | Hypovolemia / severe hemodynamic instability |
| Elevated intracranial pressure | Fixed cardiac output states (severe AS, HOCM) |
| Prior spine surgery at injection level | |
| Neurologic disease (progressive - use caution) | |
| Severe spinal deformity |
Note: Existing neurological disease is NOT an absolute contraindication; however, it is prudent to avoid spinal anesthesia in progressive neurologic diseases.
- Goodman & Gilman's, p. 519
12. Spinal vs. Epidural - Key Differences
| Feature | Spinal | Epidural |
|---|---|---|
| Space | Subarachnoid (CSF) | Epidural space |
| Onset | Rapid (2-5 min) | Slower (15-30 min) |
| Dose of LA | Very small (mg range) | Much larger (mL range) |
| Block quality | Dense, complete | Patchy possible |
| Catheter possible? | Rarely (continuous spinal) | Yes - allows titration |
| PDPH risk | Present (dural puncture) | Low (unless accidental) |
| Duration | Fixed single shot | Unlimited via catheter |
| Use in labor | Saddle block / CSE | Preferred for labor analgesia |
The combined spinal-epidural (CSE) technique uses spinal for rapid dense block at surgery start, then epidural catheter for postoperative analgesia.
13. Dermatomal Levels Required for Common Surgeries
| Procedure | Required Level |
|---|---|
| Lower extremity | L2-L3 |
| Hip arthroplasty | T10 |
| TURP / bladder surgery | T10 |
| Appendectomy | T6 |
| Cesarean section | T4-T6 |
| Peritoneum (visceral pain) | T4 |
| Uterus / ovaries | T10 |
- Miller's Anesthesia, 10e, p. 6044
14. Special Situations
Cesarean Section
Spinal anesthesia is the preferred technique in the US for elective C-section. Advantages over general anesthesia: lower maternal morbidity, reduced aspiration risk, mother awake for birth, better postoperative analgesia. Typical: hyperbaric bupivacaine 0.75% + fentanyl 10-25 mcg + morphine 0.1-0.2 mg.
Obstetric Saddle Block (Operative Vaginal Delivery)
Small-dose hyperbaric anesthetic (e.g., bupivacaine 2.5-5 mg + fentanyl 12.5-25 mcg), patient seated for 3 min post-injection to limit block to lumbosacral roots.
Elderly Patients
More sensitive to intrathecal agents; reduce doses by 20-30%. Hypotension more severe and less well-tolerated. No difference in postoperative delirium vs. general anesthesia (recent evidence).
TURP
Awake patient allows detection of TURP syndrome symptoms (hyponatremia from irrigation fluid absorption) - a key advantage over GA.
- Morgan and Mikhail's Clinical Anesthesiology, 7e; Miller's Anesthesia, 10e
Summary Mnemonics
"SPINAL" for key factors affecting block level:
- Solution baricity
- Patient position
- Injection level (vertebral level)
- Needle speed (injection rate)
- Age and anatomy
- Local anesthetic dose
Complications - "HUNT Back Please Tomorrow":
- Hypotension
- Urinary retention
- Neurological injury (rare)
- TNS (transient neurologic symptoms)
- Bradycardia / cardiac arrest
- PDPH (post-dural puncture headache)
- Total spinal
Sources:
- Miller's Anesthesia, 10e (Elsevier, 2023)
- Goodman & Gilman's Pharmacological Basis of Therapeutics, 14e (McGraw-Hill)
- Morgan and Mikhail's Clinical Anesthesiology, 7e (McGraw-Hill)
- Sabiston Textbook of Surgery, 21e (Elsevier)
- Fuster and Hurst's The Heart, 15e (McGraw-Hill)
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