Spinal anesthesia: Introduction, Procedure and Complication

Nerve block can be divided into central neuraxial block and peripheral block. Central neuraxial block are further of two types viz. spinal (subarachnoid) and epidural.  

Spinal anesthesia is a form of neuraxial regional anesthesia involving the injection of a local anesthetic or opioid into the subarachnoid space, generally through a fine needle. First planned spinal anesthesia was performed by August Bier in 1886 AD. It is indicated in surgeries of lower limb, perineum, pelvis and lower abdominal, inguinal, urogenital and rectal surgeries. It acts on nerve root of sensory nerves and dorsal root ganglia.

Agents

Agents used are hyperbaric bupivacaine heavy (0.5% in 8% dextrose), ropivacaine, levobupivacaine, tetracaine, procaine and 2-hloroprocaine.

Lidocaine having rapid onset and shorter duration of action is not used because it can cause cauda equina syndrome (CES) and transient neurological symptoms. Ropivacaine has less CNS and cardiac toxicity than bupivacaine. Hyperbaric solution is used because its specific gravity is 1.0227 to 1.0278 which is higher than CSF (1.003-1.008), the drug settle down and the chance of high spinal is reduced.

Needles

Needles used are dura cutting and dura separating. Quincke is a type of dura cutting needle with sharp end and Whiatcre and Sprotte with blunt end and side injection are dura separating needle. Blunt end and smaller gauge needle reduces the incidence of Postdural Puncture Headache (PDPH).

Structures pierced by the needle

Skin→ subcutaneous tissue→ supraspinous ligament→ interspinous ligament→ ligamnetum flavum→ duramater→ arachnoid mater.

Significance of surface anatomy

It can be given below L₁ in adult and below L₃ in children as the spinal cord ends at these level respectively. There is no damage of nerve below these levels because cauda equina floats in CSF and is pushed away by advancing needle.

Important landmarks that we need to know is that while performing spinal anesthesia needle is directed more cephalad in thoracic block than cervical and lumbar because spinous process of thoracic region slanted in caudal direction and nearly overlap. Also, C7 is the most prominent spinous process, T7 corresponds to inferior angle of scapula, L4 or L4 L5 interspace corresponds to the highest line drawn between both iliac crests (Tuffier’s line), line connecting posterior superior iliac spine crosses S2 posterior foramina.

Differential blockade

Differential blockade is seen in neuraxial anesthesia because the concentration of anesthesia decreases with increase in distance from level of injection. It results in sympathetic blockade judged by temperature sensitivity which is 2 segment more cephalad than sensory block judged by pain which is 2 segment more cephalad than motor blockade.

Procedure

Patient positioning

 It can be given in sitting position, lateral decubitus(fetal position) with patient’s hips and knee flexed, neck and shoulder flexed towards knees and in Prone or Buie’s(Jackknife) position patient lies in prone position with OT table flexed under his flanks(used for anorectal procedures).Most commonly done in sitting position with flexion of spine.

Approaches

Approaches are midline and paramedian approaches.

In midline approach, spine is palpated and L₄ and L₅ vertebrae are identified along Tuffier’s line. Then, sterile field is established using chlorhexidine solution. After the solution has dried up, a skin wheal is raised in the interspace between L4 and L5 vertebrae and the needle is injected in the midline slightly cephalad. As needle courses deeper, first resistance is felt due to needle encountering supraspinous and interspinous ligament. The second resistance is felt when it encounters ligamentum flavum. Loss of resistance indicates that needle has reached subarachnoid space. After that, the stylet is removed and free flow of CSF is observed. Then the anesthesia is injected slowly and the needle is removed.

In paramedian approach, needle is inserted 2 cm lateral to midline and the needle is directed at an angle of 25  to midline. It is done for difficult procedures like severe arthritis, kyphoscoliosis where patient can’t be positioned easily.

Factors affecting the spread of anesthesia

Some of the factors affecting the spread of anesthesia are:

• Baricity of solution: A solution is made hyperbaric by adding glucose and hypobaric by adding sterile water or fentanyl. In head up position, hyperbaric solution moves caudal while hypobaric moves cephalad. Isobaric is made by adding CSF in solution in1:1 ratio. Hyperbaric solution settles at most dependent area of spine T4-T8 in supine position and produces block below it.
• Position of patient: In head up position, hyperbaric solution settles caudal whereas in head down it moves cephalad. In lateral position, it moves towards dependent area.
• Drug dosage: Larger the dose, more cephalad is the level of anesthesia.
• Site of injection: Higher the space chosen, higher is the level of block achieved.
• Other factors: Age, CSF, curvature of spine, drug volume, intra-abdominal pressure, needle direction, patient height, pregnancy etc. CSF volume is inversely proportional to spread of anesthesia. Increase in intra-abdominal pressure cause engorged epidural vein thus decreasing CSF volume like in pregnancy and ascites. CSF volume also decreases with age.

Complications

Due to excessive response to drugs

• Hypotension: Sympathetic blockade from T₅ to L₁ reduces vasomotor tone which cause pooling of blood in lower extremities and decrease venous return.
• Profound hypotension and bradycardia: High spinal-combined effect of inhibition of cardio accelerator fibres (T₁-T₄ blockade) and no compensatory vasoconstriction above block causes profound hypotension and bradycardia.
• Respiratory paralysis: Due to hypotension and hypoperfusion to medullary center.
• Cardiac arrest
• Anterior spinal artery syndrome: Due to compression of spinal artery following hematoma, abscess, epidermoid tumor.
• Urinary retention: It is due to blockage of S2-S4 root fibres which decreases urinary bladder tone and inhibit voiding reflex.

Due to needle

• Backache
• Dural puncture or leak: Postdural puncture headache, bilateral frontal or retro-orbital or occipital acute in onset, throbbing, may radiate to neck, aggravated by sitting or standing and relieved in supine position. Risk groups include larger needle, cutting needle, young age, female and pregnancy. Mechanism includes leakage of CSF from dural defect→intrcranial hypotension→loss of CSF faster than formation→traction on structures supporting brain→headache.

• Diplopia: Due to traction on 6^th cranial nerve due to loss of CSF.
• Tinnitus
• Neural injury: Nerve root damage-pain during injection and paresthesia, Spinal cord damage, Cauda equina syndrome
• Intraspinal or epidural hematoma: Usually in presence of bleeding disorder.
• Inadequate anesthesia
• Inadvertent intravascular injection: High serum level can affect CNS and lead to unconsciousness and CVS leads to hypotension arrhythmia. Early signs include tinnitus, lingual sensation.
• Arachnoiditis: Due to use of drug with preservative, non-infective type occurs.
• Meningitis: Aseptic due to use of detergent in anesthesia in past, infective due to streptococcus viridans (oral flora) which made use of face mask mandatory.

Chlorhexidine is the most effective antiseptic used for its prevention. Headache due to chemical meningitis is a high pressure headache which should be differentiated from PDPH by applying pressure on jugular vein→increase in meningitis but relieved in PDPH (Queckenstedt’s test).

Due to drug toxicity

• Systemic toxicity
• Transient neurological symptoms: Mostly due to hyperbaric lignocaine and in male undergoing surgery in lithotomy position. Symptoms is back pain radiating to legs.
• Cauda equina syndrome

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References

1. Morgan and Mikhail’s Clinical Anesthesiology, 6th edition.
2. Ajay Yadav’s Short Textbook of Anesthesia, 6th edition.
3. Spinal Anesthesia, Wikipedia.
4. Research article “The centennial of spinal anesthesia”, Wulf HF.