Neuraxial Anesthesia

POA EXAM 4

Vertebral Canal extend from ______ to __________?

foramen magnum to tip of the coccyx

Total number of vertebrae in the human spine

33

Which spinal region has a concave curvature?

Cervical and lumbar

Which spinal region has a convex (posterior) curvature?

thoracic and sacral

Number of cervical vertebrae

7

Number of thoracic vertebrae

12

Number of lumbar vertebrae

5

Number of sacral vertebrae

5

Number of coccygeal vertebrae

4

Anatomical opening containing the spinal cord and nerve roots

vertebral foramen

Poster vertebral structure that defines the anatomic midline + bony projection used as landmark

spinous process

Paired structures connecting the vertebral body to the lamina

pedicles

Paired posterolateral bony projections of a vertebra

transverse processes

Paired posterior bony plates that fuse to form the spinous process

laminae

Anatomical direction of cervical and thoracic spinous processes

inferiorly (downward); longer and more acutely angled

Anatomical structure formed by stacked vertebral notches and articulating surfaces

intervertebral foramina

Primary function of the intervertebral foramina

protects and provides passage for spinal nerves

Structures that innervate the facet joints

branches of adjacent spinal nerves

Region of skin innervated by single spinal nerve, clinical manifestation of facet joint injury-related pain

dermatomal distribution

Group of muscles innervated by single spinal nerve, clinical manifestation of facet joint injury-related muscle spasms

myotomal distribution

Lumbar vertebra characteristics (size, spinous process shape, overlap)

largest size, shorter/broader spinous processes, least overlap

Anatomical structure that typically hinders needle access to an upper thoracic interspace (e.g., T5 space)

inferior portion of the superior spinous process (e.g., T4)

Anatomical structure formed by the incomplete lamina of the last sacral vertebra (usually at the S3-S4 vertebral level)

sacral hiatus (key clinical landmark for accessing the epidural space)

Bony horn-like projections that guard the sacral hiatus

sacral cornua (cornu)

Primary clinical purpose of palpating the sacral cornua

landmarks for caudal anesthesia

Clinical concern prompting decreased use of pediatric caudal blocks noted in the handwritten text

risk of spinal injury

Total number of spinal nerve pairs and their anatomical range

31 pairs (C1 to S5)

Bundle of lower spinal nerves extending beyond the terminus of the spinal cord

cauda equina

Three major posterior spinal ligaments serving as neuraxial landmarks

supraspinous, interspinous, and ligamentum flavum

Spinal ligament that connects the apices of the spinous processes

supraspinous ligament

Three structural layers of the supraspinous ligament

superficial, middle, and inner

Spinal ligament that runs directly between adjacent spinous processes

interspinous ligament

Spinal regions where the interspinous ligament is poor/absent versus very thin

cervical: absent/poor, lumbar: very thin

Strongest posterior spinal ligament, composed of yellow elastic tissue

ligamentum flavum

Anatomical structures joined together by the ligamentum flavum

adjacent laminae (vertebral arches)

Average thickness of the ligamentum flavum at the L2-3 interspace

3 to 5 mm

Clinical/tactile significance of the ligamentum flavum during neuraxial blocks

thick, distinct tactile feel (indicator to slow down)

Superior anatomical boundary of the spinal cord

medulla oblongata

Inferior spinal level terminus of the spinal cord in most adults vs in children

L2 - L3 vertebra (adults) & L3 vertebra (children)

Length range of the adult spinal cord

42 to 45 cm

Common pediatric neuraxial block levels

L3-L4

Terminal tapered tip of the spinal cord

conus medullaris

Two regions where the spinal cord is physically enlarged

C4 to T1 and L2 to S3 (supply the upper and lower limbs)

True or False: Spinal cord levels directly correspond with vertebral levels

FALSE

Vertebral body levels spanning the adult lumbosacral enlargement (L2-S3)

T11 to L1

Outermost, thickest, and most protective spinal meningeal layer

dura mater

Tactile sensation expected when a needle punctures the dura mater

pop (leather feel)

Middle, delicate, nonvascular, spiderweb-like spinal meningeal layer

arachnoid mater

Anatomical structure where the arachnoid mater ends inferiorly

S2

Potential space located between the dura mater and the arachnoid mater

subdural space

Anatomical space located between the arachnoid mater and the pia mater (Primary target space for a spinal anesthesia injection)

subarachnoid space

Four major structures contained within the subarachnoid space

CSF, spinal nerves, blood vessels, support ligaments

Anatomical range of the subarachnoid space

brain to S2

Innermost, delicate, and highly vascular spinal meningeal layer. (Meningeal layer in thin, direct contact with the outer surface of the spinal cord)

pia mater

Anatomical boundaries of the epidural space & Anatomical range of the epidural space

outside the dural sac but inside the vertebral canal from the base of the cranium to the base of the sacrum

Average width of the epidural space

~5 mm

Five major structures contained within the epidural space

epidural veins, fat, lymphatics, segmental arteries, and nerve roots

Clinical complication risk associated with the vascular structures in the epidural space

bleeding and hematoma formation

Key physiological characteristic of epidural veins

valveless

Two patient populations where epidural veins become engorged and swollen

obese and pregnant patients

Clinical risk associated with utilizing a lateral approach for an epidural block

increased risk of hitting blood vessels

Clinical presentation of an accidental local anesthetic injection into the subdural space

patchy and oddly high blocks

True or False: The subdural space is NOT a desirable target for intentional anesthetic injection

TRUE

Primary goal of neuraxial anesthesia

block pain transmission from areas of injury, disease, or surgery

Spinal nerve levels providing cutaneous innervation to the umbilical area

T7 to T11

Vertebral body level located directly posterior to the umbilical area

L3 vertebra

Clinical methods used to assess the level of a neuraxial block

motor or sensation tests

Target ion channel and specific state required for local anesthetic binding

Blocks sodium (Na+) channels on the cytoplasmic (inner) surface of the nerve. LA selectively binds to channels in the Activated Open and Inactivated Closed states.

Relationship between nerve firing rate and local anesthetic blockade sensitivity

faster firing increases blockade (frequency-dependent)

Number of consecutive nodes of Ranvier required to be blocked for myelinated axons

3 adjacent nodes

Anatomical definition of a differential block

interruption of autonomic nerve transmission without sensory or motor blockade

Dermatomal level relationship: Sympathetic block vs. Sensory block

sympathetic is 2 to 6 dermatomes higher

Dermatomal level relationship: Motor block vs. Sensory block

motor is 2 dermatomes lower

Why are smaller, lightly myelinated/unmyelinated nerve root fibers blocked first?

located closer to the nerve root surface

Why are larger, myelinated motor and sensory fibers harder to block

located deeper within the nerve bundle

Nerve fiber type possessing the highest susceptibility to local anesthetic blockade (++++ sensitivity)

B fibers (preganglionic autonomic)

Nerve fiber type that is largest in diameter, lowest in sensitivity (+), and hardest to block

A-alpha (motor)

Nerve fiber types corresponding to the second wave of contact and blockade

C and A-delta fibers

Specific temperature sensation lost first during local anesthetic onset

cold before hot

Nerve sensation that is rarely lost due to its profound resistance to blockade

deep pressure

Initial anatomical site of action where epidural local anesthetics act first

dural cuffs (sheath)

Structures epidural local anesthetics must diffuse through after longitudinal spread to reach nerve roots

meninges and dural cuffs

Three clinical variables that can be increased to achieve a denser epidural block

concentration, volume, and time

Primary mechanism of initial drug distribution in the epidural space vs. the spinal space

bulk flow (epidural) vs. diffusion (spinal)

Order of Nerve Fiber Blockade (Onset):

B→ C → A-delta → A-gamma→ A-beta → A-alpha

(Autonomic → Sensory → Motor);

But Can Delta Give Better Actions?

Order of Blockade by Clinical Sensation

superficial pain > sympathetic (T1-L2) > Temperature > Motor > touch > proprioreception > deep pressure

Blockade Assessment correlates with sympathetic block

loss of temperature (ice test)

Bloackade Assessment used to determine the sensory level of a block

distinguishing sharp vs. dull sensations (blunt needle, sharp object)

Spinal levels evaluated by assessing a patient's ability to raise their knees

L2-L3

Spinal levels evaluated by assessing a patient's ability to tense their abdominal muscles

T6-T12

Spinal levels evaluated by assessing a patient's ability to dorsiflex their foot

S1-S2

Dermatome level corresponding to the nipple line

T4

Dermatome level corresponding to the xiphoid process landmark

T6

Dermatome level corresponding to the inferior angle of the scapula

T7

Dermatome level corresponding to the umbilicus landmark

T10

Dermatome level corresponding to the inguinal ligament

L1

Dermatome level corresponding to the iliac crest landmark (Tuffier's line)

L4

Required sensory level for a hemorrhoidectomy

S2-S5

Required sensory level for foot surgery

L2-L3 (knee)

Required sensory level for lower extremity surgery

L1-L3 (inguinal ligament)