Page 480 - Basic _ Clinical Pharmacology ( PDFDrive )

466 SECTION V Drugs That Act in the Central Nervous System

Subarachnoid space
in spinal canal

Epidural block

T12

Base of
spinal cord L1
(Conus medullaris)

Dura L2

Filum terminale
Epidural space
L3
Spinal block
Cauda equina
L4

L5

S1

Epidural space S2
in caudal canal

Caudal
block

FIGURE 26–4 Schematic diagram of the typical sites of injection of local anesthetics in and around the spinal canal. When local anesthet-
ics are injected extradurally, it is referred to as an epidural block. A caudal block is a specific type of epidural block in which a needle is inserted
into the caudal canal via the sacral hiatus. Injections around peripheral nerves are known as perineural blocks (eg, paravertebral block). Finally,
injection into cerebrospinal fluid in the subarachnoid (intrathecal) space is referred to as a spinal block.

Clinical Block Characteristics anatomic considerations noted earlier for peripheral nerve trunks,
onset with peripheral blocks is more variable, and proximal motor
In clinical practice, there is generally an orderly evolution of block weakness may precede onset of more distal sensory loss. Addition-
components beginning with sympathetic transmission and pro- ally, anesthetic solution is not generally deposited evenly around a
gressing to temperature, pain, light touch, and finally motor block. nerve bundle, and longitudinal spread and radial penetration into
This is most readily appreciated during onset of spinal anesthesia, the nerve trunk are far from uniform.
where a spatial discrepancy can be detected in modalities, the most With respect to differential block, it is worth noting that
vulnerable components achieving greater dermatomal (cephalad) “successful” surgical anesthesia may require loss of touch, not just
spread. Thus, loss of the sensation of cold (often assessed by a wet ablation of pain, as some patients will find even the sensation of
alcohol sponge) will be roughly two segments above the analgesic touch distressing during surgery, often fearing that the proce-
level for pinprick, which in turn will be roughly two segments dure may become painful. Further, while differences may exist
rostral to loss of light touch recognition. However, because of the