Subarachnoid Neurolytic Block

Relevant Anatomy

This procedure is suitable for patients who have significant cancer pain that is not responding to any other modalities. The pain should be localized into few sensory segments of the spinal cord. Sensory fibers arise from the dorsal aspect of the spinal cord while the motor fibers arise from the ventral aspect. The main goal of this block is to keep the neurolytic material around the sensory portion of the spinal cord (dorsal surface). This can be accomplished by a combination of positioning the patient in a way that the neurolytic materials (either hypo- or hyperbaric) remain localized in the dorsal surface of the spinal cord. Table 28.11 differentiates between the use of hypobaric and hyperbaric solution for neurolysis. The goal is selective destruction of the dorsal root ganglia. The physician should keep in mind that lower thoracic and lumbar nerves leave the spinal cord at a higher level when they exit the neural foramina. This is extremely important for the lower thoracic and lumbar neurolysis. For example, pain at the level of T9 nerve distribution requires neurolysis at T6, T7, and T8 levels. Volume to be injected is 0.6 ml within 80-90 s for each level with either agent. Diagnostic block using local anesthesia with the same baricity is essential prior to the actual block, to help evaluate the effects of the block, including any possible side effects. Complete neurological examination before and after the block is mandatory to evaluate for any neurological deficits. Extensive discussion with the patient and possibly family members about the risks and benefits of the block should take place, and possible side effects or complications should be explained at this time.

Table 28.11 Differences between hypobaric and hyperbaric solutions in subarachnoid neurolysis.

Hypobaric neurolytic block

hyperbaric neurolytic block

Agent and concentration

Alcohol 100%

Phenol 6%

Additives usually used

None

Glycerin

Uses

Intractable unilateral cancer pain involving few

Same

dermatome

Patient position

Lateral semi-prone position with area to be

Lateral semi-supine position with area to be

injected placed uppermost

injected lower most

Physical characters

Unstable at room temperature. Vials should

Stable

opened just prior to injection

Baricity

Hypobaric (i.e., will raise up in CSF). Specific

Hyperbaric (i.e., will sink in CSF). Specific

gravity of alcohol is 0.8 compared to 1.007 of

gravity of glycerin is 1.25 compared to 1.007

CSF

of CSF

Effect

2-4 days

1-2 days

Position after the block

Keep patient in the same position for 45 min

Same

after the procedure

Fluoroscopy use

Mandatory

Same

Number of needles

Based on the level of pain

Same

Type of needle

22 or 25 G 3.5 or 5 in. based on the patient

20-22 G needle (glycerin is very viscous and is

size

harder to inject) 3.5 or 5 in. based on the

patient size

Level of needle insertion

Based on the level of pain (see text)

Same

Final position of the needle

Free flow of clear CSF

Same

CSF=cerebrospinal fluid

CSF=cerebrospinal fluid

Indications

Subarachnoid block may be suitable for unilateral intractable cancer pain not responding to conventional treatment, including treatment with opioids. The best location of pain for this block is unilateral cancer pain in the chest wall or abdominal wall, but it can be done for other locations such as lower and upper extremity pain. Pain should be somatic in nature. If the pain is visceral in nature, sympathetic block is advisable (see above).

Complications

The most common complication is motor weakness due to unintentional spread of neurolytic agents to the ventral surface of the spinal cord, and temporary or permanent weakness of the urinary or rectal sphincters. Post-dural puncture headache, especially in young patients, can occur.

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