Phenol is also known as carbolic acid. It is an aromatic compound with the chemical formula C6H5OH. It is comprised of a hydroxyl group bonded to a phenyl ring. It is prepared as a sterile, commercial-grade phenol in a maximum concentration of 6.7% solution in water. Although it has poor solubility in water, it is highly soluble in organic solvents such as alcohol and glycerol. The addition of small amounts of glycerol may increase its concentration to 15%. The shelf-life of phenol exceeds 1 year when the solution is refrigerated and not exposed to light. Phenol turns red on exposure to sunlight and air because of oxidation.

Phenol can be diluted with saline and is compatible when mixed with radiocontrast dye to allow fluoroscopic guidance during injection of the agent
and to monitor spread of the solution. When mixed with glycerol, it slowly diffuses from the solution. With glycerol, phenol forms an extremely viscous hyperbaric solution from 4% to 10%. This is an advantage when injected intrathecally because it allows for limited spread localized in a small target area. For the injection of intrathecal phenol, the patient is placed in lateral decubitus position with the painful side down. When mixed with water, concentrations of 3% to 10% are possible. However, in this formulation, it is a more potent neurolytic with a wider spread.

The choice of diluents depends on the extent of neurolysis desired. At low concentrations, phenol has local anesthetic properties (the use of higher concentrations of phenol also predisposes to a higher incidence of vascular injury). With increasing concentrations, phenol causes increasing neural damage. At concentrations >5%, there is protein coagulation with segmental demyelination and Wallerian degeneration. At 6%, nociceptive fibers are lysed. With higher concentration, axonal abnormalities and nerve root damage are seen. Concentrations <5% may only provide a sensory block. Some studies have shown that 5% phenol is equipotent to 40% alcohol. Degeneration takes 14 days, and regeneration is complete in 14 weeks. Effects of the block cannot be evaluated until after 24 to 48 hours, to allow time for the local anesthetic effect of phenol to dissipate. The neurolytic effect may be clinically evident only after 3 to 7 days. If inadequate pain relief is obtained after 2 weeks, this may indicate incomplete neurolysis and require repetition of the procedure.

The subsequent fibrosis that occurs following phenol injection makes nerve regeneration more difficult, but not impossible. Nerve regeneration can occur as long as the nerve cell body is intact, at a rate of 1 to 3 mm/day. Nerve arborization and neuroma formation can occur at the site of nerve disruption and can be a focus of a neuropathic type of pain.

A wedge pillow is used to help roll the patient 45 degrees posteriorly so the dorsal column of the effected side is most inferior, or the painful side down. The bevel should face down and the injection must be done slowly because of the high viscosity of the drug. Warming the phenol can make it easier to inject. Phenol has systemic side effects, including central nervous system (CNS) stimulation, cardiovascular depression, nausea, and vomiting. Systemic doses of >600 mg can cause convulsive seizures and CNS depression. Doses < 100 mg are less likely to cause serious side effects. Accidental intravascular injection causes tinnitus and flushing. Chronic toxicity can cause hepatic and renal damage. Phenol is metabolized in the liver by conjugation to glucuronides and oxidation to equinol compounds or to carbon dioxide and water. Excretion of metabolites is via the kidneys.