The incidence of cauda equine syndrome is 32 in 1.2 million subarachnoid blocks and 32 in 450,000 epidurals. The syndrome is a combination of bowel and bladder dysfunction with paresis of the lower extremities and sensory dysfunction in a patchy distribution. It is often associated with hyperbaric lidocaine (lidocaine combined with dextrose) or highly concentrated lidocaine (5%) and suggests neurotoxicity caused by the agent itself. However, this syndrome has also been observed with other local anesthetics, such as 0.5% hyperbaric bupivacaine. Therefore, the etiology may be related to insufficient mixing of local anesthetic with cerebrospinal fluid, with resulting neurotoxicity. Cauda equina syndrome has also been linked to the use of microcatheters for repeated or continuous dosing where concentrated, local anesthetic pools around the sacral nerve roots (microcatheters are now prohibited by the FDA).

Transient neurologic symptoms consist of back pain radiating to the lower extremities, without sensory or motor deficits, occurring after resolution of the spinal block and subsiding after several days. These symptoms are
most commonly seen with lidocaine use and in outpatients having surgery in the lithotomy position and are thought to be secondary to neurotoxicity (probably not related to concentrations of <5%). Whether the true etiology is neurotoxicity or soreness from muscle strain is unclear.

The incidence of other neurologic deficits is 20 in 1.2 million subarachnoid blocks and 20 in 450,000 epidurals. Permanent or transient neurologic deficits probably represent direct damage to the nerve roots. These deficits are commonly associated with multiple attempts at accessing the intrathecal space. Paraplegia can result if there has been direct contact between spinal cord and needle. Frequently observed are loss of bowel and bladder function; paralysis of the biceps femoris muscles; and sensory loss in the dorsal thigh, saddle region, or great toes as often seen with damage to the conus medullaris. Deficits may also be due to neurotoxicity of the local anesthetic agent.

The incidence of meningitis or arachnoiditis is 29 in 1.2 million subarachnoid blocks and 29 in 450,000 epidurals. Meningitis and arachnoiditis may be infectious or noninfectious; present as pain coupled with other neurologic symptoms; and are identified using radiography, which shows clumped nerve roots. Meningitis and arachnoiditis have been previously attributed to detergents used in procaine or solutions used to clean reusable spinal needles.