An epidural is a form of regional anesthesia where medication is delivered into the epidural space, a narrow corridor just outside the spinal cord’s protective membrane. A thin, flexible catheter is placed here to continuously bathe the spinal nerves with a mixture of local anesthetic and often a low-dose opioid. This process blocks pain signals from the lower body to the brain, providing highly effective pain relief. While this procedure is successful for the vast majority of people (approximately 98%), a small percentage experience an inadequate or failed block. This failure can be attributed to factors related to the placement technique, a person’s unique internal anatomy, or the distinct pattern of pain relief itself.
Issues Related to Catheter Placement
The epidural space is filled with fat, connective tissue, and blood vessels, making the exact positioning of the catheter a delicate process. Even small deviations in the catheter’s tip can prevent the medication from spreading evenly along the nerve roots, which is necessary for uniform pain relief. Incorrect initial placement, such as being mistakenly placed in the layer of fat just beneath the skin, will cause a complete absence of block.
A common issue is catheter migration, where the tube moves after a successful initial placement, often due to movement during labor. The catheter tip may shift into a lateral position or migrate out of the space entirely, leading to a sudden cessation of pain relief. In approximately 5% to 7% of placements, the catheter tip can thread into an epidural vein. This intravascular placement results in no pain relief, as the drug is absorbed systemically before it can block the nerves.
The needle used to access the space can accidentally puncture the dura mater, the tough membrane surrounding the spinal cord, a situation sometimes called a “wet tap.” If the catheter is then threaded through this puncture, it enters the intrathecal space (where spinal fluid is located), which can lead to a very dense but often too high or too localized block. Misplacement into the subdural space results in an unpredictable, often patchy, and delayed onset of pain relief.
Individual Anatomical and Physiological Factors
Even when the catheter is positioned perfectly, a person’s unique anatomical structure can create physical barriers to the medication’s spread. The epidural space contains naturally occurring dorsal midline septae, which are thin sheets of connective tissue that partially divide the space. These septae can physically block the local anesthetic from crossing the midline, forcing the medication to pool on only one side and causing a unilateral block.
Individuals with a history of prior back surgery or trauma may have scar tissue or calcification within the epidural space. This scar tissue can distort the normal path of the nerve roots and create adhesions that prevent the anesthetic from reaching all the necessary areas. Furthermore, the ligamentum flavum, a thick ligament the needle passes through, can have natural midline gaps, which may cause the catheter to deviate laterally instead of remaining in the center.
Individual response to the medication also plays a role in block effectiveness. The volume of fat within the epidural space varies significantly between people and has an inverse relationship with block effectiveness. A higher volume of epidural fat may absorb more of the local anesthetic, potentially reducing the concentration that reaches the nerve roots. Additionally, the speed of labor progression can sometimes outpace the onset of the epidural medication. If contractions intensify rapidly, the pain signals can breakthrough before the drug has had sufficient time to take full effect.
Understanding Different Types of Inadequate Blocks
When an epidural is reported as not working, the failure is often categorized by the specific pattern of pain that remains.
Unilateral Block
One common pattern is a unilateral block, where the patient feels complete numbness and relief on one side of their body, but the pain remains intense and unaffected on the opposite side. This indicates that the medication is concentrated against one wall of the epidural space, likely due to a physical barrier or the catheter tip’s position.
Segmental Block
A segmental block occurs when pain relief only happens in a narrow, horizontal band across the abdomen or back, leaving segments above and below the band still painful. This limited vertical spread is often accompanied by “sacral sparing,” meaning the lowest nerve roots that transmit pain from the vagina and rectum remain unaffected. The sacral nerves are larger and covered by a thicker layer of tissue, making them resistant to the local anesthetic.
Patchy Block
A patchy block, sometimes described as “window pane” pain, is characterized by areas of near-complete numbness interspersed with small, intense spots of pain. This pattern suggests the local anesthetic has spread inconsistently, missing specific nerve rootlets as it diffuses through the epidural space.
Breakthrough Pain
When a patient initially experiences good pain relief followed by a return of intense pain, this is classified as breakthrough pain. This typically happens when the continuous infusion rate is insufficient for the patient’s current labor intensity or when the catheter has migrated, leading to the drug no longer reaching the target nerves effectively.