A “wet tap” is the common term for an accidental dural puncture (ADP) that occurs during epidural placement, typically for labor analgesia. This complication involves the epidural needle inadvertently passing beyond the epidural space and puncturing the dura mater, the protective membrane surrounding the spinal cord. It is called a “wet tap” because the clinician may see cerebrospinal fluid (CSF) leak back through the needle, confirming the puncture. Occurring in an estimated 0.5% to 4% of procedures, this puncture allows CSF to leak out, which is the source of the main risk: a severe headache known as a post-dural puncture headache (PDPH).
The Mechanism of Dural Puncture
Understanding a wet tap requires knowing the anatomy of the spinal column. The spinal cord is protected by three layers of membranes called the meninges; the dura mater is the outermost layer. The epidural space, the target area for the anesthetic, lies just outside the dura mater.
The epidural space is narrow, requiring precise needle placement. An accidental dural puncture occurs when the needle advances too far, passing through the dura mater and the underlying arachnoid mater. This breach opens a path from the subarachnoid space, which holds the CSF, into the epidural space.
The fluid flowing back through the needle is cerebrospinal fluid (CSF), which bathes the brain and spinal cord. CSF provides a cushioning effect and maintains pressure within the cranium and spinal column. The dural puncture allows CSF to leak out, reducing this internal pressure, which directly causes the subsequent symptoms.
Understanding Post-Dural Puncture Headache (PDPH)
The main consequence of a wet tap is the development of a post-dural puncture headache (PDPH), occurring in up to 60% to 80% of affected patients. The headache results directly from CSF leakage, which causes intracranial hypotension, or low pressure inside the skull. With reduced CSF volume, the brain is no longer adequately supported by the fluid cushion.
When the patient sits or stands upright, the brain shifts or “sags” downward due to gravity, causing traction on pain-sensitive structures like the meninges and blood vessels. This mechanical pulling is the source of the severe pain characteristic of PDPH. The headache is positional, worsening significantly within 15 minutes of sitting or standing and improving within 15 minutes of lying flat.
The pain is bilateral, felt in the front or back of the head, and can be accompanied by neck stiffness, nausea, or changes in hearing and vision, such as tinnitus. PDPH usually manifests within 24 to 72 hours following the dural puncture, though it can be delayed up to a week. Its characteristic positional nature distinguishes it from other postpartum headaches or migraines.
Management and Treatment
The management of PDPH begins with conservative measures, as the condition may resolve spontaneously within one to two weeks. Initial treatment focuses on symptom relief and encouraging the body’s natural healing process. This involves ensuring the patient stays well-hydrated and administering standard pain medications.
Caffeine is included in the conservative plan, either orally or intravenously, because it causes cerebral vasoconstriction, which can increase intracranial pressure. While these steps manage discomfort, they do not seal the dural leak itself. If the headache is severe, persistent, or significantly interferes with the patient’s ability to function, a more definitive treatment is required.
The gold standard for treating persistent, severe PDPH is the epidural blood patch (EBP). This procedure is similar to the original epidural placement, where a needle is inserted into the epidural space near the leak site. The patient’s own blood, typically 15 to 20 milliliters, is drawn from an arm vein and injected into the epidural space.
The injected blood forms a clot that seals the hole in the dura mater and stops the CSF leakage. The blood patch is highly effective, with success rates for complete or partial relief exceeding 75% after the first attempt. Patients are advised to lie flat for a period following the procedure to maximize the patch’s efficacy. While some back pain is common afterward, headache relief is usually immediate.