A lumbar puncture (LP), commonly known as a spinal tap, is a medical procedure used to collect cerebrospinal fluid (CSF) for diagnostic analysis. This clear fluid surrounds and cushions the brain and spinal cord, and its composition can reveal signs of disease. A specialized needle is inserted into the lower back to access the subarachnoid space and obtain a sample. Analyzing the CSF is a standard method for diagnosing conditions such as meningitis, certain autoimmune disorders, and subarachnoid hemorrhage.
Pre-Procedure Planning and Preparation
The clinician must perform a thorough safety evaluation and logistical check before starting the procedure. This involves securing informed consent after fully explaining the risks and benefits. A detailed review of the patient’s medical history is mandatory, focusing on conditions or medications that may affect blood clotting. Patients taking blood thinners, such as warfarin or certain non-steroidal anti-inflammatory drugs, may need to stop them days in advance to minimize bleeding complications.
The clinician must confirm the indication for the LP and rule out contraindications, such as signs of increased intracranial pressure, which may necessitate a preliminary head CT scan. Equipment preparation requires ensuring all necessary sterile components are available and functional. A standard lumbar puncture tray includes:
- Sterile gloves
- Antiseptic solution
- Local anesthetic (typically lidocaine)
- The spinal needle with a stylet
- A manometer for pressure measurement
- A set of four numbered collection tubes
The tubes should be arranged in numerical order to facilitate rapid and sequential collection once the needle is placed.
Patient Positioning and Site Identification
Proper patient positioning helps maximize the space between the vertebrae for a successful and safe lumbar puncture. Two primary positions are used: the lateral recumbent (lying on the side) and the sitting position. In the lateral recumbent position, the patient lies on their side, curling into a fetal position by flexing the neck, hips, and knees. This extreme flexion spreads the spinous processes, making the intervertebral space wider for needle entry.
The lateral position is preferred when measuring the opening pressure. Alternatively, the patient may sit upright, leaning forward over a tray table or pillow, which can be advantageous for patients with obesity or respiratory issues. The precise insertion point is located by palpating the superior aspects of the iliac crests, which corresponds to the intercristal line. This line intersects the spine at the L4 vertebral body or the L4/L5 interspace. The L3/L4 or L4/L5 spaces are the safest and most common targets for needle insertion, and the site is marked before the skin is cleansed.
Executing the Procedure
After landmark identification, the site is prepared using an antiseptic solution, cleaning the area in a widening circular fashion to establish a sterile field. Sterile drapes are then applied to maintain asepsis. Local anesthesia is administered next, beginning with a small superficial injection of lidocaine to create a skin wheal. This is followed by a deeper injection along the anticipated path of the spinal needle using the “walk-down” technique. This technique involves advancing the anesthetic needle, aspirating to ensure a blood vessel has not been entered, and then injecting the numbing agent while slowly withdrawing the needle.
The spinal needle, with its stylet in place, is inserted into the anesthetized interspace, usually at a slight cephalad angle, aiming toward the patient’s umbilicus. The needle is advanced through several layers of tissue, including the skin, ligaments, and epidural space. The clinician will often feel a distinct decrease in resistance or a subtle “pop” as the needle tip penetrates the dura mater and arachnoid membrane, entering the subarachnoid space. The stylet is then removed to check for the flow of cerebrospinal fluid.
Once CSF flows freely, a manometer is connected to the needle hub to measure the opening pressure, provided the patient is in the lateral recumbent position. This measurement is recorded in centimeters of water and is an important diagnostic indicator. Following the pressure reading, the CSF is collected by allowing it to drip passively into the sterile collection tubes. Approximately 1 milliliter of fluid is collected in each of the four tubes, which are sequentially filled. Sequential filling ensures accurate laboratory analysis and helps account for any blood contamination from the initial puncture. The collection process must be unhurried.
Immediate Post-Procedure Management
Once the final collection tube is filled, the stylet is reinserted into the spinal needle before withdrawal. Immediate pressure is applied to the puncture site with sterile gauze for several minutes to prevent local bleeding or CSF leakage. The area is then cleaned of any remaining antiseptic solution and covered with a small adhesive bandage.
The patient is instructed to lie flat on their back following the procedure to help minimize the risk of a post-dural puncture headache caused by continued CSF leakage. The collected CSF samples must be accurately labeled with the patient’s information and corresponding tube number before being sent to the laboratory. Proper handling ensures the samples are processed for the correct tests, such as cell count, glucose and protein levels, and microbiology cultures. Initial monitoring focuses on the puncture site for bleeding and on the patient’s neurological status, watching for the onset of a severe headache or changes in vital signs.