Clinical Exercise: Paramedian Lumbar Puncture

Scenario:

A 19 year old patient is helped to the emergency room complaining of a high fever, headaches, neck stiffness, nausea, photophobia (sensitivity to light), and signs of confusion. The patient's friends indicate that they board at college and describe the symptoms developing over the last day or so. Upon physical examination you conclude neck stiffness is typical of 'nuchal rigidity' (spasms of the posterior muscles of the neck limiting flexion of the head).

Diagnosis/Considerations:

The combination of symptoms/signs are a classic presentation of meningitis (infection of the meninges of the brain). The two most common microorganisms leading to meningitis are viral and bacterial. To definitively diagnose meningitis and the microorganism involved, you decide to perform a lumbar puncture to retrieve a sample of the cerebrospinal fluid for evaluation.

Mechanism:

Meningitis is an acute inflammation of the brain and spinal cord meninges. Bacterial meningitis is serious and can be deadly, with death occurring in as little as a few hours.

A lumbar puncture involves inserting a needle into the lumbar cistern to take a biopsy of cerebrospinal fluid (adults contain 125-250ml of cerebrospinal fluid). In 94% of people the spinal cord ends at vertebral level L1-2 and in 6% of people at L2-3. Thus, inserting a needle into the vertebral canal at, or inferior to, L3-4 avoids risk to the spinal cord. Lumbar punctures are commonly performed at L4-5, but levels L3-4 or L5-S1 are also acceptable. The loose nerve bundles of the cauda equina float in the cerebrospinal fluid within the lumbar cistern and are 'bumped' out of the way without damage as the needle moves amongst them.

A lumbar puncture needle can be inserted at the midline (median approach) or to the side (a paramedian approach). The needle is angled such that the tip passes the gap between the lamina of L3 and L4, then penetrates through the ligamentum flavum and through the dura of the spinal cord to enter the lumbar cistern.

Treatment (perform these steps on the donor in a mock-procedure):

PREPARATION: Send a member of your team to the instrument supply tables to fetch one syringes and one long 3-4" needle (please take only the supply numbers indicated to ensure sufficient supplies are available for all tables).

The needle can be inserted multiple times so each team member can attempt the procedure (if the needle becomes blunted badly by repetitively impacting bone switching to the second needle may be needed).

Note: We will perform the procedure with the skin reflected away from the lumbar region. In the donor, increased skin density due to embalming makes palpation and positioning artificially difficult compared to the living.

1) Open the syringe packet and remove the syringe.

2) Open the needle package and remove the needle (note the protective sheath and keep that over the needle till it is to be used).

3) Place the needle onto the syringe, locking it in place with a twisting motion into the threads on the syringe (this is a luer-lock, a common syringe connection style and one of the most stable designs).

4) Using your fingers palpate for the iliac crest on each hip and move your finger in an imaginary line between the iliac crests to find the tip of the spinous process of L4 (typically approximately in line with the iliac crests).

5) Work a finger superiorly to find the spinous process of L3, this identifies the target region for needle insertion between L3-4.

6) Ready the needle by removing the protective cap from the needle.

CAUTION: exercise care when handling needles to prevent impaling yourself or a colleague. Needle-stick injury is the leading cause of accidental blood-borne pathogen infection in hospital staff.

7) Place the tip of the needle halfway between the spinous processes of L3-4 and 1cm lateral to the midline (in the living, the area would be cleaned with a disinfectant and a local anesthesia injected).

8) Angle the needle 10-15 degrees inferiorly (so that the needle points slightly cranially) and 10-15 degrees laterally (so the needle points towards the midline) which aims the needle at the gap between the vertebral laminae.

9) Inset the needle through the deep back muscles into the inter-laminar gap, entering the lumbar cistern.

In the living, these are felt as a slight 'give' (reduction in resistance to advance) as you penetrate through the ligamentum flavum and dura into the lumbar cistern, but in the donor this is rarely felt distinctly.

The insertion depth will be ~6-7cm deep on most individuals. If the needle encounters bone before that depth you may have missed the gap and impacted on the lamina. Withdraw the needle slightly off the bone and pull back on the plunger. If no fluid enters the syringe it is likely you are superficial to the lamina and the needle tip is in deep back muscles.

To correct, pull the needle out several centimeters and change the angle to make a second entry (or third...or fourth...). This lets you 'walk' the needle contact point up the bone of the lamina until you enter the gap. 

If you are at the 6-7cm depth and contact bone, the tip may have penetrated across the vertebral canal and impacted the body of the vertebra. In that case withdraw the needle slightly so the tip returns to be within the lumbar cistern.

10) When you are inserted into the lumbar cistern, pull back on the plunger gently to draw 0.1-0.2ml of fluid into the syringe.

You may need to rotate the tip slightly to dislodge any tissue pressed against the needle opening so you can draw up a fluid sample. In the living, the cerebrospinal fluid is under pressure and pushes out through the open end of a sampling needle.

In the donor, there is usually a distinct red tint to the fluid from residual red blood cells that were not flushed during embalming. Inject any fluid back into the lumbar cistern to allow the next member of your team to try the lumbar puncture.

Sometimes, you may have a donor with little to no fluid in the lumbar cistern. If you are sure you're at the correct depth, but cannot withdraw any fluid, insert a second needle/syringe parallel to the first one at a point slightly superior. If they are at the same depth when contacting bone you are not in the spinal canal. If the second syringe contacts bone earlier than the first, then you have entered the lumbar cistern with the first syringe.

NOTE: when finished the procedure ensure that the syringe and attached needle are collected in one of your trays and transported to the sharps containers for disposal.
Do not attempt to re-cap a needle - Re-capping is a leading cause of needle-stick injury.
Never transport needles by hand - Always transport entire syringe/needles in a tray. 

Outcome:

While testing of the cerebrospinal fluid sample is conducted, you place your patient on prophylactic wide-spectrum antibiotics as initial treatment should not be delayed for the lab test. The laboratory report comes back with a positive for bacteria and you adjust the antibiotic to one with higher efficacy against the detected microorganism. Following treatment the patient makes a full recovery and returns to their college studies.