Residency · Residency · Emergency Medicine
Lumbar Puncture and Arthrocentesis
Introduction
Lumbar puncture (LP) and arthrocentesis are essential diagnostic and therapeutic procedures in emergency medicine. LP is critical for the evaluation of meningitis, subarachnoid hemorrhage, and idiopathic intracranial hypertension. Arthrocentesis is the definitive test for septic arthritis and crystal arthropathies. Proficiency in both procedures, including ultrasound-guided approaches, is a core EM competency.
Lumbar Puncture
Indications
The most time-sensitive indication for lumbar puncture is suspected meningitis or encephalitis. LP is also indicated for subarachnoid hemorrhage when CT of the head is negative but clinical suspicion remains high (CT sensitivity declines after 6 hours). Other indications include idiopathic intracranial hypertension (pseudotumor cerebri), where LP is both diagnostic and therapeutic, and less common ED indications such as CNS lymphoma, carcinomatous meningitis, and demyelinating diseases.
Contraindications
Absolute contraindications include infection overlying the puncture site and clinical signs of uncal herniation (unilateral fixed dilated pupil, decerebrate posturing). Relative contraindications include coagulopathy (INR greater than 1.5 or platelets below 50,000), anticoagulant therapy, and an intracranial mass lesion. CT before LP is indicated in patients with immunocompromise, a history of CNS disease, new-onset seizures, papilledema, altered mental status, or focal neurological deficits. A normal CT does not completely exclude the risk of herniation but significantly reduces it.
Positioning
The lateral decubitus position has the patient curled in the fetal position at the edge of the bed, which maximizes the interspinous space opening and allows accurate opening pressure measurement. The seated upright position makes landmark identification easier in obese patients, but the opening pressure is unreliable in this position. The intercristal line, connecting the iliac crests, crosses the spine at approximately the L4 spinous process or the L3-L4 interspace. The target is the L3-L4 or L4-L5 interspace, which is below the conus medullaris (the conus ends at L1-L2 in adults).
Technique
After sterile preparation with chlorhexidine and draping, the skin and deeper tissues are infiltrated with 1 percent lidocaine. The spinal needle (20 to 22 gauge, 3.5-inch standard, with a longer needle for obese patients) is inserted with the bevel oriented parallel to the longitudinal dural fibers, which reduces the incidence of post-LP headache. The needle is advanced through the skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, and ligamentum flavum (where a "pop" or decrease in resistance is felt), and then through the dura. The stylet should be removed to check for CSF flow after each 2 to 3 mm of advancement. Opening pressure is measured with an inline manometer (normal is 6 to 20 cm H2O in the lateral decubitus position). CSF is collected in 4 sequentially numbered tubes: tube 1 for cell count and differential, tube 2 for glucose and protein, tube 3 for Gram stain, culture, and additional studies, and tube 4 for a repeat cell count to help differentiate a traumatic tap from SAH.
Ultrasound-Guided LP
A curvilinear probe is used in transverse and longitudinal planes to identify the midline, interspinous spaces, and depth to the posterior dura. This technique is particularly valuable in obese patients and those with difficult landmarks. The optimal insertion point and depth are marked before sterile preparation.
<image>Anatomical cross-section illustration showing a lumbar puncture needle traversing the skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, and dura mater to reach the subarachnoid space at the L3-L4 level, with the cauda equina visible</image>
CSF Interpretation
Bacterial meningitis typically shows a WBC count above 1000 cells per microliter with neutrophil predominance, glucose below 40 mg/dL (or a CSF-to-serum ratio below 0.4), protein above 250 mg/dL, and a positive Gram stain in 60 to 90 percent of cases. Viral meningitis shows a WBC count of 10 to 500 with lymphocyte predominance, normal glucose, and mildly elevated protein. Subarachnoid hemorrhage produces elevated red blood cells that do not clear from tube 1 to tube 4, xanthochromia (present after 12 hours), and elevated opening pressure. The key distinction between a traumatic tap and SAH is that decreasing RBCs from tube 1 to tube 4 favor a traumatic tap, while xanthochromia is the most reliable distinguishing feature.
| Diagnosis | WBC (cells/μL) | Predominant Cell | Glucose | Protein | Other |
|---|---|---|---|---|---|
| Bacterial meningitis | > 1000 | Neutrophils | < 40 mg/dL (CSF:serum < 0.4) | > 250 mg/dL | Gram stain positive 60–90% |
| Viral meningitis | 10–500 | Lymphocytes | Normal | 50–100 mg/dL | Enterovirus most common |
| SAH | Elevated RBCs | RBCs (not WBCs) | Normal | Elevated | Xanthochromia after 12 hrs; RBCs do not clear tube 1→4 |
| TB/Fungal meningitis | 10–500 | Lymphocytes | Low | Very elevated | Elevated opening pressure |
Complications
Post-LP headache occurs in 10 to 30 percent of patients and is positional, worsening when upright and improving when supine. The risk is reduced by using atraumatic (Sprotte or Whitacre) needles and smaller gauge needles. Treatment includes bed rest, hydration, caffeine, and an epidural blood patch (90 percent effective) for refractory cases. Epidural or subdural hematoma is rare but carries higher risk with coagulopathy. Iatrogenic meningitis is exceedingly rare with proper sterile technique.
Arthrocentesis
Indications
Arthrocentesis is indicated for acute monoarticular arthritis, where septic arthritis must be ruled out in any acutely swollen, painful joint. It provides definitive diagnosis of crystal arthropathies: gout (monosodium urate crystals) and pseudogout (calcium pyrophosphate crystals). Traumatic hemarthrosis with a lipohemarthrosis on aspiration suggests an occult intra-articular fracture. It is also performed therapeutically to drain large effusions causing pain or limiting mobility.
Contraindications
The absolute contraindication is overlying cellulitis or skin infection at the insertion site. Relative contraindications include coagulopathy, though the need for a septic arthritis workup generally overrides this concern. For prosthetic joints, strict sterile technique should be used and orthopedics should be consulted.
Knee Arthrocentesis Technique
The superolateral approach is the most common. The patient is positioned supine with the knee fully extended. The needle is inserted 1 cm superior and 1 cm lateral to the superolateral pole of the patella, directed beneath the patella toward the intercondylar notch. The medial approach inserts the needle at the midpoint of the medial border of the patella. An 18-gauge needle attached to a large syringe (20 to 60 mL) is used for aspiration. Ultrasound guidance improves success rates, especially for small effusions.
<image>Illustration showing the superolateral approach to knee arthrocentesis with the patient supine, needle entering 1 cm superior and lateral to the patella directed under the patella, with an inset showing ultrasound probe placement and the anechoic effusion in the suprapatellar recess</image>
Synovial Fluid Analysis
Gross appearance provides initial guidance: clear fluid is normal or suggests osteoarthritis, cloudy or turbid fluid indicates an inflammatory or infectious process, bloody fluid suggests trauma or coagulopathy, and milky fluid suggests crystal disease. The WBC count further stratifies the fluid: normal is below 200 cells per microliter, non-inflammatory is below 2,000, inflammatory is 2,000 to 50,000, and septic arthritis typically exceeds 50,000 though it may be lower. Crystal analysis under polarized light reveals negatively birefringent needle-shaped crystals in gout (monosodium urate) and positively birefringent rhomboid crystals in pseudogout (calcium pyrophosphate deposition disease). The Gram stain has a sensitivity of only 50 to 75 percent for septic arthritis, so cultures should always be sent. Glucose that is markedly low (below 50 percent of serum glucose) supports the diagnosis of septic arthritis.
Other Common Arthrocentesis Sites
The shoulder is approached posteriorly, 2 cm inferior and medial to the posterolateral acromion. The ankle uses an anteromedial approach, medial to the tibialis anterior tendon at the joint line. The elbow uses a lateral approach at the center of the triangle formed by the lateral epicondyle, radial head, and olecranon. The wrist uses a dorsal approach between the third and fourth extensor compartments.
Clinical Pearls
Atraumatic (pencil-point) needles significantly reduce the incidence of post-LP headache and should be used routinely. Antibiotics should never be delayed for LP in suspected bacterial meningitis, as CSF cultures remain positive for hours after antibiotic administration. A WBC count of 50,000 in synovial fluid does not definitively confirm septic arthritis; clinical context, Gram stain, and crystal analysis are all essential to the diagnosis. Ultrasound guidance improves first-attempt success rates for both LP and arthrocentesis, particularly in obese patients. Gout and septic arthritis can coexist, so cultures should always be sent even when crystals are identified.
References
- Straus SE, Thorpe KE, Holroyd-Leduc J. How do I perform a lumbar puncture and analyze the results to diagnose bacterial meningitis? JAMA. 2006;296(16):2012-2022.
- Anavian J, Guthrie ST, Giles JB. Arthrocentesis technique. J Am Acad Orthop Surg. 2014;22(4):243-253.
- Nath S, Koziarz A, Badhiwala JH, et al. Atraumatic versus conventional lumbar puncture needles: a systematic review and meta-analysis. Lancet. 2018;391(10126):1197-1204.
- Carpenter CR, Schuur JD, Everett WW, et al. Evidence-based diagnostics: adult septic arthritis. Acad Emerg Med. 2011;18(8):781-796.
