This week, we are sharing another USMLE® Step 2-style practice question to test your knowledge of medical topics. Today’s case focuses on a 56-year-old man that has an occipital headache that progressed and is now associated with nausea, vomiting, and fever. The patient has a past medical history of diabetes, hypertension, and kidney transplant, for which he takes tacrolimus daily. Physical examination is notable for photophobia, nuchal rigidity, and scattered petechiae over the upper and lower extremities. Based on the image below, which of the following is the most appropriate anatomical location to perform the procedure to diagnose this patient’s clinical condition? Can you figure it out?
A 56-year-old man presents for evaluation of a headache. The patient was in his usual state of health until several days ago, when he noted an occipital headache that has progressed and is now associated with nausea, vomiting, and fever. The patient has a past medical history of diabetes, hypertension, and kidney transplant, for which he takes tacrolimus daily. Temperature is 38.0 °C (100.4 °F), pulse is 105/min, respirations are 17/min, and blood pressure is 90/65 mmHg. Physical examination is notable for photophobia, nuchal rigidity, and scattered petechiae over the upper and lower extremities. Based on the image below, which of the following is the most appropriate anatomical location to perform the procedure to diagnose this patient’s clinical condition?
A. A
B. B
C. C
D. D
E. E
Scroll down to find the answer!
The correct answer to today’s USMLE® Step 2 Question is…
D. D
Before we get to the Main Explanation, let’s look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
The incorrect answers to today’s USMLE® Step 2 Question is…
A. A
Incorrect: Choice A represents the thoracic spinal vertebral interspace, which is too high to perform this procedure safely. Lumbar puncture at this interspace risks damage to the thoracic spinal cord.
B. B
Incorrect: Choice B represents the thoracic interspace between ribs 9 and 10. This area may be appropriate for performing a thoracentesis, not a lumbar puncture.
C. C
Incorrect: Choice C represents the L2 interspace, which is too high to perform a lumbar puncture safely. Performance of this procedure at this interspace risks direct injury to the spinal cord.
E. E
Incorrect: Choice E represents the sacrum, which does not have an interspace for the performance of a lumbar puncture.
Main Explanation
This immunocompromised patient with headache, fever, and associated nuchal rigidity and photophobia most likely has meningitis. The most appropriate test to diagnose this condition is a lumbar puncture performed at the L4/L5 intervertebral space, denoted by choice D in the diagram above.
Indications for the performance of a lumbar puncture (LP) are many, namely the evaluation of infection in the central nervous system, subarachnoid hemorrhage, central nervous system malignancies, and demyelinating disorders such as Guillain Barre syndrome. LP is generally safe; however, it is contraindicated in patients with increased intracranial pressure, bleeding diathesis, or suspected spinal epidural abscess.
Performance of the procedure requires sterile technique and positioning the patient in either a lateral decubitus position or sitting upright. In an average adult, the spinal cord tapers and ends at the level between the L1 and L2 vertebrae. In neonates, it ends at L3, so the procedure is safely performed via needle insertion at the L4/L5 interspace in both populations. This space is most easily identified via direct palpation of the superior iliac crests and tracing an imaginary line from this point to the lumbar spine. Palpation of the lumbar spine at this location will reveal the L4/L5 spinous processes, and the area for needle insertion is between the two.
The insertion sequence includes skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, and finally, the subarachnoid space. Once in this space, removal of the stylet will reveal the flow of cerebrospinal fluid through the proximal end of the needle.
Major Takeaway
A lumbar puncture (LP) is most safely performed at the L4/L5 intervertebral space level, which is best identified via direct palpation of the superior iliac crests and tracing an imaginary line from this point to the lumbar spine. The insertion sequence includes skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, and finally, the subarachnoid space.
References
Doherty CM, Forbes RB. Diagnostic Lumbar Puncture. Ulster Med J. 2014;83(2):93-102.
Sternbach G. Lumbar puncture. J Emerg Med 1985; 2:199.
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