During morning rounds on the inpatient medical ward, a physician is urgently called to eva… — USMLE Step 3

During morning rounds on the inpatient medical ward, a physician is urgently called to evaluate a patient who developed new-onset bilateral lower extremity weakness and urinary incontinence overnight. The patient was admitted three days ago for methicillin-resistant *Staphylococcus aureus* bacteremia secondary to an infected foot ulcer. His temperature is $38.8^\circ\text{C}$ ( $101.8^\circ\text{F}$ ). Neurologic examination reveals $2/5$ motor strength in the bilateral lower extremities, absent patellar and Achilles reflexes, and markedly decreased rectal sphincter tone. Palpation of the lower back elicits exquisite focal tenderness over the L4 spinous process. The patient's past medical history is notable for severe ischemic cardiomyopathy, for which a dual-chamber permanent pacemaker was placed $12$ years ago. Medical records confirm the implanted device is a legacy model that is not MRI-conditional. Which of the following is the most appropriate next step in imaging this patient?

A
Non-contrast computed tomography (CT) of the lumbar spine
B
Technetium-99m bone scintigraphy
Computed tomography (CT) myelography of the spineAnswer
D
Magnetic resonance imaging (MRI) of the lumbar spine with and without gadolinium
E
Plain radiography of the lumbosacral spine

Answer

Computed tomography (CT) myelography of the spine

The clinical presentation (fever, focal back pain, and progressive neurologic deficits including bowel and bladder dysfunction) in a patient with MRSA bacteremia is highly suspicious for a spinal epidural abscess causing cauda equina syndrome. This is a neurologic emergency requiring urgent decompression. While magnetic resonance imaging (MRI) of the spine with contrast is the gold standard diagnostic test, this patient has an absolute contraindication to MRI due to a legacy (non-MRI-conditional) permanent pacemaker. In patients who cannot undergo MRI, computed tomography (CT) myelography is the imaging modality of choice. It involves the injection of contrast dye into the subarachnoid space followed by CT scanning, providing excellent visualization of the spinal cord, nerve roots, and any extrinsic compression by an epidural mass.

Step-by-Step Solution

Analyze the clinical presentation.

The patient has fever, focal back pain, and progressive neurologic deficits (lower extremity weakness, urinary incontinence, decreased rectal tone) in the setting of MRSA bacteremia.

This classic triad combined with a known hematogenous source highly suggests a spinal epidural abscess (SEA) causing cauda equina syndrome.

Determine the gold-standard diagnostic imaging.

Urgent MRI of the spine with and without contrast is typically required to visualize the epidural space and extent of neural compression.

MRI provides the best soft-tissue resolution of the spinal cord and surrounding structures.

Assess the patient for contraindications to the preferred imaging.

The patient has a 12-year-old legacy (non-MRI-conditional) permanent pacemaker.

Subjecting this device to the strong magnetic fields of an MRI can cause device malfunction, heating, or dislodgement, making MRI absolutely contraindicated.

Select the best alternative imaging modality.

CT myelography of the spine is chosen.

It involves the injection of intrathecal contrast followed by a CT scan, providing the necessary detailed visualization of the spinal canal to diagnose an epidural abscess when MRI cannot be performed.

Key Concept

Alternative imaging modalities for spinal cord emergencies in patients with MRI contraindications

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