An independent 69-year-old gentleman is brought to the emergency department by emergency m… — USMLE Step 3

An independent $69$ -year-old gentleman is brought to the emergency department by emergency medical services after his wife found him unable to walk. He reports severe, unrelenting pain between his shoulder blades that started $3$ days ago. Over the past $8$ hours, he developed profound weakness in both legs and an inability to void.

His medical history is significant for type 2 diabetes mellitus, hemodialysis-dependent end-stage renal disease, and sick sinus syndrome. He had a permanent dual-chamber pacemaker implanted $15$ years ago; device interrogation reveals it is a legacy, non-MRI-conditional model.

Vital signs include a temperature of $38.9^\circ\text{C}$ ( $102.0^\circ\text{F}$ ), blood pressure of $112/68\text{ mm Hg}$ , heart rate of $104/\text{min}$ , and respiratory rate of $16/\text{min}$ . Physical examination demonstrates marked point tenderness over the T9 and T10 spinous processes. Neurologic evaluation reveals $2/5$ strength in the proximal and distal lower extremities bilaterally. There is absent sensation to pinprick and temperature below the level of the umbilicus. Patellar and Achilles reflexes are $3+$ bilaterally with sustained clonus, and a Babinski sign is present bilaterally. Post-void residual volume by bladder scan is $650\text{ mL}$ .

Laboratory studies show a leukocyte count of $19,200/\mu\text{L}$ and an erythrocyte sedimentation rate of $115\text{ mm/h}$ . The physician suspects a spinal epidural abscess with acute cord compression.

Which of the following is the most appropriate next step in imaging?

Computed tomography (CT) myelography of the thoracic spineAnswer
B
Magnetic resonance imaging (MRI) of the thoracic spine with intravenous gadolinium
C
Non-contrast computed tomography (CT) of the thoracic spine
D
Intravenous contrast-enhanced computed tomography (CT) of the thoracic spine
E
Radionuclide bone scintigraphy

Answer

Computed tomography (CT) myelography of the thoracic spine

The correct answer is Computed tomography (CT) myelography of the thoracic spine. This patient presents with the classic triad of a spinal epidural abscess (SEA): fever, focal back pain, and neurologic deficits. The acute onset of paraparesis, a T10 sensory level (indicated by anesthesia below the umbilicus), and urinary retention point to an emergent thoracic spinal cord compression. The gold standard for evaluating SEA is an MRI of the spine with intravenous gadolinium. However, this patient has a legacy, non-MRI-conditional pacemaker, which is an absolute contraindication to MRI due to the risk of device movement, thermal injury, and fatal arrhythmias. In patients who cannot undergo MRI, CT myelography is the diagnostic imaging modality of choice. It involves the injection of radiopaque contrast directly into the subarachnoid space, followed by a CT scan. This technique provides excellent delineation of the thecal sac and reliably identifies extrinsic compression from an epidural abscess, allowing for emergent surgical planning.

Step-by-Step Solution

Identify the clinical syndrome

The patient has the classic triad of a spinal epidural abscess (SEA): fever, localized back pain, and progressive neurologic deficits (paraparesis, sensory level at T10, urinary retention).

Recognizing acute spinal cord compression is critical to guide emergent diagnostic imaging and prevent permanent paralysis.

Determine the gold standard imaging modality

MRI of the spine with intravenous gadolinium is the preferred and most sensitive study for this condition.

MRI provides superior soft-tissue contrast to visualize the epidural fluid collection and assess the degree of thecal sac compression.

Assess for patient-specific contraindications

The patient has a 15-year-old legacy pacemaker that is non-MRI-conditional.

Subjecting this patient to an MRI magnetic field could cause catastrophic device movement, heating, or lethal arrhythmias, making MRI strictly contraindicated.

Select the best alternative imaging modality

CT myelography is selected as the next step in imaging.

When MRI is unavailable or contraindicated, CT myelography (using intrathecal contrast) is the most accurate alternative to visualize the subarachnoid space and diagnose extrinsic cord compression.

Key Concept

When magnetic resonance imaging (MRI) is strictly contraindicated (e.g., due to a legacy pacemaker), computed tomography (CT) myelography is the alternative imaging modality of choice to evaluate for a spinal epidural abscess and acute spinal cord compression.

Estimated Time:1m 30s

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