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Difficulty: HardOrdering Imaging Studies

A 54-year-old male is evaluated in the emergency department for severe, progressive lower back pain over the past 48 hours. He notes that the pain now radiates down the posterior aspect of both legs. This morning, he developed numbness between his legs and has been unable to urinate for the past 14 hours. His medical history is significant for ischemic cardiomyopathy, for which he underwent placement of a permanent dual-chamber pacemaker in 2008. Current medications include aspirin, metoprolol, and lisinopril. Vital signs are within normal limits. Physical examination reveals palpable suprapubic fullness. Neurological assessment demonstrates decreased sensation to pinprick over the perineum, perianal region, and medial aspects of both thighs. Ankle dorsiflexion strength is 4/5 bilaterally. Digital rectal examination reveals decreased anal sphincter tone. Bedside ultrasonography of the bladder shows a postvoid residual volume of 800 mL. Which of the following is the most appropriate next imaging study for this patient?

  1. Computed tomography myelography of the lumbar spineAnswer
  2. B
    Magnetic resonance imaging of the lumbar spine without contrast
  3. C
    Non-contrast computed tomography of the lumbar spine
  4. D
    Plain radiography of the lumbar spine
  5. E
    Radionuclide bone scintigraphy

Answer

Computed tomography myelography of the lumbar spine
The patient presents with classic red-flag signs of cauda equina syndrome (CES), including lower back pain, bilateral lower extremity weakness, saddle anesthesia, decreased anal sphincter tone, and acute urinary retention (demonstrated by a large postvoid residual volume). CES is a neurologic emergency that requires urgent imaging to confirm the diagnosis prior to surgical decompression. While magnetic resonance imaging (MRI) of the lumbar spine is the gold standard modality for assessing spinal nerve root compression, this patient has a permanent pacemaker placed in 2008. Older (legacy) cardiac devices are generally not MRI-conditional. Subjecting this patient to an MRI scanner poses severe risks, including dangerous thermal tissue injury, inappropriate pacing, and complete device failure. In situations where MRI is contraindicated, computed tomography (CT) myelography is the most appropriate alternative study. This procedure involves injecting iodinated contrast directly into the subarachnoid space, followed by CT imaging, providing the high-resolution visualization of the thecal sac and nerve roots required to accurately diagnose CES.

Step-by-Step Solution

1
Identify the primary clinical syndrome from the patient's presentation.
The combination of severe back pain, saddle anesthesia, acute urinary retention, and decreased rectal tone strongly indicates cauda equina syndrome.
Recognizing this neurologic emergency dictates the need for immediate definitive imaging of the spinal cord and nerve roots to guide surgical decompression.
2
Determine the gold standard imaging test for the identified syndrome.
Magnetic resonance imaging (MRI) of the lumbar spine is the test of choice.
MRI provides the highest spatial resolution for evaluating soft tissue structures, including the thecal sac and cauda equina.
3
Evaluate the patient's medical history for any contraindications to the gold standard imaging.
The patient has a permanent pacemaker placed in 2008, which is a legacy device and is generally not MRI-conditional.
Performing an MRI on a patient with a non-compatible electronic device can lead to thermal injury, lead displacement, or life-threatening device malfunction.
4
Select the optimal alternative imaging modality.
Computed tomography (CT) myelography is the correct next step.
It involves the intrathecal administration of contrast followed by CT scanning, offering excellent visualization of the subarachnoid space and nerve root compression when MRI cannot be safely performed.

Key Concept

Alternative imaging modalities for spinal cord and nerve root evaluation when MRI is contraindicated.
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Ordering Imaging Studies

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