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Difficulty: HardOrdering Imaging Studies

An inpatient on the internal medicine ward, a 69-year-old male being treated for community-acquired pneumonia, acutely develops intense lower back pain radiating down both legs. The nursing staff notes that he has been unable to void urine for the past 10 hours and he complains of numbness in his groin. His medical history includes ischemic cardiomyopathy with a legacy implantable cardioverter-defibrillator (ICD) placed 12 years ago that is documented as non-MRI-conditional. Vital signs are within normal limits. Neurologic evaluation demonstrates diminished strength in bilateral ankle plantar flexion, absent Achilles reflexes, and loss of pinprick sensation in the perianal region. Digital rectal examination reveals markedly decreased sphincter tone. Postvoid residual volume by bladder scan is 750 mL. The clinical team strongly suspects an acute compression of the lumbosacral nerve roots. Which of the following is the most appropriate next step in imaging to confirm the diagnosis?

  1. Computed tomography myelography of the lumbar spineAnswer
  2. B
    Magnetic resonance imaging of the lumbar spine
  3. C
    Non-contrast computed tomography of the lumbar spine
  4. D
    Computed tomography of the lumbar spine with intravenous contrast
  5. E
    Plain radiography of the lumbosacral spine with flexion-extension views

Answer

Computed tomography myelography of the lumbar spine is the most appropriate next step in imaging.
Computed tomography (CT) myelography is the diagnostic imaging study of choice for patients with suspected cauda equina syndrome who have absolute contraindications to magnetic resonance imaging (MRI). By introducing contrast directly into the subarachnoid space, CT myelography provides excellent delineation of the thecal sac and nerve roots, allowing for precise localization of the compressive lesion when an MRI cannot be safely performed.

Step-by-Step Solution

1
Identify the clinical syndrome based on the patient's acute presentation.
The patient exhibits severe lower back pain, bilateral lower extremity weakness, saddle anesthesia, urinary retention, and decreased rectal tone, which are classic signs of cauda equina syndrome.
Recognizing cauda equina syndrome is critical because it is a neurosurgical emergency that requires prompt diagnosis and surgical decompression to prevent permanent neurologic deficit.
2
Determine the standard first-line imaging modality for the suspected diagnosis.
Magnetic resonance imaging (MRI) of the lumbar spine is the gold standard for evaluating suspected cauda equina syndrome.
MRI provides the highest resolution of soft tissues, including the spinal cord, thecal sac, and individual nerve roots.
3
Evaluate the patient for any absolute contraindications to the standard imaging modality.
The patient has a legacy (12-year-old) implantable cardioverter-defibrillator (ICD) that is documented as non-MRI-conditional.
Placing a patient with a non-conditional ICD into an MRI scanner can cause catastrophic device malfunction, severe tissue heating, and potentially lethal arrhythmias.
4
Select the best alternative imaging study that provides comparable diagnostic information.
Computed tomography (CT) myelography of the lumbar spine is the correct choice.
By introducing contrast directly into the subarachnoid space via lumbar puncture prior to imaging, CT myelography accurately outlines the nerve roots and demonstrates the level and degree of compression.

Key Concept

When magnetic resonance imaging (MRI) is absolutely contraindicated (e.g., due to a non-conditional pacemaker or ICD), computed tomography (CT) myelography is the imaging modality of choice for diagnosing cauda equina syndrome.
Estimated Time:1m 30s
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Ordering Imaging Studies

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