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Difficulty: HardOrdering Imaging Studies

An emergency medical services team brings a 72-year-old man to the emergency department due to severe, escalating lower back pain and urinary retention over the past 14 hours. The patient reports a heavy, numb sensation in his groin and inner thighs, as well as new-onset difficulty ambulating. His medical history includes hypertension, prostate cancer treated with radiation therapy five years ago, and sick sinus syndrome. For the latter, a non-MRI-conditional permanent dual-chamber pacemaker was placed three weeks ago. Vital signs are within normal limits. On physical examination, there is pronounced focal tenderness over the L4-L5 spinous processes. Neurologic assessment shows 4/5 strength in bilateral ankle dorsiflexion and plantar flexion. Sensation to pinprick is diminished in the perianal and perineal regions. Achilles deep tendon reflexes are absent bilaterally. A bedside bladder ultrasound reveals a post-void residual volume of 850 mL. Which of the following is the most appropriate next step in diagnostic imaging for this patient?

  1. Computed tomography myelographyAnswer
  2. B
    Lumbosacral magnetic resonance imaging
  3. C
    Unenhanced computed tomography of the lumbar spine
  4. D
    Radionuclide bone scintigraphy
  5. E
    Anteroposterior and lateral lumbar radiographs

Answer

Computed tomography myelography
The patient presents with classic red-flag signs of cauda equina syndrome (CES), including severe lower back pain, saddle anesthesia, lower extremity weakness, absent Achilles reflexes, and acute urinary retention (evidenced by a post-void residual volume of 850 mL). This is a surgical emergency that requires prompt imaging to localize the level of nerve root compression, likely due to epidural metastasis from his history of prostate cancer. While magnetic resonance imaging (MRI) is the gold standard for evaluating CES, this patient has an absolute contraindication due to the presence of a recently implanted, non-MRI-conditional pacemaker. In clinical scenarios where MRI cannot be performed, computed tomography (CT) myelography is the most appropriate alternative. This procedure involves the intrathecal administration of contrast followed by a CT scan, providing excellent visualization of the spinal canal, thecal sac, and nerve root compression.

Step-by-Step Solution

1
Identify the primary clinical syndrome based on the patient's neurologic deficits and autonomic dysfunction.
The combination of severe back pain, saddle anesthesia, lower extremity weakness, absent Achilles reflexes, and acute urinary retention indicates cauda equina syndrome.
Recognizing this neurologic emergency dictates the urgency and type of diagnostic imaging required.
2
Determine the standard imaging modality for the suspected diagnosis.
Magnetic resonance imaging (MRI) of the spine is the gold standard for visualizing thecal sac and nerve root compression.
MRI provides superior soft-tissue contrast without ionizing radiation.
3
Evaluate the patient's medical history for any contraindications to the standard imaging modality.
The patient has a recently implanted, non-MRI-conditional permanent pacemaker, which is an absolute contraindication to undergoing an MRI.
Subjecting this patient to an MRI could lead to device malfunction, lead heating, or fatal arrhythmias.
4
Select the most appropriate alternative imaging modality that provides equivalent diagnostic information.
Computed tomography (CT) myelography is selected.
By injecting contrast into the subarachnoid space prior to the CT scan, this modality effectively outlines the spinal cord and nerve roots, serving as the best alternative when MRI is contraindicated.

Key Concept

Computed tomography myelography is the imaging modality of choice for suspected cauda equina syndrome or spinal cord compression in patients with absolute contraindications to magnetic resonance imaging.
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Ordering Imaging Studies

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