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Difficulty: HardOrdering Imaging Studies

A 59-year-old man is brought to the emergency department complaining of severe, worsening lower back pain and an inability to urinate for the past 14 hours. He also reports numbness in his groin area and bilateral leg weakness. His past medical history is significant for hypertension and a history of a metallic intraocular foreign body from a welding accident 20 years ago. At the time, the metal fragment was deemed too deeply embedded to be safely removed and has remained in his left eye. Vital signs are within normal limits. Neurologic examination reveals decreased sensation over the perineum, absent anal wink reflex, and 3/5 motor strength in the bilateral lower extremities. A bedside bladder ultrasound shows a post-void residual volume of 650 mL. Which of the following is the most appropriate next step in imaging for this patient?

  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
    Plain radiography of the lumbar spine
  5. E
    Computed tomography angiography of the abdominal aorta and iliac vessels

Answer

Computed tomography myelography of the lumbar spine
The patient's presentation of acute lower back pain, bilateral leg weakness, saddle anesthesia, absent anal reflex, and acute urinary retention (confirmed by a high post-void residual volume) is highly suspicious for cauda equina syndrome. This is a surgical emergency. The gold standard imaging modality for evaluating suspected cauda equina syndrome is an MRI of the lumbar spine. However, this patient has a retained metallic intraocular foreign body, which is an absolute contraindication to MRI due to the risk of magnetic field-induced movement causing permanent ocular damage. In patients with suspected spinal cord or nerve root compression who have absolute contraindications to MRI, computed tomography (CT) myelography is the imaging modality of choice. It involves the intrathecal administration of contrast followed by a CT scan, providing excellent visualization of the spinal canal, subarachnoid space, and nerve roots.

Step-by-Step Solution

1
Identify the primary clinical syndrome.
The patient has severe back pain, bilateral lower extremity weakness, saddle anesthesia, decreased anal sphincter tone, and acute urinary retention.
These findings are the classic red flag signs of cauda equina syndrome, a neurologic emergency requiring immediate evaluation and surgical decompression.
2
Determine the gold standard diagnostic imaging for the identified syndrome.
Magnetic resonance imaging (MRI) of the lumbar spine is the imaging modality of choice to visualize the soft tissues of the spinal canal and diagnose cauda equina compression.
MRI provides the highest resolution of the spinal cord, nerve roots, and surrounding soft tissues without exposing the patient to ionizing radiation.
3
Assess the patient's specific history for any absolute contraindications to the standard imaging.
The patient has a retained metallic intraocular foreign body.
Metallic objects in the eye can move or heat up in the strong magnetic field of an MRI, potentially causing blindness or severe hemorrhage. This is an absolute contraindication to MRI.
4
Select the most appropriate alternative imaging modality.
Computed tomography (CT) myelography is the correct alternative choice.
When MRI is absolutely contraindicated or unavailable, CT myelography (which uses intrathecal contrast to outline the nerve roots and thecal sac) is the best alternative for evaluating spinal cord or nerve root compression.

Key Concept

In patients with suspected cauda equina syndrome or spinal cord compression who have an absolute contraindication to MRI (e.g., retained intraocular metallic foreign body, certain pacemakers), CT myelography is the preferred alternative imaging modality.
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Ordering Imaging Studies

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