A 65-year-old man is brought to the emergency department by his daughter due to three days… — USMLE Step 3

A $65$ -year-old man is brought to the emergency department by his daughter due to three days of progressively worsening lower back pain and new-onset difficulty walking. He reports that the pain began abruptly after he attempted to lift a heavy piece of furniture. This morning, he developed perineal numbness and has been unable to urinate for the past $12$ hours despite feeling a strong urge. His medical history includes type 2 diabetes mellitus, hypertension, and sick sinus syndrome managed with a permanent dual-chamber cardiac pacemaker (implanted $14$ years ago; documentation confirms the device is strictly non-MRI compatible). His temperature is $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ), blood pressure is $145/90\text{ mmHg}$ , heart rate is $78/\text{min}$ , and respiratory rate is $16/\text{min}$ . Physical examination reveals exquisite tenderness to palpation over the lower lumbar spine. Neurologic examination demonstrates $3/5$ strength in bilateral lower extremities, decreased sensation to pinprick in the perineal and perianal region (saddle anesthesia), and absent Achilles reflexes bilaterally. Digital rectal examination reveals significantly decreased anal sphincter tone. Bedside bladder ultrasonography shows a post-void residual volume of $600\text{ mL}$ . Which of the following is the most appropriate next step in diagnostic imaging?

A
Magnetic resonance imaging of the lumbar spine with and without intravenous contrast
Computed tomography myelography of the lumbar spineAnswer
C
Non-contrast computed tomography of the lumbar spine
D
Computed tomography of the abdomen and pelvis with intravenous contrast
E
Transrectal ultrasonography of the prostate

Answer

Computed tomography myelography of the lumbar spine is the correct diagnostic step.

Computed tomography myelography of the lumbar spine is the correct choice. This patient presents with classic signs of cauda equina syndrome (CES), a neurologic emergency most commonly caused by a massive central lumbar disc herniation. The standard of care for diagnosing CES is an urgent MRI of the lumbar spine. However, this patient has a documented legacy (non-MRI compatible) cardiac pacemaker, which is an absolute contraindication to MRI. When MRI cannot be performed, CT myelography is the preferred alternative. It involves the intrathecal administration of contrast dye followed by a CT scan, providing excellent visualization of the spinal canal, thecal sac, and nerve root impingement necessary for surgical planning.

Step-by-Step Solution

Analyze the clinical presentation and identify the primary syndrome.

The patient has severe lower back pain, bilateral lower extremity weakness, saddle anesthesia, decreased rectal tone, and acute urinary retention.

These findings are the classic triad/presentation of cauda equina syndrome, an absolute neurologic emergency requiring immediate decompression.

Determine the gold standard diagnostic imaging for this condition.

Magnetic resonance imaging (MRI) of the lumbar spine is the definitive modality of choice for suspected cauda equina syndrome.

MRI provides superior soft-tissue contrast to clearly visualize the spinal cord, conus medullaris, cauda equina nerve roots, and surrounding epidural space.

Evaluate patient-specific factors that might alter standard management.

The patient has a

14

-year-old permanent cardiac pacemaker that is explicitly documented as non-MRI compatible.

Placing a patient with a non-conditional pacemaker in an MRI scanner risks fatal arrhythmias, thermal injury to the myocardium, and device malfunction.

Select the most appropriate alternative imaging modality.

Computed tomography (CT) myelography of the lumbar spine.

CT myelography involves injecting contrast into the subarachnoid space followed by a CT scan. It outlines the nerve roots and thecal sac, making it the best alternative to MRI for diagnosing central canal stenosis, massive disc herniation, or other causes of cauda equina compression.

Key Concept

In patients with suspected cauda equina syndrome or spinal cord compression who have absolute contraindications to MRI (e.g., non-MRI compatible pacemakers), CT myelography is the diagnostic imaging modality of choice.

Estimated Time:1m 15s

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