A 62-year-old man returns to the emergency department due to urinary retention. Two days ago, the patient was moving furniture when he suddenly had sharp, shooting pain in the lower back and right leg. He came to the emergency department yesterday after his pain had not improved with naproxen. No work-up was indicated, and the patient was discharged with a short course of acetaminophen-hydrocodone. He took the medication and rested. When the patient woke up today, he had to strain to urinate. He has no chronic medical conditions. Vital signs are within normal limits. Examination of the right lower extremity shows that plantar flexion is impaired, sensation is reduced on the posterior aspect of the leg and the lateral edge of the foot, and the ankle reflex is absent. The straight-leg raising test markedly increases pain in the right posterior thigh and leg. Perianal sensation is decreased. The remainder of the neurologic examination is normal. What is the most likely cause of this patient's urinary retention?
Conus medullaris syndrome | Cauda equina syndrome | |
Vertebral level |
|
|
Spinal level |
|
|
Presentation |
|
|
Physical |
|
|
LMN = lower motor neuron; UMN = upper motor neuron. | ||
This patient's urinary difficulty, saddle anesthesia (eg, perianal numbness), and unilateral lower extremity motor/sensory/reflex abnormalities are concerning for cauda equina syndrome (CES). The cauda equina is composed of nerve roots for L2-L5, S1-S5, and the coccygeal nerve. Most cases arise when a large, central lumbar disc herniation (eg, following moving furniture) compresses multiple spinal nerve roots in the cauda equina, causing symptoms consistent with lower motor neuron dysfunction. Other causes include malignancy, inflammatory processes, and (rarely) spinal stenosis.
CES often presents with severe radicular pain involving one or both legs. Lower extremity weakness and sensory loss are usually asymmetric and reflect the affected myotomes/dermatomes. Additional findings may include the following:
Straight-leg raising testing is positive in most cases (>90%) of acute lumbar herniation and may aid in the diagnosis. Confirmation requires urgent MRI of the lumbosacral spine. Because patients with CES are at high risk for permanent neurologic sequelae, urgent surgical decompression is usually necessary.
(Choice B) Conus medullaris syndrome occurs due to compression of the terminal end of the spinal cord (ie, conus medullaris). It can cause bladder dysfunction, back pain, numbness, and weakness. However, severe pain is usually limited to the lower back (vs radiating into the leg), and numbness is typically symmetric and limited to the perianal area (vs extending into the leg and foot). Motor weakness is usually symmetric (vs asymmetric), and upper motor neuron findings such as hyperreflexia (vs absent ankle reflex) are typically present.
(Choice C) Opioids can cause urinary retention by reducing parasympathetic tone within the bladder, leading to decreased detrusor tone and urge to void. However, they do not cause perianal numbness or unilateral lower extremity neurologic deficits.
(Choice D) Unilateral S1 nerve root impingement could result in decreased right lower extremity plantar flexion, posterior leg numbness, and loss of the Achilles reflex. However, the S1 nerve is not involved in bladder function or perianal sensation; this patient has urinary retention and perianal numbness, suggesting CES.
Educational objective:
Severe radicular pain involving one or both legs with accompanying saddle anesthesia and bladder dysfunction is concerning for cauda equina syndrome (CES). CES is commonly caused by a large lumbosacral disc herniation. It requires urgent MRI of the lumbosacral spine and, in most cases, urgent surgical decompression.