A 65-year-old man presents to the emergency department with low back pain. The pain started 6 hours ago when he was gardening and carrying a heavy bag. The pain is rated as 10/10 in severity and radiates down the posterior aspect of the right thigh. He reports no fevers or chills. His only medical problem is hypertension for which he takes amlodipine. The patient is a non-smoker, uses alcohol infrequently, and has never used illicit drugs. His temperature is 99.6°F (37.6°C), blood pressure is 140/70 mmHg, pulse is 90/min, and respirations are 20/min. Physical examination reveals 3/5 strength to hip extension, knee flexion and extension, and plantar flexion bilaterally. Sensation to pinprick is diminished over the posterolateral legs and lateral aspects of both feet. Ankle and knee reflexes are absent bilaterally. The patient’s underwear is wet and a bladder scan reveals 800 mL of urine. Which of the following is the most likely underlying diagnosis?
Acute inflammatory demyelinating polyneuropathy
L5/S1 disc herniation
Spinal stenosis
Cauda equina syndrome
Cauda equina syndrome
This patient presenting with acute, severe, radiating low back pain with sensorimotor deficits (3/5 strength in hip extension, knee flexion/extension, plantar flexion, reduced sensation to pinprick) in multiple nerve root distributions (L3-S5), lower motor neuron signs, urinary incontinence (wet underwear), and acute urinary retention (bladder scan showing 800 mL of urine) most likely has cauda equina syndrome.

Cauda equina syndrome is characterized by deficits in 2 or more nerve roots that comprise the cauda equina of the spinal cord. It has multiple etiologies, including disc herniation, epidural abscess, tumor, or lumbar spine spondylosis. Cauda equina syndrome presents with severe low back pain radiating into the legs, flaccid paralysis or lower extremity weakness which can be symmetric or asymmetric (often asymmetric), bladder and/or rectal sphincter paralysis, impotence/erectile dysfunction, sensory loss in affected dermatomes, and saddle anesthesia (sensory loss in the perineum). Urinary retention occurs due to bladder paralysis. Diagnosis is aided by emergency magnetic resonance imaging. Cauda equina syndrome is a surgical emergency; the goal of surgical treatment is to decompress the cauda equina to prevent permanent neurological injury, such as with a lumbar laminectomy. This can be differentiated from conus medullaris syndrome, which has perianal numbness without numbness through other dermatomes (down the leg in the S1 or S2 patterns), less severe pain (cauda equina presents with more severe pain), and hyperreflexia in the knees (cauda equina presents with lower motor neuron signs in the legs which lead to decreased reflexes - though conus medullaris may present with absent ankle reflexes, the knees are typically hyperreflexic).

Gardner et al. review the symptoms, diagnosis, and medico-legal landscape of cauda equina syndrome. They discuss how the most frequent etiology is a large central lumbar disc herniation. They recommend being vigilant for urinary retention in patients who may have compression as many patients have urinary retention on presentation.

Incorrect Answers:
Answer A: Acute inflammatory demyelinating polyneuropathy, also known as Guillain-Barre syndrome, occurs after respiratory or gastrointestinal infections and presents with ascending paralysis and diminished reflexes. While bladder dysfunction can occur, back pain is not typical. Guillain-Barre syndrome can be treated with intravenous immunoglobulin.

Answer C: L5/S1 disc herniation affects the traversing S1 nerve root. S1 radiculopathy presents with posterolateral leg and dorsolateral foot sensory loss and absence of the ankle reflex. Typically, bowel/bladder symptoms are not present. Treatment includes decompression of the affected nerve root.

Answer D: Overflow incontinence is caused by either bladder outlet obstruction or poor bladder tone. While this could explain this patient’s urinary retention, it does not account for his back pain or sensorimotor deficits. Isolated overflow incontinence can be treated with catheterization.

Answer E: Spinal stenosis presents with lower extremity claudication, characterized by pain with standing or ambulation. The pain of neurogenic claudication is characteristically relieved by spinal flexion (“shopping cart sign”). Sensory loss and motor weakness can be present; however, bladder dysfunction is atypical. Treatment includes surgical decompression of the area of stenosis.

Bullet Summary:
Cauda equina syndrome is characterized by impingement of 2 or more nerve roots of the cauda equina, resulting in radicular pain, bilateral lower extremity weakness, lower motor neuron signs, sensory loss in affected dermatomes, bowel/bladder dysfunction, and saddle anesthesia.