Conus Medullaris Syndrome
This patient has conus medullaris syndrome, a neurologic emergency caused by metastatic spinal cord compression at the L1-L2 level. The constellation of urinary incontinence, erectile dysfunction unresponsive to sildenafil, perianal sensory loss, and absent rectal reflex in a patient with lung cancer undergoing chemotherapy is pathognomonic for this condition.
Clinical Reasoning
Key Diagnostic Features
The clinical presentation points definitively to conus medullaris syndrome:
- Sphincter dysfunction manifesting as urinary incontinence is the hallmark feature, occurring in up to 50% of patients with spinal cord compression at presentation 1
- Erectile dysfunction that fails to respond to PDE-5 inhibitors (sildenafil) indicates neurologic rather than vascular etiology—sildenafil requires intact neurovascular pathways to work 1
- Perianal sensory loss (saddle anesthesia) reflects damage to sacral nerve roots S3-S5 that innervate this dermatome 1
- Absent rectal reflex confirms lower motor neuron involvement at the conus level 1
Why Not the Other Options?
- Central cord syndrome affects the upper extremities predominantly with hand weakness and sensory loss, sparing bowel/bladder function initially
- Anterior cord syndrome presents with motor paralysis and loss of pain/temperature sensation but preserves proprioception and light touch
- Posterior cord syndrome causes loss of proprioception and vibration sense while preserving motor function and pain sensation
Pathophysiology in Lung Cancer
Spinal cord compression occurs in lung cancer through several mechanisms 1:
- Direct vertebral metastases growing into the epidural space (most common)
- Paraspinal mass extension through neural foramina
- Vertebral collapse with bony fragment displacement
- Vascular compromise of the epidural venous plexus leading to cord edema and ischemia
The conus medullaris (L1-L2 level) is particularly vulnerable because lung cancer commonly metastasizes to the thoracolumbar spine 1.
Critical Management Considerations
Immediate Actions Required
This is a neurologic emergency requiring urgent intervention 1:
- Immediate high-dose dexamethasone (96 mg/day) should be started even before MRI confirmation if clinical suspicion is high 1
- Urgent MRI of entire spine (sagittal T1-weighted with or without gadolinium) must be obtained to confirm diagnosis and identify all compression sites 1
- Expedited referral to radiation oncology and neurosurgery is mandatory 1
Prognostic Factors
Ambulatory status at presentation is the strongest predictor of outcome 1:
- Patients who are ambulatory before treatment have significantly better outcomes than those with paralysis
- Paralysis at presentation or after treatment correlates with much shorter life expectancy 1
- The devastating nature of permanent paralysis, sphincter dysfunction, and sensory loss makes this a true oncologic emergency 1
Common Pitfall
Delaying workup until Monday is a critical error—30% of spinal cord compression referrals occur on Fridays, and physicians must expedite evaluation regardless of day of week 1. Any delay risks permanent neurologic damage.
Answer: A. Conus medullaris