What is the most appropriate initial test to order for an elderly man with a history of prostate cancer presenting with left leg weakness, decreased sensation in the perineum, and reduced sphincter tone?

Urgent MRI of the Spine is Required

This patient requires an immediate MRI of the spine (answer c) to evaluate for metastatic spinal cord compression, which is a neurologic emergency requiring diagnosis and treatment within hours to prevent permanent neurologic damage. 1

Clinical Presentation Indicates Cauda Equina Syndrome

This elderly man presents with a constellation of findings that strongly suggests cauda equina syndrome secondary to metastatic spinal cord compression:

• Unilateral leg weakness (3/5 strength with left hip flexion and leg extension) 1
• Perineal sensory loss (saddle anesthesia) 1
• Reduced anal sphincter tone 1
• Urinary retention (700 mL on bladder scan) 1
• History of prostate cancer (treated with radical prostatectomy) 1, 2

The combination of these neurologic deficits in a patient with prior prostate cancer creates high suspicion for metastatic epidural spinal cord compression, which occurs in approximately 10% of patients with metastatic prostate cancer. 2

Why MRI is the Diagnostic Test of Choice

MRI of the spine is the gold standard imaging modality for evaluating suspected spinal cord compression because it:

• Provides superior soft tissue contrast to visualize the spinal cord, nerve roots, and epidural space 1
• Can identify the exact level and extent of compression 1
• Detects epidural disease, vertebral body metastases, and paraspinal masses in a single study 1
• Allows immediate treatment planning for radiation therapy or surgical decompression 1

The urgency cannot be overstated: neurologic outcomes correlate directly with the degree of neurologic function at the time treatment is initiated. Patients who are ambulatory before treatment have significantly better outcomes than those who are paraplegic. 1

Why Other Options Are Inadequate

Plain radiographs (answer b) are insufficient because they:

• Only show bony abnormalities and cannot visualize the spinal cord or epidural space
• Miss up to 50% of vertebral metastases
• Provide no information about neural compression 1

PET-CT (answer a) is inappropriate as an initial test because:

• It is used for staging metastatic disease, not for evaluating acute neurologic emergencies
• Does not provide adequate detail of spinal cord compression
• Delays definitive diagnosis and treatment 3

CT myelogram (answer d) is only considered when:

• MRI is contraindicated (e.g., pacemaker, severe claustrophobia)
• MRI is unavailable
• It is more invasive and carries risks of post-procedure headache and infection 1

Immediate Management Considerations

Once MRI confirms metastatic spinal cord compression, treatment must be initiated urgently:

• High-dose corticosteroids (typically dexamethasone 10 mg IV bolus followed by 4 mg every 6 hours) should be started immediately to reduce spinal cord edema 1
• Radiation therapy is the primary treatment modality for most patients with metastatic spinal cord compression 1
• Surgical decompression may be considered for patients with spinal instability, radioresistant tumors, or rapid neurologic deterioration 1
• Continuation of androgen deprivation therapy is recommended in patients with hormone-sensitive prostate cancer 1

Critical Time Window

The window for neurologic recovery is narrow. Patients treated within 24-48 hours of symptom onset have significantly better functional outcomes than those with delayed treatment. Every hour of delay increases the risk of permanent paralysis and loss of bowel/bladder function. 1