What is the differential diagnosis and appropriate workup for a 53-year-old male patient with a history of back injury and groin pain, presenting with sudden onset of inability to move his right foot, heavy sensation in the leg, and no urinary or bowel incontinence, fever, or other systemic symptoms?

Differential Diagnosis for Acute Right Foot Drop with History of Back Injury and Groin Pain

This patient most likely has lumbar radiculopathy from disc herniation at L4-L5 or L5-S1, with the foot drop indicating L5 nerve root compression, and requires urgent MRI of the lumbar spine without contrast to evaluate for disc herniation, nerve root compression, or other structural pathology. 1

Key Clinical Features Supporting Radiculopathy

The presentation of sudden inability to move the right foot with a heavy sensation, combined with months of groin pain and prior back-to-leg radiating pain, strongly suggests lumbar nerve root compression 1. Specific features include:

• Foot drop indicates L5 nerve root involvement, which controls foot dorsiflexion and is commonly affected by L4-L5 disc herniation 1
• Groin pain can originate from L4-L5 disc herniation, particularly in elderly patients with central disc protrusion, as the sinuvertebral nerve innervating the posterior annulus fibrosus and posterior longitudinal ligament can refer pain to the L2 dermatome 2, 3
• History of back-to-leg pain that has since resolved suggests prior radicular symptoms, with the current foot drop representing progression to motor deficit 1
• More than 90% of symptomatic lumbar disc herniations occur at L4-L5 and L5-S1 levels 1

Critical Red Flags to Exclude Immediately

The absence of certain symptoms is reassuring but must be explicitly confirmed:

• Cauda equina syndrome must be ruled out by specifically assessing for urinary retention (not just incontinence), fecal incontinence, and saddle anesthesia, as this requires urgent surgical decompression 1, 4
• Spinal cord compression from malignancy is less likely given no cancer history, but age >50 years warrants consideration 1, 4
• Infection is unlikely without fever, IV drug use, or recent infection 1
• Compression fracture should be considered given workplace injury history, though absence of acute trauma makes this less likely 1

Complete Differential Diagnosis

Most Likely Diagnoses:

• L5 radiculopathy from L4-L5 disc herniation - explains foot drop, groin pain, and prior radiating symptoms 1, 3
• L5-S1 disc herniation with L5 nerve root compression - second most common site 1
• Far lateral disc herniation - can cause isolated motor deficits without typical radicular pain pattern 1

Less Likely but Important to Consider:

• Lumbosacral plexopathy - would cause weakness and sensory loss in multiple nerve distributions beyond isolated foot drop 5
• Common peroneal nerve compression at fibular head - causes isolated foot drop but would not explain groin pain or back pain history 6
• Peripheral neuropathy - typically presents with stocking-glove distribution and bilateral symptoms, not acute unilateral foot drop 6
• Spinal stenosis - more common in patients >65 years and presents with pseudoclaudication, not acute foot drop 1

Rare but Serious Causes:

• Spinal tumor or lymphoma - can present with groin pain and neurologic deficits, particularly if symptoms are progressive despite conservative management 4, 7
• Pelvic mass or tumor affecting lumbosacral plexus - would cause plexopathy pattern 5

Immediate Workup and Interventions

Urgent Physical Examination Details:

Perform focused neurologic examination assessing:

• Motor strength: Test foot dorsiflexion (L5), great toe extension (L5), foot plantarflexion (S1), and knee extension (L4) 1
• Reflexes: Check ankle reflex (S1) and knee reflex (L4) 1
• Sensory examination: Assess dorsal foot (L5), lateral foot (S1), and anterior thigh/groin (L2-L4) 1
• Straight leg raise test: 91% sensitivity for herniated disc 1, 8
• Crossed straight leg raise: 88% specificity for herniated disc 1
• Rectal examination: Assess sphincter tone and perianal sensation to exclude cauda equina syndrome 1

Immediate Imaging:

MRI lumbar spine without IV contrast is the imaging study of choice and should be ordered urgently given acute motor deficit (foot drop), which represents a severe neurological finding requiring immediate evaluation 1, 8. Do not wait 4-6 weeks for conservative management when progressive motor weakness is present 1, 8.

• MRI accurately depicts soft-tissue pathology, disc herniation, nerve root compression, and spinal canal patency 8
• MRI is superior to CT for evaluating disc herniation and nerve root compression 8
• Contrast is not needed unless there is concern for infection, tumor, or prior surgery 5

Laboratory Studies:

Initial labs should include:

• Complete blood count - to assess for infection or malignancy 6
• Comprehensive metabolic panel - baseline assessment 6
• Fasting glucose - diabetes is a common cause of peripheral neuropathy, though acute foot drop is atypical 6
• Vitamin B12 level - deficiency can cause neuropathy 6
• ESR/CRP - if infection or inflammatory process suspected 4
• Calcium level - if malignancy suspected given groin pain 4

Immediate Management:

• NSAIDs are first-line for pain control while awaiting imaging 1, 8
• Avoid bed rest - advise patient to remain as active as tolerated, as bed rest leads to deconditioning 1
• Urgent neurosurgery consultation if MRI confirms significant nerve root compression with motor deficit, as progressive motor weakness may require surgical decompression 1

Treatment Algorithm Based on MRI Findings

If Disc Herniation with Nerve Root Compression Confirmed:

• Surgical decompression should be considered given acute motor deficit (foot drop), as this represents a severe neurological finding 1
• Do not delay with prolonged conservative management when progressive motor weakness is present 1
• Avoid routine fusion for primary disc herniation, as fusion shows no improvement in functional outcomes and worse return-to-work rates (70% without fusion vs 45% with fusion) 1

If No Significant Compression Found:

• Consider electrodiagnostic studies (EMG/NCV) to evaluate for peripheral nerve lesion, plexopathy, or peripheral neuropathy 5, 6
• Referral to neurology for further evaluation 6

Common Pitfalls to Avoid

• Do not attribute groin pain solely to hip pathology without imaging the lumbar spine, as discogenic groin pain from L4-L5 herniation is well-documented 2, 3
• Do not delay imaging when motor deficits are present, as this represents a severe neurological finding requiring urgent evaluation 1, 8
• Do not order epidural steroid injections for this patient, as the most recent BMJ guidelines strongly recommend against epidural injections for radicular pain 1
• Do not assume peripheral nerve injury without excluding central causes, as foot drop from L5 radiculopathy is far more common than peroneal nerve compression in this clinical context 1
• Do not miss cauda equina syndrome by failing to specifically ask about urinary retention and perform rectal examination 1, 4