What is the appropriate diagnosis and treatment for a young adult male with sudden onset bilateral knee weakness, unable to stand?

Sudden Bilateral Knee Weakness in Young Adult Male

This presentation of sudden bilateral knee weakness with inability to stand in a young adult male without trauma should immediately raise suspicion for functional neurological disorder (FND) with functional limb weakness, though urgent exclusion of Guillain-Barré syndrome and other neurological emergencies is essential before making this diagnosis.

Immediate Assessment Priorities

Rule Out Life-Threatening Causes First

Assess for Guillain-Barré Syndrome (GBS):

• Check for ascending weakness pattern starting distally and progressing proximally over hours to days 1
• Examine deep tendon reflexes—absent or decreased reflexes in affected limbs strongly support GBS 1
• Evaluate for recent preceding infection (respiratory or gastrointestinal illness 1-4 weeks prior) 1
• Assess for cranial nerve involvement, particularly bilateral facial weakness 1
• Check for autonomic dysfunction (blood pressure instability, cardiac arrhythmias) 1
• Perform lumbar puncture if GBS suspected—elevated CSF protein with normal cell count (<50 cells/μl) supports diagnosis 1

Critical red flags that rule OUT functional weakness:

• Fever at onset suggests infection or inflammatory process 1
• Bladder or bowel dysfunction at onset indicates spinal cord pathology 1
• Sharp sensory level indicates spinal cord injury 1
• Hyperreflexia, clonus, or extensor plantar responses suggest upper motor neuron lesion 1
• Altered consciousness (except in specific brainstem variants) 1

Distinguish Functional from Organic Weakness

Key features supporting functional limb weakness:

• Sudden onset without clear precipitating trauma 1
• Bilateral symmetric involvement affecting ability to stand or transfer 1
• Inconsistent weakness patterns—strength may vary with distraction or different examination techniques 1
• Normal reflexes and no pathological reflexes 1
• Absence of muscle atrophy (in acute presentation) 1
• May demonstrate "give-way" weakness where effort suddenly collapses 1

Diagnostic Workup

If trauma history is unclear or patient reports any knee injury:

• Obtain bilateral knee radiographs (AP and lateral views) to exclude fracture, particularly if focal tenderness, effusion, or inability to bear weight present 1, 2
• Apply Ottawa knee rules: obtain X-rays if age >55, focal tenderness at patella or fibular head, inability to bear weight for 4 steps, or inability to flex knee to 90 degrees 1, 2

For suspected neurological cause:

• Electrodiagnostic studies (EMG/NCS) to evaluate for polyradiculoneuropathy—though may be normal if performed within first week of GBS 1
• CSF analysis showing elevated protein (>0.45 g/L) with normal cell count supports GBS 1
• MRI of spine if concern for cord compression or radiculopathy 1

Management Approach

If Guillain-Barré Syndrome Confirmed:

• Immediate hospital admission for monitoring 1
• Respiratory function monitoring—forced vital capacity measurements 1
• Intravenous immunoglobulin (IVIG) or plasma exchange as disease-modifying treatment 1
• Supportive care including DVT prophylaxis, autonomic monitoring 1

If Functional Neurological Disorder Diagnosed:

Therapeutic interventions for functional limb weakness:

• Engage patient in tasks promoting normal movement, good alignment, and even weight-bearing 1
• For bilateral lower limb weakness: joint sessions with physical therapy using standing frames while performing upper limb tasks 1
• Focus on functional activities: transfers, sit-to-stand, standing, perch sitting during personal care or kitchen tasks 1
• Employ anxiety management and distraction techniques during task performance 1
• Video recording (with consent) to demonstrate symptom changeability and build confidence 1

Avoid harmful interventions:

• Do NOT use splinting or adaptive aids that may prevent restoration of normal movement 1
• Avoid immobilization which leads to muscle deconditioning and learned non-use 1
• Do not reinforce illness behavior through excessive medical testing once organic causes excluded 1

Critical Pitfalls to Avoid

• Never assume functional diagnosis without excluding GBS—progression can be rapid with respiratory failure occurring within 24-48 hours in severe cases 1
• Do not delay lumbar puncture if GBS suspected, but note that normal CSF protein early in disease does not rule out diagnosis 1
• Avoid telling patient symptoms are "not real" or "psychological"—FND involves genuine neurological dysfunction requiring specific rehabilitation 1
• Do not continue extensive imaging or testing once organic causes excluded, as this reinforces symptom focus 1
• If patient has marked persistent asymmetry, bladder/bowel dysfunction, or sensory level, reconsider diagnosis as these cast doubt on both GBS and FND 1