What is the differential diagnosis and possible site of lesion for a 14-year-old male with bilateral lower limb weakness, asymmetrical, lower motor neuron (LMN) type with areflexia, no fasciculation, loss of sensation over both lower limbs, and autonomic dysfunction, including inability to feel bladder and bowel sensations, without a history of trauma?

Differential Diagnosis and Site of Lesion

This 14-year-old male with bilateral asymmetric lower limb weakness (LMN pattern), areflexia, sensory loss, and autonomic dysfunction (bladder/bowel) most likely has acute inflammatory demyelinating polyradiculoneuropathy (Guillain-Barré syndrome) affecting the cauda equina and lumbosacral nerve roots, though spinal cord pathology (conus medullaris or cauda equina syndrome) must be urgently excluded with MRI spine. 1

Primary Differential Diagnoses

1. Guillain-Barré Syndrome (GBS) - Most Likely

• Classic presentation: Progressive bilateral ascending weakness starting in legs, with areflexia and sensory changes 1, 2
• The asymmetric pattern is atypical but can occur in GBS variants 1
• Autonomic dysfunction (bladder/bowel involvement) occurs commonly in GBS 3
• Typical onset is acute to subacute (days to 2 weeks) 2
• Site of lesion: Peripheral nerves, nerve roots (polyradiculoneuropathy) 1
• Approximately 20% develop respiratory failure requiring urgent monitoring 2

2. Cauda Equina Syndrome - Critical to Exclude

• Red flag features present: Bilateral leg weakness, sensory loss, bladder/bowel dysfunction 4
• Can present with LMN signs (areflexia, flaccid weakness) due to nerve root involvement 4
• Site of lesion: Lumbosacral nerve roots below L1-L2 level
• Requires urgent MRI spine to exclude compression 1

3. Conus Medullaris Syndrome

• Presents with bilateral leg weakness, saddle anesthesia, and early bladder/bowel dysfunction 4
• Mixed UMN/LMN signs possible depending on exact level
• Site of lesion: Distal spinal cord (T12-L2 level)
• Requires urgent MRI spine 1

4. Transverse Myelitis

• Acute/subacute bilateral weakness with sensory changes 1
• Typically shows increased reflexes (UMN pattern), but early stages may show areflexia (spinal shock) 4
• Autonomic dysfunction common 1
• Site of lesion: Spinal cord (thoracic or lumbar segments)

5. HIV-Associated Neuropathy (Given PLHA Status)

• Can cause distal symmetric polyneuropathy with sensory predominance 4
• Usually chronic/subacute, not acute presentation
• Autonomic involvement less prominent
• Site of lesion: Peripheral nerves (length-dependent axonopathy)

Urgent Diagnostic Workup

Immediate Imaging (Within Hours)

• MRI entire spine (cervical, thoracic, lumbar) without and with contrast is the critical first test 1
  • Evaluates for cord compression, transverse myelitis, nerve root enhancement (GBS) 1
  • Thin axial cuts through suspected abnormal regions 1
  • In GBS, may show cauda equina nerve root enhancement 1

Laboratory Studies

• CSF analysis: Cell count, protein (elevated in GBS), glucose, oligoclonal bands 1
• Complete blood count, vitamin B12 level (can cause subacute combined degeneration with similar presentation) 5
• HIV viral load and CD4 count (given PLHA status)

Electrodiagnostic Studies

• Nerve conduction studies and EMG to confirm polyradiculoneuropathy or polyneuropathy pattern 1
• In GBS: Prolonged distal latencies, conduction block, reduced amplitudes 1
• Should be performed but should not delay treatment if clinical suspicion high 1

Site of Lesion Analysis

Based on clinical features:

• LMN pattern with areflexia → Indicates anterior horn cells, nerve roots, or peripheral nerves 4
• Bilateral asymmetric distribution → Suggests multifocal nerve root involvement (polyradiculoneuropathy) or peripheral nerve involvement 1
• Sensory loss in both lower limbs → Indicates sensory nerve/root involvement 4
• Autonomic dysfunction (bladder/bowel) → Suggests involvement of sacral nerve roots (S2-S4) or autonomic nerve fibers 3, 6

Most likely anatomical site: Lumbosacral nerve roots and peripheral nerves (cauda equina region) 1

Immediate Management Priorities

If GBS Confirmed:

• Initiate treatment urgently: IVIG 2 g/kg over 5 days OR plasmapheresis 1
• Respiratory monitoring: Vital capacity and negative inspiratory force measurements 1
• Dysautonomia monitoring: Blood pressure and heart rate instability common 1
• Pain management: Muscular, radicular, or neuropathic pain frequently reported 1

If Spinal Cord Pathology Found:

• Urgent neurosurgical consultation for decompression if indicated 4
• High-dose corticosteroids if transverse myelitis suspected 4

Critical Pitfalls to Avoid

• Do not delay MRI spine - Spinal cord compression requires urgent surgical intervention 1
• Do not wait for CSF or EMG results to initiate GBS treatment if clinical suspicion is high and imaging excludes structural lesion 1
• Monitor respiratory function closely - 20% of GBS patients develop respiratory failure 2
• Consider vitamin B12 deficiency - Can present with similar neurologic findings and is reversible if caught early 5
• Recognize spinal shock - Early spinal cord injury may present with areflexia before reflexes become hyperactive 4