What are the differential diagnoses and treatment approaches for a patient presenting with monoplegia?

Differential Diagnoses for Monoplegia

Monoplegia—weakness or paralysis of a single limb—demands immediate evaluation for vascular, structural, inflammatory, and peripheral nerve etiologies, with spinal cord stroke, brachial plexus injury, and demyelinating neuropathies being the most critical diagnoses to exclude.

Vascular Causes

Spinal Cord Infarction

• Anterior spinal artery occlusion can present as acute monoplegia, particularly affecting a single lower extremity, though this is rare compared to the typical bilateral presentation 1
• MRI findings include T2 hyperintensity in the anterior cord with diffusion restriction and possible contrast enhancement 1
• Fibrocartilaginous embolism should be suspected in young athletic patients with disc herniation or Schmorl's nodes on imaging, especially following heavy physical activity 1

Peripheral Vascular Injury

• Traumatic subclavian artery pseudoaneurysm can cause sudden upper limb monoplegia through brachial plexus compression, typically occurring 2-3 days after shoulder trauma 2
• Contrast-enhanced CT demonstrates pooling of contrast agent near the subclavian artery with associated clavicle fracture 2
• This requires urgent endovascular treatment to prevent rupture and reduce nerve compression 2

Peripheral Nerve and Plexus Disorders

Brachial Plexus Injury

• Pan-brachial plexus monoplegia can occur following trauma with adjacent hematoma, even without initial neurological deficits 3
• MRI reveals enhanced asymmetric signal in the trunks and cords of the brachial plexus 3
• Polytrauma patients with first-rib fractures are at particular risk for delayed presentation 3

Mononeuropathy

• Single peripheral nerve lesions (carpal tunnel syndrome, peroneal neuropathy) present with weakness limited to one specific nerve distribution 4
• Diagnosis requires nerve conduction studies, electromyography, and ultrasound or MRI in selected cases 4

Multiple Mononeuropathy (Mononeuritis Multiplex)

• Affects multiple individual nerves sequentially or simultaneously, which can initially present as monoplegia before additional nerves become involved 5
• Nerve conduction studies and EMG are essential to confirm the diagnosis and differentiate axonal from demyelinating pathology 5
• Common causes include vasculitis, diabetes, and cryoglobulinemia 5

Demyelinating Disorders

Acute Inflammatory Demyelinating Polyneuropathy (AIDP/Guillain-Barré Syndrome)

• While typically bilateral and ascending, atypical presentations can begin with isolated limb weakness 6
• Progression occurs over days to 4 weeks with absent or decreased reflexes in affected limbs 6
• CSF shows cytoalbuminologic dissociation (elevated protein with normal cell count) 6

Chronic Inflammatory Demyelinating Polyneuropathy (CIDP)

• Progression beyond 2 months distinguishes CIDP from GBS 6
• Presents with progressive bilateral weakness with areflexia, though asymmetric presentations occur 6
• Electrophysiological studies demonstrate demyelinating features 6

Multiple Sclerosis and Related Disorders

• Spinal cord lesions in MS are typically short-segment and multiple, which can cause focal weakness 7
• Red flags against MS include longitudinally extensive lesions (>3 vertebral segments), prominent central grey matter involvement, and significant cord swelling 7

MOG-Associated Encephalomyelitis

• Can present with isolated longitudinally extensive transverse myelitis causing monoplegia 7
• CSF typically shows pleocytosis without oligoclonal bands 7
• MOG-IgG testing via cell-based assay is the gold standard for diagnosis 7

Neuromyelitis Optica Spectrum Disorder (NMOSD)

• Longitudinally extensive spinal cord lesions (≥3 vertebral segments) with preferential involvement of central grey matter 7
• AQP4-IgG serology is diagnostic in most cases 7
• Prominent cord swelling is characteristic 7

Structural Causes

Radiculopathy

• Follows a single dermatome distribution pattern, distinguishing it from plexopathy 4
• MRI of the spine is the preferred imaging modality 4

Plexopathy

• Affects multiple nerve distributions simultaneously, crossing dermatome boundaries 4
• Causes include trauma, entrapment, inflammatory processes, neoplastic infiltration, or radiation injury 4
• MRI of the plexus with superior soft-tissue contrast is the preferred imaging 4

Functional/Conversion Disorder

• Conversion disorder should be considered when symptoms do not correlate with anatomical patterns of neurological deficit 8
• Normal reflexes, muscle tone, and EMG findings despite reported paralysis suggest this diagnosis 8
• Often occurs in patients with psychosocial stressors or history of somatization 8

Diagnostic Algorithm

Initial Assessment

1. Establish time course: Days to 4 weeks suggests acute demyelinating process (GBS spectrum); beyond 2 months suggests CIDP spectrum 6
2. Identify trauma history: Recent falls, shoulder injuries, or heavy athletic activity raise suspicion for vascular or structural causes 1, 2
3. Examine for associated findings: Reflexes (absent in demyelinating disorders), sensory loss pattern (dermatomal vs. nerve distribution), and signs of trauma 6, 4

Imaging Strategy

• Brain and spine MRI with and without contrast is the initial imaging of choice for suspected central causes 7, 1
• Contrast-enhanced CT or MRA if vascular injury suspected (trauma history, acute onset) 2
• MRI of brachial plexus for upper limb monoplegia with trauma history 3

Laboratory and Electrophysiological Studies

• CSF analysis: Elevated protein without pleocytosis suggests demyelinating neuropathy; pleocytosis without oligoclonal bands suggests MOG-EM 7, 6
• Nerve conduction studies and EMG: Essential to differentiate axonal from demyelinating pathology and localize lesions 6, 5
• Serology: AQP4-IgG and MOG-IgG via cell-based assay for suspected inflammatory demyelinating disorders 7

Treatment Approaches

Acute Demyelinating Neuropathy

• Pulse methylprednisolone 1g IV daily for 3-5 days PLUS IVIG 2g/kg over 5 days for severe or progressing symptoms 6
• Rituximab may be considered for refractory cases 6

Vascular Causes

• Endovascular treatment for subclavian artery pseudoaneurysm with percutaneous drainage 2
• Aspirin may be effective in myeloproliferative disease-related mononeuropathy 5

Inflammatory/Vasculitic Causes

• Corticosteroids and immunosuppressors as first-line therapy when inflammatory or vasculitic etiology confirmed 5
• Early treatment prevents irreversible neurological damage 5

Symptomatic Management

• Gabapentin, pregabalin, duloxetine, or tricyclic antidepressants for neuropathic pain 5

Critical Pitfalls to Avoid

• Do not delay corticosteroids in inflammatory cases, as this leads to irreversible damage 5
• Do not assume bilateral presentation for demyelinating disorders—atypical monoplegia presentations occur 6, 1
• Do not miss trauma history even if seemingly minor (falls 2-3 days prior can cause delayed vascular complications) 2
• Do not reduce corticosteroids too rapidly, as this causes symptom recurrence requiring restart of higher doses 5
• Do not diagnose conversion disorder without thorough exclusion of organic causes through imaging, EMG, and laboratory studies 8