What are the possible causes of right-sided weakness lasting 15 days with a normal brain MRI?

Right-Sided Weakness for 15 Days with Normal Brain MRI

With right-sided weakness persisting for 15 days and a normal brain MRI, you must urgently pursue vascular imaging (CTA or MRA of head and neck) to evaluate for large vessel occlusion or stenosis, and strongly consider repeat MRI with advanced sequences including DWI, as initial MRI can miss acute stroke in rare but critical cases.

Immediate Diagnostic Approach

The 15-day duration of symptoms indicates this is an ischemic stroke (not TIA, which by definition lasts <24 hours) 1. The normal brain MRI is concerning but does not exclude stroke—this represents a diagnostic pitfall that requires immediate action.

Why the MRI May Be Normal

• False-negative DWI occurs in documented stroke cases, particularly with:

  • Very early imaging (within hours)
  • Small lacunar infarcts
  • Posterior circulation strokes
  • Technical limitations 2

• A recent case report demonstrated a 75-year-old with left-sided weakness and normal initial DWI, but ASL perfusion imaging revealed hypoperfusion and follow-up confirmed large infarct 2

Critical Next Steps

1. Vascular Imaging (URGENT)

• CTA head and neck is essential and has >90% sensitivity for identifying culprit vascular lesions including arterial occlusions, stenosis, and dissections 3
• MRA is an alternative if CTA contraindicated
• Look specifically for:
  • Left internal carotid or middle cerebral artery occlusion/stenosis (causing right-sided weakness) 1
  • Vertebrobasilar insufficiency if ataxia or cranial nerve findings present

2. Repeat Brain MRI with Multimodal Sequences

• Standard DWI alone is insufficient 2
• Must include:
  • DWI (most sensitive for acute stroke within 1-2 weeks) 4
  • FLAIR (for chronic changes)
  • Susceptibility sequences (SWI or GRE) (for microhemorrhages, chronic infarcts) 4
  • ASL perfusion imaging (can reveal hypoperfusion when DWI negative) 2
  • MRA (intracranial vessels)

3. Spinal Cord Imaging if Appropriate

• If upper motor neuron signs present (spasticity, hyperreflexia), consider cervical spine MRI to exclude myelopathy 5
• Contrast-enhanced spinal MRI recommended if transverse myelitis suspected (though typically bilateral) 5

Differential Diagnosis Beyond Stroke

Neurological Causes

• Small vessel disease/lacunar stroke: May show minimal changes on standard MRI; look for white matter hyperintensities on FLAIR 4
• Transverse myelitis: Typically causes bilateral symptoms but can be asymmetric; requires contrast-enhanced spinal MRI 5
• Multiple sclerosis: Unlikely with purely motor symptoms for 15 days, but consider if younger patient with relapsing symptoms 6
• CNS vasculitis: Requires catheter angiography if CTA/MRA show vessel irregularities 7

Systemic/Autoimmune Causes

• Systemic lupus erythematosus with CNS involvement: Can cause recurrent strokes despite normal initial imaging; check ANA, anti-DNA, lupus anticoagulant 5, 7
• Antiphospholipid syndrome: Causes arterial thrombosis; check anticardiolipin antibodies, lupus anticoagulant 5

Peripheral Causes

• Brachial plexopathy: Would cause flaccid weakness without upper motor neuron signs; requires EMG/NCS 5
• Peripheral neuropathy: Check for sensory involvement, reflexes; EMG/NCS diagnostic 5

Clinical Examination Clues

Localize the lesion by examining for:

• Upper motor neuron signs (hyperreflexia, spasticity, Babinski sign) → central lesion
• Associated findings suggesting left hemisphere stroke:
  • Aphasia (if dominant hemisphere)
  • Left gaze preference
  • Right facial droop (upper motor neuron pattern)
  • Right homonymous hemianopsia 1

Common Pitfalls to Avoid

1. Accepting normal MRI as excluding stroke: Up to 30% of acute strokes may have negative initial DWI, particularly in posterior circulation 8
2. Delaying vascular imaging: Large vessel occlusion requires urgent identification even at 15 days for secondary prevention
3. Missing spinal cord pathology: Right-sided weakness with sensory level or sphincter dysfunction suggests myelopathy 5
4. Overlooking autoimmune causes: Particularly in younger patients or those with systemic symptoms 7

Management Priorities

While pursuing diagnosis:

• Initiate secondary stroke prevention if vascular etiology suspected:
  • Antiplatelet therapy (aspirin or clopidogrel)
  • Statin therapy
  • Blood pressure control (target <140/90, or <120/80 if high risk) 4
• Arrange urgent neurology consultation
• Consider catheter angiography if noninvasive vascular imaging negative but clinical suspicion remains high 3

The key message: A normal brain MRI does NOT exclude stroke at 15 days, and vascular imaging is mandatory to identify treatable causes and prevent recurrence 2.