Differential Diagnoses for Unilateral Weakness and Numbness
Acute ischemic stroke is the most critical diagnosis to consider first when a patient presents with unilateral weakness and numbness, as it is the most common cause of new focal neurological deficits and requires immediate intervention within hours to prevent permanent disability or death 1, 2.
Primary Differential Diagnoses (Organized by Urgency)
Immediate Life-Threatening Causes (Evaluate First)
Cerebrovascular Events:
- Acute ischemic stroke - Most common cause of acute unilateral deficits 1
- Transient ischemic attack (TIA) - Particularly if symptoms are fluctuating or resolved 2
- Cerebral venous sinus thrombosis - Especially in postpartum women or those on oral contraceptives 3
- Spinal cord infarction - Can initially mimic peripheral neuropathy but progresses to hyperreflexia and spasticity 4
Key Clinical Distinction: Stroke presents with unilateral face, arm, and/or leg weakness with or without language disturbance. Patients with these symptoms within 48 hours are at VERY HIGH risk (up to 36% seven-day stroke risk with multiple risk factors) and require immediate emergency department evaluation with brain imaging within 24 hours 2.
Neurologic Causes (Central vs. Peripheral)
Central Nervous System:
- Cortical infarction - Can present as isolated hand weakness with "inverted omega sign" on MRI 5
- Seizure with Todd's paralysis - Identified as common stroke mimic (≈3% of patients treated with fibrinolytics) 1
- Complicated migraine - Another frequent stroke mimic 1
Peripheral Nervous System:
- Guillain-Barré syndrome - Can present atypically with unilateral facial weakness before progressing to bilateral extremity involvement 6, 7
- Nerve compression/entrapment syndromes - When symptoms confined to restricted portion of single extremity 8
- Radiculopathy - Associated with pain more commonly than central causes 9
- Brachial plexus injury - Consider with upper extremity involvement 10
Other Important Causes
Metabolic/Endocrine:
- Dyskalemic myopathy - Rapid onset weakness over hours to days 11
- Thyroid disorders - Check TSH in workup 9
Rheumatologic/Inflammatory:
- Myasthenia gravis - Requires EMG for diagnosis 9
Functional:
- Functional neurological disorder (conversion disorder) - Now a "rule-in" diagnosis requiring strict positive criteria, NOT a diagnosis of exclusion; accounted for stroke mimics in some series 1, 3
Critical Clinical Pitfalls to Avoid
Do not dismiss symptoms as functional without imaging - CVST and spinal cord infarction have been misdiagnosed as conversion disorder with devastating consequences 4, 3
Asymmetric weakness favors neurologic causes over metabolic or rheumatologic conditions 9
Evolving symptoms require repeat imaging - Initial normal CT does not exclude stroke; consider MRI if clinical suspicion remains high 4, 3
Isolated hand weakness can be cortical - Don't assume peripheral etiology without considering cerebrovascular disease in at-risk patients 5
Stroke mimics (seizures, complicated migraines, conversion disorder) comprised 21% of patients treated with tPA in one registry, though no symptomatic hemorrhages occurred 1
Algorithmic Approach to Narrowing the Differential
Step 1: Timing and Tempo
- Acute onset (seconds to minutes) → Stroke/TIA most likely
- Rapid progression (hours to days) → Consider GBS, dyskalemic myopathy, acute inflammatory processes 11
- Subacute (days to weeks) → Compression syndromes, metabolic causes
Step 2: Distribution Pattern
- Face + arm + leg involvement → Cortical stroke 1, 2
- Isolated hand → Consider cortical infarction or peripheral nerve 5
- Dermatomal distribution → Radiculopathy
- Peripheral nerve distribution → Entrapment/compression 8
Step 3: Associated Features
- Language disturbance → Dominant hemisphere stroke 2
- Pain predominant → Neuropathy or radiculopathy more likely 9
- Hyperreflexia/spasticity → Central lesion (stroke, spinal cord) 4
- Areflexia → Peripheral process (GBS, neuropathy) 6, 7
- Respiratory insufficiency risk → GBS, botulism, myasthenia 11
Step 4: Risk Factor Assessment