Leading differential diagnoses

1. Focal aware motor seizure
   • Sudden, brief, stereotyped, rhythmic jerking, tonic posturing, or clonic movements of one leg.
   • Awareness may remain fully intact.
   • May show a Jacksonian march, such as foot to leg to arm spread.
   • Consider epilepsia partialis continua if repetitive focal clonic movements persist for hours or recur almost continuously.
   • Focal seizures can present with focal limb movement while consciousness is preserved. Frameworks for Internal Medicine, focal seizures section, lines 10043-10086.
2. Hemichorea or hemiballismus
   • Chorea: irregular, random, flowing, non-rhythmic movements.
   • Ballismus: large-amplitude, flinging proximal limb movements.
   • Usually reflects dysfunction in the contralateral basal ganglia/subthalamic region.
   • Important causes: ischemic/hemorrhagic lesion, demyelination, autoimmune disease such as SLE/antiphospholipid syndrome, hyperglycemia, infection, and rarely tumor.
   • In a young woman, specifically consider SLE/antiphospholipid-associated chorea, pregnancy-related chorea if relevant, and drug-related causes.
3. Focal dystonia
   • Sustained or intermittent twisting/posturing of the foot, ankle, or leg.
   • May be task-specific or episodic.
   • Can be primary, drug-induced, functional, or seizure-related.
4. Myoclonus
   • Very brief, shock-like jerks.
   • Can be cortical and focal, metabolic, drug-induced, or functional.
   • Review medicines and substance exposure: dopamine-blocking antiemetics/antipsychotics, SSRIs, stimulants, antiepileptic toxicity, recreational drugs.
5. Tic disorder
   • Recurrent, stereotyped movements often with a premonitory urge and partial suppressibility.
   • New isolated unilateral leg tics at age 21 are less typical, so exclude neurological causes first.
6. Functional movement disorder / functional neurological disorder
   • Consider only after careful positive examination findings, such as marked distractibility, variability, entrainment, or incongruity with recognized movement phenomenology.
   • It should not be diagnosed solely because the patient is young, anxious, or has normal strength. Organic and functional movement disorders may coexist.
7. Limb-shaking transient ischemic attack
   • Less likely at age 21 but possible with severe carotid or intracranial stenosis or vasculopathy.
   • Often precipitated by standing, walking, exertion, or hypotension, and may improve on sitting/lying down.
   • Needs urgent vascular assessment if suspected.
8. Less likely but important
   • Spinal segmental myoclonus or clonus, especially if hyperreflexia, spasticity, sensory level, bladder symptoms, or back pain.
   • Peripheral nerve/root irritation generally produces pain, sensory symptoms, fasciculations, or weakness rather than complex involuntary movements.
   • Paroxysmal dyskinesia, often episodic and triggered by movement, exertion, stress, caffeine, or alcohol.

Features that help distinguish them

Immediate evaluation

• Detailed history: onset, duration, rhythmicity, triggers, spread, prior similar episodes, sensory aura, post-event confusion, sleep deprivation, fever, headache, trauma, medications, toxins/substances, menstrual/pregnancy history, autoimmune symptoms.
• Full neurological examination: tone, reflexes, clonus, plantar responses, sensory exam, coordination, gait, cranial nerves.
• Bedside glucose.
• CBC, renal/liver profile, sodium, calcium, magnesium, glucose; pregnancy test; toxicology/drug review as indicated.
• Urgent MRI brain with diffusion, particularly for new focal motor phenomena.
• EEG, ideally early or prolonged/video EEG if events recur.
• Depending on phenotype: ANA, dsDNA, antiphospholipid antibodies, ESR/CRP, thyroid tests, HbA1c, ceruloplasmin/copper if clinically indicated.

Red flags requiring emergency transfer now

• Persistent movement or repeated attacks without recovery
• New headache, vomiting, fever, altered behavior, seizure generalization, or reduced consciousness
• New weakness, numbness, facial asymmetry, aphasia, ataxia, visual symptoms
• Pregnancy/postpartum state
• Recent head injury, anticoagulant use, or suspected intoxication
• Severe hypertension, hypoglycemia, or metabolic abnormality