Synopsis
Distinguish true loss of strength or sensation from pain, fatigue and impaired motor planning, then map the deficit to central, peripheral, junctional, muscular or functional mechanisms.
- Clarify what 'weak' means: loss of force, fatigability, heaviness, pain-limited effort, stiffness, incoordination, breathlessness, low endurance and impaired initiation describe different physiology.
- Establish onset, maximum severity and trajectory before detailed examination. Sudden focal weakness is vascular until assessed; progression over hours to days raises inflammatory, compressive, toxic, metabolic and neuromuscular emergencies.
- Map function through tasks—buttons, jars, overhead activity, rising, stairs, foot clearance and falls—then test the individual movements that reproduce the reported limitation.
- Upper motor neurone evidence comes from the whole pattern: distribution, drift, tone, reflexes, clonus and plantar response. Acute central lesions may have normal or reduced tone initially.
- Lower motor neurone localisation combines weakness with wasting, fasciculation, reduced tone and depressed reflexes, then uses myotomes, dermatomes and named-nerve territories to separate root, plexus and nerve.
- Fatigable ocular, bulbar or proximal weakness with preserved sensation suggests neuromuscular-junction dysfunction; symmetric proximal weakness with preserved sensation suggests myopathy, but overlap demands specialist testing.
- Test sensory modalities separately. Pinprick and temperature sample small-fibre/spinothalamic function; vibration and joint position sample large-fibre/dorsal-column function; light touch alone is an incomplete screen.
- Move from normal toward the symptom boundary with the patient's eyes closed, compare sides and repeat inconsistent responses. Avoid drawing a precise dermatome from one uncertain pinprick answer.
- Pain and guarding can create apparent weakness. Optimise position and analgesia, test around the painful joint and describe the limitation instead of converting it automatically into an MRC grade.
- Functional weakness or sensory disturbance is diagnosed through positive internal inconsistency interpreted by an experienced clinician, not through normal imaging or a belief that symptoms are voluntary.
- End with a syndrome statement including tempo, symmetry, proximal versus distal distribution, upper versus lower motor neurone signs, sensory modality and territory, cranial involvement and respiratory or autonomic risk.
Key red flags
Sudden unilateral face, arm or leg weakness with language, neglect, gaze or field disturbance is a cerebral vascular syndrome until proven otherwise. Pure motor stroke remains possible without cortical features and early CT can be normal.
Upper motor neurone signs below a level, impaired hand dexterity, gait stiffness, a truncal sensory boundary and bladder dysfunction support cord disease. Segmental wasting or reduced arm reflexes can coexist at the lesion level.
Rapid, usually symmetric ascending weakness with areflexia, sensory symptoms, facial or bulbar involvement and autonomic instability suggests Guillain–Barré syndrome. Reflexes can be preserved in variants, and respiratory decline may occur without dyspnoea.
Variable ptosis, diplopia, nasal speech, chewing fatigue, dysphagia, head drop or proximal weakness that worsens with sustained activity, without sensory loss, supports neuromuscular-junction disease and demands bulbar-respiratory assessment.
Investigation priorities
Identify immediate metabolic mimics or causes and complications of profound weakness.
Management branches
Abrupt face or limb power loss, drift or loss of function, whether persistent, improving or discovered on waking.
- Perform ABCDE, glucose and a concise stroke examination; establish last known well, functional impact, seizure activity, headache, trauma, anticoagulants and pre-event disability.
- Activate urgent stroke imaging and specialist assessment rather than waiting for routine blood results or symptom resolution; include posterior-circulation and dissection clues.
- Follow the stroke team's reperfusion and antithrombotic plan, document swallowing and serial deficit, and investigate mimics only without delaying treatment eligibility decisions.