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| Test | When to order | Yield for specific differentials | |------|---------------|---------------------------------| | Non‑contrast CT head | < 6 h from symptom onset, any acute focal deficit | Excludes SAH, large ICH, gross mass effect | | MRI brain with DWI | < 24 h for suspected stroke, atypical presentations | Detects early ischemia, demyelination | | Lumbar puncture | Normal CT after thunderclap headache, suspicion of infection | SAH (xanthochromia), meningitis (cell count, culture) | | EEG | New onset seizure, altered mental status without structural lesion | Diffuse slowing (encephalopathy), epileptiform spikes | | Nerve conduction studies/EMG | Progressive weakness with sensory loss | Peripheral neuropathy vs motor neuron disease | John Patten Neurological Differential Diagnosis Pdf Free
